A class hulls for amas

Some comments on your main hull

The hull looks very 'swoopy'. My advice is spend less effort and weight making it an arrow. I would match the bow to the amas for aesthetic reasons and for spray (plan on more features to cut spray after the first sail).

The simple flat bottom is good but plan to radius the bottom edges. Zero radius means zero fluid velocity.

You show some good rocker,the more rocker the better the tacking. Some finer points, you show the static waterline reaching the transom, you need some overhang for squat in fast displacement mode. The rocker also appears to be a radius to the transom -you want the bottom to go flat toward the transom. Curvature=suction which is just what you don't want semi-planing. Do you plan to use the cat rudders or a central one?

The self draining cockpit is nice, but remember the stresses on the hull -the forward end of the leeward beam takes all the load of the craft and the windward end of the aft beam lifts the entire load of the craft. Torsional stiffness is the most common mistake of novices designing main hulls. Sitting in the main hull is the worst case and stress and deflection multiply by the beam. I would raise the floor and put a big X beam below.

 

Hi there!

Basis for my concept are the statements from the initiator for the topic. If someone is happy to buy a 150$ A-Cat it bis clear that this will be a lbp-project (lowest budged possible). Then he wants to have a trimaran because he wants his girlfriend to sail with him. So I think its good to have a cockpit.

The very big questionmark is the generation of the A-Cat, the general condition and of course the wheight of the cat ... it can be spread from 75 kg of an actual boat to 120kg of a really old one made from solid glass ... and everything in between.

Doug: I would estimate the wheight of the 19 1/2' mainhull between 45 to 65 kg. This is based on my experiences building a number of small plywood boats. The 6mm ply is a little "overscantled" but this is due to the very cheap and simple building method. A 4mm ply and glass boat could be a little lighter, but would require more details, stringers, stiffeners, glass sheeting and so on. I tried to avoid that for a more simple, faster and cheaper build.

Beams are aluminium tubes or profiles (from broken - or old masts from abandoned boats) and waterstays, made from 3x12mm stainless steel straps. No idea of custom made carbon tubes and connectives. These are expensive. The beams would be continuos and bolted to the mainhull .... simple and strong.

Skyak:

Thank you for your comments!

What you see here is a lot of concept and just little design. If you want to have a cockpit in a hull so narrow, the "dart wings " looks better to me than the boxy "square wings" of other designs. They have nothing to do with the hulls form. Of course the asthetics of the A-Cat bows will be mirrored in the mainhull ... I think I wrote that.

I love the sharpie bottom and the simplicity/cheapness in the build. On the hydrodramatical side the sharpie hull offers good performance above 2 BFT and acceptable performance below that. The hull is aimend to be a "planniung hull", so there is no ultimate need to have rounded sections underwater.

Starting point is a 150$ (possibly) very old A-Cat. There ist nearly no chance to devellop a high performance package out of that, but it is possible to make a cheap and fair fun performer trimaran for club races. Comparisons to Motive Trimarans and other high end products are silly in this context.

Sorry, but englich is not my native language. What do you mean with "swoopy"?

For the other design details you mentioned: Iam aware of the stresses and torsional loads in a trimaran, able to calculate them and transfer the results into a solid wood structure. I know how a rockerline has to look and it will be definetly different than in the concept scetches shown.

Best Regards, Michel

 

Luckystrike Michel, I like the look and the idea of a day sailor. Is it possible to email you to discuss further. On some other forum sites it is possible to private message but I cant see how to do it here. I looked at this boat the other day as a main hull. It is a 18ft sports boat but it may be to wide at the water line. Not sure why tri hulls are so narrow at the water line but they all are.
Cheers Mike

 

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Thr "traditional" trimaran main hull is usually narrow to reduce wetted surface but mainly to reduce wave making resistance. Length/beam is 10/1 or higher for best performance.
But the Weta 14' tri utilizes a planing main hull and has a L/B of around 6/1. And it sails well but is not a high performance design. Ian Farrier uses main hulls that he describes as planing because they allow more interior room.

PS- You have to have at least 5 posts before you can PM here-if you have more than that contact Jeff, the moderator.

 

Hi Mike, I will send you a pm with my E-Mail adress and it should be possible for you to read it.

Best Regards, Michel

 

Michel,
your english is very good, but not improved by reading my posts. Swoopy is not a word in any language. I made it up and define it as 'having curves because I like curves'. It is important that you like the looks and it will cause people to ask 'who designed her?' To which you can proudly reply 'I did!'.

