Wiki Sail by GRF

you can put as much structure as you like above the water line but other than lowering the water line it wont effect the displacement/ bouyancy of the craft (other than reserve bouyancy)nor the under water shape. so how do you propose this will make the craft faster? remember that air is much thiner than water and for most if not all dinghys air resitance is not important (it is, it seems for the ac72's).

Originally posted by iGRF At displacement speeds it's been my experience that long chorded low aspect sails are faster than higher aspect short chorded rigs and the reverse is true in faster planing situations, the reason is couched in complex lift drag calculations that it would be boring and tedious to go into and you wouldn't understand anyway, it's just true, accept it. So a jib combines with the main to produce a very low aspect and extra long chord which works very efficiently in low winds to produce more power with little heeling moment.

Originally posted by Owenfackrell you can put as much structure as you like above the water line but other than lowering the water line it wont effect the displacement/ bouyancy of the craft (other than reserve bouyancy)nor the under water shape. so how do you propose this will make the craft faster? remember that air is much thiner than water and for most if not all dinghys air resitance is not important (it is, it seems for the ac72's).

Which illustrates just how you lot go round with your eyes closed, half way up the gunwale of a laser is below the water line for the most part with a sailor of 85 kgs and above in it.

So the increased volume that doing that would do to the waterline with an 85kgs sailor would be to keep it nearer to the bits that are efficient, so it would be faster.

Seriously it's like a class of 'special' people on here sometimes, so busy trying not to see it..

Originally posted by Owenfackrell Originally posted by iGRF Originally posted by Owenfackrell you can put as much structure as you like above the water line but other than lowering the water line it wont effect the displacement/ bouyancy of the craft (other than reserve bouyancy)nor the under water shape. so how do you propose this will make the craft faster? remember that air is much thiner than water and for most if not all dinghys air resitance is not important (it is, it seems for the ac72's). Which illustrates just how you lot go round with your eyes closed, half way up the gunwale of a laser is below the water line for the most part with a sailor of 85 kgs and above in it. So the increased volume that doing that would do to the waterline with an 85kgs sailor would be to keep it nearer to the bits that are efficient, so it would be faster. Seriously it's like a class of 'special' people on here sometimes, so busy trying not to see it..   So you  are talking about a situation where the deck is at the water line and the boat is effectivly submerged?

 

If this is not the case then how is just rasing the deck higher (ie increasing the freeboard) going to make it faster?

Graeme,
You're getting a bit confused old chap.
The craft displaces the volume of water that is equivalent to the total mass of the craft and its equipment.
Detailed hydrostatics like you have posted are given in "proper" ship design specs over a variety of trim, heel and displacements, but only up to the design displacement, so rarely is the entire bouyant volume of the vessel considered, unless something like a 2 million barrel VLCC which is 330m long with only 7m of freeboard.

A laser would support approx 1000kg before it sinks, i.e is a 1000 litre volume craft. With one bloke aboard it still only displaces about 150kg, at a draft of about 11cm. Another 50 mm or so may get wet with small heel / bow wave etc, but until you are dealing with wave impacts then whatever you do above this area is totally irrelevant to the hydrostatics and hydronamics of the craft. You could stick in a foot of topsides above 160mm above keel , and  if you ignore the effect of weight and windage, and ergonomics it will not make a blind bit of difference to the hydrostatics or hydrodynamics of the boat.

The reason that the 250l board becomes quicker inland if it is beefed up to a 300l board, despite a displacement of only say 100kg could be for many reasons. Anyhow, the 250l board will draw, say, 50mm, and be 100mm deep. even if the increase in volume was simply in extruding the sides upwards, i.e not affecting beam, length any of the hydrostatic particulars, the new, taller parts of such a small craft would still get wet due to the bow wave and heel etc, so there would be a difference in performance. The wash would not be curling over the rails so much etc which is really inefficient. On the sea with constant planing winds and waves to knock it about, that extra volume just makes the board bouce around more, and usually has an associated weight increase.

All boats (well, properly designed ones) consider the envisaged displacement (or weight) of the craft as it hits the water from the earliest design stages - this is very important in order to optimise many facets of nthe design for resistance, and stability, and sometimes seakeeping.
But whatever happens, the displacement is equal to the weight, it is the shape and distribution of this dislaced volume that has an effect on the performance. The extra 850 litres on a laser that isn't part of the displaced volume is almost totally irrelevant, until you get into wave impacts or consider windage / ergonmics.

Hope that helps, Daniel Holman MEng CEng MRINA