Join Date: Dec 2004
Location: Portugal, West Coast
Thanked 112 Times in 95 Posts
Rep Power: 13
Re: Full or fin keel?
The paper I was referring is from the Australian Maritime College. I cannot post it since it not free but I guess I can post this one By Richard Birmingham from the School of Marine Science and Technology, Newcastle University, UK (2004). On this paper he explains why a boat with an intact mast returns easily to its feet:
Another result of recent research has caused considerable discussion due to its unexpected nature:
Should a capsized vessel retain its rig intact it is usually assumed the damping effect of the rig would impede any action tending to right it. In theory a vessel capsized by breaking waves may be righted again by another wave with similar energy. However as the mast in the water would have a damping effect not present if the vessel had been dismasted it has been assumed that this would slow the righting action.
Experimental result from the towing tank in the Australian Maritime College, Tasmania, found that a dismasted vessel required more waves to right it than
the vessel with the intact rig . ... the effect can be demonstrated, and it is argued that the damping effect is in fact helping the righting process.
The explanatory theory is that the breaking wave force the hull of the vessel at the surface sideways (in sway), while the rig, deeper in the water is
left behind. After the wave has passed the vessel starts to return to 180 degrees of heel (i.e. fully inverted) but the damping effect of the mast slows this such that it is still heeled when the next wave arrives. Each wave arrives with the vessel at a reduced angle of heel (160 degrees, 140 degrees etc.) until the angle of positive stability is reached, and the vessel re-rights.
In contrast the dismasted yacht returns to 180 degrees more quickly, so the progressive righting process does not occur.
Off course this is pretty meaningless other than from the theoretical point of view because almost all sailing boats break the mast when rolled to the inverted position.
There is a small lapse on this paper: when it is said :
"In theory a vessel capsized by breaking waves may be righted again by another wave with similar energy. "
This would only be true if the area under the positive part of the RM curve was equal to the area over the negative part of the RM curve, or saying in a simpler way if the energy required to capsize a boat was the same as the energy to roll it back to its feet.
A modern sailboat stability curve has almost always at least 2 times more area under the positive part and many times about 3 times more positive area.
This means that almost any sailboat will only requires a wave with half the energy of the one that capsized it and many will requires only about 1/3 of the that energy to get back to its feet.
Last edited by PCP; 03-30-2012 at 08:07 PM.