Definition of Planing

QUOTE=DCockey
There appears to be agreement that "a vessel's weight is supported more by hydrodynamic forces than by buoyancy" is necessary for a vessel to be planing.
its a play with word time again i see!!
Lets take a brick !! has no buoyancy what so ever !! it will plane across the surface of the water yes !! towed of cause !! but it will plane so is that hydrodynamic forces doing what they do and making it plane ???


A claim has been made that when a vessel is planing the trim angle must to go down if the speed increases.

why must the trim angle have to go down when it is increasing speed ?? some boats dont and the air at high speed holds there trim angle nose up and they just sit on the very back edge if the bottom of the hull . so that shoots that theory down the loo!

Just those two seem to answer the rest of whats written below . why is it so hard to determin if a boat is planning ?? it either is planing or its not !! whats so hard about that ??


What is a vessel, perhaps one with other than a prismatic hull, doing if above a certain speed its weight is supported more by hydrodynamic forces than by buoyancy but the trim angle does not go down if the speed increases? What term should be used for that situation?

Perhaps there will be a reply that this question is moot because the situation can never happen. If so then there is no need to use "trim angle must to go down if the speed increases" as a necessary condition to define planing. All that is needed is "a vessel's weight is supported more by hydrodynamic forces than by buoyancy".

But I'm not convinced that there are no vessels, including ones with non-prismatic hulls, which above a certain speed have their weight supported more by hydrodynamic forces than by buoyancy and their trim angle does not go down if the speed increases. So back to the first question - what is the vessel doing if not planing?

 

I don't know if there's an agreement about that planing requires more than 50% dynamic lift, even if it is agreed that dynamic lift is needed for planing... I think the more than 50%-rule is just a hypothesis, inspired by what I was taught in the -70s' by my hydrodynamics professor.

I think Yellowjacket explains here quite well how fundamental and inevitable the decrease in trim is when planing starts. To me it's fundamental like the decreasing leeway angle when boatspeed is increased.

At the very beginning of this thread, Don MacPherson HydroComp, Inc. presented a graph, showing how the trim decreases and VCG starts to rise and is higher than static. I assume that these both always happen when planing, but since the rise of VCG is so difficult to observe or measure, maybe trim-only is a sufficient criteria?

 

These both always happen for typical planing hull, which is prismatic (or flat) monohull with horizontal keel line and rather neutral thrust line. But there are several cases, which do not show the typical trim pattern. VCG rise is always present (maybe aerodynamic force could counteract that in some case?), since increasing hydrodynamic lift means decreasing buoyancy and displacement.

Has anybody considered wave pattern? What about defining the requirement for planing as monotonic decreasing wave height with increasing speed. That would be quite easy to see and measure. Would any displacement or semi-planing vessel be able to do that?

I also think that decreasing waterline is just as fundamental as trim change. Waterline works also for hydroplanes (defined for each contact point) and stepped hulls.

The decreasing wave height and waterline are both more or less caused by VCG rise.

 

Do you know that for a fact? With no change in external moments, only a change in speed? What I understand from Yellowjacket, trim change is fundamental, like angle of attack vs. lift for a wing, or leeway vs. speed (?)

 

Yes. Trim change is fundamental for a prismatic hull, but not for stepped hull or hydroplane or curved hull.

A clear example of this is a classical three point hydro used in many racing classes. It has three contact points to water and is planing on all of them individually. There is no trim change or trim change can be to any direction depending on the areas and angles of these three contact surfaces.
http://www.teampowerboating.co.uk/hydroplane/

 

OK, for many "hulls" planing individually, understood. What about a curved bottom hull? A hull with rocker, like most classic dinghies at least to some degree?

 

If high speed boats are considered to be planing, even though they may be just hitting the top of the waves and be airborne the rest of the time, how do we compare them to the ball analogy? Do they stop planing when the waterline becomes zero and there is no contact with the surface?

 

This is clearly more difficult to analyse. As an extreme example you could make the hull curved/stepped in a way that mimics the hydroplane.

Probably you could also play with selection of VCG, LCG and/or rocker in order to keep the trim high even at high speeds. E.g. some prismatic hull racing boats quite have high trim at high speeds, even with constant engine trim. Their trim goes first down but then back up again.

 

If there is no contact with the water surface, no forces from the water act on the hull. It means no planing, and no displacement either. In those instants just the aerodynamic forces are at work - the boat is in nearly-ballistic flight. It should be obvious...

 

We have never considered the waterplane area in the previous discussion. However, in the general hydrodynamic lift equation: L = 0.5 rho V^2 S Cl two things can change with speed - the lift coefficient "Cl" (depends on the trim angle) and the reference area (which can be either the waterplane area, or the wetted surface) "S".
futhermore, also in view of Leo's theoretical work on 2D planing plates and Ad Hoc's post #64, there's the known fact that the wave drag becomes progressively less predominant with speed when the boat is on plane.

So, perhaps hydroplanes, stepped hulls etc. could be included in the general definition of planing by stating that a boat of a fixed weight and fixed configuration of the propulsive device is in the planing regime if the following 3 conditions (and their combinations) are satisfied:

1) the wave drag decreases with increasing speed
2) the trim angle decreases with increasing sped, for constant waterplane area
3) the waterplane area or the wetted surface decreases with increasing speed, for constant trim angle.​

Are they sufficiently comprehensive, measurable and consistent with the practical observations, or do you folks have counter-examples which could proof them insufficient?

 

Just as devils advocate: is a lifting plate a hydrofoil with flow on both sides?
On power boats they would be trim tabs able to hydralicly push down to raise the back and lower the front
not a foil and not a foil shape water flow is under not over !!
If so, what if there was such a lifting plate forward?

 

You mean a hydrofoil with flow on just one side (the bottom)?
I don't see it as a problem. Considered individually, at sufficiently high speeds a lifting plate should behave like a planing object - and according to the above 3 conditions.

 

To answer this in such a way you either pulling my leg and playing !! or you never seen and or used trim tabs !!
Cant believe that !!

 

I am not pulling your leg and I know how trim tabs work.
But perhaps I didn't get the point of your question. Can you please be more specific - what exactly are you asking?

 

CL (lift coefficient) and CM (trim moment coefficient) will depend on trim angle and speed and draft/vertical height. While prismatic hulls have been studied most extensively I expect that other, fundamentally different hull shapes behave considerably differently.

What is wrong with defining planing as when hydrodynamic lift is a significant portion of the weight of the boat? What is the purpose of the complications?