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Old Nov 07, 2015, 04:07 PM
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Non-foiling trimarans and catamarans

Based on all of the various threads about foiling multihulls, I thought I would step back and begin one about multihulls that DON'T fly or foil. Somewhere out there I feel there are sailors who would like to try a multihull, but have not much interest in foils and one-off boats. Thus this thread.

Foiling Guys - feel free to post if commenting on building techniques, process, design plans, fittings, etc. But keep the foil stuff in their appropriate thread.

Thanks, Dick L.
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Old Nov 07, 2015, 04:19 PM
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To kick this off, back in the late 1990's NACRA had a design they referred to as the "INTER" Class - from 17 foot to 20 feet, their design at the time was cutting edge. The hulls were very narrow, sharp bows for wave cutting action, and the design proved excellent in competition at the time against their older designs, as well as their competitors - namely Hobie, and Prindle.

I started on a project to see if I could build a multihull, with slab balsa sides, but a shaped foam hull bottom and deck. The concept isn't new, and it saves a bunch of time (and work) gluing up balsa strips to get a nice round hull.

Attached is a photo of the Inter NACRA 17 cat, and also a couple of photos of hulls shaped from expanded foam (the pink, blue or gray stuff for house insulation) and later covered with glass along with the hull sides.
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Old Nov 07, 2015, 04:41 PM
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As noted, in my post above, I have elected to call my first effort an "Economaran" of which two identical hulls or two with a larger center hull could be easily built in catamaran or trimaran configuration. You will notice, part way through, I drew some bow shapes similar to the wave piercing bows currently in favor.

No - you won't have sexy round sided hulls, basically the hull is a simple box section, but you can proceed to build a series of templates and glue up strips, or even use foam sheets and bend/glue to the shapes of stations if you prefer. The idea is to give a home builder who doesn't want time, work laying up strips of balsa, who can probably assemble a complete boat in a weekend or two (with sufficient forethought) and have something that will probably out perform most monohulls on the pond or lake.

Basically, one starts with two pieces of thin balsa sheet - 1/8 thick suggested. A couple of pieces are cut and inserted to give you a "pointy" end and a "square" end. The photos are pretty self descriptive. Once you glue on the slab bottom, it's time to shape. Then one must fabricate a daggerboard trunk and fasten into the bottom of the hull. There are some places that I would suggest some extra strength. Where shrouds attach, where cross beams attach, and the underside of the deck for jib boom attachment.
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Old Nov 09, 2015, 06:33 PM
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Did some further work on the Economaran. The hull bottom foam has been added, and I have drawn in the approximate side profile of the hull. There is still foam to add to the top (deck) which may change the profile considerably - especially where and how I mount the crossbeams.. Trying to determine if I like the rounded bows (ala current AC Cup boat designs) or if I like the straighter bow shape (green line) popular with a lot of the A Class cat builders.
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Old Nov 09, 2015, 06:44 PM
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Waauw nice project!

Hello Dick,

Nice project, I sail the rc Robbe Topcat catamaran, I sailed also prindle and now hobie, so I want now to reproduce the boat in polyester with the same shape of a prindle and hobie and personalise the boatsails. Already a picture in photoshop how they need to look like..

Do you have other tips for connection hulls,..?

Thanks to help me out.
Karel
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Old Nov 09, 2015, 07:09 PM
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will help if I can........ some questions:

1) What is the final size of the finished boat?
2) Are you trying to build to a scale size?
3) Do you plan to radio control - or for display only?

The big problem that will take some time, is that both the Hobie and the Prindle have asymmetric shaped hulls - they don't (as you are aware) use daggerboards for leeward resistance - instead the outside of each has a hull that is close to flat that acts like leeward resistance. This means you would have to build a mold (or shape) two hull halves - four in total - in order to get a starboard hull inside and outside, and a port hull inside and outside.

You might be able to fashion a solid "plug" of wood or MDF board and use it to have someone vacuum a styrene sheet over the "plug" - depending on answer to numbers 1 & 2 above.

Again, depending on what you want when they are done, you could actually make the corner connectors of wood for the Hobie 16 trampoline, and then make a plaster of paris mold and cast some alloy to look like the real aluminum trampoline frame. Also depends on which size boat, as the Hobie 16 has the "Banana" shaped hulls while the Hobie 18 is a symmetrical hull, full deck and daggerboards. The Hobie 16 is a "funky" one since the tramp framework also acts to hold both hulls together, while the Hobie 18, Prindle 16 and Prindle 18 all use metal cross beams that are bolted through the deck, or use straps bolted to deck to hold the cross beams. There is a "saddle" in the decks of some, and the round cross tube drops into the saddle. A stainless strap wraps up and over the tube and is bolted to deck in front and back of beam. Some of the beams are actually pieces of aluminum boom shaped stock that were used to keep prices down since the extruded boom "profile" could be cut at whatever length is needed.

