Eruttan, of course it is because of the location of the center of effort in the sails =)
It is many metres above the water, so if that force is aimed forward, the boat will pitch forwards. And if there is not enough weight in the back of the boat, or bouyance in the bows, the boat falls over forwards.
When sailing into the wind, the apparent wind is usually like 25 degrees from forward, so most of the force (cos(25°)) is aimed sideways, and only a small part (sin(25°) if we neglect drag) is aimed forwards. So the heeling moment is large, pitching moment small. If the boat falls, it falls sideways.
When sailing off the wind, on a broad reach for example, the apparent wind angle can be 90°. This means no force is aimed sideways (cos(90°)) if we neglect drag, and all force is aimed forwards (sin(90°)). No heeling moment, large pitching moment. If the boat falls, it falls forwards/pitchpoles.
Regarding dynamic lift in the latest Harryproas, as I understand it, there is a "kink" on the bottom towards each end. The flat bottom changes angle from 0° to maybe 5°. So when the boat moves, some of the flow gets angled downwards, which, if the speed is high enough, creates lift/positiv pressure. And in the back, negative pressure..
But if the boat is standstill, and the sails are sheeted in fast, or the boat turns by itself (you can't steer a standstill boat with the rudders) into a position where the wind is creating a strong force in the sails, there is no dynamic lift from the hulls, and the inertia of the standstill hulls when trying to accelerate increases the pitching moment. This has happened to me on my Hobie many times in rough weather. Doesn't help to sit in the very back, and lean backwards as far as possible. The trick is to slowly accelerate the boat so the pitching moment is not increased by the acceleration/hull inertia, and then you can very carefully turn into the direction you want to sail. But the bows are just cm above the surface, and the waves are flooding them all the time, slowing the boat, and making things more difficult.
You should just buy/rent a small beachcat and try some sailing. It's fun!
On Wed, May 30, 2018 at 3:07 PM, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
| Just as a guess, it looks like it might. I think Rob said he could see something. Or that the bows were not pushing down on Bucket.A very light and long boat like bucket may not go bows down as much anyway.
Why do the bows go down? Is it because the center of thrust of the sail is higher than the bows, and that causes a bow down rotation?
Do we put a slight upward pointing bottom on the crush bows?
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