Definition of Planing

Yes, as I said in one post, you can very readily modify the hull to create either powered lift (the aviation term for using power to augment the natural lift of an airfoil) or using tabs or auxilary surfaces (like a whale tail on an outboard) to alter where the hull would begin to plane.

And I am sure that added power can not only reduce the speed at which planing is achieved, but it can drastically alter the trim angle. The question that it begs is what would happen if the speed was increased and the powered lift, or the tabs weren't reconfigured?

The short answer is that it would bury the nose as it came on a plane, and without a configuration change the trim angle would go down, which is consisten with what I was calling planing, the point where the trim angle come down as speed increases.

So really, if the configuration is changed as planing is achieved, that's ok. I'm just saying that if the configuration is held constant and the trim angle goes down, then you are planing. I don't see anything in the system that you described that is inconsistent with that.

 

The 50% lift definition is difficult to apply because one doesn't really know just what the breakdown is with regard to buoyancy vs dynamic lift. The Froude number definitions don't work well because different hulls can be traveling at the same speed and have such different amounts of dynamic lift that one would be planing and the other not. A Farrier trimaran is a good example of the latter. Trim angle is not a reliable indicator, either, when different hulls are considered. Similarly, a high speed displacement hull will have a dry transom but not necessarily be planing.

I think the drag hump is the best indicator of whether a boat is planing or not. I'm talking about a hump in the total resistance curve, not the resistance coefficient. I realize wave drag can cause humps in the resistance curve, too, but the experimental data I've seen shows the planing drag hump to pretty distinctive.

If the boat is going faster than hump speed, it can be considered to be planing. I think this definition has the advantage that it is experimentally observable, does not require characterization of hull forms, and is widely applicable to different types of boats. Displacement and semi-planing hull forms are included.

If a slender displacement hull has a monotonic drag curve, then it can be considered to be of a non-planing variety. There will generally be a change in slope of the resistance curve even for a displacement hull, but if there's no reduction in resistance then planing can probably be considered to be a minor actor in the boat's performance.

The reason this definition makes sense is because the drag due to dynamic lift decreases with speed, which is contrary to most other sources of drag. It's the decrease in lift-induced drag with speed, along with a reduction in wetted area due to dynamic lift, that causes the hump in the resistance curve. It's also the whole point of designing planing boats in the first place. If you need a precise definition, then this one at least gets at the overall objective of the design.

 

What will the definition be used for? That's not a rhetorical question. Rather it should have a direct bearing on the choice of definition.

 

Ok, so after 6 pages of various proposals and counter-proposals, we are still at the point zero.
Leo, looks like you won't have a mathematically defined and universally accepted solution this time...

 

"I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that."
Potter Stewart, US Supreme Court Justice, Concurring, Jacobellis v. Ohio, 378 U.S. 184 (1964).

 

Maybe planing is like time: we all know what it is, until we think about it

 

I agree that the resistance curve is a good indicator and, as Ad Hoc noted,
the skin-friction becomes dominant once planing sets in.
I also think that the Froude number based on the distance of the centre of
pressure from the stern is the correct one to use as a base for predicting
the onset of planing. The standard length-based Froude number is not quite
as useful in this context.

 

From a non-expert's angle, it could be pointed out that Uffa Fox's International Canoes seem to prove that boats with no transom and no "sharp corner" (however one wants to define that) can plane, ergo such factors cannot be in any definition of "planing".

There were also longboard windsurfers with U shaped hull sections that were almost, or definitely, double enders that planed. Most had "sharp corners" but at least one did not and it felt pretty much the same as the planers. And it would not be hard to round off the "corners" on many craft and still have something that would plane (is a racing skiboat driven by a supercharged 427 really going to stop planing if you round the lower edge of the transom off over a 1' radius?) surely the "corners" are irrelevant.

As others have pointed out, commercial and sailing cats can have "clean" or separated flow at transoms and not be planing hulls, and some longboard windsurfers have the same effect at very low speeds, so surely that can also not be a requirement?

 

A clean flow exit imo is a requirement. Matematicians would call it a "necessary but not sufficient" condition for planing. What could be dropped, in view of your experiences with non-chined hulls, is the requirement to have a hard-chined exit line along the bottom. Though I'd suspect it is not beneficial for the high-speed performance.

