Question on boat shape

[John Martin]

Hello all,

I have a question regarding bottom shape on a boat designed for cruIsing around 5 knots or so. I have become quite taken with the look of the slipper launches on the Thames River and so has my wife. I am moving soon to a community that has its own several hundred acre lake and a boat like a slipper will allow us to take advantage of the calm waters of that lake and the local waterways. I would like to try my hand at designing the boat myself to see if I can do it. If not then no biggie and I will move on to something else. But the exercise of trying is where the fun lies. I know I can get slipper plans from Selway-Fisher but this is more about the process than anything.

So with that said I am moving to design the hull. Since this is a slow leisurely boat (5-7 knots) I am looking to make the hull with a flat bottom much like many Atkins designed boats. The stem will have a slight rake to it but very little. While I was browsing this years Small Boats issue the ocean pointer caught my eye. One thing that I noticed was that the bottom of the boat was flat from stem to stern the whole way. I made me wonder from a design standpoint would this work for a slipper launch, especially considering the slow operating speeds. Also, with a boat like a slipper and a flat bottom, what sort of draft would I be looking at? My biggest concern is how it sits in the water.

Any ideas on this are appreciated. I am attaching a quick image file showing the overall look of the boat. If this idea won't work that that is fine. It is just a learning process. Thanks for any help.

John

[Thorne]

Welcome to the Forum! There are lots of great books on boat design available from our kind hosts here at Wooden Boat, and the ones covering speedboat design may be closest to what you are looking for.

You'll need to give us a lot more info before anyone can answer questions about draft -- like the dimensions of the hull, materials, weight of boat all up, what type of power, etc.

Here's how to post photos on this forum:

FIRST - Don't attach photos. Only a tiny version will display.

SECOND - Post the photos on the web. Use your own website or a free image hosting service like www.flickr.com, picasaweb.google.com, picturetrail, photobucket, etc.

THIRD - Once posted on the web, right-click the photo to "Copy Image Location", or drag the photo to another browser window, then copy the image URL (web address) which will end in ".jpg". You can test by pasting the photo URL into the location field (http://* ) of a web browser and see if the photo displays. Remember that this process will not work for photos only located on your computer, or uploaded to members-only Yahoo groups.

(In Flickr - You usually have to first click the photo to bring up the black-framed viewer, then click the "View All Sizes" link near the top right. Then you can get the image URL by right-clicking the image. Alternately you can go to the Actions menu on the upper left, then select "View All Sizes".

FOURTH - DO THIS EVERY TIME TO POST IMAGES IN THREADS:
A. In any "Reply" window you can click the "insert image" icon --> a little yellow square icon with a dot at each corner, a tiny tree in the center.

Depending on browser version and Reply/Edit status, this may bring up a simple window with a field to paste the URL into, or the "Add an Image" window described below.

B. If the window titled "Add an Image" comes up, click the "From URL" tab, paste the URL of the photo in the field, deselect the box for "Retrieve remote file and reference locally", then click the "INSERT IMAGE" button. The Forum software will resize some large images, so look at your post to see the actual displayed images.

YouTube video on how to select the image URL in Firefox, navigate the "Add an Image" tabs and paste the image URL -
"http://www.youtube.com/watch?v=AkfB_eDmBRw&feature=player_embedded"

[John Martin]

Well I guess I had that coming. I was thinking I shouldpost some of the particulars about the boat earlier but for some reason I never did. Some of these I am not sure I can answer yet since I have no idea how to calculate them but here goes.

Boat length: 20'-0" LOA
Beam: 6'-0"
Freeboard bow: 24"
Freeboard stern: 4"
All up boat weight: I anticipate about 1200 pounds but not sure how to calculate it
Draft: Anticipated 9" but not sure how to calculate it
Materials: mahogany frames with ply gussets spaced at 16", cold molded construction using 1/4" meranti or similar, mahogany deck and interior
Capacity: 6 persons max
Power: electric motor with straight shaft under front deck, (8) AGM 200AH batteries or so in (2) banks under front deck. Still need to look up weights and sizes

Here is a larger image of what I want to achieve.



Again, I am new at this but it is a fun process so far. I will not have any trouble with framing and such once I get the design of the hull down so that is my focus now. Thanks again for any help of comments. I am very thick skinned and take criticism well

[mike hanyi]

I think if you get your design ready in your eyes, and ask a navel architect of your choice to look at it for you as a consult to make sure you did your calculations correctly, it would cost you a small sum and you can still say it is your design.

