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Thread: atkins seabright skiff hulls and reliable power to speed predictions

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    Default atkins seabright skiff hulls and reliable power to speed predictions

    Hi all,
    i posted a few weeks back on some sail conversions done by Atkins on the seabright skiff hull, but not much information was forthcoming. Boat hulls such as the New Jersey Sea Skiff dont exist on this side of the Atlantic,so it is hard to come up with first hand experience of this type of hull shape and its performance as a power boat,or indeed,under sail.

    Atkins has drawn a 19ft motor sailor, called "surprise" which he states will do 15mph on an 8hp motor.

    The largest 28ft "two brothers",with a 12hp motor is reckoned to be 11mph.

    Neither of these boats can be considered to fall into the normal displacement or LWL speed predictions as is normal for a displacement hull.

    My question is,and im hoping someone out there has experience with a seabright type hull,is if these speeds on these HP predictions can be called reliable. Obviously weight issues etc etc come into play,but i find it hard to believe that 12 hp will push even a semi displacement hull this size at this speed.

    I have read that Atkins was sometimes a little optimistic with his speed calculations. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Put it this way:

    Hullform doesn't matter as much as the combination of weight, length of waterline, and beam. If she's very long for her width, and very light, she's going to scoot along faster than you might think. Atkin had many boats built, and they tend to hit their performance numbers exactly when built EXACTLY to plans. When they are "slightly" modified, they tend to miss their mark because people add too much weight or manipulate the lines for more beam or more overall space, then fill that space with more crap.

    Look at the Rescue Minors out there. 20 feet of boat, 18 mph, 20 hp engines. Many note that they are seeing full speed at less than 3/4 throttle, which means you are using 12-15hp.

    Long light and lean is right!

    E

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    i totally agree with you. However, the design "two brothers" is not a small craft,a LWL of 26ft6in and a beam of 8ft8in. No displacement is recorded,but she has a draft of 1ft9and a half inches. Im not saying its impossible,the hull is rather fine below the waterline,but i would be surprised by such performance. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    just for a bit more interest, she has a WL speed length of 6.9 knots (at 1.34 squre root). 12 horsepower will only plane 600lbs of hull. i have not worked out the displacement, but going through the timber scantlings,this is not a light boat. The question remains.....is a seabright type hull that efficient that it will get 11mph with 12 hp?? If these figures are correct,then i dont understand why boats like the Macgregor 26 "power sailer" with a 50hp motor were a success.....seems to me to be a very inefficient design. Is there any software someone could put this hull through to come up with some specific answers?? Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    For those that may be interested..... i worked the displacement out to be 2.41 tons. This gives a DL of 128. A block coeficient of 0.26 and prismatic coeficient of 0.40.......if i have crunched my numbers correctly.

    At an SLR of 1.55 she will be doing 8 knots and require 10.25hp to do so. Atkins quotes 11mph/or 9.5 knots which i am now guessing is a possibility IF the aft underwater lines do actually provide enough lift to overcome displacement speed.

    She could certainly built a lot lighter,and a Beta 16hp would more than adequate,and economical to run at cruise speeds. I think she will make a fine boat. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    For Two Brothers to manage 9.5 knots on 12HP she will have to be a lot lighter than 5400 pounds, something like less than 4000. Keep in mind these are designed as very simple boats, a D/L of 96 is not impossible given TB has no deck house, four canvas bottom berths and a 260 pound engine. At 5400 pounds she will require about 19-20Hp for 9.5 knots. I don't have the lines for Two Brothers but looking at those of Surprise I would guess she'll be standing on her tail at 22 MPH, 19 knots, a S/L of 4.5! Two Brothers looks to be wider at the transom than Surprise, and so may not trim as much as the smaller boat.

    Your Block Coefficient of .26 seems rather low, I would expect something more like .3-.4, and the Cp of .4 should be something more like .6......
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Thanks for the response Tad. Im not sure i got the numbers bang on,but they were all worked out from the displacement figure i came up with via graph paper and drawing sections. TB has does have a wider stern than surprise, and surprise also has more of a downward curve aft than TB. I looked hard at Surprise, i just dont see 15.5 mph on 8 hp,and agree that at 22mph she must be "interesting".

