# Planing speed

Discussion in 'Hydrodynamics and Aerodynamics' started by 7228sedan, Nov 26, 2013.

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### 7228sedanSenior Member

Looking for input on what has more impact on planing speed, weight, or hull shape? Should a planing hull with a single engine plane at a lower speed as there is less weight than it's twin engine counterpart? Or is it more of a function of the shape of the hull which would be the determining factor of the planing speed? If the hull in question is sub plane with a large bow wake at 10 knots, would the same hull with a single engine be sub plane with less wake at the same speed due to the lower weight?

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### 7228sedanSenior Member

Or, in the equation of (1.34*LWL^.5), how can I calculate the actual factor for the hull in question? FYI The boat is a Maxum 2900 SCR. Does the displacement of the boat come into play when calculating the "1.34"?

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### BarrySenior Member

You mentioned in your second post the equation that many contributors use to determine a theoretical maximum hull speed. This equation does not apply to planning hulls. It is a number that pertains to displacement boats only. Displacement hulls are slow moving hulls which move the water slowly out of the way, and then let the high pressure area, that it has built up ahead of say the midpoint of the boat, the boat recovers some of this energy toward the stern. For illustration purpose only, if you were to drop a ping pong ball ahead of the boat on the centerline, the pingpong ball will stay against the hull and exit at the centerline at the stern. Pressure recovery,

But you have a planning hull at planning speed, and this fictitious ping pong ball would run down the side of the boat until it hits the transom and separates from the hull at this point, The hull is supported by hydrodynamic forces only, and the weight of the boat is supported by this hydrodynamic force,

So by planning speed, I guess you are meaning at what speed will your boat plane. The definition that I would use for a boat that has a planning hull design, square transom, straight lines in the last half of the boat (could be warped but lets leave this out) is the speed at which you can see the entire transom of the boat. Ie the water that was turbulent behind the transom but still offered some displacement buoyancy is gone and the hull is completely supported by the hydrodynamic forces.

What has more impact?
Take your 29 foot planning hull
I assume that it is a deep v which gives a good ride in rough water. If you were to flatten the bottom of this boat, a lower v, this boat would plane at a lower speed

If for your current boat, you take 2,000 pounds out of it, the boat will plane at a lower speed as you do not need to create the same amount of hydrodynamic lift

If for your current boat, you could add say an additional 1 foot in width, you would increase the length of the high pressure stagnation line, the boat would plane at a lower speed.

So hull shape is paramount, as is weight.

Re what has more impact on planning speed, weight or hull shape

A planning hull

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### PARYacht Designer/Builder

A kitchen table turned upside down, with an outboard bracketed to it can plane. It can actually do it fairly efficiently, if getting to plane quickly is your goal. All three aspects of your premise come to play as Barry has mentioned. The percentage of which is more important isn't as important as the "concert" they voice with each other.

Simply put, you can have a light, well shaped, yet under powered set of shapes, that don't do well and the other possible combinations, are just as important. It's a package deal, kind of like in-laws. You don't get one, without having to drag along and accept the other portions of the equation.

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### gonzoSenior Member

Basically, if you use the same standard for defining planing, all boats of the same waterline length will plane at the same speed regardless of bottom shape. They will require different power to do so though. However, planing is a dynamic condition where, depending on the hull shape, the waterline length may change dramatically with changes of speed.

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### Mr EfficiencySenior Member

Well, if that is so, there should be a simple scale we can peruse that shows the "planing speed" for a given waterline length. I can't say I've ever seen such a table.

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### 7228sedanSenior Member

I understand that there are different factors based on hull shape, and other factors. As the equation inicated the 1.34, I assumed that there was a set of standard to calculate an appropriate factor for the particulars of the hull. The purpose of this exercise is do determine if there is a way to increase the planing speed in order to increase the boat's efficiency sub plane. Can the hull support a hull speed of say 10 knts pre plane as opposed to the 6.9 knots current experiencied by decreasing weight or converting to a single engine?

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### JSLSenior Member

A quick 'seat of the pants' formula I have used for absolute minimum planing speed is

V (knots) = 4 x LCG/ square root of B

LCG is for'd of the transom. (usually about 40% of the LWL). B is chine beam
eg: a 25'wl x 10' chine
Vmin = 4 x 10/3.16 = 12.6 kn. 13 knots
Of course, faster is better. And with more input you can get a more accurate figure
for SI (metric) replace the constant 4 with 7.2

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### gonzoSenior Member

That constant in the planing formula is the source of endless arguments. The fact is that there in no consensus of what is the minimum planing limit. However, at 4 times the hull speed we would most agree the boat is planing.

