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Thread: Fins VS props

  1. #1
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    Default Fins VS props

    Props are expensive, they tend to be designed to spin really fast, and there efficiency, although it can be up to about 90% is typically more like 50%. At least for anything I can afford. So for displacement speeds I keep thinking that fins might work out. OK they tend to use a more complex mechanical system but, they wouldn't have to move very fast = less friction. They could be easily DIY.

    i'm thinking something like this
    http://www.youtube.com/watch?v=DtD09...feature=relmfu

    but not a planing hull.

    Its called a swashdrive but thats kinda a misnomer cause there is no swash plate in it.

    On any drive system the business end is going to end up taking a beating eventually. So I keep thinking replacement cost. Fins could be wood, ply or whatever, props I gotta buy, and they're spendy. So the cheaper option is the fin. Chalk up one for fins

    Considering efficiency the typical prop sucks, 50% loss to friction and cavitation is nothing to be all that proud of. OK it is possible to build a prop with about 90% efficiency but its a huge prop, ends up looking more like an airplane than a boat. Fins on the other hand already look like half an airplane prop, and the half part means they only stick down half as far into the water, IE shallower draft. Chalk up two for the fins.

    Mechanicals, The prop wins this one hands down. It spins, couldn't be much easier. Fins on the other hand have to oscillate on two axis, definitely more complex and definitely not as efficient. I'm thinking any mechanical system looses about 1% pr gear/chain/belt connection so a prop with a couple bearings and a reverse gear might have 6 or 7% losses in the mechanicals. The fin drive on the other hand has roughly twice as many, but they move slower, which does seem to make a difference in the longevity of the parts. Props are more efficient mechanically but they have parts that wear faster. Fins would have more parts that move slower. I'm thinking a fin might move at 100 rpm where a prop would spin at 1000 rpm +. I still think the prop wins this one but the slower moving system does have its merits.

    weight, Prop wins this one as well. The fin system is complex and ends up weighing a lot more.

    Anyway just some late night thoughts on drive systems. I'm thinking eventually we'll come up with something better than props and who knows, maybe fins will be it.

    I like the idea that they already have some good fin based stabilization systems out there. Seems kinda reasonable that they eventually develop a good fin drive system as well.

    Cheers
    B

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    Default Re: Fins VS props

    I like the video, but I'm afraid I've only got questions - no answers.

    Isn't a prop just a set of rotating "fins"?

    You point out the low efficiency of props, but what's the efficiency of the fin?

    John

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    Default Re: Fins VS props

    Build it, test it, compare to prop drive in same situation, report back.
    Tom L

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    Default Re: Fins VS props

    Fun. There's a very good commercial one fin pedel powered version for use on Hobie and other kayaks that works a champ. Also, as a recent WBM article on pedel-powered small shanty-boats and other such - home-made of wood prop and readily assembled pedal drive from hardware store materials - shows, slow turning human powered propellers are also perfectly wonderful.

    Each version has advantages and disadvantages. A key element of getting propellsion into the water is blade area. Given what you lose in prop hub, it's easier to get more blade area into shallower water with the swash fin, given how inefficient even three much less four or five bladed props are at low speeds. On the other hand, the props are possibly able to withstand more abuse and deliver continuous power rather than pulsating power. It's all in the choises for a given vessel.

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    Default Re: Fins VS props

    Reverse?

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    Default Re: Fins VS props

    Isn't the Hobie Mirage Drive the most advanced fin-type drive for small displacement boats?


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    Default Re: Fins VS props

    Two blades. I was wrong.

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    Default Re: Fins VS props

    Except for induced twist a fin and a prop are basically the same thing. The longer and narrower a prop as well as the slower it goes, the better right ? So a fin is basically a props best characteristics. Problem is in achieving the motion within the range of the benefits. IE get the mechanical losses down bellow the advantages so when you add it all up you get something better than the basic outboard.

    Oh reverse could be by simply rotating the whole unit, could be easy if you start with a vertical shaft anyway.

