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  #1  
Old 12-11-2007, 07:08 PM
kmeastman kmeastman is offline
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Canoe Catamaran

I am a Mechanical Engineering student at the University of Nebraska-Lincoln. For my senior design semester project I would like to design a catamaran that is sail powered and uses two canoes as its two hulls. I have never designed a boat so I was hoping that more experienced people could help me out.

The purpose of using canoes is mostly because the cargo/people capacity would be much greater than with most catamarans. I realize that this would make the boat slower than the average catamaran but I am ok with that.

I was thinking along the lines of using 17' aluminum canoes, which usually come with two seats (each canoe), but a third would likely be added. It would likely be built to the maximum trailerable width of 8'6". I was thinking of having only one mainsail and probably adding a captain's seat between the two canoes toward the rear.

I have done some preliminary research and here are the problems I forsee:

1. Since I am going to buy the canoes instead of building them, I can't get very good estimates of factors like wetted area and lateral plane area. I think I could figure them out if I had a canoe to take measurements on and then model it in a 3D CAD program. The problem is that I don't want to buy one and then find out that I need a larger or smaller one to make this work.

2. The major difference between a canoe hull and a catamaran hull is its width at the water level. Since the canoe is much flatter, it will have a large wetted area compared to its lateral plane area. I'm not sure how big of a problem this could be. I could increase the lateral plane area with the use of a center board but I would definitely have more overall wetted area and drag than an average catamaran. Again, I am not overly concerned about the speed I can get out of it, but it will hardly be worth putting a sail on it if its going to be too slow.

3. Catamarans that I have seen have similar L/B ratios to what I am talking about here. However, because a canoe is much wider than a catamaran hull, the center to center distance between the canoes would be much smaller than the center to center distance on a catamaran given the same L/B ratio. Is this a problem?

4. I was thinking that the boom of the sail would be mounted high enough that it would clear the head of the person in the middle seat (much lower than the other two seats) and the boom would not be long enough to reach the person in the back seat. In order for it not to hit the person in the front seat the rotation of the sail would have to be limited. Since the pivot of the sail and the front seat will likely be at about the longitudinal placement, this is about 180 degrees of rotation. From sailing books I have read, you should never need your sail to be rotated more than this but is there any reason why limiting its travel would be a problem?

Any help is appriciated. If you have any advice on these problems or can think of any other possible problems I would love to hear what you think.

Thanks
Kevin
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  #2  
Old 12-12-2007, 12:11 AM
masalai masalai is offline
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Research on polynesian ocean voyaging and Melanesian ocean voyaging and some of the catamaran threads.
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  #3  
Old 12-12-2007, 03:37 AM
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Pericles Pericles is offline
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I concur.

http://pvs.kcc.hawaii.edu/

http://www.pbs.org/wayfinders/polynesian2.html

http://www.janeresture.com/voyaging/main.htm

http://www.moolelo.com/hokulea.html

http://starbulletin.com/2007/03/18/news/story04.html

Masalai,

Two minds with but a single thought.

Pericles
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  #4  
Old 12-12-2007, 04:20 AM
masalai masalai is offline
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Seasons greetings to you Pericles (God of ? remind me. I was endeavouring to maintain a "grumpy old bastard" attitude and make him look for himself. Good exercise for the young wipper-snapper. eh eh eh
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  #5  
Old 12-12-2007, 09:06 AM
Village_Idiot Village_Idiot is offline
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A coupla things I would consider:

1) Canoes generally aren't used for catamaran hulls because they can take on waves/water and list/sink/capsize. If you're going to sail this thing in anything with over foot-high waves, you need to make the hulls shed water. Maybe try the FRP solid (sit-on-top) kayaks that have no interior. However, you will lose your room/capacity that you are trying to gain with the open hulls.

2) You can get around the 8.5' beam limitation by making the hulls movable under their scaffolding - once the boat is launched, move the hulls outboard to gain your stability. Depending on how wide you want the scaffold (deck), you can even go narrower than 8.5' to make trailering easier.

