856-678-2186 - The online community for enthusiasts of trailerable (and cartopable) trimarans moreinfo@smalltrimarans.com

I received the following info and pics from sailor David Kagan. He has taken a narrow small Hobie cat hull and then added outriggers to it.

Will it work?

David promised to share how his sea trials go with us. I can’t help but admire his boatbuilding skills. This unique craft sure looks nice and is sure to draw attention at the boat ramp!

I’ll let David take it from here.


Unique Small Quadmaran Concept
By David Kagan

I’m just about done building a trimaran where I built outriggers and a sliding beam system to fit onto a Hobie Bravo. The system straps onto the Bravo with ratchet straps, so no modifications of the original Bravo were needed at all.

More correctly, this perhaps is a quadmaran, since the Bravo although narrow has a catamaran hull shape.

It remains to be seen how this will work on the water. One issue is whether the Bravo hull will handle the new stresses. I’m optimistic on this one. The other issue is whether the boat will still sail balanced in the water and not want to pitch pole. Fingers crossed on this one.

I did this for a few reasons. The Bravo is a very powerful little boat about 12 feet long. But, it’s narrow and doesn’t necessarily fit two adults in total comfort. Plus, where I launch, bluffs block the wind most days, so it’s hard to get to and from the launch dock under sail alone. The outrigger system provides side seating for two adults easy. It provides a place to mount a small motor. Likely, the boat will be less tippy.

I designed the hulls using Powerpoint. Yup, Powerpoint. I drew out some hull shapes. Then printed those and glued to thin cardboard. Then cut those out and taped together. It took 4 tries to get the hull shape I wanted. The cardboard is a close but not perfect match for how the plywood will behave. It took around 4 tries to get the A shape I liked.

I used a modified stitch and tape construction approach. So, a third issue is whether this approach will be strong enough for a small boat. I would not do this on a bigger boat or any kind of power boat. I used mainly PL construction adhesive and drywall fiberglass tape to make the interior seams.

I used only PL on the outside (no fiberglass), smoothed out, to make the outside seams. With the deck and bulkheads holding the shape fixed, my working theory is that fiberglass tape is not needed on the outside seams for a small boat. The deck and bulkheads won’t allow the hull to change shape. I also skim coated the inside and outside with a thin coat of PL for waterproofing. When thin, this actually dries smooth and clear. It’s not abrasion resistant like epoxy would be, but it is very waterproof per testing.

PL needs 4 to 5 days of curing to reach impressive strength. It will not be impressive if you test it after only 24 hours. For example, in the early stitching stage, I placed small blobs in the stems to close the hull up at the bow. After 24 hours, the PL did not develop sufficient tensile strength to hold the hull closed if stiches were removed. But, wait one more day, 48 hours, and the hold is very strong. A couple days later, it’s much, much stronger. So, I was careful not to attach the deck for 7 days after all the glue work inside the hulls was finished to give the glue several days to cure.

The hulls are tortured plywood with three “planks” per side. One of the blanks broke during torturing. Lesson is to fiberglass the outside stress region first. The hulls took a total of only 20 hours to make. Very fast and easy. Each hull is close to 20 pounds, maybe a little less. They are easy to lift individually. The hulls have 4 bulkheads inside. Only two are needed, but I put the first pair in the wrong place initially relative to where I wanted to attach the beams.

I didn’t use heroic efforts to make the hulls glass smooth since this is an experiment after all. So, I sanded, filled the worst spots, and then sprayed primer and some finish coats using a good quality house paint. Then, I used green and red duct tape to make some accent stripes on the hulls. Sounds funny, but the striping looks pretty good. If the sun fades the tape after the season, it’s pretty easy to replace the tapes later. The duct tape grip gets more tenacious after its been on the hulls a while. I think it should stay on the boat while sailing.

The sliding arm system took about 10 hours to fabricate from metal fence poles and unitstrut. It took this long because I designed it as I went, stumbling a few times. The system allows the boat to compress to 7.5 feet wide for trailering and expand to 10.5 feet wide for sailing. Turns out the metal fence poles are a very nice sliding fit around the unistrut. The hard part for me was devising simple stops so the tubes can’t go in or out too far. Not such an easy problem to solve for a tube that slides inside another tube where I didn’t want to make a long slot in the outer tube (I don’t have tools to do that in a metal fence tube). The overall sliding beam system is not unduly heavy. Each of the front and rear beam systems is about 20 pounds. I think that’s bearable for a trimaran.

