Longer Amas and Increased Beam?
Question:
Ron: I have a question for you Mike. Someone bought an 18-foot Sailbird like mine but it has been modified with longer amas, set to give 14' beam vs the original 11'. Will that help the boat be faster? Also, will sailing qualities be improved?
Answer: (see below for warning note added Jan 2023, re imploding risk)
Yes, it's almost certain that longer amas will speed up your boat. Think of it like this. Most of the time you're sailing as a cat on 2 hulls—yet one hull is much shorter! That is surely slowing you down as longer, slimmer hulls are inherently faster. (The greater drag can cause lee helm also.) I've long thought that having all 3 hulls the same length, made the most sense just considering speed, but this needs compromising to allow more space in the center hull and also to help the boat turn. Personally, I like to have amas about 90% of the center hull length though—that's what my W17 and W22 have.
By contrast, those who convert mono hulls by adding short amas for stability are really just adding 'training wheels' to their boats and the results will not give good performance .. just a little added security against heel.
Now, as far as increased beam is concerned. Spacing them out will also add significantly to the stability and enable one to accept more driving sail power and therefore be faster. But this will put more load on the mast and also a lot more load on the akas (beams) that may well need to be beefed up—or have waterstays added. Check here for how to calculate the load involved and there's another article in the Design section re Waterstays too.
Many would also consider that the wider boat is not as safe—especially for novices—as the narrower one; the reason given is that the amas on the narrower boat will be pushed under*, giving an early warning of possible capsize and also allowing the resulting increased angle of heel to spill wind from the sails. The wider boat DOES require more vigilance and skill, but for some (like myself), the extra power and speed is worth that effort.
*WARNING: Although not a form design issue, its important to remind designers that IF you are planning to use smaller amas to take advantage of the ability to push it under to release sail pressure, or even make a boat easier to get upright again, that such an ama could be pressed under to a depth of approximately half the full beam. As an example, if this were a boat of 12ft beam, the ama could be pressed down to 6ft under the surface. Down there, the pressure on the structure will be 6 x 64lbs per sqft or 384 lbs/sqft. So when designing the ama structure, you'd need to imagine two heavy individuals standing on each and every sqft of the ama surface to get a realistic image of what the ama structure must resist. Quite a number of trimarans with low buoyancy amas have had one pressed deep under, only to implode and disintegrate. One easy way to add strength is to fit internal horizontal struts from side to side about half way down the depth, but these need to be connected in a way to lower stress concentration so are typically placed between stringers. But if the buoyancy is enough to keep the ama at the surface, then this imploding risk can be totally negated. The W17 is designed like this and when correctly sailed, the ama deck is always above the water. IF one ever did need to rotate an ama deep under water, the deck access hatches should be removed to allow full flooding, which would then remove all skin pressure.
Sailing qualities of the wider boat will change a little but whether they are improved or not, will depend on how the boat handles at present. The boat will likely be a better sea‑boat and also more stable on a straight line but possibly a little harder to turn. The extra beam and increased ama length will likely make the boat drier but all these positive things can encourage a sailor to push the boat faster and the end price for that is a pitchpole, as the higher stability of the wider boat will better resist a normal athwart-ship capsize. So the price required for more speed and overall performance, is more vigilance, better skills and quicker reactions.
Mike ... Jan 2023
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