Either way, you will have a lot of work making plugs or molds from which to fashion the three parts of each hull (inside of hull, outside of hull and hull deck).

Hope this helps.

Dick

BTW - you may want to look around and see if you can find a Barbie and Ken (doll) sailboat. they were made like the Hobie 16 and I remember my daughter having one. Was about 18 inches long and was big enough to support both dolls when they were hooked to trapeeze wires. Might be expensive now as collector's item.

Another thought - both the Prindle and Hobie 16's, if built to scale or look-alike probably won't have a lot of room inside the hulls in which to fit your sail winch and rudder control servo.
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Last edited by Dick L.; Nov 09, 2015 at 07:13 PM. Reason: Added: Another thought
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Old Nov 09, 2015, 07:33 PM
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@ Karelro -

I found this set of line drawings for the Hobie 16. Maybe if you contact Prindle @ Performance Catamarans they might share the Prindle 16 lines since it is no longer in production. Just be sure to tell them why you want/need them and they may release them to you. I would ask for hull cross sections only, as the side view will become evident if you can get the section template lines. Also, the Prindle has a similar cross section as the Hobie - it just doesn't have anywhere near thee amount of rocker the Hobie has.
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Old Nov 10, 2015, 07:12 AM
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Dick, aren't those crossbeam locations going to be too close to the waterline? Looks like a mild wind chop is going to hit the beams.
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Old Nov 10, 2015, 10:12 AM
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Very possible, Jim.

Actually, the front one was drawn to locate the beam while shaping it so I didn't have to keep looking inside for the location - same with the rear beam. Actually, the foam deck is at a rough 3/4 inch to add on to top of hull profile, and then the deck will have cupped saddles for the beams. Of course if I cheat and build a trimaran by adding a third hull between, I can go with some formed foam and wood cross beams that are arched higher between the hulls.

For right now, I really want to keep this as a catamaran format I think, as the build might be more interesting and easy for the beginning home builder, and much less expensive.

The time frame is really in the shaping of the foam, and in shaping the dagger boards and trunks. An electric planer would greatly speed up the shaping of foam, but I am doing it with some 80 grit sandpaper and a very coarse file that was too dull for horse hoofs. A Belt sander would also help speed up the shaping of foam, and is what my son and I used back when when we were doing his sailboards from foam blanks. Currently, the bottom of the drawn circles are about 3-1/2 inches above the bottom keel of the boat, which is still in rough shape and finish.

Dick
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Old Nov 11, 2015, 03:22 PM
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Spent some time working on shaping of the foam bottom hulls. This shaping may force the project into two (guesstimate) weekends, as it is running a bit slower than I recalled for my other Formula48 (a trimaran).

Below are two side profile photos as foam shaping continues. One is the stern section, the other is the forward section from front beam location forward.
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Old Nov 12, 2015, 04:39 PM
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Had a few minutes so decided to trim the bow into a sharp profile. Still not sure if I will keep it as shown, or if I will cut a radius in the bow's profile - much like the AC cats. Radius is drawn in with pen for reference.

My initial reaction is that I like/prefer the sharp, straight reverse bow.
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Old Nov 12, 2015, 05:07 PM
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cat

I like it as drawn!
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Old Nov 13, 2015, 10:02 AM
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I'm still undecided (more or less) I really like the aesthetics of this one, and upon which I was basing my profile. I will probably go this way - as a bandsaw will do quick work to convert mine over to the curved bow if/when I should decide to change the "look". I don't see any advantage of one over the other.

I also am going to keep a fairly large stern transom. Was going to go small, with a beam taper, but then thought about a possible prevention to a back flip - the wide stern giving more buoyancy.
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Old Nov 16, 2015, 09:30 PM
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Finished rough cut on top of hull profile, and bottom of hull (foam) is about 45% shaped. Maximum beam is located under front beam location and is 3-5/8 inches wide. (approx. 9 cm). Yellow measuring stick is one yard (36 inches) in length to provide idea of final size of hull. Actual finished size will be a Formula48 sized hull (same as Mini 40) measuring 48 inches in overall length. Rudders will be mounted inboard. Transom mounted rudders too easy of target for damage.

I still have deck to shape yet. Add blocks for cross-beams and get all "hard" attachment points for the rigging cut/fabricated and glued in place. Also need to finalize design/type of beam - straight across, or shaped with a riser curve.

Finally, did a quick masking tape mock-up of sail set for this hull. Wing shaped mast is wood/foam/glass and at maximum height. I may wind up cutting length to bring the foot of main and jib down closer to "deck" level. Sail has a large luff pocket so may try sliding it over the wing mast (which will be free to rotate) or use a round carbon tube mast if the wing proves a bit too heavy. Another thought is to use the wing mast as a mold and lay up a hollow carbon or glass shaped mast with some internal beam structures for rigidity.
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Old Nov 17, 2015, 08:18 AM
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It's good looking, Dick!
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