Hobie/J24 Trimaran Conversion

Tom/Frosh,

Thanks for your input guys. By the way Tom, I was checking out your web site today, very interesting. This past weekend the J went on a crash diet of about 950lbs after a lot of cutting with speed grinders and a chop saw. In the end I may realize your predicted result or at least in part. I'm commited to seeing the project through though I expect the result wont be optimum. Having said that, and now that you see I,m really stuborn please bare with me. I,m thinking about projecting the mast about a foot or so higher to give a higher sail projection and higher boom clearance. Good or not worth the effort? Also reading forum threads on lifting foils, Moths and others. Here's a ?, would it be feasible (barely Ok) to build a foil or foils to the keel stub fixed or variable pitch to achieve lift, speed and all that stuff. Or possibly a V foil in the front with a T-rudder and or T-daggers. Consider the total weight at approximately 2700lbs with 3 soles. Just looking a options to enhance performance of a non ideal solution.

 

Initial stability and rigging tensions

Along with the genral consenses, I have to relutantly agree.

What is really being done here is the design of a new boat, not, as it appears at first glance, the combination of two old boats. This can create some interesting problems. One of which is rigging stresses.

When the J24's rig was designed it was designed for the very limited initial stability of a keel boat plus a few hundred pounds of moveable ballast (the crew).

This determined everything. It determined the size of the shrouds, the size of the chainplates, The size of the rigging screws, not to mention the amount of reenforcement the sections of the hull that take the chain plates needed.

Now this careful engineering is being thrown out the window. This is because a trimaran can have up to three times the righting moment of a similerly sized mono. So even if the sail plan is not changed, the failure of rigging parts can be confidently predicted.

Early multis were well known for rigging failures as their builders tried to ape the rigging sizes of the monos of comparable sizes they were sailing against. After all, length for length the early multi's were from 1/3rd to 1/2 lighter than the monos.

But the absolute loads were considerably higher and easily overwelmed the safety facters put into the same rig on a mono. Then, of course, there were the shock loads. These accured when the multi was hit with a gust of wind.
The mono, in the same situation just rolled over on its side until the gust passed, shook itself off, then continued on. The multi just stood there and took it (and converted it to additional speed, if the gust lasted long enough).

You can probably frankestein this thing together and it may actually perform reasonably close to your expectations, but as the weather worsens the nasty surprises can too. And the worse part is, that without a careful load analysis of the whole structure, there is no way to predict which part will fail first. Will the mast buckle, the shroud snap, or will the crossbeam fold?

I do, however, have great sympathy for the idea that multis are designed too much for speed and too little for payload. Going very fast as opposed to going sorta fast is not much fun when you have to weigh your provisions with a gram scale.

 

Delane, have it your way, it is your boats and money. It worries me that not only are you going ahead to morph a unhealthy hybrid which will inevitably break ( but not when conditions are smooth, but when you you sailing in horrid wind and water conditions) and put human lives at risk, but you are now talking lifting foils. As far as I know no craft no matter how well designed has ever benefitted from lifting foils when the power to weight ratio is as low as the boat you are creating. You say that you need a sea worthy craft for inter-island passages. Sounds like you have a death wish or you believe that you are right with your concept no matter what others who may have more experience than you might advise.

 

Well, now you guys have me a little worried or sobered up anyway, but I have the keel cut off and there's really no turning back. To address the issue of shock load, I'll up the size of the upper shrouds (or all?) and possibly run shrouds out to the hobbie hulls as well for additional support. I've already replaced the chain plates, thicker, longer and well supported on the front side. Not too worried about the cross beams folding as they are really beefy. Also adding foam blocks to several compartments to insure positive flotation. Also well equipted with VHF, GPS, Epirb, Cell, flares, and all the other usual stuff. When completed, I'll test structural integrity with baby steps close to shore to get a feel for handling, performance or any signs of stress or weekness. I appreciate your concerns for safety and performance but this is something I've wanted to do for some while. The hobie I rescued off of a beach, minus sails, daggers, rudders, boom and some hardware. Just need dagger boards. By the way, I'm retired Air Force (Jet Engines) and have enjoyed sailing here in Okinawa for 17 years. Started with a Hobie 16, J24, Yamaha 36 (sunset cruise business, racing, etc) and now back to a J24. We frequent a small chain of islands about 25 miles away called the Kerama's. Great sailing when we aren't having typhoons.