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### PARYacht Designer/Builder

Most boats will be considered planing in some form once above S/L 2, though it's debatable if this is really a full plane, while most agree 2.5 to 3.0 the range most agree full plane occurs.This is correct, there's isn't a definitive answer to minimum plane speeds, though most will agree it's somewhere around 2.7 to 3.0 S/L, with 4.0 being well into a full plane regiment. Again there are several functions to consider, with some shapes completely circumventing any formula, constant or descriptor.

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### BarrySenior Member

As Gonzo suggests there are endless discussions about what the definition of planning

I want to take a stab at it, though I know there is an area that will cause some debate.
Just to keep this effort clean, I will only refer to a planing hull, lets leave the other displacement hull issues out of the discussion. Planing hull, say 20 degrees deadrise, 8 foot chine width, 30 feet long, parallel chines to keel in the aft 2/3, at rest sits 2 degrees bow up, transom square to the keel

We would all agree that a non moving vessel floats due to the buoyant forces acting vertically upward on the hull.

At this point the only forces on the hull are buoyant forces.

We apply power, the boat begins to move at 3 knots, the speed causes some hydrodynamic forces ( planing forces) to act on the hull.

The boat is now supported by small planing forces and larger buoyancy forces

We put increase the boat speed to 6 knots. The planing forces become higher, and the buoyancy forces become less as the boat is now beginning to lift out of the water. The boat is higher out of the water than at rest when only buoyant forces acted on it.

There is still water against the transom but the pressure in this area is approaching atmospheric. The water coming down the side of the boat has not separated at the transom sides

We apply more power to 10 knots at which point the water separates at the transom, the pressure on the transom is atmospheric, ie no water pressure. The planing forces are large enough to support the boat and the buoyancy forces ZERO ( and this is where the discussion will get interesting)

This is the point that that I would call full planing. Not efficient planing as the bow will still be high, as the Center of lift is quite far ahead of the CG.

The reason that I suggest that there are no buoyant forces at full planing, the hull is being supported by dynamic forces only is that by definition only, the body of the boat is not immersed as Archimedes principle states. One side, the transom is not immersed in the water. There are some naval architects who say there are still buoyant forces but cannot quantify them and some who say dynamic forces only.

In any case, if you use my definition of when the boat becomes full planing at the speed at which the transom becomes exposed to the atmosphere and loses any benefit from buoyant forces by definition of an immersed object, we have a numerical value for a specific hull full planing speed.

I expect some strong response to my statement, that there are not buoyant forces acting when the water separates at the transom. My question to this will be " show me some calculations that will put a value on dynamic forces vs buoyant forces at different planing speeds"

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### DCockeySenior Member

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### Mr EfficiencySenior Member

Isn't it all just pressure, as modified by the motion of the vessel ? I'm sure the hull doesn't know the difference, it just encounters pressure.

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### gonzoSenior Member

It is and interesting observation that the transom only has atmospheric pressure. I think that there still is displacement though. The difference is that the hole left behind the boat is also displaced water which uses requires power.

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### BarrySenior Member

David, I read quite a bit of the 47 page discussion on planing looking for a quantitative analysis to define planing, and the other ongoing thread about semi planing, semi displacement etc. and have come to a conclusion that there are as many ideas on a definition as there are contributors. Some with marine design credentials and some with opinions and yet a quantitative answer eludes them as well as us or we would not be having to try to get a handle on it

Mr E, there is no doubt that there is pressure acting on the hull, but if buoyant forces by definition result from immersion, with all sides of the water in contact below the water line, would there be buoyant forces on the hull when I the water is moving past the hull at speeds say from 10 knots up to 100 knots and one side of the hull is exposed to atmosphere?

Gonzo the transom would actually a little less than atmospheric as exhaust gases existing through the transom will often stay with the transom.

In Sedans question, and I fear that I have lost site of what he is trying to do. But in his last post it appears that he is preplaning at 6.9 knots, though I do not know his definition of preplane, ie is this the speed at which his transom goes air only. And now he wants to find a way to have this move to 10 knots, to gain efficiency in this subplane speed.
He wants to do this by reducing weight but if he can plane at my definition at 6.9 knots, loses some weight, then he should be able to plane at less that 6.9 knots
Hard to understand without his definition

But what we do know from experience is that as the boat is approaching my definition of planing which usually occurs around 10 knots, the bow comes up and the fuel consumption is high, the miles per gallon is higher than when the bow is coming down to a more favorable attack angle. But sedan feels that he wants to operate in this sub plane/ preplane mode for a fuel efficiency I do not believe is there.

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