I would consider some kind of pod drive system, you loose so much energy thru the shaft system.


Also consider a minnkota trolling motor installed in a rudderpost with a more efficient aftermarket prop.
I have seen it done and it is simple and easy and efficient.

[Woxbox]

If 5 knots it the top speed, the hull will be far more efficient if the water can leave the stern cleanly -- meaning that you want some degree of rocker in the hull, so that the transom just touches the surface of the water. But of course, it's not that simple -- in a small boat trim is affected greatly by how many people pile in and where they sit. And then as soon as the boat starts moving, the trim will change again. This is what the professionals get paid to figure out.

But not to discourage -- the best starting point is too look at lots of boats that do what you want to do, and take note of what they have in common. But don't confuse the planing hulls designed for bigger motors with the displacement hulls that are driven with very little power. They are two very different types of hulls. And to my eye, the efficient but slow boats are far prettier. These include anything that's human powered, the old steam launches, and most sailboat hulls.

[Thorne]

Can you post links or photos of the slipper launch designs? If they all have V-bottoms with keels, I suspect that building one with a significantly different bottom shape will result in a very expensive mistake.

From slipperlaunch.net -



These do not look like slow boats to me!

[aldebaran]

These boats are not planning. I dont think its allowed.

Here you have the drawings of a 18´slipper launch designed to be used with a 4 - 5hp electrical or petrol outboard engine
http://www.selway-fisher.com/McClassic.htm#LEY

[aldebaran]

And here you have a 20 or 22´launch for a 20hp inboard diesel engine

http://www.selway-fisher.com/McClassic.htm#KENNET

[John Martin]

That is the basic look of virtually all slippers I have seen. Very sharp bows with flattened sterns below the water line. Now I have to say that looks can be deceiving. These boats are based of an older faster design but they are NOT used at speed. These boats are used as day cruisers on the Thames and at slow speeds ( I believe the Thames has an 8kmh speed limit above the locks so these are crusing boats to be sure). Check out the link below to see many in action.

http://www.tradboatrally.com/gallery5slipper/index.htm

Many are low powered and many are electric. Slow speeds are what I'm after to be sure and these boats look good and are designed for putting along.

John

[Peerie Maa]

Couple of points. The cut up at the stern is too abrupt, and if you are planning a cut up aft, why not curve the keel line like an American skiff? The first slipper launches were built that way, no rise of floor and a rockered keel in a gentle curve. They did not originally have that twisted bottom with the chine rising above that water, that came in later.

[John Martin]

Nick,

That first boat in you post shows quite well how I think mine will look when finished. I like the fact that the chines are below the water line the whole length. Considering most slippers were 25' to 30' it is hard to shrink that down to a garagable size while maintaining the look so important to them. Max I can go is 22' LOA plus trailer due to depth of my garage. I might have to go ahead and design a vee bottom.

[John Martin]

Here is an updated look for the side view. I think this works a bit better

[Thorne]

The hull design seems to be a deep V-bow twisting to shallow-V, then to slightly concave at the transom. If this is the boat you want, why not build to the plans?

[John Martin]

I just may build to existing plans. This is more an exerise in learning about what does and does not work. Working from published plans offers the assurance (not always) that the design will work as intended. Working as I am is allowing me to learn why things work and why they don't. I will never do this from my own stuff unless I was 110% positive it would work. That is a lot of money to waste on a design that doesn't do its job. BTW, where did you get those pics? They look like they are from the Ariadne build log but not sure.

John

[Thorne]

From the build logs at www.slipperlaunch.net

As I said, there are lots of great books on hull/boat design. Since you want to learn basic design elements, that will be your best route. Many interesting things go on when wood meets water...

[John Martin]

Any recomendation on which books most prefer? 6 pages of books to choose from at the store :-)

[Woxbox]

John -- When you shorten a design, it's not just about losing the elegant shape. You lose a lot of displacement, too. So if a boat that sits on its lines with four adults aboard works at say, 25' length, then at 22' it's going to have be to a deeper boat, which means more rocker or other shape to the bottom keep the flow clean. A really good way to get a feel for this is to learn to use one of the 3D hull programs. I like Delftship because it's free. This allow you to punch in the numbers of the boat you have in mind and quickly get an idea how much it will carry at the designated waterline. You can make subtle changes in the curves and find out how much (often a lot) difference a small change can make. You also get a nice 3-D model of the boat, which you can spend hours fine-tuning if you get carried away with it. Simply going to a V hull can complicate things as much as it solves them. You still want a clean exit, and getting the transition from the midsection to the transom can produce a boat that's harder to build.

