Planing speed

Barry, what are your definitions of "buoyancy" and "buoyant forces"?

 

"buoyancy is an upward force exerted by a fluid that opposes the weight of an immersed object due to the volume of water ( or alternative fluid) that it displaces

A focal point from me is that at transom water separation, the object is not immersed, ie all the surfaces below the water line are not in contact with the water.

So therefore, are buoyant forces non existent?

Certainly there are upwards forces, but these are just caused by the depth of the water. Ie take a one cubic foot balloon, hook it to a bourdon style pressure gauge which the whole unit weighs 63 pounds, so it just sinks, ie the buoyant forces would be 62.5 pounds due to the water displaced at sea level , and toss it over board.



The pressure guage will begin to read pressure based on the water column, this is the pressure at that depth, The buoyant forces will decrease because the balloon will decrease in volume due to pressure of the water column,

Sorry, have to add in that the balloons internal pressure is zero gauge, ie forgot about the elasticity of the balloon causing an internal pressure. Alternatively, hook up the guage and zero the scale. oops



Additionally, with water flowing at say 30mph across the bottom of the hull, how would a static buoyant upwards force add further pressure to the hull due to displacement of a volume of water

 

I have thoroughly enjoyed the discussion on my original post, thank you to all who have educated me quite a bit. I have been away for the Thanksgiving Holiday, and am just getting caught up I feel a bit more comfortable with my quandary as it appears to be an almost mystical principal . The goal is to somehow enable the boat to be more efficient at sub plane speeds. The boat has a fairly significant bow wake, and the bow just begins to rise at around 10 knots. She is not on what I would consider a full plane at that point by any means. If by converting the boat to a single screw with the same power as the twins, thus reducing the weight, and drag associated with the second outdrive; would that likely result in greater sub plane efficiency? Or would the boat plane at a lower speed possibly based on the reduced weight & drag?

 

You could try trim tabs, generously proportioned ones, they will give the possibility of keeping the running attitude less bow-up at hump speed, what overall gain you might get is questionable though, they generate drag as well.

 

The short answer is yes to both, although the sub-plane efficiency at speeds like 10 knots won't change very much.

If the hull is lightened, and the CG is kept in the same place, it will, of course, plane earlier, and with lower drag from a single engine installation it will go faster for the same power, which is saying that the efficiency improves.

If the weight is reduced you need also to pay attention to what that does to the center of gravity. If the engine and outdrive weight is reduced, and that weight is all at the very back end of the hull, this will move the center of gravity forward and that will also improve the low speed efficiency by reducing the trim angle at these lower speeds.

Mr. Efficiency is also correct that good sized planing flaps could help to reduce drag at lower speeds. The objective of their use is to increase planing surface area, which provides additional lift. These would be flaps that are the full width of the hull and would have a length of 8 to 12 inches or more. In addition, this lift is provided at the very back of the hull which reduces the planing angle and reduces the overall drag. If the flaps are kept level with the planing surface there won't be a significant drag penalty and the additional lift will reduce the overall drag.

The objective of these steps would be to reduce the trim angle and provide efficient cruising at a planing speed that is lower than it is now. That does not mean that you will cruise at a better efficiency than it is at displacement speeds, but it will be much more efficient than it was previously at this speed.

While the reduced weight and reduced lower unit drag will help the high speed efficiency, the other changes noted may increase drag at high speed, depending on how fast the boat is now. If the hull could get down to the most efficient planing angle previously, you will most likely lose top speed, but that is the price for optimizing the hull to cruise on plane at an efficient planing angle in the low speed regime.

 

Yellowjacket,
You say that the " objective of planing flaps is to increase surface area" and "if the flaps are kept level with the planing surface"
If you were merely to add planing flaps onto the back of the boat to increase planing surface area, you will have added planing surface area to the boat where the pressure is at the least amount. The longitudinal pressure distribution is highest at the stagnation line, prox 1/3 back from the leading edge of the wetted surface and a relatively linear slope to the transom. ie the back of the boat has the least amount of lift per square inch. I say zero, yellowjacket has said less than zero.


The purpose of planing flaps to is to provide additional lift at the transom. In order to maximize lift the flaps should have a higher level of attack than the hull. That is why they make them hydraulically adjustable.
When you drop the flap down wrt to the hull, the water at the flap changes direction with a corresponding increase in vertical lift which lifts the back of the boat. Certainly there is a drag component but the expense of this small drag, is nothing compared to the reduction of the drag due to the bow dropping down.