Don't know if you guys are familiar with the sport of waterpolo. I've played it in my young days. There is a type of shoot in which the player throws the ball towards the water surface at a small angle and imparting it a very high speed. The ball hits and skims on the water surface (in a fraction of a second) and then bounces back often at a much higher angle, thus deceiving the goal keeper. It is called "skip shot". In that short time period the ball planes on the water surface and then leaves it again, pushed upwards by the lift force. It is a dynamic effect, not the hydrostatic one, because a high forward speed is required for skipping to happen. I would love to have a slow motion of a high-frame rate video of the moment it skims on the water, to see if there is a rooster tail behind it or is the flow cleanly separated along a clearly defined line on the rear part of the submerged spherical cap. And, being a ball, it has no chines.

P.S.:
Found one video of a skip shot, with a nice slow motion at 0:25 : http://youtu.be/Y9WjRfOe2-4 .
Didn't like the fact that it shows the Croatian national WP team receiving the goal but, hey, we have to let others make few goals too, or they won't want to play with us anymore!

 

The shorter answer is that..it works like it does. Simply an example where the "complete boat" achieved planing from zero speed without a "large bow up trim" transition occurring and so yet another example where a simple definition is potentially hard to craft.

I do agree with the suggestion of a "some percentage of displacement is supported by dynamic lift" ....

 

I suggested the idea of 50% hydrostatic, 50% hydrodynamic being an
indicator of planing.
And then I thought, maybe it might be useful because it would indicate
semi-planing, e.g. 70% hydrostatic, 30% hydrodynamic.
But now I'm wondering if that would be misleading, or difficult to estimate.
For example, 100% hydrostatic suggests a simple displacement vessel.
But what is a vessel that is supported by 100% (hydrodynamic) lift, and is it
achievable, or is it just a limit?

 

You miss the point.

Planing is by definition the use of a (relatively) flat surface to generate hydrodynamic lift to support the weight of the hull and reduce the overall drag.

You can always install hydrofoils, large trim surfaces or as you noted a means of directing thrust vertically to drastically alter the speed at which planing occurs and the resultant trim effects can be changed using those means to attain any trim angle you want.

What I'm saying is that for a prismatic hull in any one specific configuration, if you increase the speed and the result is a nose down change in trim angle you are planing.

Of course, you can apply thrust and drastically alter the trim angle. Heck, you could apply enough thrust and just vertically lift the entire hull out of the water. But that's not planing. You could, with vectored thrust take enough weight out off of the planing surface so that you can plane at a much lower speed. And that is fine.

But once you plane (by definition, you are supporting a given weight with hydrodynamic lift generated by the flat surface of the bottom of the hull). The fundamental relationship between lift and trim angle at speed must apply. If you go faster, then the trim angle must go down to remain in equilibrium. That is, as I said, fundamental to the concept of using a flat surface in the water to generate lift.

If you change the configuration (move tabs, change hydrofoil angle, or change vertical thrust) then that's a different case and of course you could make it so that the bow came up as the speed increased. You could do the same thing with moving weight forward and aft, but then that's also a different case.

What I'm saying is that for any specific configuration (and that implies a given FIXED configuration, trim tab or after plane setting, thrust line or CG position), the system can be considered planing if for a speed increase, the trim angle goes down. Just because you can jack around with other factors and change the trim angle doesn't alter the basic definition.

 

You can't get to 100%. There will always be some displacement effect although as you go to very high speeds it will be very small.

Since it takes some trim angle to generate lift, from the point where it touches the surface to the rear of the hull there is a small amount of water dispaced. This displacement is real, and the heavier the hull and the larger the trim angle the greater it is. In Savitski calculations it is always included as part of the overall lift. You shouldn't ignore it unless you are traveling at really high speed and have a very light boat.

To put it another way. If you had 100% of the lift coming from hydrodynamic lift, you wouldn't be touching the water, so yes there is a limit. Any small amount of draft below the surface at the trailing edge is, by definition displacement lift, and it counts.

 

I was half joking about the 100% limit.
However, if you wanted to get really nerdy, you could argue that at 100%
the only portion in contact is the boundary layer which, technically, is not
part of the displacement of the hull. Therefore, 100% dynamic lift is possible,
and perhaps, even 100.0001%.

 

The complication arises when you consider many boats that also use aerodynamic lift at high speeds. Three point hydrofoils and sea sleds come to mind.