Planing Instability

3D PDF of the bottom surface less chine flats attached. It can opened, rotated and panned with most PDF viewers. Remember that this is based on the drawing Tad posted which did not include details of the bottom shape, just the keel and chine curves.

 

Nice hull, pity about the 15,000 lbs !

 

Only guessing because Tad's drawings don't indicate one way or the other, and I know ally can be forced to pretty much any shape you want, but I would have expected the sections to be developable and as such have some convexity, particularly up fwd..

 

Bottom re-worked with close to "exact" developable surface. The developable surface appears to be more convex.

One attachment has lines of the developable surface, the other is a comparison between the lines of the developable surface and the initial quick surface I created with a simple sweep.

Edge curves needed to be reworked slightly which is reason for difference in them. These lines are intended should not be considered as representing actual shape of the bottom. I don't know what Tad based his posted drawing on nor how accurate it is, and then I had to decide where to put the curves relative to the aliasing of the curves in his images.

 

This article from Professional Boatbuilder is relevant:

Donald L. Blount and Dean M. Schleicher, “Correcting Dynamic Roll Instability,” Professional BoatBuilder, August/September 2003

Dynamic Roll Instability as described in the article is the same behavior Tad described. Potential causes include:

Boat too heave for its size
LCG too far forward relative to longitudinal distribution of chine beam
Hook in hull bottom near transom
In profile, the buttock curvature is too extreme near the bow quarter length or extends too far aft
Rudderpost ventilation
Rudder ventilating near hull
Rudder toe-in/out not properly set
Trailing edge of rudder too close to or extends aft of the transom​

One of the solutions suggested for "buttock curvature is too extreme" is "add-flow separating wedges to hull bottom".

 

Is the loll-over associated with the aft sides going from dry to wet as the boat flattens out? The heel in this case could be due to aft suction on the hull sides.

Although the duoprop may balance some torque about the prop axis, there may still be a propwalk force acting about the center of lateral resistance. Can you try a flatter prop set?

When the boat lolls over and turns, steering to resist the turn causes even more heel, and steering into the turn does what, exactly, besides scaring the crap out of everyone? What ever mods you do, I think they should work towards getting the boat to steer in the direction it leans.

 

The Codega, Blount and Schleicher explanation in the Pro Boat articles is the "suction" occurs forward where the buttocks curve upward. Experimental results as well as analytical models show bottom pressure aft on planning hulls with straight buttock lines aft is positive relative to atmospheric pressure, not the negative "suction" as postulated by Lindsay Lord sixty-plus years ago.

 

Hi Tad
looking at the hull lines, you have a slightly warped planing hull there (ie. decreasing deadrise towards aft). With hulls like these the trim tends to flatten out substantially as speed is increased. I suspect the trim flattens out a lot to the point where the heavily curved parts of the buttock lines in the bow are in water contact. When water flows over such curved parts, it creates suction. This suction effect pulls the boat over to one side - hence the 8-10 deg heel you mention. The resistance is now assymetrical longitudinally and you need apply starboard rudder to keep the boat going straight. I suspect that as the speed is increased further more and more rudder is applied to keep it straight and then at some point you hit a small wave which allows the bow to break free of the water, suction and assymetrical resistance disappears and the rudder angle forces the hull into a sudden turn to starboard.

To solve this problem you need to increase the running trim of the boat to prevent the curved parts of the bow touching the water. I would propose you add some rocker plates to the transom or alternatively add some interceptors transversely near the bow to prevent the suction effect there and create positive pressure to keep the bows up.

Or if you are going to remove ballast remove the ballast furthest forward so the LCG moves back. That may help.

Please keep us posted on how you solve this problem...

 

I think the obvious place to look is the deadrise and faking the hull out with wider and deeper chine flats, also with the addition of more stakes, can effectively reduce the deadrise, as far as flow is concerned (the boat will think the deadrise is less). On such a burdened craft, lacking bearing area aft, it seems surprising she's running her chines dry, but I'll bet this is her problem. When you run her numbers, does she come up with a negative chine length?

 

Lots of input and ideas....many thanks to all

This is where I think the boat lies on the Instability chart published by Lou Codega in ProBoat #31.

 

I think the obvious place to look is the deadrise and faking the hull out with wider and deeper chine flats, also with the addition of more stakes, can effectively reduce the deadrise, as far as flow is concerned (the boat will think the deadrise is less). On such a burdened craft, lacking bearing area aft, it seems surprising she's running her chines dry, but I'll bet this is her problem. When you run her numbers, does she come up with a negative chine length?

Sottorf raises an interesting point, though I don't think it's an issue of her run, which is pretty much monohedren, but possibly pressure waves flowing unevenly around her chines, in the lower trim angles also screwing with the issue(s). This would tend to be random, so she'd flop from one side to the other, but since she has a distinct preference, I'm betting on prop walk forcing the issue.

I'll bet she's driving over her patch and drying the chines.

 

Here's running trim I could measure. This is taken relative to the overhead, inside, running uphill in a chop. Her at rest keel angle is at least 2 degrees and the cabintop is probably (guessing) parallel to the keel. So this is representative rather than exact. But it appears to be a typical planing hull coming over the "hump" and dropping onto plane. Beyond this peak RPM the bow drops another 2 degrees as she starts to heel.

 

My understanding was dry chines can result in oscillatory chine walking rather than non-oscillatory flop to one side.

Blount and Schleicher in their Pro Boat article describe this behavior in heavy boats as occurring because the boat doesn't lift enough at high speeds and the curved portion of the bow is immersed and with an area of negative pressure. The result is an unstable situation so the boat flops to one side. They have a diagram of positive and negative pressure areas in their article. A consequence of this is bow needs to be raised rather than lowered.

 

- What was the trim at rest?
-any sister-ships with/without the problem?
- this is a narrow boat so any significant topside weight (large cabin with stuff on top) and/or windage may contribute to the instability: the mv "Ei Lean" could become the mv "Fall-Over".

 

Consistent with what the articles describe with the immersion of the curved portion of the bow as the cause of the problem.