    Im drawn to the design purely on an economy basis, she is a very basic boat,but with accomodation for 4, and it was the performance im interested in if it was correct. Even if she does require 20hp for 9.5 knots, thats still only one gallon an hour (diesel).

    Im thinking she could be bought down in weight with a ply hull (lapstrake or moulded), as designed, she may lose a few inches of water line,but she is semi planing anyway. I already have a 14hp engine suitable, but would be very dissapointed to find she did need a 20 to perform as stated instead of 12hp as Atkins suggested.

    How easy would it be to test a scale model for HP requirements? Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by skaraborgcraft View Post
    How easy would it be to test a scale model for HP requirements? Cheers
    Single model testing will provide little in the way of accurate required HP data. This is due to troubles scaling speed, wave resistance, skin friction, water viscosity, and surface tension, not mention the boundary layer problem.......This is why we pay big bucks to test tanks for accurate results, which for novel hull forms are not that accurate. Which is why outfits like the US Navy then spend millions on full scale prototypes....which turn out to be failures on occasion.

    On the other hand testing a series of models, changing one model and measuring the effects, or testing one hull against another of known quantity can yield useful info on relative drag and trim. This will not give you accurate HP requirements. You could build two models of the same length, beam waterline, weight, and surface finish. One would have sections matching Two Brothers, the other a standard shallow vee bottom. Tow them at various scale speeds on a yoke (one against the other) or against a spring scale. The main problem is that the models must be very accurately finished, the slightest imperfection in the hull will have an exaggerated effect. Models should be about 3'-4' long.

    Some days I could ring Robb White's neck....ever since he published stories (maddeningly short of detail) of his version of Atkin's Rescue Minor, the Seabright tunnel hull has been imbued with magical powers of minimal resistance. IMO this is not the case. As mentioned by Spook above, the important factors are length, weight, and HP, sectional shape follows from those. If you build a light shallow draft boat, some portions of her bottom will be fairly flat, this aids in providing some lift at speed, reducing resistance.

    I believe that a shallow vee bottom of identical weight, length, beam, and depth as any of Atkin's Seabrights, will be as fast or faster than the Seabright form. But the shallow vee hull will be of deeper draft with prop and rudder hanging below the hull, and she wouldn't have a keel to protect these as that would add drag with increased wetted surface.....

    I would love to see someone test one against the other in an accurate test series....or provide some accurate data on the Seabright form.....
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Tad, IIRC, the articles I have and some of the documentation of the boat noted that Robb's all-up weight was 550lbs with engine. Obviously she's very, very lean, has essentially flat sections aft (as you describe), and only has to force a pirogue shaped piece of deadwood/box keel through the water.

    Did you ever get a look at Shoals Runner, the later, longer iteration of the hullform? She's almost completely flat bottomed amidships, with a small sharp forefoot, a deeper tunnel, and anti-trip chines out back. Definitely adds some fuel to your idea.

    Lastly, what relationship do you give to weight placement in the hullform v. performance? A good example is my little beater 16' aluminum work boat, 7' of beam but only 6' at the chines, deadrise of 6 degrees, LWL around 14.5', and a Yamaha F50TLRC outboard on it, which doesn't get as good of fuel economy as RM was quoted to have by any stretch. Would centralizing the weight to an inboard and moving the helm from the far-forward jet sled position to the middle of the boat aid in regaining some fuel economy? Reason I think of this is some of the RM's built from the Atkin plans never saw the strong side of 12kts, but they had things like the operator aft of the engine, or layout changes that put weight towards the transom.

    Thoughts I ponder over that RM, she just seems too good to be true.

    E

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    I did chat with Pat Atkins via mail, she confirms a few TB designs were built , and even sailed on one ,but cant be specific on the performance data. Tad: thanks for the words re models,pity really.....cheaper to build as a test run. The alternative is to build a full scale model from cheap timber and see what happens....... buying a new diesel doesnt come cheap, and i would be much happier knowing what to expect before i started building. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by Spokaloo View Post
    she just seems too good to be true.
    Don't get me wrong...it's not that I don't believe what Robb wrote.....only that his version was extremely light (a D/L ratio of something like 40-50 with Robb aboard) and thus exhibited amazing performance. I submit that any form built that light will exibit similar performance. But the boat has no deck, no interior fittings, tiny fuel capacity, and a thin skin with no framing. All the RM's built by others seem to have decks, interiors, framing, etc, and weigh considerably more, thus performance is reduced.....no surprise.....