The late great Phil Bolger did create one design that was intended to work well both slow and fast -- and have a very simple shape -- the Sneakesy. But it is long and skinny and very light -- the only way he could see accomplishing the job.



But by all means, keep on working the sketches, but make sure the math is good, too.

[Thorne]

We saw one of those on a TSCA row in the California Delta last year - very impressive!

[CWSmith]

I didn't realize until now that a "slipper?" is not just a speed boat moving slowly (for the moment).

What kind of fuel consumption do they get? They look like they might go a long ways on a gallon.

[aldebaran]

I like the idea of long sleek motorboats. Driven at hull speed by an electrical engine. No noise and no annoying waves for the rest of us.

I can imagine that the idea of the slipper is to look cool. Like an old movie. But wouldnt it be a much more efective hull with less eletrical or whatever consumption if the the hull was doubleended?

[Woxbox]

It doesn't have to be double ended to be efficient. It just has to be designed so that the water flow is smooth across the full length of the hull. That can be accomplished with lots of different shapes. If it's a box section hull, the two simplest approaches are to have a point at both ends, or to have the stern sweep up gently to the waterline -- which is what the Slipper does. But there are endless solutions, and those that eliminate the chines alltogether can take the efficiency up another notch or two. It can get this elaborate, for example, and very complex to build. One advantage of the more complex shapes is that they can be less sensitive to how the boat is trimmed. But then again, hard chines will give a light boat much better initial stability and be a bit more secure in a light boat for people to get in and out. There's no best answer to the puzzle, just lots of solutions and you pick the one that looks best to you.

[John Martin]

OMG. I had never heard of the Sneakesy. That boat is simply steller. I love the long narrow look and it can really be decked out to look fabulous. Not real fond of the tall windshield but some mods to that to lower the overall profile would really take on a new look. I was browsing the plans a minute ago and it looks to be a 2-seater. I wonder if 4 could fit? Me, my wife and my 2 kids although most time it would be just 3 of us. I saw a video of it running on a lake and it was moving out nicely. It looks perfect for a small lake or costal river use like the Carolinas. Cost to build would probably be less than $10K overall including power so that is a HUGE plus in my book.

Anyone know of similar designs like this? I was looking at some of the Atkins boats last night and many of them look perfect for a small lake cruiser. His Russel R. caught my wife's eye. While I love the slippers there are a lot more boats out there good for what we want that I was never aware of. Any other recommendations are welcome. The Sneakesy is a bit long for me at 26' and I'd like to keep under 22'.

[mcdenny]

A great feature of "building" your hull in a 3D program like Delftship (same thing as Freeship, I'm a big fan too) is that you can put in a boat like sneakeasy then independently scale x y and z axes and see what happens to design hydrostatics like displacement, CG LCB, etc. It will take a bit of time to learn the software but then you have a terrific tool.

I'd bet sneakeasy would work with x (but not y or z) shrunk to 22' with a little more rocker in the bottom to get the displacement back where you need it. I'm not sure but sneakeasy might have a flat bottom to help planing performance (which you won't be doing with electric power). Adding some rocker to get the load water line right at the transom edge would help displacement performance.

Have a look at my website for a 25' boat similar in performance (but not appearance) to what you are after. There a lot of details about speed, power and range that might be of interest. I did the first one with (8) 6v batteries and a conventional inboard electric drive as you describe. PM me if you want some pics of that drive system.

[John Martin]

Denny,

How do you like that EPIC motor? I have looked at it before but was curious how it performed based on actual experience. I downloaded Delftship and the tutorials and will install it tonight at home. It does not look to awfully complicated to use.

[mcdenny]

The EPIC motor works as advertised. It seems well built, I understand the company was started by a couple of torpedo engineers from Electric Boat. EPIC is now out of business however. I think the combination of the great recession and Torqeedo competition did them in.

