On Sun, Mar 8, 2009 at 6:01 PM, Peter Stockdale <[email protected]> wrote: > --- In [email protected], dvdob...@... wrote: >> >> It's seems to be a rule of thumb as well as a characteristic of boats in >> general. It is only necessary to be aware that the pivot point exists and >> noting >> where it is for any individual boat to allow you to steer properly - and >> directional stability doesn't really seem to come into it. >> > Why is the pivot point (lengthways) not the lengthways centre of gavity? >
The pivot point on a boat is more a function of water drag than any other matter (but wind, and moving water can of course effect this). The hull shape of a boat even a narrowboat has a "keel" effect, and so when moving forwards in the water provides resistance to sideways movement. To turn a boat a lateral force needs to be applied, this is normally the force of directed prop wash. This force of course is on the rear of the boat, and if static the boat would turn at it's centre of gravity if the cross section along the hull is identical. Rarely is a boat a perfectly level loaded square/oblong, so this centre point isn't always exactly in the centre of the length. Now if we consider the above while moving, the effect of the boat moving through the water adds to the keel effect, which is why you don't have to crab sideways with strong cross winds so much when going fast. Now my brain can't quite think why the pivot point moves forward, maybe because the front is acting as a keel and trying to hold course, and the turning force is applied at the rear which moves easier? I do know that if a boat is loaded bow heavy the pivot point can be made to be very close to the bow (trialed in a small dingy :-) Victoria in her unloaded state turns not far in front of the engine room, as most weight is in the back, the bow only a couple of inches in the water moves easily sideways - sometimes too much (especially in reverse...) Of course, so as you add external forces - wind and tide, the boat can move in almost any direction including sidewards, shown here at 6min 40: http://www.youtube.com/watch?v=6BeyyrD_bMc Cheers, Mike -- Michael Askin http://shoestring_DOT_zapto_DOT_org/