    Absolutely trim plays a huge role in performance, there is an ideal LCG related to a hull's speed through the water (S/L). What the exact effect is in the Seabright form is unknown due to the lack of testing or accurate data.....Some hulls are faster with more trim, others are faster when running flatter, generally I prefer flatter......just looks better to my eye. Shorter and wider hulls tend to trim more than longer narrower forms......

    I have advocated testing with 3/4 size self propelled models of throw-away construction. For Two Brothers this would result in a 21'6" by 6'6" boat, power could be a 5-6 HP outboard......
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Tad, like the idea of a 21 ft model. Im guessing you would scale the weight with the appropriate scale of horsepower and see what happens? I was just thinking of making the hull and adding weight to get to the design waterline. Is running an outboard on this behind the transom going to give a true guide,im guessingl the model will have to be trimmed to counteract the weight aft. So, if this is a 3/4 size model, is it going to weigh 3/4 of the original design when on the lwl??

    I have not seen any other designs with this much room,supposedly with this performance. Economy is the watchword. I could build it lighter,could even flatten the run aft, but then im starting to do stuff and ending up with a different boat altogether......and i dont even want to hear about putting trim tabs on the back...ugh! Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by TR View Post
    Some days I could ring Robb White's neck....ever since he published stories (maddeningly short of detail) of his version of Atkin's Rescue Minor, the Seabright tunnel hull has been imbued with magical powers of minimal resistance. IMO this is not the case. As mentioned by Spook above, the important factors are length, weight, and HP, sectional shape follows from those. If you build a light shallow draft boat, some portions of her bottom will be fairly flat, this aids in providing some lift at speed, reducing resistance.

    I believe that a shallow vee bottom of identical weight, length, beam, and depth as any of Atkin's Seabrights, will be as fast or faster than the Seabright form. But the shallow vee hull will be of deeper draft with prop and rudder hanging below the hull, and she wouldn't have a keel to protect these as that would add drag with increased wetted surface.....

    I would love to see someone test one against the other in an accurate test series....or provide some accurate data on the Seabright form.....
    Tad,

    You insist on injecting some sanity in the RM discussion. I was beginning to suppose that the apparent magical RM performance was due to Dark Energy.

    On the other side of the argument, I have questioned two builders that have built an RM to plans. Both Mark Bayne of Sea Island Boatworks and Timm Schlieff of Schlieff Boatworks do say that they liked the boat a lot. Timm reports 16 to 18 kts at typical 0.5gal/hr with a Yanmar. How does that compute?
    Tom L

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    skarab.......

    The model would be run at the same speed/length ratio as the parent. Two Brothers has a LWL of 26.5', the sq rt of that is 5.15, speed is 9.5 knots, 9.5/5.15 = a S/L of 1.84.

    Our 3/4 scale model is 26.5 * .75 = 19.87' LWL, sq rt of that is 4.45 * 1.84 = 8.2 knots becomes the model speed.

    Ideally one would test at a range of speeds (fixed RPM) and measure precisely the real fuel use (thus HP required) at each speed/length ratio. These can be graphed against the vessel weight to create powering or resistance curves. Commonly these are calibrated in lbs/SL/HP.

    Displacement scales with the cube......(.75 * .75 * .75 = .42), so the 3/4 scale version will have 42% the displacement of the parent.

    As this scale version is smaller, the crew weight becomes significant. I would really like to see the prop in the intended spot aft of the box keel. This would require a engine well with fairing plate. Not impossible. The crew would compensate for the engine weight aft by sitting forward.

    Tom.......

    Unfortunately it doesn't really compute at all.....

    This is the difficulty....inexact or incomplete data....with these boats the numbers are so small that a small difference is interpreted as a huge result. Timm states on his website that the RM weighs 6-700 pounds.....frustrating.....I assume that's without anyone aboard....But with a 15% variation in weight and the same in speed nothing significant can be discerned.....rash statements without hard data to back them only add fuel to the fire.....which I'm trying to avoid.

