Deep vee hulls, center of pressure vs center of lateral resistance?

[dustman]

How does one determine the center of lateral resistance of deep vee hulls(like a wharram) and thus decide where to place the sails(center of pressure)?



How much pointing ability would I be giving up by using the hulls as my underwater lift devices rather than boards? I imagine this has a much poorer lift to drag ratio than boards.

[mikereed100]

I expect that any marine oriented CAD program will calculate center of lateral resistance for you. I have used Rhinocerus 3.0 for this. You might want to ask this question on boatdesign.net. There are a lot of naval architects on that site and someone might be able to steer you towards an equation that would let you calculate lateral resistance without investing in an expensive CAD program.

Wharrams, while awesome boats, are not known for their weatherliness. Some folks have added daggerboards to help, but I have no experience with this and can't comment on how effective it would be.

[Kettlewell]

How does one determine the center of lateral resistance of deep vee hulls(like a wharram) and thus decide where to place the sails(center of pressure)?

Since traditional Wharram hulls are basically symmetrical, the center of resistance should be in the center! Actually, the rudders probably move it a bit aft of center. You'll note that Wharram masts are typically right in the middle between bow and stern. A crude way to figure this out for another design would be to take a similar Wharram plan and see where he put the center of effort or pressure.

[catsketcher]

You are using the wrong terminology - it matters because centre of pressure is not centre of area.

The centre of lateral resistance (CLR) is a measure of the centre of the underwater area of a hull. This is a helpful bit of data, but not that useful on its own. It is NOT the point at which the boat pivots. That IS the centre of pressure. A hulls centre of pressure (not that most people call it that) is forward of the CLR by about 7% of the waterline.

CP and CLR can vary a lot, especially on foils. Have a look at a bird feather. The shaft is about 20% aft of the leading edge. Any further aft and it may not feather properly. Getting this right is a bit tricky and I have been on a boat with a rudder designed by a proper foil designer who got this wrong and it needed to be held in the centre, which was bad.

The Centre of Effort of the rig (CE) should usualy be about 3-7% in front of the CLR in monos, but cats are trickier. Look at lots of plans to get an idea.