3) For truly innovative engineering design, use flat-transom canoes (that you can put an outboard on if you wanted), then design a configuration where you can swing one of the 17-foot canoes around backward so you can join the two canoes at the transoms and make a nice 34-foot canoe (remember, longer displacement hulls are faster and more efficient). This would give you a l/b ratio of around 11:1-12:1. You can add an outrigger for stability. Properly designed, you can turn the sailing cat into a fast outrigger canoe for when the wind isn't blowing. Maybe you could even design the mast, and/or the deck, to do double-duty as an outrigger. Or maybe just have an efficient fast-sailing canoe with a proper outrigger and sail combination...
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  #6  
Old 12-12-2007, 12:41 PM
kmeastman kmeastman is offline
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Thanks for your suggestions,

I have seen the polynesian sailing canoes that you are talking about, which is what convinced me that my idea would work.

I have checked out some of the other threads on catamarans and have gotten some valuable info from them. I'll check out the specific websites that you reccomended.

As for the canoes taking on water, I do not intend this to go on real rough water. It will probably always be used on inland lakes and probably not on water rougher than you would normally take a single manually powered canoe on. If the water became an issue, do you think you could put some kind of water deflector on the top edge of the canoe that would make splashing waves bounce away from the hull?

As for the trailering, I have already thought about making the boat wider by sliding the hulls out after it gets trailered. I would like to avoid this but I haven't done enough calculations yet to make sure it would be stable without any extra width.

I really like your idea idea about the 2 flat transom canoes. I had anticipated using the double ended ones but one of the whole purposes of this project is to have a very versatile boat. One of the design requirements is that it needs to be dissassembled to portage between two lakes. This way the canoes could still be used individually too. I like this idea of having an extra long canoe as another option. It could be used maunually powered or if I decided to add an outrigger could be sail powered. Also, I didn't really want it to have an outboard motor on it, but if I had the flat transom style canoe I would have that option in the future.
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  #7  
Old 12-12-2007, 08:11 PM
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rwatson rwatson is offline
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James Wharram designs sailing catamarans that are canoes - with a kyak like covered in seat in each hull
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  #8  
Old 12-13-2007, 02:08 PM
Petros Petros is offline
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Sounds like a fun project. How about one really large cargo carrying canoe (better l/w ratio), with a kayak outrigger (or even two kayaks to make a trimaran). The kayaks can be attached to the outrigger booms with quick and simple bungee cord mounts. That way you do not have to unpack the canoe to take one of the kayaks out to scout ahead or find a campsite, etc. You would have more flexibility as to the use of the hulls together or apart too. Under sail the passengers and cargo are all together, where your single rudder is located on the center hull. The sea kayak outriggers could also store cargo was well, or an extra passenger.

It would also be easy and light to attach the outriggers after trailering so you can go much wider which is much better. You can carry more sail this way too. There is less drag with a larger distance between the hulls, so not only can you carry more sail, but lower drag too due to interference effect from closely spaced hulls.

the rotation molded sea kayaks (called "Tupperware" kayaks) are cheap, get them used from craigslist. Or better yet, build skin-on-frame kayaks, quick, light and inexpensive. I've built seven, they have cost me $30 to $50 each to build (using salvaged lumber), even buying all the materials new would still only cost about $100 or so each. Mine weight only about 20 lbs complete (attaching something this light after trailering would be an easy task). The canoe can be built skin-on-frame as well, a lot less expensive and lighter than aluminum.

BTW it is common to use fabric splash covers on river canoes to keep the water out, no reason that could be done on your canoes as well.

Good luck.
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  #9  
Old 02-05-2008, 04:26 PM
kmeastman kmeastman is offline
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Well, the project has been approved and is underway. My group was able to model the canoes and get some measurements like lateral plane area, Longintudinal center of buoyancy, LWL, and wetted area for different weights.

I have done some estimates on D/L and SA/D ratios and they seem within reason for the kind of boat I would like to build.

Most of the reccomendations for these ratios that I have seen are for monohulls. Would they be any different for a catamaran? Is there any books or websites that anyone can reccomend that give calculations or reccomendations that are specific to catamarans?

What I would like some help with is how to determine if the lateral resistance area that I calculated is enough or if I need to add a centerboard.

Also, what calculations can I make with the wetted area?

