simple catamaran hull design

[Ward]

I've been trying to design a simple-to-build catamaran, and I've come up with this hull design. This is just a sketch to give an idea of what I have in mind. I have never built a catamaran before, so I have no idea if this would be very efficient. The dimensions I have in mind are about 15' in length, and about 18" wide (beam). I would like to use 5mm ply for the entire hull as well. I've tried searching on google, but I really can't find much information on building catamarans, and especially not simple ones. Also, due to the deep-vee design, do you think it would be possible to have no daggerboard/keel and only have rudders? I imagine it would need oversized rudders however, since it probably wouldn't want to turn very well. Any comments/suggestions/information is greatly appreciated.

[Ward]

Might help if I attach pic

[emubo]

good idea - keep things simple

I have just some suggestions:
in my opinion the hull looks like one from a big boat (about 40 feet) - but on a 15' boat you will face other problems.
The weight of your body relatively heavy on such a small craft. the v-shape plus the straigt sheer gives not very much bouancy for and aft. That means you would place yourself carefully on the boat and/or sail area is very small.

Can you tell more about the intended use of the boat, can make things easier to discuss...

[Ward]

Well, I live by a good sized lake, at its deepest part its probably 30 feet deep. Theres more than enough room to cruise for hours, and theres plenty of people sailing larger boats (20 to 30 feet) on it. I'm pretty much just looking to build a nice cruiser that can carry 2 people, but can be sailed by one. I would also like to fish from it, but thats not really going to effect the boat design very much. I'd prefer it to be as fast or a little faster than my friend's canoe with the 2 of us rowing. Is 10-15 knots unreasonable? I would also like to have something that can be disassembled easily and either car-topped or truck-bedded. My firebird doesn't have a trailer hitch and I dont plan to add one because it would just look like crap I can however borrow my dads pickup (also no hitch on it) for launching. Im open to all ideas, I am by no means set on this hull design. My ultimate goal is to simply have a working design, whether its of my own creation or not.

[icetreader]

a cross section shaped more like a U than a V. It will reduce the wetted surface area, and consequently the drag.

[Ssharpsjc]

shape is very important in hull design. The pic I saw posted would be easy to build but extremely difficult to turn and would have a terrible draft.

For a simple hull construction in wood that is very inexpensive go to Kurt Hughes website at www.multihulldesigns.com and look at his cylinder molded process. This is not his exactly because I made a Tornado with this method in the early 70's from instructions I received in the mail form someone. This was a typical method for a quick construction of simple hull designs. I own a multi built this way and if I was going to build in wood, I'd use this method. Very inexpensive and very fast. A 30 foot hull will take a few days.

[emcmia]

I have a P43 Privilege, 25 ft beam, 26000 lbs displacement with fixed keels. Each keel is approximately 14 ft in length and extend about 24 inches below the rounded hull form resulting in a total draft of 48 inches. The keels are essentially flat slabs with virtually no foil shape and average about 6 inches wide, flared into hull and tapered at the bottom to about 4 inch width.

Underway, either under power or sail, especially in shallow water and hull speed over 8 knots, the boat tends to squat, progressively until the bow raises and the water line at the transom (i.e. swim platforms) is submerges as much as 12 inches.

I have extended the swim platforms 48 inches which has resulted in adding at least 2 knots to the hull speed. However, the squatting effect has not changed.

At the next haulout, I am considering changing the shape of the keels to a foil shape, similar to a conventional aircraft wing but symetrical on both side and tapering the trailing edges to less than 1 inch. The objective is reduce the negative pressure that I believe is occurring aft of the keels due to the turbulence created by the existing keel shape. That being said, I am concerned by the prospects of increasing the drag due to the increased width that will be inherent with new foil shape.

Your comments and suggestions would be seriously appreciated.

Thanx, in advance.

[gonzo]

James Wharram desings cats that have a section and don't have keels or centerboards. I had a 34' and it sailed OK. Of course it doesn't point like a 12M. I think that you need less rocker and more freeboard.

[betelgeuserdude]

Here's one simple catamaran design that sounds similar. It's supposed to be a pretty decent little boat.

http://woodsdesignssailingcatamarans...attrobuild.htm
Building a Quattro 16

http://www.sailingcatamarans.com/quattro16.htm
Quattro 16

DC

[sblevins]

Plans are available to build the Taipan 4.9 from primarily 4mm ply. For its length it has very broad performance characteristics. A number have been homebuilt and the builders have posted their suggestions. Phillip Brander (Aus) has a site with extensive descriptions and pictures.

You may well find that it is cheaper to buy a production catamaran. Hobie 14s (only for sailing solo) and Hobie 16s (for sailing solo or double) can be had for a couple hundred dollars, usually with a trailer (put a hitch on your dad's pickup). Check out E-Bay and BoatTrader.

I have the Quattro 14 plans and they are very thorough. I bought them, not to build the boat, as a general "how to design and build a catamaran" guide. I'm currently designing a Formula 14 racing catamaran to be built in plywood. It will have canted hulls and dagger boards and hopefully blow every other 14' design (and some 16' designs) away.

