Originally Posted by codprawn
As to hull performance I am not so sure. I always used to believe in heavy displacement hulls being the best but some RIBs with planing hulls can cope with horrendous conditions. Bit like a cork bobbing on the sea!!!
My broadband is down at the moment so I'm stuck on dial up and can't watch the vids, but horrendous conditions for the RNLI means waves up to 14 to 15 metres high, or perhaps more critically the sort of confused seas you get in areas like the Pentland Firth.
In big seas, any boat traveling at speed is going to tend to take off from the wave crests, particularly a planing hull, and when it lands the flatter the hull the more it is going to slam and the more it is going to hurt (just the same as shallow vee vs. deep vee on a RIB).
You can overcome this to an extent by driving on the throttles, but this is not always possible, particularly at night when you can't see what is coming up and anticipate it.
25 knots in a semi displacement hull is not a problem, but requires more power and therefore weight than a planing hull. Not quite such a problem for a slipway launched Tamar to weigh 35 tonnes, but FCB2 has to be towed on a tractor and trailer so weight is critical.