Thanks for the help,
Kevin
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  #10  
Old 02-05-2008, 06:49 PM
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rwatson rwatson is offline
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"Also, what calculations can I make with the wetted area?"
You can calculate how wet the wetted area is ? :-)

Years ago, an old yacht designer told us kids about how to determine the centre or resistance without any math.
He cut out a profile of the underwater hull from a piece of cardboard, balanced the shape on the edge of a ruler, and where it balanced, marked it as the centre of lateral resistance.

It seemed to work back then. I always wondered if the profile he cut should have been the profile of the heeled yacht (would have had a curved top edge and a shorter keel), but that would not apply to catamarans.

Does anyone else still use this or a similar technique these days?
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  #11  
Old 02-07-2008, 06:32 AM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
There are plastic pontoons made that can be put together to most any length to build your basic boat..

The shape of the weted surface would be better than the canoe , and might even be cheaper.

FF
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  #12  
Old 02-07-2008, 06:53 PM
Petros Petros is offline
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The wetted area is used to calculate the drag. The less the wetted area the better, but in a canoe or monohull the smallest wetted area is a semi-circular cross section, which means no resistance to heeling moments. On a cat this matters little, so many cat hulls are semi-circular in cross section (for least drag) but if you use the hull as a canoe it will simply roll over. You need a flatter bottom so it has some inherent stablity,
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Old 02-07-2008, 07:00 PM
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rwatson rwatson is offline
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There are very few cat hull that are semi circular in cross section! They are either bell shaped or sharp edged.

The sides are flattened to resist leeway, and avoid the necessity of big daggerboards. Certainly the forward part of the hulls are almost knife shaped, for easy entry to waves and wind. Examples, Hobie Cat, Tornado (which has a semi circle for a small section of the stern), Nacra etc

These features to enhance sailing ability will be far bigger considerations than a bit of friction on a small 16ft hull. due to wetted area.
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  #14  
Old 02-07-2008, 09:53 PM
Guest625101138 Guest625101138 is offline
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Kevin
Get hold of a hull surface rendering program called Delftship. You can get a free version of it.

Look at the hulls you are thinking of buying and draw approximate models in Delftship. This should take all of 10 minutes if you are a slow learner. If you are having trouble then post a couple of photos and I will spend the 5 minutes to do them for you.

Delftship will provide you all the hydrostatic information. It will give you crap hydrodynamic data. However you can export the hull into Michlet format and then use Michlet to get accurate drag information - within 5% for a canoe type hull.

With a sailing cat you can design for many cases. The two extremes is where you have both hulls equaly loaded on say a beam reach. The other extreme is where the entire weight is carried on a single hull when you are pushing hard to windward. There are obviously a whole series of load cases in between but looking at resolving the forces under these two extremes will give you a good understanding.

If you want help on the detail after you get started, just post specific questions that are giving you difficulty.

Rick W.
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  #15  
Old 02-08-2008, 02:45 PM
Petros Petros is offline
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Quote:
Originally Posted by rwatson View Post
There are very few cat hull that are semi circular in cross section! They are either bell shaped or sharp edged.

The sides are flattened to resist leeway, and avoid the necessity of big daggerboards. Certainly the forward part of the hulls are almost knife shaped, for easy entry to waves and wind. Examples, Hobie Cat, Tornado (which has a semi circle for a small section of the stern), Nacra etc
For simple (and common) beach cats this is true, the asymmetric hull is to simplify the design by eliminating the dagger board, though this not a very efficient design and you still can not use an asymmetric cat hull as a canoe. These hulls are not optimized for low drag, but to make recreational cat sailing less work and less complicated.

The bell shaped hull of the larger cats is to satisfy a number of other operating conditions such as pitch damping and rough water handling, but the actual wetted surface on smooth water for the bell shaped section is more or less semi-circular. And as you pointed out, sharp bows are again an accommodation to real open water conditions for wave piercing performance. But that does not mean the semi-circular cross section hull is not ideal for reducing drag. Many high performance cats DO have semi-circular hull cross sections as their basis to minimize drag, some do make alterations to this ideal for other considerations but it is not relevant to my statement. Why confuse a simple question?

The question was why do you need wetted area, and it is because it used to calculate hull drag. The less wetted area, the less drag, semi-circular cross sections have the least wetted area, and therefore have the least drag. That does not mean there are not other design conditions you also have to accommodate, it is a simple statement of fact.

And Toranado's do have near semi-circler cross sections for most of the hulls length from the dagger board aft.
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