[John Martin]

Unfortunate. I had thought of contacting Duffy and seeing if they would sell one of their power rudder systems individually. It would work well in this application too with the added rudder effect

[mcdenny]

Search the interweb and you can find descriptions of home made electric outboards, where people have grafted an electric motor on top of a lower unit from a donor gas outboard. That's kind of what Duffy's power rudder is. I'd love to hear Duffy's price for the drive system. It won't be cheap.

It is hard to beat a Torqeedo if you can afford it.

[Gerarddm]

OMG. The launch Thorne posted in #6.

I.
Want.
One.

< reaches for paper towel to catch the drool>

[John Martin]

OMG. The launch Thorne posted in #6.

I.
Want.
One.

< reaches for paper towel to catch the drool>

Now you see why I like slippers so much :-) They are quintessentially English but very modern as well.

[John Martin]

Search the interweb and you can find descriptions of home made electric outboards, where people have grafted an electric motor on top of a lower unit from a donor gas outboard. That's kind of what Duffy's power rudder is. I'd love to hear Duffy's price for the drive system. It won't be cheap.

It is hard to beat a Torqeedo if you can afford it.

I was watching a video of the Torqueedo and it was quite powerful. I saw one video in which the motor was very loud though but it may have bee the video amplifying the noise. To use something like the Torqueedo I would need to develop a way to hide it in the stern and a slot to raise the motor but height above transom is quite small. Overall it is probably a really good option for the boat.

I am going to spend some time this afternon developing the outlines in Autocad and export them as a JPEG. Tonight I am going to go into Delftship and inport the lines drawing and see what it tells me after I develop the knuckle lines and such. I can probably get the whole boat drawn in a basic configuration this evening if all goes well.

John

[John Martin]

I contacted Elco today to enquire about the cost of one of their systems and was surprised at the cost. I had expected much higher. The EP-600 system which is 36 volts runs $5,985 not including batteries or charger. But if you have never seen an Elco motor it is a work of art. Check out this link for a video showing the motor in action.

http://www.elcomotoryachts.com/elco-motors.shtml

This is pretty reasonable and the crusing time seems spot on. Using 220Ah worth of battery capacity (3 batteries in series) you can cruise for 7-10 hours at 4 knots with a total discharge of about 50% battery capacity. That is really good. While this system is more expensive than a Torqueedo, the benefits are a small packaged inboard that connects to a straight shaft for those applications that use one. Elco uses it on their launch that weighs about 3,500 lbs. I think the added cost is warranted for the type of application I want.

[Thorne]

How quickly can you recharge the batteries from 50->100% after 10 hours of cruising? I'd be concerned that you couldn't get them back to full charge just overnight unless you charge them at so high a rate that they risk damage - but I'm not an electric boat guy, this is just a guess.

[John Martin]

Elco states about 5 hours on their sheets after max crusise to a discharge of 50% capacity. Using AGM batteries you could charge at about 25A safely, and probably higher than that. 25 A is pretty reasonable considering I charge model airplane batteries (Lithium Polymer) at 2-3 times the capacity (5Ah batteries at 10 Amps and recharge them in 30 minutes). Batteries have come a long ways along with smart chargers that makes battery power even more appealing for smaller craft.

[mcdenny]

You could easily recharge a 36v 220ah bank over night with a 110v charger in the $500 range.

John, Start with your boat's weight and waterline length, calculate how much power it takes for 4, 5 knots, whatever. Define the battery bank and calculate range. PM me and I'll send you an excel spreadsheet to do this.

There is nothing magic about the Elco system - they are very short on details but it is most likely a DC brush type shunt wound motor mounted in a housing incorporating a thrust bearing. Their speed and range performance numbers look waaay optimistic, especially since they don't specify what size, weight boat.

Electric Yacht, Solidnav and Propulsion Marine make similar systems. I believe they are about $5k for a 5 or 6 kw system.

[Don Chapin]

There are tons of choices for electric inboards these days. I am currently putting together a 48 volt AC system with a curtis controller for my 26 foot torpedo stern launch. A lot less wires and contactors with the AC systems. Propulsion Marine is an AC system definitely worth looking at. Simplicity, no reversing contactor.

[John Martin]

There are tons of choices for electric inboards these days. I am currently putting together a 48 volt AC system with a curtis controller for my 26 foot torpedo stern launch. A lot less wires and contactors with the AC systems. Propulsion Marine is an AC system definitely worth looking at. Simplicity, no reversing contactor.