    Certainly a 900 pound boat, 19' on the waterline, at 16 knots is within the realm of possibility. .5 gph is about 10 diesel hp, but if it's really .6 or .4, the difference is a couple HP either way, so again a 20% possible error......Nice round numbers like .5 are really suspect....not that anyone is intentionally misleading....just imprecise measurements.

    I take it all with a grain of salt until we get precise fuel measurement at precise weight and fixed RPM both ways over a measured mile. Don't want much do I ?
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Interesting numbers here... This thread and the other thread on the other forum about efficent hulls, one advocate being a narrow hull versus basically a conventional planing hull with the beam more in proportion with the length seems to almost prove that narrow does not always rule the day. For sure comfort does require some additional fuel but by comparison its worth it if you look at the comparisons here.

    Over the weekend I did an open water test using exact measuring methods to see what my new 24 foot full cabin hull would do after some rough estimates from about eight different trips. After getting up on plane I can run in almost any water that I may experience if I need to and require if needed anyway which is pretty darn shallow. Anything less there is really no need anyway to be on plane.

    I can at least skate across where I need to for sure and keep going in the most extreme places that I have been to date to check it out.

    The boat and engine weighs 2260 lbs plus three people on board that weighs a combined weight 490 lbs and 105 lbs of assorted gear. In a three foot ground swell on the stern quarter both ways to my distination I ran 15 mph using 2.3 gallons and ran on cruise 4,800 rpm for 1.8 hours total and a tad bit of puttering around for about 15 minutes. The engine is a 40 hp four stroke outboard. How this computes to all the expert figuring I don't know. But by comparison its not much different than what I did in the 16 foot open fishing skiff with the same engine.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Tad, many thanks for the data, fortunately i can test on a lake and so can build a test hull with only a small amount of freeboard. I have a 6hp outboard i can fair into the bottom. Is prop size on this going to make a big difference to the speed and fuel burn figures? are fuel burn figures that accurate to calculate HP used? Will a petrol 2 stroke consume the same amount of fuel as a fourstroke at the same HP ? Seems modern outboards are far more economical.......

    Tom, passed details of Bluejacket on to client, but again fuel burn is too expensive for what he wants...... at least 8 knots on 10hp/ half gallon diesel an hour. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Erster...interesting figures.....looks like your boat uses only just over a gallon an hour.....but if petrol only gives 10hp per hour, then your 40 hp outboard is either very efficient or your boat planes very easily...and hence doesnt need so much power to keep moving.

    Obviously there are some laws in design that can be broken....... Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by skaraborgcraft View Post
    Erster...interesting figures.....looks like your boat uses only just over a gallon an hour.....but if petrol only gives 10hp per hour, then your 40 hp outboard is either very efficient or your boat planes very easily...and hence doesnt need so much power to keep moving.

    Obviously there are some laws in design that can be broken....... Cheers
    Well I know nuttin when it comes to fancy programs that says what things are supposed to do. With the wheel now the engine turns 5,400 rpm on top. Four strokes get their majority of its getup and go on the top end. The new E-Tecs are really the way to go if you want to run in the midrange areas.

    I was a bit surprised early on and during a conversation with a well versed individual concerning fuel burn, we both agreed that we needed more imformation and more imput to determine that infact the numbers were correct.

    That was after probably the second good run and two weekends ago I attempted to get a better handle on the hard numbers running inside in depths around 15 foot tops and in semi exposed waters and go about one gallon an hour.

    This past weekend I had a chance to do an offshore run also wanting to see how the boat handled too with measured fuel amounts. I have also now tweaked the engine angle a bit more so that I can get the back tip of the cavitation plate down a bit and plan on doing another test this coming weekend. I am probably just a bit underpowered in the ocean from what I experienced in running hard downhill but I am using what I have for power.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Tad, after your comment about V bottom , i took another look at some designs by Atkins and found Little Silver 25ft6in X 24ft X 2ft draught and 7ft 7in in the beam. What she does have and needs is a keel that protects the prop . Other than the underwater shape ,which would be an easier build than TB, she is a small boat evreywhere else in comparison, though sheer line heights are very similar to the skiff Lena, and she has a raised sheer to gain a little more room. Station spacings could be stretched by 3 inches to bring the waterline to 26ft6in as TB. Depends on your viewpoint of bastardising a design by Atkins !

    Both these designs would work,but LS would need modifying. I dont have a problem with the box keel on TB as long as it performs at it should.