Additionally, if you put the flaps on correctly, you can optimize the lift component up at the stern vs the horizontal drag on the hull

We have discussed transom ventilation and at 8 - 10 knots, it should be ventilated. If you were to attach the flaps so they are say 1/2 inch up the transom, not more, This will leave the bottom of the transom ventilated, so when you drop the flap into the water stream, you create a new stagnation line on the flap. As I said earlier, a stagnation line has the highest pounds per square inch pressure than other spots on the hull. As Yellowjacket stated, make them as long as possible, so that you maximize the length of the stagnation line.
Additionally, the horizontal drag component will also create a moment around the center of lift which helps push the bow down.

In summary
Mount the flap slightly up the transom to ensure that you have transom ventilation just before the flap, which will produce a new max pressure stagnation line with minimum drag

This concept is not new, stepped hulls benefit from the introduction of more than one stagnation lines.

Our current boat at 44 feet cruises at 20 to 22 knots depending on loading conditions.
knots, with the tabs up. If I play with the tabs and drop the bow 2 - 3 degrees, I can often pull up to another 2 knots out of it at the same rpm.

As it sounds like you are in a high angle of attack, your efficiency should exceed this.

Please look at the link that I sent you for the larger maxum as you really do not want to be running bow up as the mpg close to the so called hump speed is not very good.
The best mpg usually occurs a few knots past the point where the bow becomes to come down due to its speed. ie if you are thinking that you are efficient at speeds just below the hump, you are not correct

 

The pressure on planing flaps at high trim angles are positive. Only at high speeds does the pressure get lower. And yes the area isn't as as effective as it might be, but lift at the very back of the hull can have a significant effect on the resulting trim angle. The objective here is to essentially move the hull back under the center of gravity a bit and reduce the planing angle.

Your idea of moving the flap up and the running a higher local angle on the flap won't provide anything but additional drag as compared to a flap that is flat with the bottom at the speeds we are talking about here. If the trailing edge of the flap is in the same place the flaps will work essentially the same, except there will be a separation zone early in the face of the flap that will simply cause additional drag.

In order for stepped hulls to work properly the step has to be properly ventilated, and the step has to be designed to control the incidence angle on the front and aft parts of the hull. Neither of that is happening here. If this was a high speed hull and you had a relatively narrow trim tab, then mounting it above the hull line would make sense. In this case it does not.

Most planing hulls have a cg further aft to improve high speed performance. The OP is trying to gain fuel efficiency at a much lower speed. In order to do that he needs to reduce the weight and move the CG forward relative to the rear edge of the planing surface. That will reduce the trim angle and provide lower drag at these lower planing speeds. In addition, some slight downward deflection of the flaps could also reduce the trim angle and increase lift at the rear of the hull. This will do pretty much the same thing as an "Interceptor" and increase the lift at the transom.

 

Yellowjacket says the object is to move the center of gravity forward to reduce the planning angle.
That would be the empirical way of solving the bow up attitude. Take an engine out of a twin, do all the work necessary to plug up all outdrive holes, remount an engine on the center of the boat, new controls, whew! sounds expensive to me
Alternatively, move the fuel tank ahead, this also sound expensive
The inexpensive fix is a set of trim tabs that are relatively cheap and a proven method

You say " my idea of moving the flap up will not provide anything but drag"
This is not true, if you read my post, I said that if you move the tab mount up so that the area behind the transom and ahead of the flap is ventilated, this provides another stagnation line. Like a stepped hull. This will be ventilated.

Say my flap set up will give you 400 pounds of vertical lift with a 10 degree down angle wrt to the keel. The drag component of this is Tangent 10 which is .176, times 400 which means the drag is 70 pounds.

A small amount drag to the 400 pounds of lift. And I do not know what the value of lift would be with a couple of tabs all the way across an 8 foot chine width.

But what will happen here
The 400 pound force times the distance between the center of lift of say 12 feet, just a guess, will give a bow down moment if 4800 foot pounds. That is considerable
Additionally the 70 pound drag times the distance this thrust is below the center of lift will contribute to the bow down attitude

 

The flow pattern really depends on the frame of reference. In the first attachment, a model (Sysser72) in a water channel, with the model constrained from moving (in the X-direction), and the water in the channel moving. This corresponds to the case where the observer is travelling with the boat moving in still water - the sailor's point of view.

In the second attachment, a "tank test" with the model moving in a channel, and the water staying still. It corresponds to the case of an observer watching the model sail by in the channel. The flow pattern in a plane just below the surface is very different indeed for the 2 cases, yet the forces on the body are similar.

 

Mikko, thanks for the excellent example.

The velocity as seen by the observer watching the model sail by is frequently called the perturbation velocity in hydrodynamics and aerodynamics.

