Mast heights, boom lengths and aspect ratios

[h_zwakenberg]

Originally Posted by Allan Jennings

What happens if I increase the length of the foot and shorten the mast to maintian sail area,- do I lose performance?

if you lengthen the boom and shorten the mast, to end up with the same mainsail area, the following happens:
- the aspect ratio of your mainsail planform is reduced a bit, which will only be detrimental to performance when you sail upwind.
- mass is reduced up aloft and some is added to the boom. Also, the shrouds will be a bit shorter. This means that the CG of your rig will be a bit lower; the CG of the hoisted mainsail will also be a bit lower, both of which enhance stability somewhat.
- since your mainsail's centre of pressure will also be somewhat lower, the heeling moment at certain wind speeds will be a bit lower too. This might produce a slightly smaller heeling angle, hence less hull resistance. For some wind angles, this might even result in a slight speed increase...

Summary:
the somewhat lower CG of your rig and the somewhat lower centre of pressure on the mainsail stiffens your boat up a bit. You loose some performance when sailing upwind.

This much from theory. Whether these changes are significant for a rather heavy displacement cruiser like the one you're building can only be found out by a full VPP analysis.

take care,

Hans

[RHough]

Originally Posted by Allan Jennings

OK So I will keep the original sail plan and just make the boom a little bit longer so the mainsheet drops behind the cock pit seat and fit a tripping reef so that the boom is lifted when heeling in strong winds to stop the boom from dipping in the sea.

The only thing to work out is how much longer do I have to make the boom, does the mainsheet need to drop vertically from the end of the boom when the boom lies on the centre line of the boat or can it be inclined parallel to the topping lift?

In line with the topping lift is fine. When the boom is centred the mainsheet pulls the boom down, since the mast and boom do not compress the change in load is on a line between the clew of the sail and the head. If the mainsheet angled forward of that line part of the sheeting force is compressing the boom towards the mast.

As Bob says, the angle makes little difference.

[RCSail]

One other concern that I have seen with shortening a rig is that the majority of the sail area is closer to the water and thus you lose wind speed due to the boundary layer effect. This would primarily concern light wind performance, as the stiffer boat would probably make up for the difference in heavy winds.