Can you point to a source that suggests aft fullness reduces hobby horsing?

http://www.shuttleworthdesign.com/NESTalk.html

"As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem, because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. "

Fat aft hulls are a problem, as I understand it it seems. Thats why I like HP's skinny ones.

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Posted by: "." <eruttan@yahoo.com>

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I won't go down this point by point, but I stand corrected,  however you left out an important part of the quote:

"However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull, and aft in the lifting hull (ref 2 and 3 ). This effect can be achieved in both cats and tris, giving a very comfortable and easy motion upwind. At the same time windward performance is improved, because the apparent wind direction is more stable across the sails."

    What does "finer" mean?   How much finer?  Note the portion about the center of buoyancy being moved forward in the immersed hull...........     I am not even remotely considering dropping my size constraint, so we must work within this constraint.   One can only compress the interior space so much without the result being cramped.   In lieu of increasing length to achieve this space, which is entirely off the table for me, if not for you, the alternative would seem to be moving the center of buoyancy forward.   The ideal compromise hull shape would then be a sharp knife edge bow increasing in beam fairly rapidly to a maximum submerged beam at perhaps 1/3 of the total length followed by a gentle decline in buoyancy of the submerged hull.   In the drawing below, you will see at a glance that the max buoyancy of the submerged hull of the KD 860 is well forward, peaking at BH4, and rapidly decreasing behind BH 5 to zero at the transom.    BH4 is at station 4770, which is aft of center, which would be about station 4300, 370 mm forward of BH4.  While this may not be perfect, it takes into account the more important consideration of beginning the bridge deck well aft, about station 2800, between BH2 and BH3, this being the curved leading edge starting quite high, and curving down to actual bridge deck level at around sta 4000.    ST 3-4 comprises a pair of lateral double berths, right on the pitch axis of rotation.  Really the best location for a berth.  Minimal vertical heave motion, and your head is not constantly being raised and lowered in relation to your feet, which is not good.   This is fundamentally good design IMHO.   In this case, the designer obviously strove for a balance of priorities, a decent ride combined with decent accommodations.   You can't cram everything you want into a 28' hull without making some design compromises.   Perhaps the max submerged displacement could be moved forward a bit, and the contours aft less abrupt..... Without building models and tank testing one would not know how much difference such things would make.  One trimaran designed for the OSTAR years ago resorted to sponsons at the forward end to prevent submarining.    On pitch oscillations, obviously length makes all the difference, but let's stick to the length constraint you hate so much.   The challenge every designer is faced with is to work within limits.   In this case that is 30'.  The drawing and the rendering here are the KD 860, and below them Richard Woods Tamar 31 footer.   The Tamar weighs almost twice what the KD 860 weighs.  3000kg versus 1800kg (empty), and both have the same payload, 1200kg.  Over all length is 3' more.   

                                                                                                                                                H.W.

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Posted by: StoneTool <owly@ttc-cmc.net>

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