rocker

Originally posted by NickA I was looking at an RS Aero the other day and realised that it has no-where to sit "up front".  It has a long nose and a mast that is well forward, but the crew is forced to sit quite a way back, behind the centre-board.  I guess it's designed to work well when planing at the expense of being able to hold the nose down. The D-zero, on the other hand seems to have a better range of seating positions allowing you to get well forward of the centre board ... and also seems to go better up wind.  I wonder if these things are perchance related!

Older, generally heavier and slower boats usually have the ends just kissing the water at full load. Hence not much forward trim needed in the light. Also, older boats squat and trim by the stern at speed so less need to get weight back.

Hullforms optimised more for semi displacement (5-12kts) will have their ends immersed at rest at full load as significant amounts of transom immersion is optimal from a resistance perspective, whilst maintaining waterline length.

What this means is that, along with lighter weight some Modern hp boats have a lot less “rocker” which by proxy means higher cp, and manifests itself in less squat / more dynamic lift, thus less drag in semi displacement p. In practice this was also enabled by the designers subsequent to ok etc understanding that sailors would be happy to move fwd and aft to optimise the boats trim to boatspeed.

Modern straight low rocker hp boats actually require a higher moment to change trim (mct) ie are less sensitive in absolutes trim terms to weight placement, but the hydrostatics / transom and chine immersion changes a lot more per degree of trim than say, a firefly.

Yes understood. My curiosity was regarding the need/effectiveness of sitting forward in the light stuff. The supposition being that old boats had more rocker and deadrise you don't need to sit as far forward to avoid dragging the transom. 

Skiffy boats aren't designed with light winds in mind so I suspect the latter but we take advantage of the space in a drifter, obviously it's irrelevant once you are sitting out. 

Edited by Sam.Spoons - 16 Mar 18 at 9:20am