tomtom wrote: ↑
Wed May 15, 2019 12:30 pm
And TT its almost not even planing its semi displacement planing
This, as I read it, is wrong.
As soon as water is moving off of the tail of a TT, that hull is planing and displacement is no longer a factor.
Displacement requires volume in the hull. The volume of water displaced is excluded from entry into the hull to the depth submerged. If a hole below the waterline is created in a typical (open) boat hull, water will enter and fill the hull. If not enough water is displaced by impenetrable internal flotation (such as foam blocks), water will continue to enter the boat until it sinks.
Planing occurs when only speed and a contact area, NOT displacement, is required to support the weight of the boat. When planing, volume can be eliminated, and only the planing surface contact area needs to be watertight.
If this is unclear to you, it is easy to do some thought experiments on your own such as:
Consider a small motor boat with a near vertical transom and a drain plug at the bottom of that transom. When the boat is moving at "displacement speeds", a hole in the bottom of the boat or pulling the drain plug will sink the boat. But at planing speeds, the boat moves fast enough for the water to not be in contact with the transom. In this situation, pulling the drain plug would not allow water into the hull. And if a hole smaller than the drain plug is present in the bottom of the boat, water entering there would run out the drain plug hole.
For those of us that used to windsurf, looking at the tail of the board when moving is a clear indication of whether you are at displacement or planing speeds. If the water is coming up along the back of the board and/or on top of the tail, the windsurfer hull is functioning in displacement mode. But when the water is passing under the tail of the board fast enough come straight out without being able to move up onto the top of the tail (any water on the tail runs off), then the windsurfer hull is functioning in planing mode.