Porpoising

And I very much appreciate all the ideas input from you all thank you very much

 

We have a record !!!!!

https://en.m.wikipedia.org/wiki/Longest_English_sentence




pogo

 

This paper is of interest:
http://www.dtic.mil/dtic/tr/fulltext/u2/b240994.pdf

 

He has an atypical pod set-up, wider than usual, and with a positive angle of attack.

 

Cheers for that gonzo I have better understanding of what is going on now I moved the CG aft what increased the problem and didn't have the extra trim angle to compensate for the load Still going with a transom wedge and maybe trim tabs remove the load move the CG more towards the bow I will keep you all informed how I get on

 

There is nothing more fun than tinkering and experimenting. Good luck.

 

Yes good to work things out. work out what went wrong had another idea that may help. Instead of lowering the pod How does this idea sound make a high lift pad on the pod effectively that would be as lowering it without the extra drag Do we think that there should a negative angle can only get 2" rise over 24 ". From stern to end of pod maybe even chop bit of he pod get a little more angle may get a 4" rise Does anybody know what minimum negative angle. can be achieved over 24" Don't really want to lower the pod also could add extra two chines to the pod

 

I take it the top rough drawing resembles what you have, the lower is what may have avoided the problem, but it might not have too. That depiction is more or less representative of a full width pod, which probably adds more weight than is desirable.

 

Porpoising is caused by excessive trim of the boat while planing. You can try to reduce the trim by moving/adding weight forward, lowering engine trim or extending the prismatic hull, which effectively moves the CoG forward, since it is measured from the transom of the prismatic hull.

You can try to the moving/adding weight option by taking some friends with you and ask them go as much forward as needed to stop the porpoising. Then you can estimate, if it is a feasible option.

How low trim angles of the engine can you achieve? It may help to go more negative than usual, but that will become less efficient at extreme trims.

Since you have your pod out of the water while planing, you should be able to quite easily extend the prismatic hull. It is probably enough to extend only a portion of the length of the pod, maybe 1/4-1/3. The extension should not have any rocker, it must be straight.

An other option for extension is to make the hull into a step hull by making the "transom" part of the pod as the other carrying surface. Then it would be important to calculate the correct trim for a stable ride for the whole speed range in order to know how much higher shoud the pod be. https://www.ave.kth.se/polopoly_fs/1.169854!/Menu/general/column-content/attachment/Svahn_Thesis.pdf

 

That's not true. The line of thrust is a part of the trim equillibrium, but it is not going to go anywhere near CoG for most planing boats. CoG is well above keel line and in normal OB case the thrust line is about parallel to keel line 20-30 cm below it, thus going well below CoG and causing a moment lifting the bow up.

There may be some shaft driven planing boats that have the line of thrust going through/near CoG, but they are mostly semi-planing or suffer from too low trim and thus excessive drag, which can be cured by using IPS etc.

The longitudinal distance of CoG from the transom is clearly the most important factor here. It and the longitudal center of lift will be the basis of the trim of the boat while planing. The vertical location of CoG and the thrust line will have much less effect on the trim.

 

In the early days of raised pods you'd see a few with a positive angle of attack, today almost none. There has to be practical, experiential reasons for that. Which is what makes me think that it is a likely factor here. The lift generated is so variable with slight trim changes, it must cause unwanted trim oscillations. And exacerbated by the width of this one. I'd have thought when pods first became the fashion, one with positive attack angle would be preferable, keeping the bow down when backed off, but that is not the way they have evolved. There must be a snag.

 

Porpoising is not caused by excessive trim but rather the changing of the predominate dynamic forces that support the hull when planing.

At rest, a boat is supported by buoyancy only, at planing speeds, the boat is supported by buoyancy and hydrodynamic forces.
As speed increases, buoyant lift forces diminish and hydrodynamic forces increase.

What is important regarding porpoising is the location of the center of lift that each of these lift force impart to the hull and their relationship to the center of gravity of the hull.

The center of the buoyant forces at rest is immediately below the center of gravity of the hull.

Though part of the analysis, I will leave the thrust force out of the discussion by making it go through the center of gravity.

For simplicity's sake, I will use a flat bottom hull.

So as the boat begins to move and dynamic forces begin to lift the bow up to a maximum bow up trim angle. The center of lift for buoyant forces move rearward. The immersed profile is triangular and this center of buoyant lift force would be 1/3 of the distance forward from the transom of the immersed section.
The center of lift for the dynamic forces is say 1/3 back from the wetted surface from the front of the boat. Ie the dynamic forces lift the front up, immersing the rear of the boat, moving the center of lift for buoyant forces further back.