    Couldn't sleep last night anyway so I made a few drawings of the mechanism. Probably could use a lot of improvements but its basically a blade on a pivot with a spring, in a box/housing that swings back and forth, so it kinda automatically sets its own angle to the speed and resistance of the motion to the water. Virtical pivot on the leading edge of the upper part of the blade. Couple springs on the trailing edge. That parts in a housing that then has two more pivot points on the lower leading and trailing edge of the blade housing, connector rod to the top and away it goes.

    Not sure if it works but its simple, and it could use the same kinda pillow block throughout, connecting rods and all. The central moving piece would be a crank shaft of some sort just like in a car, actually out of a two cylinder would be perfect as the thing is already set up to move the two arms in tandem

    I'll post some pictures ;-) tonight when I get home.
    Last edited by Boston; 10-09-2012 at 11:21 AM.

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    Default Re: Fins VS props

    We (at USNA) built a 2 person, human powered submarine, using a pair of the Hobie Mirage drive uinits. With it, we won the overall event prize held every 2 years. We set new world records for a non prop driven sub and it wasn't until the last day that a propeller driven sub exceeded our top speed of over 6.2 kts.
    The advantages were (some particular to submarines) were the lack of torque (important for subs), the ability for 2 crew to drive (propel) independently. The lack of other apendages relating to the drive -ie: no shaft, strut, etc. Better interior space utilization. Put us in a non prop category with a more attainable speed record. Drive units placed ahead of the safety buoy release tether line. Primary advantage was that the "stutter step" pedaling motion uses far less room than a rotary pedal motion, allowing the sub to be smaller (less volume of water carried around - these subs are flooded and occupants breathe scuba)
    The Hobie drives worked flawlessly and were surprising in their efficiency.
    When we first considered them, we tied a standard paddled kayak stern to stern to another using the Hobie drive. The Hobie fin drive "towed" the paddler steadily backwards! No contest.
    The linear drive motion seems to be more efficient than the rotary pedalling motion. One drawback to the standard Hobie drive is that it is designed so that the work required is minimal and you could cruise around in one all day. We are working on a longer, more powerful drive and stroke and a more powerful set of fins to match the 1 minute or so burst of energy required to complete the course by some quite fit and powerful Midshipmen.
    The fins are really just reversing propeller plades, taking a half stroke. The rubber readily conforms to an approximation of the proper twist and shape of a prop blade, reversable on each "tack". We are working on improving the blade by molding it's rubber shape over a carbon "armature" that gives some better control over it's twist. The angle of attack of the root of the blade will also be more controlled. There seems to be further development possible in the drive system. The primary advantage over a propeller is the balanced torque energy that provides some efficiency over torque losses from standard props.

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    Default Re: Fins VS props

    I've seen the video of a single Hobie Mirage drive winning a tug of war against a kayak with two paddlers.

    How would a Mirage drive compare head-to-head with a pedal driven propellor unit on matched kayak hulls? Has that test ever been done?

    Brian

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    Default Re: Fins VS props

    Then there is Harry Bryan's unit, which sound more like what the OP has in mind. The drawback with this approach is that the tail wants to wag the dog. (Not to suggest that a double paddle does not.)


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    Default Re: Fins VS props

    A discussion from a few years back. Lots of images, too.

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    Default Re: Fins VS props

    Quote Originally Posted by Brian Palmer View Post
    I've seen the video of a single Hobie Mirage drive winning a tug of war against a kayak with two paddlers.
    How would a Mirage drive compare head-to-head with a pedal driven propellor unit on matched kayak hulls? Has that test ever been done?

    Brian
    I think a proper sized prop would win of course. The Hobie drive advantage lies in it's easy linear pedaling motion (vs a typical pedaling) It's certainly easier to fish from a kayak while pedaling than paddling.
    Who knows? Maybe with more development the mirage drive could be better? With no loss from torque (or balanced torque) it certainly has potential.
    Tom

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    Default Re: Fins VS props

    Boston-

    What size boat are you thinking about using such a unit on?
    “So we beat on, boats against the current, borne back ceaselessly into the past.”

