Definition of Planing

Low aspect ratio hulls, in theory, produce better planning hulls. However, wave making resistance is proportional to beam^2, thus making a hull wider may make the main hump resistance too much to get onto the plan, unless huge amounts of power...which is heavy and so goes the design spiral like little Alice...

Does anyone have data to support this?

 

Shallow water is a special case. If the depth of the water is low relative to the length and beam and the static draft is a significant portion of the depth, the problem becomes one of a hydrodynamic bearing, and not one of normal planing.

To put it another way, if the water can't get out from under the hull, it will create more hydrodynamic lift, more like a hydrodynamic bearing, than it would ordinarily create in deeper water. But unfortunately, since the water can get out from under the chines, it is much more difficult to calculate, although if you took the length, beam, depth of the water, and mass of the hull, I'm sure you could apply hydrodynamic bearing theory ad write an equation for it. The problem is, since the lift is now more related to the effect of the fluid being squeezed between two surfaces (the hull and the bottom), it isn't really planing in the same way as it would be in deeper water.

We've all seen, or been (don't aske me how I know this for a fact) a water skier cartwheeling up onto shore, who thought he was going to sink in and then just step out of the ski as he reached the beach, only to seemingly accelerate (at least that's what it seemed like I swear) when the beach was suddenly fast approaching and you weren't slowing down like you thought you would.....

So in reality, the effect of shallow water, while it does produce a planing effect, it isn't the same as it is in deeper water and therefore the response isn't the same. You need to look at hydrodynamic bearing theory as opposed to planing theory there.

 

What you're saying, Ad Hoc, is that some boats are unable to achieve planing speed. OK - but does that pose a problem for this as a method of determining what planing speed is?

Also, when you say wave making is proportional to beam^2 are you comparing boats with equal fairbody draft or boats with equal midship areas? Or are you saying it doesn't matter? I've seen lightweight flat bottom boats that were wide yet made very little wake.

 

Interesting !!

Shallow water is a special case. If the depth of the water is low relative to the length and beam and the static draft is a significant portion of the depth, the problem becomes one of a hydrodynamic bearing, and not one of normal planing.

This i very interesting Hydrodynamics bearing theory !!! i have seen this from the deck of a boat traveling over shallow water covered with weed .
So in air this would be called surface effect or squash is what airlines do every day of the week and how speed boats can end up so high up on a sand bar when they run agound ! have seen many times but never really known or thought about it much !!

 

For an introduction to the topic see http://en.wikipedia.org/wiki/Squat_effect

 

Yes, exactly. Wings in gound effect produce more lift if the total height above the ground is greater than a specific percentage of the semi span. Same for boats, it's exactly the same thing. And as many pilots of overloaded airplanes have found out, you can lift off the runway in ground effect, but that doesn't mean you can fly....

 

So at planing speeds boats experience the opposite of squat?

 

Indeed it is, however, the effects of shallow water are well understood, in that the energy of the wave system changes, owing to the maximum wave that is produced owing to the depth of water and reaches critical at Fnd = 1.0. When going from sub-critical speeds to super critical, the effects on resistance are dramatic. But this is also applicable to planing hulls too.

The point being that there are endless caveats, displacement hulls, shallow water etc etc, which makes any definition unsatisfactory, especially if the number of caveats outnumber the actual definition in words!

When I was a teenager there some that wanted to go one better than we had with our RIBs, fitting huge outboards, with loads of power. But they couldn’t plan because their boat was now too heavy and couldn’t get over the hump! They couldn't understand why...

Beam as in max beam, wherever this may occur along the hull.

 

I agree shallow water is critical,--if you draw say 2 feet and the water is only 6 inches deep planing would be difficult.

 

Excellent post and great graphic. As I noted earler, I don't know for sure if the change of trim angle to nose down is the best (although in this case it appears to be) or true defintion of planing. But in this case it is clear that when the height versus speed characteristic hits the flat part of the curve you are planing. And this set of curves shows the roundoff of the lift curve very nicely.

The shorter the hull and the further aft the CG, the steeper and more peaked the trim angle curve will be, and the more pronounced the change from the climbing curve (semi planing regime) to the planing section of the height change curve will be. For very short hulls, the trim angle curve becomes a cusp and the change from semi planing to planing is very pronounced.

For longer hulls with much more forward CG's the curves are much flatter. If the aft lines of the hull have any rocker to them, or if the CG is well forward, then the change to planing is not going to be well defined.

The reason for this is that as speed increases, the aft hull is generating negative lift. This negative lift increases the load (and drag) on the planing surface, and, since more lift must be created, the trim angle doesn't go down, and at higher speeds it could even go up. While planing is achieved, it would take a large amount of power to drive it much faster. I still think there has to be some increasing trim angle to attain planing, but the further forward the CG is, the lower the trim angle is to start with.

 

Ever tried taking a sea boat on a canal and trying to get it on the plane.

It wont do it. Works fine in open water but not on a canal.

A huge wave appears in the front and water runs past the boat so fast it dissapears almost,-- seeing bike frames and bed spreads. The boat is then facing a massive bow wave then touches the bottom of the canal. Stopping the power then lets the boat rise up and floats normally. The huge wave runs up onto the side path of as much as a 2 foot tsunami and into a field that made the cows run away..

 

Today i have learned something valuable ! something i have seen and felt the effects of but never knew the name of !
Acting a little dumb has its merits and can be used to good effect most times to stir the pot !

 

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:

 

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.

This is of course very illegal. Lancaster canal basin---sea entry Glasson dock.

 

No, if you are trying to define a specific condition, and if the fundamental mechanics are different, then you can't use the same definition. Just because the hull is being supported by hydrodynamic lift, that doesn't mean the mechanisms are the same in shallow water as it is in deep water, and even thought it might appear the same, it's a totally different thing.

Think about it. An aero engineer doesn't use the same set of equations to define lift and drag in ground effect as he does to define lift in the free stream. The forces are different, and the effects are different, and there is no rationale to use the same definition of lift or planing in shallow water as there is in deep water. Just because it looks the same doesn't mean it is.

You have to restrict your definition to the system that applies to physics of the situation. As I think we can all agree, the mechanisms creating the lift are different, and the same definition does not apply. So the definition might apply to prismatic hulls in deep water, that's ok. If you don't have a prismatic hull, or if you are in shallow water, that might not apply. Sorry, but I don't see a problem here.