    After running some fuel calculations (0.06 gall/hp/hr) the difference between running a 12hp engine and a 16 hp engine , over a 2000 hour season is 480 gallons, at the price average of $8 a gallon, thats $3840 a year,every year just to burn those
    extra 4 hp, and is why i would like to know this boat TB will perform on 12 hp and not need 16-20hp.



    Im going to start another thread to see if any other designs can fullfill the specs, but i cant imagine yet that TB is going to get bettered....... for its intended use. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by erster View Post
    its not much different than what I did in the 16 foot open fishing skiff with the same engine.
    Length is the cheapest speed you can buy........

    You didn't mention total fuel load and if that was included in your all up weight.

    But lets say total weight is 2855 pounds. Fuel use from your post is 1.28gph. From this we should be able to derive HP used, but without a fuel map or specific consumption numbers....we can't really do that. For some reason manufacturers are very cagey about publishing fuel consumption data for outboards and 4-strokes in particular. Specific consumption, fuel required to make one horsepower for one hour, varies with RPM. For these outboards it may be quite good at lower RPM....but I don't know. The old rule of thumb was .36 lbs of diesel per HP/hour and .5 pounds of gas per HP/hour. From somewhere I have a number of .46 lbs/HP/hour for a Honda 50, but I don't know at what RPM that is.

    1.28 gallons is 7.92 pounds of gas, divided by .46 is 18 HP. That doesn't add up, it's way off the scale of required HP for that weight, length, and speed. The required is up around 26 HP. Either weight is lower, speed is lower, distance is shorter, specific consumption is lower, or total fuel use is higher, or the cruising portion was shorter and the puttering was longer, or the current was with you, or the swell added speed....lots of variables....and probably a little error in each adds up to the difference.........measured mile and fixed RPM data is good......don't believe all the gps tells you.....
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by skaraborgcraft View Post
    a design by Atkins !

    Please!!!! It is Atkin, William (Billy) and his son John, and now John's widow Pat. There is no "S" in their name.......sorry, nit-picky I know......
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    when i say Atkins design, i mean a a design belonging to Atkin.......a bit like Tads boat. I may have slipped an occosional s in there though......your not the only one to get annoyed at that Tad! Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    This thread shows the initial launch and running of the boat.

    http://forum.woodenboat.com/showthre...boaton-trailer..
    I have been known to be wrong numerous times over but distance and time is how I personally learned to boat and navigate using a compass, watch, dividers and parrallels with a chart and still to this day depend on this as my main equipment with a handheld backup gps. This was also my method to fish bottom fish finding rocks on the bottom learning from some of the older guys. I do not own any other equipment and continue to hand steer the boat with no auto pilot.

    This is the actual weight of the total package without coast guard gear, humans , coolers and food. The boat actualy netted 2160 with the motor and initial gas tank after going back and looking. My puttering consisted of a slow area in an anchorage and moving around so I could be able to dig some clams before returning home.









    In addition this past weekend I added all the gear and three passenger and another one gallon fuel tank which I filled for the run out out of curiosity. I have been using one six gallon tank and filling it up to the cap rim at the filler and then fill it after using the boat, carrying an extra two gallons early on as a safety measure in the other tank.


    The boat floats in six inches of water even though the bow has a nice vee entry that works fine, keeping the boat usuable in some extremely shallow waters.


    If you have any further interest, I can also document the actual running for a measured mile this weekend with a shot of the tack and gps too.
    Last edited by erster; 08-12-2010 at 02:47 PM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Here are Suzuki's fuel consumption figures at RPM for a 40-hp Outboard like Erster's engine. Granted, they come from a biased source, but they are from a test on a much lighter boat. I am enjoying this discussion and thought perhaps they would be of some help.

    Carry On!
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    The boat shown in the article is basically a john boat which shows 9.90 mpg. I can tell you beyond a shadow of a doubt that on my 16 foot open skiff I have never burnt that much fuel and averaged running at 24 mph over the course of probably 1,200 hours on the boat with two engines of the same hp, one two stroke and this four stroke. The two stroke used about double the gas. The last time I had the engine on the suzuki computer it showed just over 420 hours.