Is this a restoration or a newly built boat? I'd love to see pics. I spent a couple hours last nigt browsing the Port Carlington boat site watching video of the boat shows up in the northeast and Canada and some of thos launches are VERY nice. I love the Minett's.

[Woxbox]

The guy in the Elco video says they use AC motors because they're brushless and quieter and more reliable. Does the amateur hobby community concur with that? Would it necessarily be more or less efficient than sticking with the DC feed from the battery bank? I notice they also step up the voltage as the motors get more powerful.

[mcdenny]

AC motors are smaller, simpler and cheaper than brush type and electrically more efficient especially if they use permanent magnets for the field instead of electromagnetic windings, which take battery power to energize. The controller that makes DC into three phase AC and varies speed and direction has been the challenge. Curtis, Sevcon and Kelly all make consumer AC controllers now, probably more but those companies come to mind.

I know AC motors are cheaper, I believe they are more efficient and sometimes they may be quieter. In my personal experience the noise of the brushes scraping on the commutator is undetectable in the real world and the brushes will last probably decades in recreational boat service. DC controllers are cheaper than AC but most require an expensive DPDT contactor to get reverse.

AC motor noise is quite dependent on properly matching the controller to the motor. The controllers can be field programmed to suit the individual setup. IIRC the Sevcon PMAC in my first boat had 50 or 60 parameters that could be adjusted, even with that it took three tries for the factory to get the firmware right to enable reverse while coasting forward.

Both of my 25' launches have had AC motors. I bought all the bits and built the first system. It worked fine but took a long time to get there. The most expensive parts are in the $500 range but the $$ add up fast. The second boat has a "package", an outboard pod that just plugs into its speed control and throttle. Were I to build a third one I'd go for the Torqeedo 4 running on 48v and figure out how to hide the 'powerhead'.

I looked at the Elco website, too. The motors are very heavy for their output but they may be using an industrial gear reducer which could weigh more than the motor. Their small 3kw unit weighs 85 lbs. A MARS 5kw A/C motor weighs 23 lbs.

[Woxbox]

The electric options intrigue me because I have a boat with an old 8 hp two-cycle on it. It still runs, but when it comes time to replace it, the electric would be nice. But here's the issue: the comparable Torqeedo motor is $3,700. The boat is a trimaran, so I don't have the space or the displacement to fit a big bank of lead acid batteries. This Torqeedo requires two Torqeedo lithium batteries, which cost $2,500 each. So now I'm looking at a very sweet system, but the bill is $8,700. Add odds and ends for charging, etc., and I'm over $9,000 for sure. I can fit an 8 hp. Honda 4-stroke for about $3,000, and that, of course, will charge the house battery as well as push the boat.

Can we expect the costs to come down? I would expect that as more lithium batteries come on the market for marine use, they'll get less expensive. But these rigs are so much more than gas, it's really hard to justify. Even if I converted the current motor with an electric powerhead, the battery cost would stop me.

[mcdenny]

You don't need the Torqeedo lithium batteries. They save weight but at a very high cost. They are $2500 for a 44# 2700 watt-hour battery. Top of the line AGM batteries would cost about $600 for the same electrical storage but weigh 160# and take up more volume. You pays your money....

How far do you need to go and at what speed? That is the place to start defining the battery capacity. Note the lead vs lithium weight penalty for a 5400 w-hr bank is the weight of one crewman laying deep in the bilge.

The Honda is a good option and surely less $$ no matter what batteries use use - it is more powerful to boot.

Electric power is a different set of compromises from gas or diesel. E power is relatively low power, less range and more money but has several plusses (no noise, no oily mess, no maintenance, easier to stow an OB aux, no trips to the gas station or gas dock). Your weighting of those + and - determines the best power for you.

[Thorne]

Don't know if anyone else has suggested it, but you may want to joint the Electric Boats forum on Yahoo Groups. Lots of really good information and experience there if you want to go that route.

[Woxbox]

They are $2500 for a 44# 2700 watt-hour battery. Top of the line AGM batteries would cost about $600 for the same electrical storage but weigh 160# and take up more volume.

The Torqueedo motor actually requires double this -- two of their batteries, or 6 AGMs, which I don't have room for. I'm going to watch on the sidelines for a year or two to see who else enters the marketplace and how the prices go. There is big investment in lithium batteries and factories right now, so the prices should drop.