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    Default Re: Fins VS props



    The top drawing is looking at it from overhead, plan view, the lower drawing is looking at one elevation. Whats shown is the mechanism for obtaining the side to side motion. The angle of attack on the fins is set by a spring that will have to be "tuned" to the size of the blades and the optimal cruising speed. But the spring makes it kinda automated, I think ;-)

    This next one shows the the fin the pivot and the spring set up inside the housing



    I gotta go check out how the hobbie thing works. I've seen the fins and the boats but I've no clue how they rigged it to peddle.

    The system I have in mind would be 30 hp max and use fins maybe 4' long. Low rpm system pushing or pulling a 26' Redwing. I want the mechanism to be robust enough to break the fin rather than the fin break the mechanism. The whole thing is just a thought but who knows, I'll play with it for a while and see what comes of it. I like the Redwing though, trailerable but big enough to be comfy for two on an extended trip to the inside passage pnw.
    Last edited by Boston; 10-09-2012 at 08:42 PM.

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    Default Re: Fins VS props

    Just read the peddle powered designs thread, interesting stuff. But I want some power behind it. Looks simple enough though, simple = cheap. Cheap and rocks go together really well. Nice new million dollar prop and rocks on the other hand ?

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    Default Re: Fins VS props

    Other theoretical work has led to the conclusion that a single bladed prop is the most efficient. This, and a desire to remove all unnecessary drag, brought the Hickman Sea Sled design to life. The design is said to have been very successful - the shaft exits the transom, so only one blade of the two-bladed prop is in the water at any time. Plus the bottom of the boat has no appendages to create drag. This is a planing hull, of course, but it might work with a slow-turning prop out the back of a skiff. Don't know if that's ever been tried.




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    Default Re: Fins VS props

    Have you considered paddle-wheels? For displacement speeds they can be reasonably efficient, though not as good as a well engineered prop (that is, one that is spec.ed out for the load, power available, and desired speed). They can be quite robust, are pretty simple to home build, and do have one overiding advantage over just about every other propulsion system; that is that the wheel can actually be shallower than the hull draft. So for riverine cruising on snag infested shallow waters, they can work very well.

    Bob

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    Default Re: Fins VS props

    Don't work in a swell. The png definitely develops a swell from time to time. I love paddle wheels tho, would be perfect on the right design if I was planing on sticking to just rivers, but I'd like to do some coasting and with the box keel option Redwing seems like its ok for my purposes. Not perfect, but in calm weather it should be OK. Although aside from a box keel I've often thought a forward fixed fin, something like where a bulb keel is on the bigger boats, might do the trick to reduce slamming. Rather than a box keel.

    I was just kicking around different options to help with the costs. If I change those housings from square to round on the fin drive and change the connecting rod bracket to something more like a tie rod end, like whats on a cars steering assembly, then I could rotate the whole thing about and eliminate the need for a rudder or reverse gears. Just a clutch and away we go on fin drive.

    Also the fins are apposing so the forces that might "wag the dog" should be eliminated. Unless that is you consider the angle of attack has a slight downward thrust at the apex of each arch. But the frequency would be such that boat wouldn't have time to respond. I think ;-)

    Its all a guess till I try a mock up, assuming that is someone doesn't come up with some reason it wouldn't fly anyway.

    Wox sounds interesting, wet, but interesting, I was hoping I could hide whatever Rube Goldberg might be proud of under the hood so to speak. The Redwing has a nice enclosed engine housing and I'm not sure a surface drive would spray water everywhere. Although it would be conducive to wood props used in say small ultralight airplanes. If any could be found that were cut right for the job, otherwise I'd be looking at something much more complicated to DIY than the fin.

    Damn those things are cheap
    http://www.google.com/url?sa=t&rct=j...E-gikQ&cad=rja

    It would need a trany with reverse but it does seem cheap, simple and efficient. Might just have blown my fin idea out of the water, so to speak ;-)

    Wouldn't the prop blade entering the water bring air/down with it and kinda wreck its performance ? Also all that energy wasted on splashing water around ? I'm fuzzy on how the surface drive at lower speeds, say under 10kn would work out. Any examples ?
    Last edited by Boston; 10-09-2012 at 11:08 PM.

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    Default Re: Fins VS props

    Going in reverse, or rigging the device so it will reorient, is going to be a challenge
    “So we beat on, boats against the current, borne back ceaselessly into the past.”