 

...

Need i say more.

 

Best wishes and a few suggestions

Delane:

Even though I sided with the others on this thread, I did so reluctantly. And I did so exclusively for safety reasons.

But now that you're rally going to do this, let me make a few suggestions:

1.) Find youself an experienced NA or boat designer who has experience with multi's and have him or her calculate the needed hull reenforcements. The few hundred you'll have to pay this person will be more than made up for in peace of mind. (I would offer to do such work myself, but am afraid I am not qualified. I have some training but no experience)

2.) Prepare yourself for a lot of boos and hisses from both the mono and the multi cultures. Everybody hates a half bread. (But mutts are considered by many vetrinarians to be the most healthy and often also the most intelligent dogs around.)

3.) Prepare yourself as well to laugh as you blow past a mono j24 on a broad reach (and as a similer sized multi blows past you). Speed is cool, but it aint everything.

Best of luck.

Bob

 

Bob,

I'm going to take your advice and get a proffesional opinion on reinforcements for the main hull. I found a guy (boat builder/designer), I think Ted Brewer on the web that for 300 bills will assist. I'll draft up some KAD drawings of my interpretation of strength and see how I measure up. One positive side of a through hull spar is the forces generated on either side is distributed to all four points where the spars enter the hull. I'll keep you updated on my progress and post pics when completed. Thanks

Delane

 

Hello Delaine,

How is your convertion going ?
Will be interested to hear results

regards,

 

In Progress

Hello,

Plan to seal up the keel stub this weekend, hang the motor, add all the gear and equivelant weight of spars, amas and support stuff and mark up the water. This will determine how far down I cut the holes for the spars. Initial measurement is 2 inches below the top of the hull lip. Would like to go to 3 inches if the waterline will allow, meaning enough to allow the amas to barely clear the water when the boat is level. And then again it probably wouldn't hurt if the hulls touched the water. After all when loaded, the windward hull should fly around a foot higher. Sound good to you?

 

Interseting thread, as I myslef had the idea too. Sounds as though were trying in a sence to mix oil and water and build a boat.

How did your endevor work out? From the time stamps on these I suspect your sailed her by now.

Dave
Hillsboro, Ohio

 

On Hold

Hi Dave,

Not finished with the project. Found a house with a Million $ veiw and had to do it. Have about a dozen more small projects to complete in and around the house until I resume on the boat. I'll keep you updated.

 

"a trimaran is not really "a monohull with training wheels"

Glad to here this as I just spent a chuck of money on F-22 plans

 

IT IS (a monohull with training wheels) if you buy a monohull and catamaran and join the whole damn thing together as best you can!

 

Progress

The tri conversion is moving along well after many delays doing other things. I want to use lifting foils and have several ideas I would like some feedback on. Configuration 1: Built on a small thin fin/dagger to the keel stub and add a lifting foil as well to the stub. Next add plug in lifting foils that enter the end of the cross bar pipes at the amas and led down the side of the outer hull and under the bottom, then turn inward about 50 degrees for about 3 ft. Configuration 2: Build lifting foil to the bottom of the keel stub and use original (not purchased yet) daggers for the H20 hulls and add a foil to the rudder with a flap to control pitch when needed. Not looking to go totally foil borne, just assist and increase speed a little. Any ideas are welcome.

 

I know just the right person!

Delane, If you have been reading the forums at all over the last few years you will know that there is one un-surpassed expert who regularly posts articles about fitting lifting foils (both fully, and partially). I suggest that you ask Doug how to design them; he is an expert, and he also has close relationships with at least one Naval Architect, so you might get a design from him free of charge. Best of luck!