    I have owned this engine since 2000 and only carried a six gallon tank just because but have run the same region and use about 2/3 to 3/4 gallon when I run the ocean. Inside is a shade less. I like to plan on the worse case scenerio in fuel use.
    4500 25.80 2.60 9.90
    I know what I used for this boat. I will measure the fuel tank with my fancy dipstick, [wooden measuring tape] and will log the amount of fuel as close as possible.
    Last edited by erster; 08-12-2010 at 03:47 PM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    erster......

    Please don't think I'm being critical......just trying to pin down some data that makes sense.....note that Suzuki claim this engine burns 2.6 gph at 4500 RPM, yet you burned only 2.3 gallons in 1.8 hours? I have no idea what is going on there......2.6 gph is somewhere between 32-36 HP, somewhat more than what we would expect will push your boat at 13 knots.....

    This is why you don't find this data often, it's hard to pin down.

    And thanks Kevin for the Suzuki performance data. Now what I really want is a series of curves showing fuel use and HP produced over a range of RPM's.
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    I'm cool for sure. I know nuttin about numbers and calculations past what I have experienced. This is the boat that the engine came off of which has never burnt the amount of fuel that is showing on the website. We have fished the dog out of it and used it for family outings forever until they all left home. Then mamma and I fished it exclusively while not sailing during the offseason of fishing.
    We use to carry a metal fuel tank, six gallons and it began to rust out so we replaced it with a three gallon plastic fuel tank that I used in my 12 foot skiff until I took the engine off of the boat. We never came close to running out either over the course of the hours. We had a standing joke that the engine manufactured gas. HEHE! The Suzukis are used in hundreds of fiberglass production skiffs too.




    Edited to add that one of the reasons I want to know the worse case scenerio is that I want to know that I do have enough fuel for my intended distance and a fourth more too and thats it for a long day. With the ethanol issues I do not want those big in deck tanks or even carry a lot and have a lot leftover either if I can get around it. Gasoline turns to jelly now and plugs lines and injectors and filters when stored for any period of time and getting stuck from bad gas ruins a good day on the water.
    Last edited by erster; 08-12-2010 at 05:06 PM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by Breakaway View Post
    Here are Suzuki's fuel consumption figures at RPM for a 40-hp Outboard like Erster's engine. Granted, they come from a biased source, but they are from a test on a much lighter boat. I am enjoying this discussion and thought perhaps they would be of some help.
    Quote Originally Posted by Breakaway View Post

    Carry On!
    Kevin
    Tad something to consider here. Lets look at the real numbers on the link.
    I was reading more on the website and even though its a Suzuki generated link and information I find several things interesting when considering what I know about many of the john boats and their uses. These boats run a fair amount in the course of a day of fishing and I cannot understand if the numbers are correct why a six gallon tank is listed at all with that burn rate thats listed.

    But consider something, the speed at 5000 rpm at 29 mph. I am doing 15 and burning what appears to be about half the amount tops [3.2]? plus or minus for the sake of discussion of what the John boat thats listed on the site is doing. Take the numbers thats listed now. The key element is the consumption per mile.

    The ad shows the weight of 525 lbs. for a 16 foot long by 74 inch beam which is 2 inches more than my 16' hull which also weighs about 150 lbs more without the engine too. The weight of the gear is 600 lbs. The engine weighs 243 lbs. The john boat carries a six gallon fuel tank with a fuel burn of 3.2 at 5000 rpm. I have never ever used anymore than four gallons with five people in the boat and boating for eight hours of assorted running and messing about that I can ever remember. So how does the john boat compare to what you came up with today with their numbers?.
    Last edited by erster; 08-12-2010 at 08:46 PM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    wierd science......... sometimes numbers just dont add up. ......in theory ester your boat should be burning much more fuel than you are saying......but will a modern muli-jet injection system produce more HP than an old carb motor for the same fuel burn?

    A bit off track for the reason i started this thread, but usefull information anyway. Anyone else care to comment with experience of running an old outboard to a new one?? ( i think we can establish that 4 strokes are more fuel efficient than 2 strokes,which due to EU laws we cant even buy new anymore) Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by skaraborgcraft View Post
    wierd science......... sometimes numbers just dont add up. ......in theory ester your boat should be burning much more fuel than you are saying......but will a modern muli-jet injection system produce more HP than an old carb motor for the same fuel burn?

