Froude and planing

sandhammaren
Why do you think that the bow rises?

 

That is quite shocking to imagine. Can you give an example?

 

A waterski?

 

Good answer, but the "transom" is hard to detect !

 

Lift doesn't "set in sharply", it increases with speed. The change in trim is due to bow wave, not due to stern starting suddenly to lift.

 

Correct.
And it also looks like there is a misconception about the difference between (or the effects of) the transom ventilation and the flow separation.

 

Has any experiment with pressure sensors peppered all over the bottom of a planing hull ( flush ) been done ?

 

Rastapop
That is quite shocking to imagine. Can you give an example?

Originally Posted by W9GFO
A waterski?


No, that is an incorrect view of the situation. The displacement of any craft is the amount of water it displaces, whether the ski has that much volume or not is immaterial. All that extra displaced volume of water is caused by the dynamic motion of the ski but it is displacement whether static or dynamic.

This is a tricky situation but all of the above seem to be correct if the proper physics are applied.

Edited to add: By all the above, I mean the comments posted by others are correct if interpreted properly.

 

I did not mean to kill the thread discussion. Based on earlier comments, surely some one has a opinion related to what I called dynamic displacement?

 

I've read several papers on flow separation from the bottom edges of transoms. As daiquiri suggested if the the beam at the transom is large compared to the static submerged transom depth then the Froude number based on static submerged transom depth tends to be a good indicator for of the speed at which there will be clean separation from at the transom. The speed at which the flow separates cleanly from the transom is not strongly influenced by the length of the vessel. For vessels with a static submerged transom depth which is very small compared to the length of the vessel clean separation can occur at very low Froude numbers based on vessel length.

I also saw a video of the flow behind a hull with a submerged transom which was accelerated very quickly to a steady speed. Initially a "hole" was created in the water where the boat had been sitting, and then a transverse wave formed which followed the boat and broke forward towards the boat.

 

The bow often rises to about 30 deg on an outboard or stern drive v-bottoms when you accelerate from rest. These boats typically run high rake props, which encourages that. Some racing tunnels accelerate from rest with relatively small trim angle increase. Again, the props are high rake.

 

Newman would have had no problem detecting it: it's the end of the waterski. Which brings to mind: some of the older waterskis were squared at the transom, slalom skis often were not. So they mushed aft.

 

Lift sets in sharply, exactly when the Kutta condition holds: the flow separates tangent to the bottom at the transom. At that point the lift coefficient is nonzero. The lift force increases with the squarer of the speed but is zero so long as there's backflow up the transom.

 

What exactly do you mean by transom ventilation?

 

A waterski with a high trim angle creates large form drag, large waves. Skis that run at a small trim angle are harder to control than those that mush at the transom.