Trimaran From a Tornado Cat

Where Randy surely had an advantage was not needing to have another body on board if your getting that extra righting moment from the trimaran configuration. I'd rather take the extra buoyancy than have another body on board.

Fair comment on hull shapes it would be interesting to build a hull with the flat bottomed moth style hull shape as a comparo to see how it performs in comparison the minimum wsa high fineness ratio semicircular float you would predict reduced immersion due to more volume lower in the float. I've wondered lately whether you could build one of each syle of float and sail the boat proa style on one tack before you built the second hull to see how they perform.

 

Doug,

Be true to yourself (did that come from a 70's song?). I don't expect abstinance from a foil guy.

Thanks for the comments, modern developments should be kept in mind or I would just build a Cross 18. I don't really dislike the Cross, it just represents a mindset I am not really interested in. I actually thought about building one just because it would be easy.

It is really interesting that there is a lack of daysailing tri's in the B class range. Lots of "good" boats with a cabin or cuddy. When I first saw the Weta I got realy excited until I realized it was the same length as the first boat I ever sailed and the second - a Sailfish and a Lido 14. Actually the next was a Hobie 14 and I was never interested in going back. SewSew is a little hard core for my fat butt.

Corley,

Thanks for the encouragement. Do you have a picture of the boat you were talking about - where you curved the akas down to meet the amas? I don't think I have ever seen it in all the posts I have seen from you.

It seems like it would be easier to make two different hulls and put them on the same boat. comparison testing just by changing tack.

Marc

 

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The longer the hull you're considering the easier it is to get both low wetted surface and high L/B. The Moth is interesting because they can't do both.
Small tri's under 20' "Type B's" could run into a similar problem with amas.

 

I'm sorry Marc I did not take any photos of the beams I made I felt a bit embarassed about the boat, to be honest it looked a bit of a pig but in retrospect the beams were the nicest part of the thing.

They were quite simply constructed I used WRC strips (12mm) bent over a one sided form similar to a stem lamination form the sides were made of 12mm and I inserted a plywood centre form that was the same height as the sides i filleted bulkheads into the beam at a distance of twice the beams height along its length it was about 200mm high at the centre and tapered to a height of 150mm where it curved to enter the cat hulls. I wrapped the beam in unidirectional eglass to give crossgrain strength. I used a tapered end fitting on the beam that sat into sockets on the cat hulls I made the sockets by removing the cat decks in a small area and inserting two bulkheads and filleting them into position the beams were held at the end with a stainless pin that inserted through the two bulkheads and the end of the beam. The beams were quite overbuilt but not overly heavy I think they were about 15kgs each.

The main hull was a simple draggy dory shape with a lot of rocker that was not much good at anything I built it out of 4mm ply with closely spaced bulkheads and stringers.

I'm a bit perfectionist and dont like having unsuccesful prototypes lying around I ended up sawing the main hull and beams up with a circular saw and sending them out with my hard collection a pity in retrospect. I didnt take failure well back then and had a bad habit of taking my frustration out on what I made.

The closest Ive found to the method I used would be this photo of the beams off the wavelength 780 folding trimaran.

 

Corely,

I sympathise with your dislike of projects that didn't match your desire.
I have a cat rowboat that I have thought of running over with a pickup truck. One motivation is the possibility that I could justify making it again at about 1/3rd the weight. Maybe my wife wouldn't kick my butt on hers.

Doug, I was thinking about the moth and the importance of L/B instead of semicircular (least surface area) hulls. Do you know what the L/B was with a semicircular (or what ever it was) hull? It would be interesting to get an indication of where the Moth transition worked. IE, semicircular L/B at 4? is worse than pointed rectangular at L/B = 6?. There was a discussion on the Peddal Power Boat thread as to wether a 6M boat should have a pointed rectangle or semicircular hulls. The only thing I wanted to do was a tow test of 2 comparable hulls. Opinions without data are interesting for a while. But....

Marc

 

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Marc, I can give you an approximation based on a midship section with an immersed 1sq.' area:
1) Square section:
--a. Beam at wl=1'
--b. L/B for 11' hull=11/1
--c. Wetted surface= 3'( actually the sides and bottom of the square)
==
2) Semi-circular section:
--a. Beam at wl=1.6'
--b. L/B for 11' hull= 6.9/1
--c. Wetted surface=2.51( actually the cicumference of the semi-circle)
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So the approx. reduction of wetted surface using a semi-circular section with the exact same area as a square=(3-2.51=.5 /3=) 16%
And the increase in waterline beam= 60%-not a good tradeoff....... The advantage of knowing the Moth info is that, for its size and displ. it is proven data and not speculation. There are all kinds of tradeoffs that can modify the final result a bit-this is just a rough illustration.

rough illustration-there is some slight blurriness that I can't explain and I don't have time to fool with it now-still readable-click on image:

 

Regardless of whether the comments and questions of mine and others are right or wrong, I do not care for the tone of Marc's responses.

