Will it Plane?

[TeachSB]

No worries Bill. Glad we could come to an understanding.

[blueplanetsurf]

We give a little more of a fig, Robert, since we are dealing with slow speeds, long skinny hulls, and a really weak power plant.

Understood, but do you agree that:

"If the percentage (of dynamic lift) is greater than 50%, we should be safe in saying that the boat is in the planing mode." ?

[starman]

Will it Plane?  is starting to sound like a Monty Python skit;

[TallDude]

Or even more to the point.

[Pierre]

This afternoon DW session, I cannot be sure if or  how long  I , and my board, planed, but so far, that has been some FUN level... which is more measurable that gravity effect vs planing ability.  Prolem is not: "Will it plane", it is: "will it be Fun and Fast", solution is in our heart.
#Hi-Fun Hydroworks.

[Beasho]

"No Chaos No Learning"  My father, Professor Chase, who taught at the Hotel School at Cornell for 40 Years.

Last year I read up on Bob Simmons to understand just WTF is going on under the water when planing. 

Summary:  The only 2 forms of lift available on the water are Displacement and Hyrdrostatic lift.
Displacement is how much water the vessel impose/displaces in the water.
 
Hydrostatic Lift is the more interesting component.  The word at its root describes what is going on Hydro (Water) + Static (Stationary).  The water is stationary, not flowing, we are moving or slamming through a FIXED medium.
 
Introduce the Conservation of Momentum:

“For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.”

As our boards move horizontally, nose up, we are pushing the water down using what is basically an inclined plane.  The faster we move the harder we push the water ‘down.’  This downward pressure or force can be calculated as the water goes from a resting state at the surface to being pushed ‘down’ in a fraction of a second to the depth of our wake.   

To test this theory I ran the numbers using the rough tail shape of my board and assuming a board speed of 20 mph and the depth of my tail below the surface ~ 2.5”. 

By hook or by crook the result came out to 200 lbs of hydrostatic lift.

[Beasho]

Building a table of values shows that the amount of Hydrostatic lift increases 600% from 10 to 18 mph.  When I used to windsurf with a SpeedWatch I noticed that planning started at 17 mph.  This would be consistent with the data shown. 

Add to this when we are on a wave and the required Hyrdostatic lift DECREASES as the COSINE of the angle of the wave increases and we start to fly.  Add a square tail, like on a SimSup, and the hydrostatic Lift profile can increase another 20%.

[PonoBill]

Nice, Beasho. I think that's all about right. People talk about the water flowing, but as I said, I wonder what they think is making the water flow? We are indeed banging into it, and yes, Robert, the accepted percentage of hydrostatic lift for planing is 50 percent. Of course there are hulls where that isn't enough to make them really plane. Lots of reasons. One reason that round bottom boats aren't considered to be planing with that much lift is that they fall over on their sides. There isn't any stable surface for them to plane on and be stable laterally. Tail rocker is much the same--no stable surface. A very small change in attitude or penetration of the hull into the water makes the hull pitch, and also tends to drastically reduce the area. Reduce the area Beasho posits by half or more as the board comes up to plane and most of the lift goes away.  You might bounce a few times.

the mathematics book I'm reading and rereading (lots of great material) offers very interesting insights into how people fix on specific ideas, and how a rational thought process that's mathematically and statistically defensible can be categorically wrong. the title of the book is "How Not To Be Wrong", and I highly recommend it. I'm in my second reading, and I'm going to go through it again taking notes. I think it might help you Beasho, in convincing your clients to take more notice of statistical proof, or at least to have a quantifiable understanding of the Bayesian Framework that might lead them to reject concepts that go counter to their prior experience or prejudice. It's helped me understand financial markets better, and should make the book I'm writing on retirement a little more useful.

I know conversations like this tend to bore some folks, and piss off others, but they help anyone interested in the topic to think about their assumptions and probably learn a bit. I enjoyed it, and learned a bit,  but then I like Monty Python too.

[blueplanetsurf]

Thanks for this conversation, I learned a lot and it made me really think about what planing is and what it's not and what makes a board plane.

Here is my scientific diagram on why I think that you are wrong about rockered boards not being good for planing.  We agree that a flat bottom is best for planing on a flat surface but when we start to plane on a stand up board, we are on a curved surface of water when we catch a wave or a bump that gets us up to the speed needed to lift the board out of the water.  On a curved surface, a rocker that matches the curve of the water will create more lift and plane easier because it matches the contour of the water, while a flat rockered bottom (like the one that Bill is riding in the illustration) fights the shape of the surface and creates more drag.  On a flat bottom board you have to lean further back to keep the nose from plowing, which creates a lot of extra drag.  So in my opinion, on a curved surface a flat board will NOT plane as well as a board with a rocker line matching the water surface.

[Area 10]

Yeah but when you are "gliding" or whatever it is called when you are riding a bump downwind, the whole surface of the board isn't in contact with the water surface like your diagram shows (for the rockered board). At least it isn't in the conditions I downwind in. To catch the bump you stand towards the middle to trim the board for maximum paddling speed, then as soon as the trough opens up in front of you and you've paddled into it, and the glide has started, you get back fast and ride the board with the front half of the board sticking out of the water, like when riding a wave. So there's no point when actually bump-riding that the board is positioned like you show it, is there?

[PonoBill]

Certainly true to a degree on steep waves going straight down the face, though that tends to be a pretty slow way to surf. When we go across the wave face it's less true. Having the tail "dig" into the wave face doesn't really increase drag substantially. Certainly it takes some energy to push the water away, but the board probably gains more than it loses. The skin friction of a board conforming to a wave is higher than one that doesn't. That's a good thing--a lot of the kinetic energy of the wave gets coupled to the board by skin friction. It's why high rocker boards grab short period waves so well.


I dug up some internet stuff about big wave guns. Not at my home computer so I don't have the link handy, but the gist of the article was about how the big wave board shapers developed guns with substantial rocker and plan curve (nose to tail) expressly to keep them from planing. They wanted the board to "stick" to the face of the wave, both on the drop and on the face to keep it controllable. I think Kai's big wave SUP is not designed that way. I'll ask him about it next time I see him.

Thanks for the hair by the way.  It looks like a lot more than I actually have.

[blueplanetsurf]

Yeah but when you are "gliding" or whatever it is called when you are riding a bump downwind, the whole surface of the board isn't in contact with the water surface like your diagram shows (for the rockered board). At least it isn't in the conditions I downwind in. To catch the bump you stand towards the middle to trim the board for maximum paddling speed, then as soon as the trough opens up in front of you and you've paddled into it, and the glide has started, you get back fast and ride the board with the front half of the board sticking out of the water, like when riding a wave. So there's no point when actually bump-riding that the board is positioned like you show it, is there?

True, the image shows lift created before the board releases and wetted surface is reduced, but even if only the last foot of the board is touching the water, if the rocker conforms to the shape of the water surface it should create more lift than a flat surface.  So on drawn out, almost level bumps a flatter rocker works better while in steep, short period swells having more rocker works better.

[blueplanetsurf]

Thanks for the hair by the way.  It looks like a lot more than I actually have.

Glad you like it!

[stoneaxe]

I had to fix your illustration Robert....too much hair, not enough body, and it needed leggings....

[mrbig]

Semi-displacement hull is a valid type according to a multitude of boat hull sites I checked out. New England Lobstah Boat a primary example on several sites..

But, if I am standing on the tail of my biggest sup preparing to do longboard floater rentry it is not planing - sigh!     .