As the boat moves forward after maximum trim up position, due to increasing speed, the center of lift moves rearward, the bow begins to drop. Buoyant lift forces decrease as the dynamic forces increase due to speed as the sum of the both forces only have to lift the weight of the hull. A finite value. The center of lift from buoyant forces moves forward.
(ignoring skin friction, another element)

So now the combined center of lift of both buoyant and dynamic forces are under the center of gravity and the boat is holding a fixed trim up attitude for that speed.

Speed increases, and the center of dynamic forces moves further rearward, when it moves behind the center of gravity, the bow drops. The onset of porpoising.

The first drop is slow with a small vertical movement of the bow. The drop in the bow causes the now prominent center of lift due to dynamic forces to move further forward. I will just drop out buoyant forces though the center of lift from buoyant forces moves toward the bow as well as the immersed cross section shape changes.

As the center of lift moves forward, the boats bow is pushed up. But now the boat has gained some angular momentum.
The center of lift heads toward the stern, past the center of gravity of the boat. Now the boat is unstable, as the center of gravity of the boat is ahead of the center of lift, so the bow drops again. The bow drops again, with more angular momentum in the hull, the center of lift moves further forward to the bow, the moment arm becomes longer wrt the center of gravity, the bow gets pushed up higher and the porpoising gets worse.

Others have suggested a fix and that is to experiment by moving the center of gravity of the boat forward and with buckets of water (or friends) , to see how big of problem that you have. Changing the thrust line of the motor, adding in trim tabs may help. If the pod was extended in line with the bottom of the boat, it may have helped but that might be a big job here.
I have never used a Doel fin, or equivalent, but others have used these on shorter boats with success.


I had left off the force from the drive for simplicity sake.

If you take a beam and load it up with various VERTICAL loads (downward) , you can support a combination of these vertical loads by a single upward force. The position of the upward force is the center of gravity.
For static forces.

If you take the hull and apply loads at different directions to the hull, the only way to keep the hull from turning is to apply a single load and a couple ( a torque if you will) to the structure.

When the boat begins to move non vertical loads are imparted to the hull, skin drag, the center of lift for dynamic force has a horizontal force as well as a vertical force, the thrust force rarely goes through the center of gravity so another couple (twist) is applied to the boat.

 

I think Barry made a very good review of forces that operate at various stages as a planing boats starts to move forward. Leaving the thrust line out of the analysis may over simplify the situation but does limit the variables to make the forces on the hull more understandable.

When a pod is added to an existing hull, the couple between the propeller thrust and the transom is greatly increased which will add to the initial trim angle. By aligning the pod bottom with the hull, this couple is brought back closer to its original length and can prevent porpoising unless other factors dominate. Adding a pod to an otherwise good operating boat can be a fad that cause porpoising and/or ventilation, especially in short period waves.

If a boat is well designed for maximum speed performance, the operating CG will already be well aft near to the point where the trim angle is near its optimum angle for the boat and where porpoising can be initiated if the CG goes any further aft. On such a boat, adding a pod may or will shift the CG aft of the CG an porpoising is the result. On other boats that have the CG well forward, cabin boats, cruisers, etc., adding a pod does not usually upset the trim enough to cause problems.

 

Well, it depends what is meant by "caused by". If the equillibrium trim is low enough, the boat wont porpoise. If you change something and increase the trim, the boat will start to porpoise.

There are charts and formulas for the maximum stable trim, which depends on the geometry of the hull, loading and speed. When this limit is exceeded, the boat will porpoise. Thus porpoising is caused by excessive trim. In this case caused by moving CoG quite much rearwards.

The engine was removed from the forward side of the transom and a heavy pod was installed with a heavy OB far rearward of the transom. No surprise to see wild porpoising after these changes. The boat was likely designed to be quite close to porpoising limit with the stern drive it had, since porpoising limit gives the best performance.

These are easy to calculate with the Savitsky method, if you know the needed input (chine beam, deadrise, CoG, displacement and speed or power). My Savitsky software also shows the porpoising limit and it seems to be quite accurate.

 

Actually, it was your comment "porpoising is caused by excessive trim of the boat" which is pretty self explanatory.

Trim angle is one of the variables not the only variable.

Your comment "excessive trim" is a little ambiguous. Ie for different deadrise values , there is an optimum angle for the best combination of lift and least drag. At around 30 degrees of deadrise, the most efficient trim angle is around 8-9 degrees. At around 12 degrees, it is around 4 1/2.
Efficient in this case defined as maximizing lift and minimizing drag for a specific deadrise angle. ( of course perhaps a speed should be included but I have not seen that on the tables)

A slightly different topic but pertinent I think,

While Sawitsky and others have built various tables for estimating porpoising regimes, I would suggest that using towing tanks might not be the best way to arrive at accurate data


Ie in real life, the thrust is from the back and below the center of drag, which will tend to rotate the bow up while in in the tank with the rope, or however they attach the PULLING "rope?" would tend to provide a force which would rotate the bow downward.