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

Nope. If the heel is caused by a permanent shift in transverse balance, you are probably right. If the heel had a dynamic cause like I surmised, it is much more likely to set up the oscillation like the OP has. It's called "yaw instability" and is fairly common in some boats even if the cause is not well understood by many. A highly warped hull bottom will do this at high speed also, but for a somewhat different reason.

 

When a slow turn at low speed is initiated by a slight off-center position of the drive (due to slack in steering components), the turning radius is gradually reduced, due to position of lateral center of pressure. At a specific angular velocity, the boat speed held constant, the rear of the boat is "sliding outwards", causing an increased side force on the "rudder area" of the drive leg.

This forces the servo-valve to a position, where it reacts by sending the drive leg in the opposite direction. With the right combination of wire play, valve deadband and longitudinal centers of mass and lateral area you have all that is needed to create a steady oscillation, driven by the hydraulics.

Change one of the factors, and the resonance is broken; this is the basis for my suggestion above, and the reason for your observations with passengers at different placements ("Been there, done it all, got the T-shirt.....").

 

Don't know if this makes a difference, but the more the boat is propped for speed, the bigger the effect. Power props for towing and underpowered boats oscillate much less

 

There's a good article on this "wander" in the Feb. edition of Boating Magazine. http://www.boatingmag.com/find/wander

 

Does the engine idle smoothly? My gut reaction would be an imbalance between the trim tab on the leg, and the prop torque at idle. Engine idle increases slightly, prop torque drives boat to starboard, engine idles back down and trim tab drives boat back to port. Perhaps a tug-of-war between the trim tab and the engine's loop-back computer controlled idle speed...?

 

My Donzi 16 does exactly that too. I've had the boat for a good 30 years so ignoring that funny low-speed yaw-oscillation behavior is easy for me to do.

 

Sounds like you are assuming a type of hydraulic steering with a servo valve.

 

My read also. I'm a marine controls designer by trade; my stupid-simple cable-steered Donzi exhibits the yaw wandering behavior under discussion with absolutely nothing in the entire craft moving or touched. It just does it.

 

Tom has suggested the cause might be a coupling between heeling and yawing which sounds plausible. Has anyone with a boat with this characteristic noticed any periodic heeling associated with the yawing? Does the boat continue the periodic yawing if heeled due to passengers or other weight on one side? How does the natural period for small heel oscillations compare to the period of the yawing?

 

OP couldn't be bothered clarifying the matter.

 

My deep-V Donzi exhibits the continuous yaw oscillation at idle-ahead speeds and a bit above that. There is no noticeable heeling resonance or roll-coupled-motion at the same time. To the limited extent that I've even thought about it, I attribute some of the behavior to the fact that the RH prop wants to push the stern on my boat to starboard..but once any significant yaw to port results from that action, the immersed leg of the outdrive has an appreciable angle of attack on it that creates a force opposing the prop walk direction. None of that directly explains why the yaw behavior is resonant..but there you do find at least two of the partial derivatives of the overall system matrix, and with opposite signs....and we've not even begun to add in those for the hull yet

The center of presssure for long-slender bodies in a fluid flow does a rather dramatic bit of 'traveling' fore and aft as a fucntion of angle-of-attack of the body in question. We typically focus on how the movement of the CP affects the vertical-plane 'lift-moment' balance of a craft at high speed. But...that same characterstic is also affecting the long slender body of a 'not planing' deep-V hull in the lateral plane too.

 

As the prop drives the stern to starboard, the angle of attack of the starboard gunwale provides increasing resistance to push it back to a straight tracking (since deep-Vs should tend to develop a "straight-ahead" tracking under momentum). Perhaps the inertial overshoot is sufficient to reach approx. 22 degrees each way. Otherwise, I still wonder about the idle characteristics of the engine...

However, this would not explain any similar phenomena with contra-rotating dual engines, unless one engine was predominant.

 

I did not abandon all of you.

www.wanderfin Does make a greater vertical area plate that stops the wandering instantly. It just works. Will have one put on first thing this spring.

The plate does not affect anything else at any speed.

Thanks for all the thoughtfull reasons.

 

Wanderfin - STAY AWAY FROM THEM!!!!

I have a Mercruiser Bravo II outdrive with a 454 MAG 330hp motor. At low speed the wanderfin did reduce (not eliminate) the wandering my boat
experienced, but it also produced a small vibration that did not
exist when it was not there. I took the boat out of the water to inspect the fin and noticed the force of the water at high speed from the propeller blades when passing by one of the vertical fins (the first one the propeller blades see as they rotate clockwise) literally bent it.
I work with pumps and a propeller is like a pump that pushes water thru the curved blades as it rotates. with pumps there is a phenomena that is produced in most pumps called "vane Pass Vibration" and this occurs when the blade of an impeller (or propeller) passes what is known as the "cut Water" this is where the water changes direction using a perpendicular or 90 degree fin to the rotating blade pushing the fluid (water in this case).
This action causes a hydraulic force on both the impeller and the "cut
water" or blade and fin, and in this case caused the aluminium fin to bend since this is the weakest part (the propeller is stainless steel and its tensile
strength is higher than the aluminum fin).
I ask their engineering department to closely look at pictures I sent and get back to me as soon as possible.
Their reply was; we will send you a new Wanderfin shortly... I told them that a new beefier Wanderfin would not make the problem of the forces go away and asked them to put in writing that they guaranty the fin to work and NOT DAMAGE my outdrive or motor, they categorically refused!!! They were nice enough to refund my purchase. I would think twice before putting one these Wanderfins on a piece of equipment that's worth thousands of dollars without knowing the true effects it will have on the engine and drive. If the companies that make the propulsion for the boats thought it was a good idea they would sell these type of accessories.
Think about it, and use a little elbow grease instead to keep you boat going straight!

 

Every boat is unique. Please post a photo of the fin installed on your boat and how it bent.

You don't have a written guarantee from Mercruiser that their sterndrive won't damage your transom either. Manufacturers provide equipment. You need a qualified engineer / naval architect to choose the right equipment for your application.

I am not a proponent of adding anti-cavitation plate "tails", fins, etc. However, I would point out that the propulsion companies most probably didn't "make" the propulsion for your specific boat. Your boat manufacturer chose a standard power option. Most boats don't need assistance; the added tail isn't necessary and would only add drag and snag potential on most boats. I have seen a few boats, heavily loaded or slightly underpowered, benefit from an aftermarket "fin". The sterndrive is designed for the most common application; the sterndrive manufacturer also does market a duo prop and counter rotating package, at a price.