Planing Instability

I'd like to add a suggestion that's purely intuition, I don't know a great deal about planing theory. It could be a relatively cheap fix though and would be interested to hear if it worked -
Add say 2-3" to extend the width of the chines / spray rails far forward, using ally strakes say 3 feet long up the front, at a point where it would increase the bow lift.
they could be made to reflect the look of the rubbing strips on the topsides

At the transom, fill in the hollow of the chine, or rails, to reduce the severity of the hollow at the rear. This may reduce the lift at the transom especially the lift of the highest chine.

Does this make sense? what do you reckon?

 

A friend of mine had a trailer sick boat that would heel over like that after it got on plane.
It was funny, don't know why he didn't just push the hollow out and brace it. Maybe its got too large a surface area in the stern, and too much weight in the bow.

 

So yesterday we finally ran the model of this boat. The model is 1/8th scale, about 42" overall. We towed it up to 15 knots real speed (about 42 knots scale) and could get no reliable instability out of the model....what am I doing wrong? This model is just the bare hull, without the sponsons that have been added at the suggestion of the original designer. (I have an idea these sponsons make things worse)

We got the model to spin out twice in about 10 runs. I think both times were when waves came along. Looking at it closely while running it felt really unstable directionally. But generally it towed and ran beautifully. I have a feeling she wasn't properly ballasted (still too light) and I need to check that again before the next runs. Trim seems really flat but there's no sheer in the model so hard to see.

 

In their article Correcting Dynamic Roll Instability, Blount and Schleicher (ProBoat # 84) cite these causes....

1) Boat too heavy for its size
2) LCG too far forward relative to chine/beam center
3) Hook in hull bottom
4) Buttock curvature too great forward or aft
5) Rudderpost ventilation
6) Rudder Ventilating
7) Incorrect rudder toe in/out
8) Trailing edge of rudder too close to transom

Make the model heavier(1) and try with (2, 3,4,)
Due to the water viscosity & scale effect it may still not 'roll'.
External forces can have a big effect - Make sure the tow force aligns with the shaft thrust

 

Tad

Did you have the correct LCG and VCG?...and what about its radius of gyration?...and was the tow line in-line with the VCG and shaft line?

I assume you have you also checked it against the normal criteria for instability:

 

I've been worrying about that, the towing rig adding stability.....With the outdrive the thrust line is below the keel.....I've seen pictures of towing rigs using a nail out the bottom of the boat? Then I'll guess we would need a fin off the transom to aid direction......a fake outdrive leg.....

 

JSL

Ahh..well spotted...I missed that bit in your post

Tad,

You can of course correct this by simply adding a small weight , w, at a distance , x, from the tow point and then the distance from the tow point to the thrust line, z must balance the thrust/resistance, R.

So x = Rz/w

BUT..x changes with speed and hence a change in R....so not an easy thing to duplicate.

But the nail from the bottom would be a simple solution.

 

As the others have noted - the tow line angle looks wrong.

 

did you use scale water and air?

 

The tow line needs to align with the propeller thrust vector, but that may not be sufficient for studying the instability. Let's assume the tow line attachment point is near the bow, the attachment and tow line direction align perfectly with the full size propeller thrust vector when the boat is moving at a constant speed at a constant pitch angle and zero yaw or heel. The force from the tow line on the model aligns with the full size propeller thrust vector. Then the boat yaws or rolls a small amount. The force from the tow line is no longer aligned with the full size propeller thrust vector. This misalignment may affect whether the dynamic stability of the model.

Conclusion - to study the dynamic instability of the full size boat with a model the model may need to be self propelled.

 

That is correct.

 

To bring this thread to conclusion, we altered the bottom and fixed the problem. The owner is happy and cost was roughly in the $20k range. I'm more than a bit chuffed, plus relieved......

The owner reports the boat will now achieve a top speed of 32.9 knots at 3600 RPM, and cruise at 27.4 knots at 3200 RPM, burning 14.5 gph. He also reports a "solid feel" and perfect manners running at any speed including fast through ship wakes and high-speed turns.

The model, though imperfect in many ways, was invaluable. Mostly it gave me confidence that there was nothing basically bad about the form. Even overloaded it behaved with impeccably good manners.

Building (in Rhino) comparable hulls (Bertram 28 and CC Launch, and others) and scaling them was also useful as all are within a few inches of each other and overall proportions match closely.

After watching the hull model towed at up to 30 knots, my conclusion was that she ran too flat. With her overloaded and flat trim, too much of that curved bow surface was immersed(IMO) and pulling her into the spin. I concluded the only answer was my first inclination (I have to pay more attention to my instinct), add lifting strakes forward and get the bow up.

I decided on two additional strakes each side, the outboard one full length and the lower (inboard) one ending about 2/3's of the bottom length back from the bow. I would have added another but the bow thruster tunnel was in the way.

In theory the full length outer strake narrows the planing surface and should move the center of lift pressure forward, adding positive trim. The short lower strake would add lift forward. They both act to break up the flow over the curved forward surface.

I believe there is a downside to this, the additional flat surfaces forward should make the boat harder riding and wetter. But in this particular boat those effects may be minimized. The control position is fairly far aft, and the boat is massively heavy (well insulated and solid). Noise is just not transmitted much.



View attachment Belexstrakes.pdf





Oh and the above performance was after removal of the 600+ pounds of trim ballast in the engine bay.

 

Tad, thanks for the follow-up and great to hear the problem is solved. Any idea of how much the trim angle changed at higher speeds due to the changes?

 

Unfortunately no, the boat is a couple of ferry rides and a full day of travel away. I do hope to get aboard at some point, but I have no idea when that might happen.

 

"The owner reports the boat will now achieve a top speed of 32.9 knots at 3600 RPM, and cruise at 27.4 knots at 3200 RPM, burning 14.5 lph."

That should be gph ? If not, I want one of these !