Three Symptoms of Mismatch
Updated: Apr 16, 2022
Post #04 Three Symptoms of Mismatch
In the previous post I mentioned there are three possibilities or outcomes when there is a mismatch or an imbalance between the superstructure and the hulls. I don't believe we need to do a lot of work to make the case that these three symptoms are ubiquitous in modern cruising catamarans.
The first symptom, transom immersion is OK in moderation. A little bit of immersion at rest avoids the annoying slapping you get under the hull on anchor when you're trying to sleep. Excessive immersion creates turbulence, a sure sign of compromised performance. Keep in mind that a hull at rest which is just kissing the water at the transom is more deeply immersed when it becomes the leeward hull under way.
The second outcome; preempting compromised trim in the design stage is more subjective. If you want generous deck and saloon spaces for a given length overall and you don't care so much about performance (see my first post) then this is the best approach to the problem. Be realistic about the displacement and the location of the centre of gravity. Take care of it in the design stage by increasing the rocker or making the hulls fatter, especially in the aft sections. Those who are passionate about efficient sailing hull design may be critical about the forms that are arrived at but a good "fit" between the hull design and the superstructure should take precedence in the hierarchy of design considerations.
The third outcome; remedial work like transom extensions in my view, is the least preferred but is all too commonly resorted to as a means of correcting an immersed transom.
If the boat has a flat or "flattish" rocker line then you have to extend the transoms a lot to bring the transom close to the surface.
For a more moderate rocker line it's not so difficult to bring the transom to the surface by adding length, but it will have very little effect on the trim because the additional volume enclosed by the addition is mostly above the waterline and will have negligible effect on moving the centre of buoyancy aft.
Changes made here affect other things too. A rig that was intended to be at 45% of LOA is now further forward relative to LOA after the change has been made.