 

Its interesting to note that though my tri-mosquito was a failure it wasnt a total failure. It was actually ok to sail but I think the lesson I took from it was low weight is crucial and buoyancy in the bows of a tri is very important when my boat would start getting going and driving the float down it would develop a bow down attitude my reading of it was that the bows of the floats didnt have enough volume forward of the centreline and the main hull was too fat and had too much rocker which increased the tendency to go nose down if I designed a new main hull now It would be very different. I'm also convinced that for a really good beach tri you want to be around a minimum of 20' waterline.

What I noticed was in short chop the boats leeward ama would bury excessively and slow the boat moving the mast back and trapezing at the rear of the float helped but didn't eliminate the negative alltogether. The curved beams were stiff, stayed well out of the water and coupled with a decent wave piercing float and main hull design would have been a winner.

The positives I took from it were this. A small tri like that is a much more pleasant and less tiring platform for comfortable sailing, the pitch control was much better than an equivalent cat and even with my poorly designed planing style main hull it tacked really quickly and well. The boat had good response to the helm with very little weather helm. In every dynamic sense except speed and the nose down attitude it was a better boat to sail than the cat it was based on. It would have been capable of carrying much more sail and still been manageable as there was a lot of righting moment left over as the boat was 15' wide. From a practical viewpoint it was very easy to manage on the beach for one person with every component being quite light and easy to carry across the beach and assemble. It made a narrow package on the trailer and generated a lot of interest on and off the beach from technically inclined sailors.

 

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Corley, I'm surprised at this. You don't think smaller "beach tris" have any possibilities?
This is a conceptual model(based on extensive numbers) of a 12' tri with an adjustable gantry that I think has a lot of potential. Beam 17.5' overall , 15' cl to cl. This is a high performance tri with 170+ sq.ft.SA.
Ideally, I'll build this after the SRT. But all I can do now is show that the numbers all add up and your statement baffles me.....
click on image:

 

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Alan, if there was any 'tone' to Marcs responses it might be because a couple of people continued to suggest other types and configurations after he said several times what he was looking for. Also, its hard to judge somebody's attitude over the internet-forget it and re-join the discussion.
I'm interested in hearing why you don't think a Tornado hull is a good main hull. I disagree but I haven't heard your rationale yet- I could change my mind if you have a compelling argument.....

 

The 20' waterline is arbitrary but the longitudinal stability of the boat both beach cats and tris seems to improve noticeably around that length. What I noticed with my proto and I'm by no means saying it was a triumph of design was the pitching seemed to steal a lot of power when sailing it wasnt a sudden stop but you would feel the float strike the short chop we have around here and bleed off a lot of speed. The proto used the Mosquito rig so we were definetly underpowered as well.

It could well be that it speaks more about good float design than waterline length but its why I'm not particularly in favour of unmodified donor beach cat hulls in the trimaran float application. Its possible that a fine hull like the A class cat hull we were discussing in the T20 thread would do the job though on a very light boat. Its worth noting that the Mosquito floats were just able to fly the whole boat but were heavily immersed at that point (and you needed a lot of wind). If I sat on the windward float the main hull would never lift out I do weigh 90kg's though.

It also points to the challenges of very small trimaran design everything has to be perfect on the design to see a real advantage it has to be an integrated approach that takes advantage of every facet of the trimaran format. I haven't had enough experience to comment on lifting foils on smaller than 20' boats to comment they probably make all the difference. Also local wind and wave conditions probably effect your perceptions of what is satisfactory as well. Looking forward to seeing how your new proto goes though and the 18' osprey.

 

Corley, thats a very insightful response-I appreciate it. Small trimaran design has a long way to go to catch-up performance wise with beach cats but I'm convinced it will happen-there is so much potential there!

 

Ted Warren has an interesting tube and socket arrangement on his warren 23 trimaran which allows the floats some pitch freedom and claims it glides through short chop. Nice boat too but a bit larger than what Marc wants I'll post the link though for anyone who is interested. He notes a float displacement of 200% so it should be able to fly the main hull.

http://www.warrenmultihulls.com/w-23.htm

 

Looks good and he gives more info than most small tri designers. There is one thing he does not mention: is it designed to fly the main hull?
You know, at least two of the Gougeon Brothers tris(Victor T and Adrenaline,-IIRC) had somewhat flexible ama mounts. They wouldn't work-as best I can tell-with a boat with ama foils.

 

Doug couple of questions:

what's the l/b of a moth and a tornado?

tks.