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The challenge of building a trimaran from beachcat hulls


Hi Mike!      Let me first say I appreciate the depth of information on your web site :)   

I've been ‘oogling’ it for years.     But now I have a question regarding the potential of building a trimaran out of some old 18' cat hulls.     How would you suggest I proceed and what might I expect from the result ?     

…..  Josh from Kalispell, MT.

ANSWER:   Tks Josh … always pleased to hear my work is proving of value to others.   

I first need to say that using cat hulls is not a perfect fit for either the vaka (center hull) or even the amas (floats) but sometimes we can make something work as a fun project.    The vaka will be the biggest challenge.    I could briefly answer your question with just a list of do’s & don’ts, but I think a more detailed reply will give you some specifics and cover a broader range of related questions, so here we go.

First step is to make a weight estimate starting with what a ship designer would call the “Lightship weight”(boat ready to sail, but no supplies, crew or their personal equipment).    You then need to decide what weight you wish to tack-on to form the Design Displacement.  This we call the ‘Deadweight’.   The very minimum would be for solo day sailing .. say 200 lbs added to the Lightship weight, but for an 18footer, this will likely be more like 350lbs as an absolute minimum.    

So let’s assume the boat and its rig and gear weigh 550 lbs.  Now with 350lbs added, the required Design displacement will now be 900lbs.    The volume to support that will be 900/64 = 14 cu.ft.    (1 cuft displaces 64 lbs of seawater, or use 62.4 for fresh water)

The difficulty with a trimaran over a beachcat, is that there is initially only ONE hull to provide this buoyancy volume, compared to two for a catamaran, so right away, one can see why a single catamaran hull is WAY too small for a vaka.  Let’s say these are your 3 hulls.

Place the widest one with most volume in the center.

Obviously, if you kept the hull in this level position, your trimaran would be dragging 3 hulls through the water with far more drag than for a catamaran.

So you need to lower the central hull, so that it will support your 900 lbs .. requiring 14 cu.ft volume under the waterline.   It will clearly not have the volume needed so what can be done ?   There are 3 basic ways to go.  You can lengthen it, widen it, or sink it deeper.  Using all three would be best if designing a new vaka but I suggest that sinking it deeper will be the easiest, as you can then just raise the sides.  But you will probably need some frames to support the deeper hull section as the hull will now have to accept more hydrostatic pressure.


It will now look more like this:

You can either raise the sides (C) or a better solution would be to combine this with side extensions (D) to give yourself some form of central cockpit as well as more width to later attach your cross beams (akas).  Exactly how deep this central hull will need to go, will decide how practical this whole ‘conversion’ will be, and it’s quite possible you will need to do some major surgery to this central hull (or build a new one) to get the buoyancy volume you need.    If you are not already familiar with how to calculate this, I suggest you study this page from the design section.

A very rough idea of the sectional area (marked as ‘buoyancy’ in the above sketch) can be obtained by assuming a Block Coefficient* of 0.5.    For this, double your required volume and divide by your waterline length.

So in this case, this will be 2 x 14 / 17.5 or 1.60 sqft, which will roughly require a beam and draft of (1.600.5 ) or 1.265 ft for both waterline beam and for draft.

If your hull is much smaller, you either have to go deeper than 1.3ft draft OR find ways to widen that hull to get the cross-sectional area you need.   (Typically, a central trimaran hull has a waterline width of about double that of the ama beam).

Moving forward .. let’s assume you now have chosen your best workable solution for the vaka.

As far as the overall beam is concerned, the issues involved and a way to select a suitable beam is already explained in this Q&A from earlier this month.

We now need to connect the amas with the main hull using beams.   As I understand you are planning to use a similar folding system to my W17 and using my design of fiberglass hinge and latch, I will explain what you will need.

Beam and latch strength will be defined by the maximum righting moment that the ama can apply, plus a safety factor.

As a safety factor, it is common to apply the full buoyancy of the ama (when fully underwater) to the main beam ... and multiply by the Beam Lever (shown here), which might be ~4ft in this case.  IF the full ama volume is say 800 lbs, then the bending moment on the beam would be 3200 lbs.ft

The bending moment at the hinge will be less as the lever is shorter, but it will depend on the location of that hinge.   I did notice on your own sketch that your hinge was not correctly located.  If placed in your location, the amas would collide with each other when folded.  Check out this sketch., Your B1 dimension is greater than B2, when in fact, the reverse is required … unless you really want the folded amas to be meeting each other higher up at the centerline …. an arrangement that would require blocking and would raise the center of gravity on a trailer.

Once you have fixed the hinge location, you can calculate the bending moment at the hinge, using the same 800 lbs (just assumed for this example) but with a smaller lever.   Let’s assume it is 2.5ft in this case, so the Bending Moment will now be 2.5 x 800 or 2000 ft lbs.     This would mean that IF your vertical distance between the upper hinge and the lower latch is 5” deep (0.42ft), that the latch will need to take 2000/0.42 or 4762 lbs in tension, for the hinge to be as strong as the beam.   This shows the important value of 'beam depth' with a hinge system.

