Planing Instability

I was going to ask about leg trim. With the leg trimmed out/up, the thrust would tend to turn the boat in the direction of heel, which is what you want. The question is, do you need so much trim that a Whale Tail w/ less trim beats trim alone, but more of it. And do you still have adequate helm at high power and trim? Will the boat do more than 20 if this does get fixed? 18 would seem like a reasonable cruise for the weight and power.

I would think adding lightness would be the most efficient fix. Even at $25 per pound saved. You could spend twice that on stuff up high and then loose a pound of lead, too.

I'd be switching to lexan silverware and cutting the little weights out of the bottom of the shower curtain. Buy him a great big dock box for all the junk in the boat. Better make it two dock boxes.

 

For Bellex the first part of the instability is definitely non-oscillatory. At about 18.5-18.6 knots she will run happily all day heeled 10 degrees to starboard. Answers the helm more or less normally and will not "come back upright" until the speed is reduced.

 

10 degrees is a lot ! What effect does trimming the drive leg out have, once it drops on to this leaning posture ?

 

too heavy in the bow so its bow steer?

 

Yeah, I thought about bow steer too, but it would "root" around, not stay on one tack continuously. I think there's something missing here, possibly (probably) drive related.

 

The leg is flopping back and forth and then turning hard port but self corrects when you cut the throttle? And is fine up to 18.1 knots? Anybody with this kind of experience with Volvo DP legs?

These are the results from Blount's Instability Calculator. The boat is a long way from the "recommended zone" but it's also a ways from the "instability probable" zone.....?

 

I couldn't find that leg trim made any difference in the "Bow-drop and flop-over" point. Speed seemed to be the key factor.

Well Phil.....that's what I would do...but others differ

I think 23-24 knots is possible for this weight and power. And that's the problem, a new hull will cost $20-30k per additional knot, which is a lot. Some sort of fix at $5k per knot (with increased safety and reliable control) is more likely.

 

I reckon it could as once flopped over and turning the forces on the hull will continue as the bow slows down in relation to the stern, it cant break out of that

 

Tad, one other question from the peanut gallery. Is there any hysteresis? Does she transition back to proper form at about the same speed she lays over, or do you need to drop way back to 12knots or so before she pops back up? You say she responds to helm normally, I assume there is some yaw though, and the helm is over a bit to go straight when heeled over.

I have one idea as far as cause goes. I think the chine flats are too deep to be effective due to load. When the boat starts to heel, the high side chine flat finds lots of length in its happy zone and provides additional lift and torque. On the low side, the effective chine flat is shorter and located nearer centerline near the bow. Can you fill in the chine flat for a test? It might be draggy as all get-out, but it could identify the problem. Just cut a 2 x 8 and splotch it in there. If this is the problem, I'd expect a good bit of hysteresis. It would explain why it happens at a specific speed in spite of other parameters seeming to be okay. I'd also figure it would be stable on either "tack". Can you get it to heel the other way by accelerating in a turn?

edit, actually, I'd only expect hysteresis if the high side was operating above the level of the craft at design load. If the heel is just putting the high side at its design wl, then probably no hysteresis.

 

This business of the drive leg flopping around underway worries me, time to get all the slack out the steering imo.

 

some rambling from the cheap seats

I played with a similar issue a few years ago -35' sportfisher, similar symptoms: would accelerate to a certain speed then flop over and go into a turn, pretty much as described in the literature talking about dynamic instability. We tried a bunch of things with little success.
Drive was twin inboards + rudders.
Eventually we moved the rudders further away from the transom and the problem disappeared. ie it was ventilating the bottom run aft via a rudder's proximity to the free surface.

The vessel had a few sister ships without issues, I think this one had a moved CG (fwd) and increased displacement. I figured the increased displacement meant slightly more of the bow curvature was in the water, the fwd CG tipped it a bit further still; then the increase in the bottom loading from the increased displacement was enough to suck air via the rudders.
not sure exactly how that works, but moving the rudders solved it, maybe something to do with greater range in the pressure field across the bottom, dunno.. anyway I'm thinking if it was a similar problem some kind of large ventilation plate on the I/O might help (a-la Will's suggestion)

edit -hull was about 17deg, a reasonably straight run aft - maybe a slight hook before the transom.

 

if the drive is flopping around underway then I'd say its bow steering or the drive is at some weird trim angle to make it go.
What is the transom angle? That has significant effect on an SD as steering axis is inclined and doesnt move when trimmed like an outboard

 

Thanks RT,

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

It sounds like you may have seen a combination of problem 5 and 8......

 

Yeah, that's about what I figured. The sister ships had the same rudder/transom geometry, but no dynamic instability. They must have been at the margins of stability because in the new boat when the displacement was increased (a little) and moved fwd (a little) the dynamic instability kicked in. -a fine line..

It seems the drive flopping around is not helpful and should be fixed, but is it just a catalyst for an underlying problem of dynamic instability induced by overloading the hullform? (ie 1 and/or 2 with ventilation to the run aft care of the stern drive)

If the hull was overloaded then reducing weight is obviously one answer (but apparently not viable), another could be somehow increasing the lift from the hull. Shooting from the hip - what about a horizontal foil slung under the hull; not a big one, just enough lift at speed to offset the overloading? Probably would need to stop the ventilation aft as well (with a big cav plate or something). Boat might start looking like some kind of experimental monstrosity...

 

A chine flat looks a lot like hook in the bottom to a portion of the flow, yes?