Definition of Planing

A boat skims the surface -- a plane does not.

 

Thanks Frosty, regarding your canal observations and exposing real world experience to incorrect chair bound theories.

 

I take your point, but I disagree.

If the hull is being supported by hydrodynamic lift…what difference does it make how it is generated? This is a case of cause and effect rather than a definition.

A surface wave, is still defined by it length, amplitude and period. Whether a wave is sub-critical, deep water, or super-critical, shallow water, does not alter these definitions. All that has changed it the energy and angle at which they travel…a wave is still be defined in the same “frame of reference”, its length, amplitude and period.

No he doesn’t, they are modified owing to the lower induced drag, by the vortex shedding being altered by the proximity of the surface. But the “lift” is still “lift”, an upward positive force. The proximity of the surface to the wing does not change the definition of lift, only the way you calculate the lift.

That’s the problem, since there are so many applications that fall outside such perspective definition that currently renders a general definition, rather tricky, or silly.

 

May I disagree. Low aspect (I'm making here assumption of the displacement and waterplane area being the same as high aspect counterpart) means the wave resistance hump coming at lower speed when frictional resistance is still much less than in the case of high aspect hull.. Thou this is just based on my personal observations, no data to support it.
BR Teddy

 

The changes shown for CG seem rather small for a 43 m vessel. What CG change would be equal to 50% change in displacement? I suspect that you could easily detect planing in those conditions, since the change would be much bigger.

 

planes will skim if it gets close enough to the surface of the water !!! but then its likely to sink when its finished skimming !!

 

Always nice to know there is an aeronautical engineer around.

 

Yes of course

Well, the classic Bailey series says otherwise.



Here are 3 Fn’s and highlighted L/B ratio of 3.0 and 6.0. In each case the EHP is lower ergo the resistance is lower too.

Define small..and when is a rise, not a rise?

 

If we start from beginning planing was defined hydrodynamic lift being 50% or significant amount of displacement. A 43 m hull will probably have draft in the order of 1 m and 50% reduction in displacement must be significantly more than about 0.1 m shown in the curves. If for this example the threshold for planing was defined to be say 0.3 m CG rise (relative to static) for that hull, the differences between the two curves would be quite small and it would be clear that it is not planing. And a planing hull would probably show rather small difference of Fn for that limit and also the planing threshold can be a function of water depth.

 

Quote:
Originally Posted by Ad Hoc View Post
That's because the transition from sub-critical speeds to super-critical speed requires a lot of power to over come the increase in resistance where the depth equals the wave velcoity, noted here:

Attachment 76340 From Frosty
No--there was no shortage of power the bottom of the boat touched the bottom of the canal. The water at the front could not get to the water at the back fast enough. There was no water at the sides, running past at about 1 foot deep the boat hit the bottom. To the rear was a wave following and we were in a hole.

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This illustrates a difference between theory misapplied and actual observed practice in field conditions. Both depth and channel width change things such that open water theory is not at all applicable. Even a 65 foot, 6' 10" beam narrowboat in a narrow channel at 4kts will build up such a bow wave that it sinks in the hollow created. That is why it's illegal to go that fast in such channels. No one at the livery told us about this but we quickly found it out and drove accordingly.

Makes me wonder about speed records set by sailboats in places like the French trench. How much aid do they get from the added lift of shallow water?

 

If I understand the Bailey graphs right it's rigid lwl and only the beam varies, so it's a huge difference in waterplane area. IMO it's not telling much when the transition from displacement to planning might happen.

 

What I would ask about the Bailey graphs: what is the parent hull form?
Looking at http://www.staatsgeheim.com/wp-content/uploads/Savitskyreport.pdf page 29 figure 4 I suspect the hull under consideration is following the "overdriven semi-displacement hull" resistance line, not the one for planing hulls. I don't have data ready at hand to cite, but I've seen graphs for planing powerboat series for which there is a clear crossover point where increased chine beam results in lower drag at higher (planing) speed.

 

Savitsky's method assumes a prismatic hull. THere will be plenty of hulls that can plane but are not prismatic. Making any reference to Savitksy will be too restrictive.

 

This is true of prismatic hulls or those with hook in their buttock lines. However if you add some rocker into the buttock lines you still have a planing hull but one which will continue to increase its trim until it becomes unstable. Nevertheless remains a planing hull.

I would be very cautious to add a trim criteria onto any deifinition of planing.

 

There is the notorious 1950's ad by Mercury Marine of a guy with an outboard mounted on a table cruising along. The table is planing. Without dynamic lift, the flotation is negative and the whole contraption would sink. What would the change in attitude or trim be in this case? A good definition should either cover all cases or have clear limits.