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    Default Re: Fins VS props

    Not at all Rbgar

    each fin would end up in a round housing, the connecting rod being at the center of the circle when at rest looking at it from the top. A tie rod end off a truck would be perfect as a connecting rod as it swivels a full 360° One pipe inside another is about all it would take. But it does make the whole thing heavier. Weight = death of a dream ;-( Granted it wouldn't work over the full 360, maybe just 15 or 20° off the central axis of the vehicle but still enough to steer the thing and enough to have reverse, no trany

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    Default Re: Fins VS props

    I actually have the plans for the Harry Bryan boat , and have a real interest in the Hobie drive , but I can't help thinking if you want 30hp , then maybe a Variable Pitch Propeller setup might be in order , our Scandinavian friends are very familiar with them.
    You would basically have a big 2 blade slow turning variable pitch propellor , commonly used behind a SABB .
    FWIW .
    Rob J.

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    Default Re: Fins VS props

    I think you missed the part about my being a cheap bastad. Variable speed prop sounds pricey, and like it requires a complex drive unit, also pricey. I'm aiming for cheap, bang for the buck.

    Ideally it would be something that could be spun around for reverse. Its gotta have a cheap business end so when the dope driving grinds the thing into the only rock on the river, he can smile instead of cry.

    i'd also like to be able to have all parts be from common sources, off the rack only. Pillow blocks, all the same standard size, wood props that can be bought by the case, V belt drive and things like that.

    If I was to piece a drive system together, I'd want those pieces to be the easiest pieces, on the wallet and on the mind. It sounds like Hobbie developed a lot of respect for its Mirage system because its simply, key element there about simply. I haven't seen it yet, but rumor has it there linkage is very basic.

    that 30 hp is max, I'm looking at two Yanmar 13.4 hp diesels and thinking that the Redwing 26 would probably laze along just fine on one, with the other one being clutched to the system and available when more power was needed. But at cruise, and for efficiency, I'd think one would be sufficient on smooth water or going downstream. If I remember that engine is rated for 3600 rpm with something in the 2000~2500 range being its best performance window. But the system would want to at least be able to handle 30hp, just in case.

    I was looking at the drive unit off a 30 hp outboard and thinking I could weld it up to a tube and bring the shaft up through that, pivot the tube around and again no need for reverse. But then I'm back to the prop question, basic outboard props are mind bending inefficient.

    I did really like the idea of surface piercing props, but I don't understand how they end up being more efficient when you are bound to have so much splashing and cavitation. Eliminates a ton of stuff and a simple forward reverse trany would be very easy to rig into the shaft/shafts

    maybe a little light reading ;-)
    http://www.well.com/user/pk/SPAprofboat.html
    Last edited by Boston; 10-10-2012 at 12:39 AM.

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    Default Re: Fins VS props

    Quote Originally Posted by Boston View Post
    ... maybe just 15 or 20° off the central axis... enough to have reverse...
    Can't picture that working as a reverse function. Do you envision the fin units being retractable like the Hobie and Bryan units? I'd anticipate them snapping off or that something would bend/break in a strike. Four foot fins are quite a long lever arm.

    Regarding the cost of propellers (which seemed to prompt your thinking along this line), there are tons of them of all kinds (inboard and outboard) on eBay for short money, and even if they are defective you can have a number of spares for the prices there.

    [Don't two (marinized, presumably) Yanmars mitigate against your desire for low cost, especially if one or the other is left idle? http://www.ebay.com/itm/Yanmar-Diese...item53ef1dedf9 ]
    Last edited by rbgarr; 10-10-2012 at 04:50 AM.
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    Default Re: Fins VS props

    At the Norwalk (CT) Seaport Museum there is on display a boat, "Tango", that was pedaled across the Atlantic using a bicycle-type drive train and a propeller that was basically a large model airplane propeller. It was only about 12 inches in diameter.

    Brian

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    Default Re: Fins VS props

    I found a great deal on the Yanmars. Super cheap, new units, and I'd not idle one if not in use, I'd let it sit cold. V belt clutch system most likely but who knows, maybe I'll use a PTO clutch and reverse system. Its cheap and easy and being a farm kid I'm pretty familiar with it.

