Ben
The 18m proa has bow-up trim from 12 to 20kts if hull is clean. This is due entirely to buoyancy. Its largest bow-down trim under 20kts is 50mm at 10kts. Above 20kts it starts to dive. If it had flat sections forward then they would provide substantial lift at 20kts.
These numbers do not account for the poor shape of the bow that lifts water at substantial flow rate above 14kts.
For pitchpole resistance hull length is the main factor followed closely by height of CoE. Other factors are height of CoG, pitch inertia (generally lower inertia the better so concentrate weight around the middle in a harryproa), flat sections provide greater pitch damping than round, bows that dynamically lift rather than dive. Any bow with a flat deck and round bottom will dive if pressed below the surface at speed.
I have not yet determined how much buoyancy in the ends is enough other than for any giving panel area it is better making the hull longer rather than higher for resisting pitchpole. For a length constrained hull I am yet to work out if more buoyancy is better. With a proa the bow eventually becomes the stern and a buoyant stern will increase bow-down trim in following seas.
If you look at the progression of the bows of modern racing boats like A-class and amas on large trimarans you see lower buoyancy in the ends; flatter underwater sections in the bow and peaked or rounded foredeck that is narrower than the waterline. These factors contribute to the hulls drive through green water without much drop in speed while generating uplift through both buoyancy and dynamic lift. Apart from length these are the factors I would concentrate on to reduce tendency to pitchpole when driven hard.
If you have low buoyancy in the ends then good buoyancy in the mid sections of the hull contribute to lifting the hull above the troughs. This means the ends are submerged less.
If you are intent on shifting weight then move as much as you can at the level of the keel so it stays low, as little distance as you can to keep the pitch inertia low.
Rick
On 15/10/2012, at 11:49 PM, bjarthur123 wrote:
thanks for the replies.
rick: call me paranoid, but please let's at least entertain the idea of weight shift and do some rough calculations. i agree the boom idea is a bit crazy, but a sled or a two-tank-with-pump system as arto suggested are not.
arto: 400 kg. is that really how much weight would need to be shifted? or is that the weight you estimate for the things i listed (motor, fuel, batteries, water)??
harryproa.com is still down, so i can't look it up, but i recall aroha nominally weighing 800 kg unladen, and having a design payload of another 800 kg.
let's see here: a moorings 403 has a fresh water capacity of ~500L and fuel of ~130L. a similarly equipped harryproa would then have several hundred kilograms available to shift, but that weight would diminish on a long voyage. would be nice to be able to move the motor and batteries as well.
but still, how much weight must be shifted to compensate for the pitch moment of the sail lift? more or less than the combined weight of the motor, fuel, water, and batteries if placed on the aft beam? it'll vary with wind strength of course, but let's just say 20 knots. this must be a relatively simple calculation.
ben