Deep V off plane wander. What are the forces causing it?

here are some pics.

Good point, Attached are some pictures.
Thank You

 

Could you also post a photo showing the fins' mounted position in relation to your prop.

 

It must be a very short boat at that extreme angle of attack. Try trim tabs and drop the out-drive all the way down . I have had this experience in a small skiff while sitting in the back . It would steer on the bow wave and roll back and forth .

 

Position on cavitation plate

Attached are some pics of the typical position the fin is installed on Bravo Drives. (not my actual drive)

 

This article gives a clue to a hydrodynamic cause for this effect: Von Karman vortex shedding which is common on blunt bodies. At low sped when the transom is not ventilated, there could some kind of vortex shedding creating pressure fluctuations along the hull.

 

good article. bad recomendation.

Unfortunitly this is where i got the fin recomendation.

 

I dont think the fin is going to sort the problem out if vortex shedding is the culprit as the vortexes work on a much larger scale. The attached picture shows the idea. On a blunt body the vortex's that shed off the back alternate from side to side. As this happens the pressure aft on the hull will flucuate and could cause the wandering effect. Of course this is only happens while the transom is wetted. Once the transom is fully ventilated the vortex sheedign will have disappeared.

Many of you might be familair with this vortex shedding effect on a smaller scale where it is a common cause for propellers "singing". This is commonly corrected by modifying the trailing edge of the blades which in turn changes the frequency of vortex shedding away from local natural frequencies.

All this is guess work right now but as a first step it would be worthwhile investigating (1) does the wandering occur while the transom is wetted and if yes (2) investigate if the vortex shedding frequency corresponds with the wander oscillation freqency.

 

Vortex shedding as illustrated, a vortex sheet, is a fundamentally two-dimensional process and occurs behind long, narrow object with relatively constant cross section oriented with the long dimension across the flow.

The cause of wandering may well be a non-linear flow instability with separation or "vortex" shedding from alternate sides but it would be different than a simple two-dimensional vortex street.

The formula for the vortex shedding frequency is based on the the long dimension being much greater than the short cross-flow dimension and doesn't include a free surface. This isn't the case for an immersed boat transom.

 

neverthought,
If you are still here , why not contact the manufacturer and ask someone there about it . You might have some luck with a powerboat forum. After all you are looking for results , and I am afraid you might get sucked into the vortex here .

P. S. my comment about dropping the lower unit is wrong and will most probably make things worse as the vectoring of the thrust downward will just push the *** end around more .

Maybe try to trim the out drive up just till there is no change in the attitude of the boat .

The only other fix might be to add 2 strakes , but where ?

Frank

 

Frank Smith,

What are strakes? On my RC racing boats we install vertical fins on or near the transoms edge on both sides is this similar? would a center fin (like a dagger board help?) the use these on ski boats, but I think its for short radius turning.

 

I have not ruled that out!

 

Vortex shedding is always described in the 2dimensional case in most textbooks as that is the simplest form of it. It is however present on most blunt body flows. For example see Fig.1 in the paper here:

http://www.fluid.dtu.dk/upload/fluid.dtu/hassan div pdf filer/moscow_paper.pdf Madeira Island can hardly be considered a 2D shape!

I agree that with more complex geometry the shedding will change pattern and frequency but without resorting to CFD the best one can do is approximate the problem with the simple 2D equations. It will at least give some indication if we are ballpark correct with the oscillation frequencies.

 

You might try "the hull truth" forum .

 

"Performance Path TM" is an aftermarket strake??? How does it attach and fit?
Never heard the name before.

 

Assuming that vortex shedding is the cause of the off-plane wander, then you will need a fairly large skeg on the keel to "eliminate" it. I doubt that adding spray strakes will help significantly. However all these devices will help reduce the amplitude of the oscillations to some extent.

What might also help is to trim the nose of the boat down as much as possible when it starts to wander. THis will reduce the submerged transom area and reduce the intensity of vortices.