    A bit off track for the reason i started this thread, but usefull information anyway. Anyone else care to comment with experience of running an old outboard to a new one?? ( i think we can establish that 4 strokes are more fuel efficient than 2 strokes,which due to EU laws we cant even buy new anymore) Cheers
    Well whether my science and figures are off when comparing to computer programs i have no idea. Using the Suzuki website I am for sure not that far off in fuel burn per mile even pushing a larger hull using two hulls to compare. I have not checked any consumption at speeds that you are also considering either. I could be close to the half gallon an hour and not really know it either.


    I have never considered or given a thought while filling up the "gas can" after multiple daytrips without a worry of running out of what was supposed to be according to some program when running the smaller hull and the same engine. I do understand that smart folks in the boating industry these days depends on certain knowns that helps with liability issues too.

    I initially replied after reading the perimeters and after reading that to achieve them in part may or would require someone to build a very complexed and unorthodoxed design too. You can actually build a conventional hull with a tunnel using your diesel and have comfortable accomadations inside with your perimeters of draft too. This has been successfully achieved in the marine industry in some very expensive hulls but has also worked in simple work boats on the east coast of the states.

    The Deltaville work boats also incorporated a non traditional tucked stern and was built from traditional wood too. Your biggest obstacle for a proportioned and seaworthy hull is keeping the boat trailable with the weight and beam. The outboard is still dollar for dollar the way to go using the single issue of costs by using the new four stroke HT engine .

    Simplicity is second on the lists. Short of that you may need to go back to a proper credentialed designer and pay the freight to either modify or create a bottom and a hull for you.
    Last edited by erster; 08-13-2010 at 07:15 AM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Erster,fair comment. We had pretty much decided that the Two Brothers design was suitable, but it was the prediction of 11mph on 12 hp that started my search for some hard data. Fact is,no one can tell me for sure that this boat will perform as Atkin says.

    The option of sticking a slightly larger engine ,16hp, is possible, but again, if the predictions are correct, then we have a surplus of power we probably wont use,nor be able to put through to the prop (depending on prop chosen) . Again, it comes down to economy.

    There are a few tunnel stern designs available,but this boat really needs a long keel,even if shallow, due to the way she will be used,a tunnel stern seems to require more HP than a conventional set up ,again,requiring a larger engine.

    The outboard is a no go in this situation, too expensive to operate,again,running costs are a primary factor with this boat.

    Thanks for the feedback anyway,i get blinded by numbers myself too sometimes, i understand why a local pettersson type cruiser is economical just by looking at it, but this seabright skiff type hull doesnt really compute in my eyes....looks like a lot of drag in that keel.

    Im not even sure model testing will be too accurate,having read some information by Tom Lathrop, but Tads suggestion of a disposable 21ft model is possible......maybe......owners desicion at end of day. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    More to ponder...

    Installing a bit oversized engine from your point of view is actually a cost savings in the long run if you attempting to create a semi displacement hull. Work boats do this all the time and wheel them accordingly for the intended speeds to benefit from the torgue while pushing that style through the water.

    There are ways to create a tunnel hull with a rigid skeg which also allows water to flow completely without being obstructed.
    While this is a smaller hull you can build your fixed keel and stop short which gives you the benefit of being a bit below your wheel but still allows full water flow the wheel and allow your boat to respond better to the helm at low speeds, something that you give up with full keels on single inboard drivetrains with a stern bearing and shaft.

    You can see from the shot that all of your hardware can be secured in the tunnel. The bronze skeg in the shot is a very dated style but one that is also used off of the worm shoes of full fixed keels on many solid planked wooden working hulls along the coast here.
    This can be incorporated into almost any size hull these days.


    We have also found that converting many of the old working hulls from single screw to twins that handling became an issue at high speeds and downseas. So shaving the keels completely off has been the normal procedure now and improved both speeds and helm response. Rarely does the guys worry about the bottoms and repeated groundings.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Thats interesting information on the tunnel style Erster, thanks. The owner has said he will now accept a larger 16hp engine, but he still wants assurance that TB will cruise at 8 knots.

