A three pointer is delicately balanced with most of the buoyancy in the sponsons to support the engine weight when stationary and the larger planing surfaces running from just behind the centre of gravity to well in front of it so the front lifts first to keep the full propeller buried until some speed builds then as the speed increases the propeller shaft lifts the rear until tit is planing on the third point being a small flat on the bottom of the skeg. This third point keeps the propeller at just the right height so it is fully surfaced with only the bottom half in the water and the propeller shaft and all but the planing surface of the skeg out of the water.
If the shaft angle is to steep it drives the nose down, if it is to shallow, it drives the nose up.
If the shaft axis passes through the hull to far forward, it lifts the nose, if to far back it lifts the tail.
The gap between the floor, the inside edges of the sponsons traps air and creates a ground effect. Trim angle of the floor determines mount of aero lift and longitudinal position of the sponson in contact or very close to the water surface controls where the lift applies.
If hydrodynamic lift is to great at the front the front lifts rapidly then stalls and falls, creating a porpoise like motion. Same if there is unstable suction at the back.
If aero lift is to high at the front the front climbs then as the angle of attack increases the lift, especially at front increases until the boat kites and becomes airborne and comes down upside down from a back flip or barrel rolls. These are the shots that make the news.
On a race boat you want the maximum aero lift you can get without exceeding the boat weight, so you maintain a minimum safe weight on the water surface. Waves and head winds both work against maintaining that balance and that is the razors edge of flying these machines.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