 

Thank you, David, for giving me the inspiration for these comparative runs. Since we are most of the time moving with the boat, looking at boats sailing around us with more or less similar speeds, we are exposed to the first case, ignoring what really is happening with the water around the boat. It does not matter, of course, unless you happen to be a fish .

 

A good discussion. One point in the material posted by Barry caused me to disagree though. The claim that the separation of water at the stern changes that point of the hull to go from buoyancy to lift and that buoyancy no longer is a factor.

I would say that the existence of water against the transom is not a sign of buoyancy (or not) except when the boat is at rest. As the boat moves forward the pressure on the submerged transom changes from positive to negative buoyancy, drag, suction, negative lift or whichever. At the point of separation, it was noted that the pressure is just a bit less than local atmospheric as witness the entrained exhaust gasses there. Still drag in any case.

As for the note that there is no buoyancy force once the transom is clear of water, that is counter to all the learned thinkers I have read who maintain that some buoyancy exists as long as any part of the planing hull is below ambient water level. If true, the change from buoyancy to dynamic force would actually take place much earlier due to my above claim on separation. I fought with this many years ago and am not certain, or if it even makes any real difference in results.

Barry notes that this is just the depth of the water displaced. All true, but that is what buoyancy is, is it not?

 

Hi Tom
I agree that many technically involved people who actually have done tank testing will comment on the fact that there are buoyant forces in play at all planning speeds, but I have not found a technical paper that says simply " if the boat displaces say 10 cubic feet of water (even at say 50 miles per hour planing) then the buoyant forces ARE 650 pounds and we have proved this by _______________"
So this led me to put this into the forum of learned people to see if someone could offer either a link to a paper or even some suggestions as to how buoyant forces could be proven to act on a planing hull and could quantify a buoyant factor at planing speeds.

In addition there is always the disagreement or inability of contributors to agree on at what speed is a planing hull planing.
Some citing a range of Froude numbers.

To spark some feedback, it was an easy position to take that there is no buoyant forces acting on a planing hull after the water separates at the transom because by definition " buoyant forces are equal to the weight of the water displaced when the object is IMMERSED." So my course of action was to say then when the transom ventilates, there are no buoyant forces by definition. ( or to reiterate, show me how to prove this concept)

But many believe that the proof of existence of buoyant forces after transom ventilation
lies in the fact that at an instantaneous instant, the hull is displacing some water therefore, buoyant forces are at work in conjunction with planing forces. In a totally static position, buoyant forces act at 90 degrees to the surface, but when planing the forces act at some angle to the surface.

In any case, I think that we have had some good discussion on this topic but I would still like to see a quantitative paper on the issue.

Regarding the Froude number,
Froude developed this number, which is dimensionless, using 3, 6 and 12 foot displacement models. The numbers that he developed showed that for a DISPLACEMENT HULL, the resistance increased with an increase of speed and that the resistance approached a theoretical infinite value when the Froude number approached 1. At that time his research suggested that it would be impossible to increase the speed of the boat past this theoretical speed. Of course, resistance testing data for an incompressible fluid suggested that a plane could never exceed Mach 1.

As the equation to determine the Froude number is produced by gravity, the length of the waterline and the velocity of the hull, and was developed for displacement hulls, it seems interesting that this number is so often referred to when discussing planing hulls.

It is logical to assume that there is a buoyant component on a planing hull when planing, though to me, immersed means that there is water pressure acting on all vertical surfaces of the object, not just on 3 sides of a 4 sided object.

Thanks for the input.

 

It is logical to assume that there is a buoyant component on a planing hull when planing, though to me, immersed means that there is water pressure acting on all vertical surfaces of the object, not just on 3 sides of a 4 sided object. I guess I don’t have a problem with buoyant force existing on a planing hull with part of the hull below ambient water level exposed. I first satisfied myself with this view while considering water skiers as they are first pulled up from a stop. I divorce myself from all argument of just when planing starts or when a boat can be called planing and accept any arbitrary definition if one could be agreed on. Since none could be found on that other voluminous thread, I use my own. A boat is starting to plane when the water breaks clear of the transom and is fully planing when dynamic forces exceed buoyant forces. “Hull speed” plays no part whatsoever in the determination and is only a distraction.

 

I have not been following all this in detail so the observations may have already been made or you may all fall asleep from boredom.
All of the scantling calcs I have used for planing hulls take draft into account as well as speed. So, planing hulls do make waves (wake) and therefore must displace some water when at speed. In the extreme you might use a racing boat (hydroplane) where little water is displaced and there is little wake. Another is a hovercraft (ACV) which travels fast, over the water, but still creates a wake.... from the air bubble it sits on, and displaces some water.