dihedral on a tri

[blue0106]

Hi,
I was just curious, what factors influence the amount of dihedral (height of ama's above the water when the tri is completley upright). Why would you go for more or less, purely from a racing perspective.
Thanks for any input,
Chris

[Gary Baigent]

For me it is to get the windward ama and foil clear of the wave tops. Also with dihedral the platform will slide along beautifully in very light airs because the boat is heeling to leeward and the sails fall into a sweet, curved shape. This is, unless you have an enormous amount of sail area, where the sticky catamaran fails because it is upright. The disadvantage is that at anchor, the platform will flop from side to side especially in higher winds. This is somewhat annoying but not as bad as a rolling its guts out monohull.

[Doug Lord]

Originally Posted by blue0106

Hi,
I was just curious, what factors influence the amount of dihedral (height of ama's above the water when the tri is completley upright). Why would you go for more or less, purely from a racing perspective.
Thanks for any input,
Chris

=====================
To add a bit to Gary's post: dihedral must be carefully considered on a tri designed to fly the main hull-which most high speed racing tri's are designed to do. Some designs go for skinny ,very deep main hulls with a relatively narrow overall beam. The result can be a large sailing angle when the main hull is flying depending on the dihedral and other factors.
My opinion is that keeping the sailing angle at 10-15 degrees is preferable to something like 26 degrees. Controlling that angle is,like most things in design, a result of the interaction of a number of factors-main hull rocker, overall beam, ama immersion(is it partially lifted by a foil?) and dihedral.

two different sailing angles when flying the main hull-less heel is faster:

[Pouakai]

Some interesting comment on the subject in this thread:

http://crew.org.nz/forum/viewtopic.php?f=37&t=14789

[Doug Lord]

Very interesting. When the main hull just starts to fly it seems to me the ama
should be vertical. If its not then the effective L/B ratio changes(max wl beam is greater) ,potentially increasing wave making resistance at a time when you're trying to make max speed.
Thanks for the info.