In my experience with tris, my greatest desire is fast easy setup by one sailor. Farrier's folding mechanism (patent expired) is the best. My tri sits in the driveway because it takes at least two super strong men (or 5 mortals) a day to put in the water or on the trailer.

It sounds like you know what you are doing and can calculate the deflections (more my concern) as well as the stress. I see so many trimarans with slack stays and windward floats pointing down into the water on youtube. The Motive tri is one of them. I would suspect it is less capable than it looks because flat surfaces flex too easily. A little curvature would save a lot of reinforcement and deflection.

About the main hull width -trimarans don't always have enough wind to go fast. Sometimes the main hull needs to displace water. For low wind conditions the narrow 10:1 main hull is important. Heavy cruising tris might even want a round bottom main. Small light tris with tall masts and light loads want flat bottoms to plane at lower speed. And short boats (like the weta) might even go wide to plane sooner and significantly increase load carrying capability. The center hull of a tri has much greater design freedom than an ama. Many people miss this nuance and opportunity for optimization. Done right, fast tris are much easier to sail at high speed than any other configuration.

 

Swoopy is actually a very useful word that has been around a lot longer than you Skyak. Though it is little used and should be better defined, and it once sence it is one of those kind of words that sounds like the visual you should have with it.

The root word "swoop" should give you a clue, though that is also little used, but usually refers to something the quickly and suddenly swings down out of the sky and rescues someone, used to be used for knights in shining armor rescues maidens in distress. they will "swoop them off their feet" to deliver them. Now more often used metaphoricly. So "swoopy" would the the adjective to describe the noun, and in this case visual.

so I would contend that it means the appearance implies motion, or speed. So a fast looking sailboat would be "swoopy". Even if not actually fast, it helps marketing if it a design looks swoopy.

 

Much earlier in this thread, Phil (Catsketcher) wrote about the low buoyancy of the A class hulls. He and Richard Woods in particular warned me about that when I intended to use them for my new tri and I'm glad they did. My solution might not appeal, but by adding asymmetrical foam formers then strip planking in Paulownia. plus glass cloth of course, I got the volume (and strength) up by a factor of 4 with only a modest increase in weight - 50 Kg each versus 35kg.

I got the entire A Class including mast and trailer for only $1700. It might not be cheaper, but certainly a lot less work to use Hobie 18 hulls or similar.

Hope this helps

Alan

 

Resistence is Futile!

Hello!

Ok, the A-Cat has been sold, but Tamas will continue the project. He is now seeking a alternative cat, preferbly with a little more volume. Mystere 5.5 would be my favorite for this project, but it will be difficult to find one "down under".

Skyak:
Ok, I like "swoopy" designs! *smile* Iam not british and to me square boxes as hulls are quite ugly. But with a little styling and flare even a simple built, chined hull can have good looks for my "kraut" eyes.

On folding mechanism (fm):
If you have to trailer regularly you are right, a fm is the right solution. To be honest, I don't like them and prefer the simple arrangement with tube and seastay, but i can store my boat direct at the habour and trailer only a few times.
Farrier's patent is quite old now and Iam not sure if it is still in charge or open to everyone. But for sure the patent belongs only to the double strut method, allowing a folding on water. If you use only one pivoting strut it should be patent free. For a 20 footer this will work out or use another system. The geometrie of the hull (at least geometrie of the pivot points) must be changed for a fm. But I think this is possible to design it into this boat.
I take it as the horsemen do .... don't jump before you reach the obstackle.

Like wharram does it with his tiko designs, there will be a little radius in the panels to stiffen them up, making slight compound curves. Plywood is flexible enough to allow this. (remember tortured ply construction).
To optimise the stiffness of the mainhull lengthwise I still prefer continuos chine- and sheerstringers over the "cove and rove" panel connection. As well as a diagonal stringer starting at the main bulkhead (where the front beam is).

The stiffness in the beams (or the slackness of the pointing down bows of the windward ama as you said) is important, especially the forward beam. But this is also a function of the position of the shrouds and backstays, where you put aditional power to the rig to have a tight forestay. If it is to far aft, you bring up the tail of the ama and the bow down.

With the slim mainhull I agree with you. The mainhull has to be around 10:1 for less wave making, therefore lower resistance in displacement mode.

It is as the Borg said: "resistance is futile".

Michel