    Im reminded of the super yacht that was designed by a well known european , and built at a well known superyacht yard in the South west of England that i used to work. This 23 metre motor sailer was designed to do 22knots,a request by the owner and agreed by the architect. In practice it didnt work out too well......after a 4 hour sea trail in which she burnt over 300 gallons of diesel, she was not performing as designed, one very unhappy owner, an unhappy designer and a yard worried about its reputation and litigation......a long story i wont/cant go into it. I dont want to promise this guy that this boat will do something i dont know it will,of course, im sure it will do 8 knots night and day on 20hp, but we need the minimum fuel burn to sustain this speed.......we just dont know how much hp is required....yet. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Well the first thing you need to do is to pin down the basic hull design. I cannot find the TB online either. But like all hulls clients are different. Making demands without first pin pointing the hull and study the general guildlines of that boat, you may need to seek out another client. The basic principle of a small open boat may apply to a larger one but cannot be used as a gauge to determine the best boat.

    Go back to your basic perimeters. A trailable boat from 25 to 30 feet is probably the largest crap shoot of any other length of hull since few one offs exists as a real gauge. Adding to the mix a sail rig, without the rig being just something that will move the boat foward, you may need to determine or even rule out with this guy this one perimeter for sure in the area of boat and hull designs.

    We know also how people around here feel about hardchined sailing vessels too and people that lean towards them too. HEHE! Figuring speeds and even fuel burns on hardchine boats is probably easier and drops no matter the hp of the power plant the rounder the turns of the bilges. Thats just a fact of life too.


    Some of these folks seems to feed on having people do a lot of legwork, in particular the ones that are in a way unrealistic from the beginning. There are only really two knowns in life and neither pertains to boat While we know that we will be throwing money into a rathole with a boat, we do have a choice with a boat, unlike with the other two options.
    Good luck
    Last edited by erster; 08-14-2010 at 05:27 AM.

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Quote Originally Posted by skaraborgcraft View Post
    .....we just dont know how much hp is required....yet.
    Which is why you hire an experienced NA to do a study. If you have a design and a set of scantlings (which your NA will verify as adequate) weight, speed, and power can be estimated to within 10%, and the required HP for 8 knots can be estimated very closely. As speed increases room for error does as well, but especially through the semi-displacement region due to a lack of established scientific data. And there are at least a half-dozen variations on the Seabright form just within the Atkin's work. The NA will error on the conservative side.

    Your large motorsailer project experience was probably an accumulation of small errors......weight creeps up very quickly on big boats, actual engine HP can vary a lot, gearboxes, propellers, rig windage....on and on.....often it gets out of the poor designer's control, then a scapegoat must be found......but the lawyers eat well......

    When you mentioned the coast of Norway I immediately thought of C.G. Pettersson's little double ender, Viking X. Why not a lapstrake ply adaptation of that boat? Done as a NC cut ply kit it would be no more difficult that the Akin to build and far more valuable. Viking X was built about 1924 and is 30' overall, with a beam of 7'1", and draft of 2'7", displacement about 3 tons according to historian John Leather. Power is a 9HP Albin gas engine, cruising speed about 7 knots. In 1925 she made a 3200 NM voyage around Sweden and Norway, which she retraced in 1980. It is reported they burned 1758 litres of gas on the 3200 mile cruise........about 3.8 L/hour at 7 knots......
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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    Viking 1X was recently sold and i missed her by a day.....gutted! An interesting comment Tad as we both discussed a Pettersson type,but the owner dismissed it as being too narrow gutted in the size he wants. C.G.Pettersson drew many great boats,and a few good motorsailors. I believe the client sees Pettersson types as a rich mans toy rather than the honest workboat types he is used to. It will be up for him to decide at the end of the day, but i dont see a CGP in his near future. I certainly agree a replica of Viking would probably sell here in Scandinavia,and a clinker ply version could be done,though everyone up here still prefers solid planking. Theres many designs to look at in the Stockholm Maritime Museum. The client is off on holiday and will be pondering his options......i will let you all know what he decides when/if he makes a desicion. Cheers

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    Default Re: atkins seabright skiff hulls and reliable power to speed predictions

    just to thank everyone who had an input. I heard from the owner today,and he has decided to go for an apartment in the Canary Isles rather than a new boat......so it looks like hes retiring to the south instead. Never mind, i enjoyed the disscussion, and when i learn how to put pictures on here,i will show those that are interested "two brothers". I think she has potential even today. Cheers

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