    The 15 or 20° thing.

    there are three axis of motion involved in the system. One the side to side motion of the fins lets call axis Y. If you drew it out on paper you'd see X as a horizontal line, Y as a vertical line and Z as the line coming out straight at you. In the Y axis you have the side to side motion of the fins, in the Z axis you have the ability to rotate the blades within the housing. So you'd likely not get to far into the rotation before the blades weren't perpendicular enough with the Y axis motion to do much but knife back and forth through the water. If that makes any sense. But once you got them spun enough the pivot and spring/angle of attack set by the blade angle to the water would be reversed. Thing should go backwards about the same as it goes forwards.

    The transom on the Redwings looks like it clears the water, so no underwater drag on the transom, in true sharpie fashion. A slow rotating surface drive might just be the ticket. Oh and yah I looked at the hobby props as well. But the ultra light ones seem a lot more robust. And they could be hidden in the engine housing, sorta. I keep envisioning two counter rotating props that offset and overlap slightly. Both geared to each engine. It would be much simpler than the fins and use off the rack PTO stuff found in just about any descent hardware store. Call it something marine, and triple the price. V belt or PTO clutch later and I'm bound to be saving weight over the fin system. And no custom stuff. The fin drive, would require a lot of custom fabrication, but I'd need a rudder.

    Two of those 40" props would give me about the right depth, boat draws 1'6" and run about $200 each. Case price is likely much less.
    Last edited by Boston; 10-10-2012 at 08:43 AM.

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    Default Re: Fins VS props

    Anyone have anything on surface drives at low RPMs and displacement speeds ? Rumor has it that the ventilating effect is hard to replicate at lower rpm. After a few days of thinking about the various suggestions I'm thinking the most likely thing, since I own a wood shop, is to calculate the proper pitch of the prop I need at the largest diameter practical and just make one, see how it works. I love the fins Idea, but the R&D might be a bit much for the ole wallet. Although it would be fun to build a plywood mockup and see if I can at least get the motion down.

    cheers
    B
    Last edited by Boston; 10-11-2012 at 03:17 AM.

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    Default Re: Fins VS props

    I'm still wondering about the efficiency of prop vs fin. Sure, props can be relatively inefficient if constrained to a small dia turning at high speed*. On the other hand, for how much of its stroke is the fin producing maximum thrust? I suppose there'll be a "dead" time at the end of each stroke when the fin is producing nothing but drag, and a period during mid stroke when it's producing max lift. Apart from eating away at the efficiency I wonder if this pulsing thrust would be acceptable.

    I've never had the chance to try out one of Hobie's drives, but I have tried out a pedal powered prop drive and that was disconcertingly fast. Anybody seen a Hobie vs prop test. Tug of wars are okay, but all I think they tell you is who'd be best at a tug of war.

    John

    * However if you were going to have a 4' fin.........

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    Default Re: Fins VS props

    Agreed, however there are advantages as well. The slower speed for one and the possible length of the fins vs the diameter of the prop. The losses at the hub and the drag of the structural supports of the prop being in the water, vs the fin who's mechanicals are all up in the air. I'd also like to see a side by side comparison of similar hulls. I'm been reading stuff for the past few days and still have no clear answer as to which is likely to achieve higher practical ( not theoretical ) efficiencies.

    The paddle wheel is a good example, they can be very efficient on paper, but on any kinda rough water they loose there advantages in a flash. They require huge maintenance time and they take up a ton of room.There's a reason propulsion evolved the way it did. Fins however are a new player in the propulsion game, Id say there potential has yet to be thoroughly explored.

    Cant sleep again tonight so I'm just playing with ideas of how to best build a mock up out of wood. Not sure it would work at all in the water as the wood would swell and screw up the tolerances of all the moving bits and pieces. Would be fun though, maybe once I have some down time from this window job I'm on now. But thats months down the road.

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    Default Re: Fins VS props

    Losses from the structure and hub in a displacement powerboat? In the scheme of the things (versus power available at the prop) they are truly minimal.
    “So we beat on, boats against the current, borne back ceaselessly into the past.”