Hope to be on the water in a few days with it. I haven’t built the final seats yet, but I’ll use simple planks of plywood for the first outing to make sure the boat works before building the final seat structures.

If all works as hoped, I’ll send sailing pics and more pics of the beam system and the launch. In the meantime, here are a couple construction pics and a pic of the two hulls after painting and striping.

My hope is that the outrigger hulls are short enough to be out of the water if the center hull is sitting level. I might have goofed on this one a little, but we’ll see. A couple inches off should not matter, as the center hull will still be floating and one outrigger will be out of the water when sailing. What would be bad is if the center hull is out of the water when the boat is at rest. I don’t see any way this will happen with this design. In any event, I would be able to raise the outriggers higher if needed. Hence, I’m not worried about this issue. I’m more curious about sailing balance, weather or lee helm, and pitch poling risk. The outriggers are set up to be very symmetrical fore and aft relative to the center of the Bravo. I’m thinking this might help preserve the excellent helm balance of the Bravo. A possible cure might be a leeboard if this becomes problematic.

For the akas, holes are used for in and out stops and more importantly for hitch pins that lock the sliding mechanism in place when the outriggers slide in or out. Only one hitch pin on each side is needed at each sliding beam, but I used two for redundancy. My main concern was not to lose an outrigger while cruising down the highway.

One picture that is noteworthy is the “plan” I started from and the corresponding cardboard model. The plan is the PowerPoint printout of the hull plank, and the cardboard is the model showing how the plan would look. The full size hulls and the model are very similar, so the technique is useful for design. Plus, it’s fast. The PowerPoint sheet and the model is al I worked from.

The plan picture is the entirety of the plans. The plan sheet shows that the bottom slit in the plywood is only cut from the stern up to the center of the boat (see the blue circles on each slit, which show where to stop the cut). This worked in cardboard. In the plywood, I cut the slit much farther up toward the bow. The plan also shows that the bottom of the stern would be pointed. But, I cut the plywood to make each bottom plank about 2 inches wide there. The cardboard model and the actual plywood hulls closely match in overall shape.

Once the hull was stitched up and spread at the gunwhales to create a decent shape, the bulkheads, transom and deck were then cut to fit.

Here are pics showing the assembled “quadmaran” on the trailer.

The pics show how I preassembled the outriggers and beams using a couple of wood boards as alignment adds before attaching the beams to the outriggers with a pair of lag screws (using epoxy on the threads) at each mounting location.

The other pics show the outriggers both in for trailering and out for sailing. A cockpit picture shows how the beams are x-lashed using ratchet straps around the catamaran hulls through factory provided scuppers in the cockpit. The cockpit itself remains uncluttered and note how no modifications of the Bravo hull were needed whatsoever.

As I look at the assembled boat, I think the outriggers could be 2 to 3 feet longer and maybe only 2/3 as tall top to bottom. But, otherwise, it looks promising and was an easy build totaling only about 30 hours including painting.

I was a bit surprised that the outriggers slid in and out as hoped on the first effort, showing how important it is to double check measurements and have a good plan of action.

Looking at the beams, the center tube is a fence post cut down to 58 inches. The sliding arms are 3.5 lengths of unistrut. The sliding fit of the unistrut inside the fence posts is surprisingly good. The sliding arms are locked in position using a pair of 3/8 inch hitch pins with a cotter pin in the ends. These parts are inexpensive yet very strong. The beams are not the lightest, but they are not unduly heavy (fingers crossed) either. In trailer mode, the boat is 7.5 feet wide. In sailing mode, the boat is 10.5 feet wide.

My electric trolling motor mounts easily to the beams after the beams are extended. The battery box and battery mount inside the hull just behind the rear beam.

Oops. I didn’t match the color of the stripes on the outriggers to the color of the stripes on the Bravo. If the boat sails well, I’ll fix that.

Now, all I need to do is clean up the Bravo hull after its winter storage, add seats, and then I’ll be off for sea trials.

— David B. Kagan