The first thing I learned is that 3mm is fine for the hulls. You'll use thicker on the deck and frames to strenghten the hulls. The lighter the boat, the better. Lighter boats will float higher and sail faster.
The second thing I learned is that a rounded or elliptical hull shape is most efficient. A rounded-V, however, could be easier to build and is almost as efficient as the famous rounded U-shape. The V-shape is the worst hull design to date, giving the least amount of bouyancy and the most wetted surface.
The third thing I learned is that 10" wide hulls would be better than 12" wide hulls for this racing boat. A 16' boat more concerned with cruising and carrying a load would do better with 12" wide hulls, but 18" is far too wide and will really slow the boat down. Catamarans need to have at least an 11:1 fineness ratio (11 units length per 1 unit width). The finer the hulls, the faster the boat, and the less weight it can support.
I've more-or-less figured out the rig, and now I'm working on hull-rocker (the curvature of the keel is important to the maneuverability of the boat), and cross-sections next. Then I'll be able to start building balsa-wood scale models.

[sharpii2]

Hi Ward

A long time ago, I considered the idea of a dory like hull form for a catamaran. The advantage I saw was that it would be easy to construct and it would have less wetted area than a "V". Also (it was hoped) that the flat bottoms, which would be half the beam of the hulls, would get up and plane under really blowing conditions.
Rather than bother with dagger boards, I intended to use long keels that were to be half the length of the hulls. These were to be slab sided and only tapered at the ends.
What I had in mind was not a particularily fast ship. She was to be aprox. 15ft long and have about 100sft of sail. Her hulls were to have dry storage compartments too. Eight to ten knots was her goal speed. She might have been able to do better off the wind. I planned on a low aspect ratio rig which would be simpler, cheaper, and less hassle to set up. (I don't know how they manage with those 28ft hobie masts.) I was thinking more in the area of 18ft.
A plain plywood box section which would be held up with a trystay arrangement. NO JIB. I hate jibs and usually sail without them. Besides, there is room for doubt how well one would stand with such a simple stay arrangement.
Of course, any other catamaran on the lake would blow my design away in a race (but maybe not if it were asked to cary the same payload). But my design would be twice as fast as the same size mono (with the same level of crudeness) and would be able to be so, placedly.
I applaud the idea of using the multihull concept for something other than pure speed.

Why follow the crowd.

Bob

[BIG MAC]

hobie 16's have no dagger boards or keel. they use asymetrical hulls. they are light and fast and could be made drier if desired. they are a dime a dozen but watch for delamination at the forward pylons. much stress here and some fail. if you just want to build your own, look at the hobie 16 as a guide. you could easily stitch and glue the hulls and hobie sells (or at least used to) templates for the hulls for guides in re-fairing. if you build it and cruise rather than race, give the hulls a few inches more freeboard. the 16 is a wet boat.

[brian eiland]

Originally Posted by emcmia

I have a P43 Privilege, 25 ft beam, 26000 lbs displacement with fixed keels. Each keel is approximately 14 ft in length and extend about 24 inches below the rounded hull form resulting in a total draft of 48 inches. The keels are essentially flat slabs with virtually no foil shape and average about 6 inches wide, flared into hull and tapered at the bottom to about 4 inch width.

Underway, either under power or sail, especially in shallow water and hull speed over 8 knots, the boat tends to squat, progressively until the bow raises and the water line at the transom (i.e. swim platforms) is submerges as much as 12 inches.

I have extended the swim platforms 48 inches which has resulted in adding at least 2 knots to the hull speed. However, the squatting effect has not changed.

At the next haulout, I am considering changing the shape of the keels to a foil shape, similar to a conventional aircraft wing but symetrical on both side and tapering the trailing edges to less than 1 inch. The objective is reduce the negative pressure that I believe is occurring aft of the keels due to the turbulence created by the existing keel shape. That being said, I am concerned by the prospects of increasing the drag due to the increased width that will be inherent with new foil shape.

Your comments and suggestions would be seriously appreciated.

Your squatting problem is not a result of the keels, and changing their foil shape is not going to help with that problem. I am going to introduce a discussion of trim foils to address these problems, and likely some of the solutions will be retro adaptable. But I'm affraid it will have to wait until my return from a trip to Thailand.

[brian eiland]

Originally Posted by BIG MAC

hobie 16's have no dagger boards or keel. they use asymetrical hulls. they are light and fast and could be made drier if desired. they are a dime a dozen but watch for delamination at the forward pylons. much stress here and some fail. if you just want to build your own, look at the hobie 16 as a guide. you could easily stitch and glue the hulls and hobie sells (or at least used to) templates for the hulls for guides in re-fairing. if you build it and cruise rather than race, give the hulls a few inches more freeboard. the 16 is a wet boat.

Take a look for Prindle hulls, PARTICULARLY the asymetrical 18 model. Their asymetric design and volume distribution (and no pylons) make them infinitely superior. I know from many years in that business.

I have one brand new available (left over from my old rental operations) if you're interested. That one is in Fla. And I know where to find some others in MD.

beiland@usa.net