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Converting a Monohull to a Trimaran  .. and what to consider.


I have an old monohull with sound structure that I now plan to convert to a trimaran.  What aspects should I consider when selecting suitable amas (floats) and what sort of performance might I expect ?    Mark, New Zealand


This is perhaps the most frequent question I've received over the last 10 years of running this Q&A service.  Here is some personal advice on the subject.

First, the owner must accept that the performance of the final boat will not match that of a custom-designed trimaran.  That center hull will be proportionally wider at the waterline than would be a trimaran vaka (main hull), and even the keel profile will typically be far more 'banana' shape with the result that there's less directional stability and more pitching.   Water surface will also be higher and the monohull will not be at ease being pushed much faster, so main hull waves will limit maximum speed compared to that of an equivalent length trimaran.    Upwind performance will vary a lot, depending on the final choice of amas, how broad is the central hull and the windage it creates.  The final converted boat will also be unlikely to garner much value so one cannot expect the same resale value as for a dedicated trimaran design. 

But if the owner/builder can accept all the above, it IS possible to create a comfortable trimaran that heels less than it did as a monohull and also sail, at least in some directions, a little faster than it once did at the limiting hull speed of the original boat.

Here are some steps you will need to consider.

Conversion work on the Main Hull:  Choice of Amas & Akas Sail plan:  and Rudders & Foils ... so let's look into each. 

Ideally, the Main Hull (Length 'L') should ideally be relatively narrow and light, and permit the keel to be readily removed as this will no longer be required for stability.     However, the trimaran will still need some lateral resistance so a centerboard of some type will be required.   (my article on Foils might help on this).   Obviously, the hull should be faired and closed-in the neatest way and with any lost strength compensated with the addition of adequate glass or carbon fiber to the shell laminate.

If the hull is very rounded, the low speed performance should be fine as long as the amas are not set too deep relative to the main hull (see below and read about dihedral here) .... but if the main hull is too vee'd, expect it to be wet when pushed ..., or if the keel profile is too banana shaped, expect the boat to pitch considerably in waves and be more limited in speed.

You will need to choose the position of the cross beams, and the structure and arrangement of both the main hull and that of the chosen amas must be part of this decision.   They should be a reasonable distance apart (say 35-45% of the main hull length) to keep the stresses acceptable in the connections.

Re the choice of Amas, this will depend on the type of sailing you enjoy and seek.     Smaller amas (say 0.75L) will support less sail area but will allow the boat to heel more enabling wind to escape, as well as needing less strength from their akas      If you are prepared to ease off the sail as soon as the leeward ama goes 80% under, you will stay safe and be unlikely to capsize.   However, if you want more power, amas up to 100%L and with buoyancy over 100% of the total weight will offer more power and add more speed potential, as long as the akas and their attachment to the main hull are designed with adequate strength.   See this article on Aka design.    As noted, I typically advise that the load on the forward aka beam end be taken as the maximum buoyancy offered by the ama, even if this includes a margin for most normal sailing needs.

One aspect that I consider important that others may not, is that I would personally seek a visual match of the chosen amas with the main hull.   A main hull with a large bow overhang can look ungainly with amas that have more vertical stems, even though the boat may still sail acceptably.     From the performance point of view, amas that have straighter keels (like the newer beach cats, compared to the earlier H14 and H16) work better and help to reduce pitching.   Ideally, you want the center of the ama volume (the center of buoyancy) to be well forward as this helps to prevent pitch-poling and if different from the main hull, can also help to reduce pitching.    Positioning the ama bow well forward will also help to resist pitch-poling.   

In profile, unless trials prove otherwise, it's generally a fair initial assumption to place the sheer line of the ama parallel to the sheer line of the main hull.   In terms of the ama height, this is more determined by the location of the ama keel relative to the waterline.  If the ama keel is placed slightly below the static waterline, the stability of the ama acts more quickly and the boat will therefore feel more stable.   But the additional ama drag can slow the boat in light winds.   Contrarily, if the ama keel is positioned to be above the water-surface, the boat will be fast in light winds but will feel unsettlingly jittery until it heels enough to put one ama a few centimeter's into the water.  The compromise I most often aim for myself, is to have the ama keels 'just' kissing the static water surface with the main hull fairly lightly loaded.   Ultimately it's for each designer/builder to make their choice as each decision will be a compromise.    Light racing boats may have amas a little higher, but loaded cruising boats will almost always have their ama keels underwater.    Boats with higher amas (like the Buccaneers of Lock Crowther) will allow the boats to heel much more before the amas 'get-to-work'.   While this heeling will lower the sail power that can be applied to the boat to drive it faster, it also unloads excessive wind load from the sails that can save it from capsize. With fast boats that have their amas at the waterlevel, the best way to avoid capsize will generally be to reduce sail ..., and/or to have some semi-automatic way to release the mainsheet .. but that's a whole new subject ;)

The actual Aka (or cross beam) could be made from a pipe or mast section (aluminum or carbon fiber), or from fabricated wood beams.  But either way, the deck edge of the main hull will need to be reinforced over a reasonable distance to adequately spread this upwards load, down into the side of the main hull.

The Sail Plan could either be from the original main hull ..., or even from the boat donating its amas ... such as a beach cat.

But remember that 1) sails on a multihull generally need to be cut a little flatter than for a monohull and 2) the stability of a trimaran will increase the load on both the mast and sails, so if coming from a beach cat, it's recommended to cut down its mast length to not exceed 1.35L of the original beach-cat length .... or if from the main hull, consider a max of 1.25L of the main hull.   Of course, if you have the means to calculate the actual mast strength relative to the boat's stability, then you may be able to slightly exceed these figures, but without that, I would recommend to stay within them.

The final thing noted above are the Foils.   Some means to limit lateral sliding of the boat will be required, and this need will be more with a banana-profile hull than with one that has a deep forefoot and relatively straight keel.  Some guidance on a suitable area will be found here.    The rudder of the original hull will likely be more than adequate, but you may want to modify its profile to bring a little area forward of the rudder-stock (pivot) to lighten its feel.  Just 10-12% of the total area forward of the pivot can make a significant difference in feel and the pleasure of sailing.

Good luck with your conversion if you go this route .. and feel invited to send me some pics that I can then add here if they show something of pertinent interest.



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