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    Default Re: Fins VS props

    I would guess that the efficiency of a surface drive would suffer in rough water as well.
    At high speed,the blades run in a fairly solid smooth wake.
    At slower speeds,yer bumpin'.
    R
    "Now Ron,don't you do anything stupid!" - Grandma B.

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    Default Re: Fins VS props

    yup, after some late night reading I'm no longer thinking surface drives. Apparently a large part of the efficiency comes from the difference between cavitation and ventilation, the later controlling the former, as well as some number of other things that don't apply as well at lower speeds.

    The paddle wheel is out for obvious reasons

    The fins are still a question but the mechanism is much much more complex than in a simply prop set up. Ends up being a compromise whatever I do. Chances are it will boil down to stern drive or the outboard well as drawn. One thing I'm determined to accommodate is my home brewed fuels, which means the use of diesels and hence my Yanmars being applied to the drive system no mater what direction I go. I'd consider a diesel outboard but holly molly have you seen the price on those things ? Unbelievable. Did I mention I'm a cheap bastad yet ? ;-)

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    Default Re: Fins VS props

    There are suitable propellers on ebay all the time for under a hundred dollars...

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    Default Re: Fins VS props

    You know what? For a vessel that doesn't exceed hull speed I bet it would be hard to beat a plain old fashioned paddlewheel!

    I remember as a child seeing the last of the commercial stern wheel tow boats pushing strings of barges on the Ohio River. Being somewhat fascinated by those memories, and with the advent of the Internet making the research easier, I was amazed at how little horsepower was used to turn those wheels and how much work they got done.

    Think about it, you had that big "bucket" - what they called the paddle - being pushed through the water, or more accurately being used to push against the water - and as each bucket started rotating clear of the water there was another one rotating in. Plus at the same instant you had one totally immersed and pushing with full strength.

    Here's a web site devoted to the towboat "Barbara H":

    http://www.hspsi.org/barbara_h/home.html

    The page I linked directly to is of the hull survey. Note the last column devoted to the engine. Note the last repower noted - 1940 with a 160 hp Fairbanks-Morse Diesel. Heck, the boat was built in 1925 with a 60 hp gasoline engine. And this is a boat 99 feet long with a 19'- 6" beam! And it still was intended to push a series of barges for Standard Oil Company!

    Hard to beat, huh?

    For relatively calm hallow water, I think that's the answer.

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    Default Re: Fins VS props

    I don't want to be limited to rivers, PNW inside passage is my target environment, the water is not always calm. So a paddle wheel really isn't an option. Although I do really like the ease of which they can be a very efficient form of power transfer.

    A prop of any significant efficiency looks more like an airplane propeller than the typical short fat boat prop, and most out boards or stern drives wouldn't fit anything to efficient anyway, nor turn it slow enough for things to work out. Even the variable pitch props aren't really all that efficient. I'll continue exploring the fins option but some kinda customized stern drive is beginning to sound more and more conducive to the kinda efficiency I'm after. Although I would like something that might be able to swivel all the way around and so eliminate the need for a reverse gear.

    Really I was just mussing late one sleepless night and was trying to compare fins to props, but apparently there just isn't enough research into fins to really make a good comparison. Although logic says since there are more moving parts in the process of achieving the motion, that it would end up less efficient. But my curiosity lies in if those losses are made up by the low cost of replacement blades and the higher possible efficiency of the fin itself and I'd think less slip than a typical prop. A high aspect ratio fin seems synonymous with the more efficient props at half the draft. I think, who knows, I suppose I'll crank one out of ply and see what I can do with it.

    Cheers
    B

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    Default Re: Fins VS props


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    Default Re: Fins VS props

    Quote Originally Posted by Woxbox View Post
    Then there is Harry Bryan's unit, which sound more like what the OP has in mind. The drawback with this approach is that the tail wants to wag the dog. (Not to suggest that a double paddle does not.)
    ...
    I saw an RC boat with a dual version of than mechanism -two fins side by side driven in opposite sense - no innate. tendency to wag
    Complicated problems usually have simple solutions - which are almost always wrong.

  38. #38
    Join Date
    May 2009
    Posts
    2,712

    Default Re: Fins VS props

    yup, opposing forces all the way.

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