a boat?

you could use the derivative to get the slope of the tangent line.
in this case y''(x) = cos(x)
if you need to get the angle then its a little bit trickier but you could just take arctan(y''(x)) = arctan(cos(x))
there is probably a nice simplification of that expression but its a good start

I didnt want to try simplifying it by hand so I tried to have my Ti89 do it, unfortunatly accroding to it, arctan(cos(x)) is the simplest form.
I tried arctan(1/cos(x)) just for kicks, but the result endedup being more complex. :-/

90*(pi*sign(cos(x))-pi*arctan(cos(x))/90)/pi
(if anyone cares )

yeah, cos(arctan(x)) is a different story, that simplifies rather nicely

arctan(cos(x)) is about (pi/4)cos(x), but its doesnt sound as though you will be calculating more than once per frame so its probably not worth it

I don''t meen to complicate things, but doesn''t this assume the boat is quite a bit smaller than the wavelength of the waves?

If the boat were spanning a wave it''d be more complicated, wouldn''t it? Certainly it would tilt more along it''s shortest dimension...

Yes Thanks for the Answer i wanted this answer to but isnt it possible to just take the Normal of the Triangle its on and us that to calculate angle?

yes, but how do you write derivative in c++?

do I have to include any more h file other than math.h?

dawid - yes of course, I just think the code for the derivative is simpler, cross products get messy. It depends on how the data is being stored.

dennizzz - I don''t even think c++ can do formal derivatives, you would have to precalculate them for a given wave. You could also approximate the limit but if you are going to do that it might be just as simple to use dawid''s cross product suggestion