If your hinge cannot achieve this, then you will need to use a waterstay – a cable or S/S strap down to the central hull just above the waterline.   This would remove most bending from the beam, but would change its load to then being mostly in compression.  While this is an easier load to design for, it does add the complication of a waterstay, requiring strong connections at both ends.    For my W17, I did design the latches to take the full load without waterstays, but this also means that the tensile load has to be carried through the latch and its bolts, into strong connection palms built into the upper and lower parts of the box beams ..see here

There will also be a download on the upper hinge, due to whatever crew weight you place on the flying windward ama (plus the weight of the ama and outer beam itself).   With a large volume ama, this download is typically about 70-80% of the upload, so the hinge is a little less stressed in tension than the lower latch, but with a small ama, it could be the reverse, so you need to do the maths..

In Fig 3. note that I show the amas inclined out at 10-12 degrees.  If this is not done, they are more side-loaded when heeled and this adds an unnecessary load to their beam attachment as well as making the ama more resistant to push through the water, so personally, I consider some outward angle is essential.

In addition to their location in the above sectional view, the outer hulls that will work as amas, also need to be positioned in profile view, relative to the central hull.

In the sketch below, they are shown with slight lift of the bow that some designers in the past have incorporated with the assumption of less resistance and 'to ride over the waves'.   In the 1970’s many amas were then banana shaped and even back then, Hobie 16 hulls were sometimes used as trimaran floats.   Having sailed on one a few times, I found such boats (like Newick’s Tremolino for one) were very jittery in any seaway, were very wet, and also pitched excessively.    Even using the straighter and therefore more suitable H18 hulls, their stability role for a trimaran is still compromised if installed as shown here in Fig 6.

Just compare these two sketches in Fig 7 that both show an identical assumed heeled water line (in red) and you can see that IF the bow of the ama is raised that the ama will not give any bow lift UNTIL the whole bow drops, severely increasing the risk of pitch-poling, so unless the bow is very full with a wide vee, its more effective and safer to actually LOWER the buoyancy of the ama bow so that it quickly immerses, to provide more effective lift when heeled. 

But when using a beachcat hull, the foredeck is now pretty low,  For a well designed modern trimaran, the bow forefoot can be made deeper while still maintaining a reasonable deck height. To confirm that I ‘practice what I preach’, check out the ama for my W22 for example ;)   Though designed back in 2008, this is now a far more common profile .., in fact the fastest non-foiling boat (2022) for its size in Europe (Black Marlin), sports a very similar shape and posture.

So what can be done to make the beachcat hull more like this ama?  The bow is deep and narrow, not vee’d.   Think of the Hazelett mooring buoy that is a vertical tube.   While it still offers buoyancy, it moves very little in waves.   When a bow is like that, its not easily launchied in the air to only fall again, over and over again.   It just stays quiet and cuts horizontally through waves and it certainly increases the waterline length.   Early rockered amas, had short waterline lengths and were also vee’d … two things I advise against.  After all, we are ’sailing’ on this ama and need it to act like an efficient boat hull.   But as it would be complicated to deepen the forefoot of an existing beachcat hull, its easier to lower the bow as shown above and then raise the deck forward of the forward beam. 

As a picture is a 1000 words …, check out what I would do.   3 days of 4 hours could see both done if you plan your work.

You might find a collision bulkhead inside that will need cutting across, but it should not be hard to raise that with rigid foam, so that the raised deck can be bonded to it and keep it watertight.   The ¾” holes in the sides will stop the end of the cut from splitting and cracking, and will be filled and covered over by the fill piece.

This should give one enough to make the important decisions and calculations, but as can be seen, catamaran hulls are not ideally interchangeable with Trimaran hulls, so their use like this will always be a compromise.

You also have to decide which hull will hold a centerboard or daggerboard.    It’s much easier to reach one in the center hull, but this is your choice and dilemma.    Either can be made to work but do not use ama boards unless you are operating in deep water, or you may end up IN the water while trying to adjust them from an ama, especially in rough water.  

As far as the end result, it depends on so many things that it’s hard to predict.  But it could be made to perform quite well in relatively flat water and give you some fun at least … but it will not be a dry boat like the W17, nor as comfortable or have as low leeway, as the round hulls are just not as ideal for slicing waves with minimum disturbance as the W17 hulls are.   While rounded hulls are very effective in lowering surface friction, they seldom slice waves as well and certainly do not provide the leeway resistance of the W17 hulls.

It's also frequently forgotten (or just not understood) that often, the greatest resistance from a small boat is wave resistance or wave making … being the most critical in the 4 to12kt range.  Optimizing for this range with non-round hulls can more than make up for their added skin drag.   See this article for more on this, where there is a chart showing what proportion of residuary (wave) resistance applies at what speed.  

Hope this helps to get you going in the right direction, and good luck with your project.

Also note that there was an earlier question of using the reverse ... Trimaran hulls for a Catamaran and as my graph showing the proportion of residuary resistance versus the total is also there, it may be of some interest too.

Mike ... 2022

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