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Can I increase boom height, and what are the issues ?

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QUESTION: Hi Mike:   I am very interested in the W22 but am 6'4" and would like to get clarification on cockpit seating clearance from the boom.  Can I increase boom height from cockpit floor without affecting stability?  Or if not, can I increase overall boat width to compensate?    Presently sail a Hobie18.            ..... Clint-B, NSW, Australia

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ANSWER:     This ia good example of how a seemingly small issue can go far beyond the obvious re 'effect on design' .  Nice that your question acknowledges that you personally see the possible implications.  Let's take a look

As I am personally reluctant to give away performance easily, I always lean towards protecting that, and one thing is certain, the lower the sail is to some deck or horizontal surface, the more efficient is the rig.    Just glance at the AC75's where performance is EVERYTHING.   Now the crew has to either dive in a hole, or run around the mast to change sides!

But of course, for pleasure sailing that's totally impractical.   So what IS the standard of clearance?

As designers we can set our own of course, but raising the boom and rig adds to the heeling moment as well as allowing more lost drive under the boom.

Racing dinghies ... especially the Laser type, typically allow only about 24" (0.6m) to duck under and I have seen OK dinghy sailors using even less to keep their sail in shape with a flexing mast.  But for sure, they are not over 50 like most of us.

But some new builders still ask for what I consider super-high clearances and I just figure they never learned to sail on a dinghy.   A decent sailor will ALWAYS know when the boom is coming across as they will dictate that happening .. by knowing how to position to the wind and turn their boat for a tack or gybe.   Except on a larger cruising boat with non-sailing guests, you never really need boom clearance totally over the heads of sitting crew unless they are really passengers who know zilch about sailing ... and for those you either yell 'DUCK' or sit them on the floor in the first place .. which is the best for kids anyway. 

For my boats under 8m, I have boom clearance to suit 'efficient sailing', and it's about  60% of 6ft or 43" min. on my W17.     I can still readily duck under that even at 90 ... but I am still fairly flexible and now only 5-8.    Increase that clearance to '60% of 6-4' gives 46", but as the W22 will be used for camp cruising, I already designed her with 50" designed clearance above the cockpit floor, which is about what the Sailplan measures.   In theory that gives about 35" above the seats, although it's the sail cut (plus any rake or mast flexing) that will decide the final height.

Increasing the athwartship stability by adding to beam is really not smart.   A good multihull design needs a balance with diagonal stability or you will trade-off a capsize for a pitchpole!    Think of it this way. 

Imagine your multihull is an elliptical table on a single post ... but turned upside down on flat grass, that simulates the sea. (see sketch)

Start with the long axis as the boat length, the narrow one the beam.  Now push the pole sideways in different directions and you will see that it wants to capsize more than pitchpole.   Now rotate it 90 deg and mark the wide side as a more stable beam.  Now push the pole sideways and you will see that the pole will now far more easily want to go fore & aft, as the side stability is now more than the fore & aft stability ..., ultimately relieving the forced heeling moment, almost regardless of load direction, with a pitch pole.   Your boat will react more like that if you add additional athwartship stability

Like most things,  a heeling boat will take the passage of least resistance.    If you add to the transverse stability, pitchpoling can become the easier way to go over when a high gust hits you.    Perhaps you have even experienced that sailing your 18ft cat ... add trapezes and now you will discover you are more likely to pitchpole.  Many do not think of that.

As adding beam would also add to weight, (with heavier beams and waterstays to handle the higher righting moment), I would not recommend such a change.  The W22 is already at the maximum sensible beam imho.  I recommend, if you REALLY want to raise the boom, to just accept to cut 15cm off the luff and raise the boom up the mast that much.     You will lose less than 0.5 m2 of sail and as it's at the bottom, this will affect the total drive quite minimally.

There is a more general article on Diagonal Stability posted earlier HERE.

And also touched on HERE while discussing Length/Beam Ratios.

May 2024

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