Planing Instability

Ah, ....missed that, thanks yes it wouldn't make any difference in that case.

 

She had tabs, they did nothing(hearsay)....someone talked the owner into fitting cheap plastic interceptors, they do nothing either except trim the nose down and into the spin earlier......

 

Thanks guys I've got something to stew over tonight.

 

Tad

Sounds like you're really caught between a rock and hard place!!

Well, my opening gambit would be, since the displacement is now over the limit by some 3.5 tonne (7,000lbs or so), why is that (let them answer)? Since for you to be able to make any in roads to fix what appears to be a perennial problem, it needs major surgery not a sticking plaster. Thus would the owner load up his/her pick-up (I assume s/he has one) to more than double or triple its road legally allowed payload....if not why not? Thus why do it with the boat...same quid-pro-quo.

You need a an "angle" to demonstrate that the boat is significantly under performing owing to issues which in another market...cars...would never be allowed/permitted. So why do they think it's "okay" on a boat?

Once you can begin to get his/her mind set into that acceptance, you have a chance of major surgery, otherwise, what ever you try most probably wont help matters much as all.

So, as Daniel says, turf out the weight...if they don't like it, seriously over load their truck and ask them why they wont drive it past a police station!!

Everything has consequences...

Good luck....although I fear you'll need more than just luck

 

Does the owner have any hair left, or has he torn it all out ? Could it have a "dynamic hook" where it only oil-cans under load ? Hardly seems likely though.

 

Donald Blount has done a lot of work with dynamic planing stability with these sorts of boats. He has a stability calculator available here:

http://www.dlba-inc.com/dlba_interactive.htm look for the link at the bottom of the first paragraph. It might help give some insight.

And a ProBoat article worth a read that came up when I was searching for Blount here: http://www.proboat.com/predicting-bad-behavior.html

 

This will seem to be an overly simple answer... and it may or may not work... I've seen both.
Add a cav-plate foil - like this http://www.stingrayhydrofoils.com/stingray-classic/
I've been aboard a couple of boats where these have completely transformed the performance of the boat - particularly on overweight ones. I've also been on boats where it made absolutley no difference whatsoever. At around 100 bucks it's always worth a try....

 

The 600 pounds of lead low down in the engine bay (how ridiculous is that !) suggests to me that attention has focused previously on this boat having way too high a centre of gravity, and this was an attempt to alter that. Hardly likely to make a substantial difference if it is 15,000 pounds though, and that extra weight over the 9000 lbs original designed weight is high up. The thing must be costing a bomb in extra fuel useage too. Job for Jenny Craig imo.

 

If Blount has it right, it is not hard to guess which of these three parameters he mentions as the most likely suspects applies here:

■the longitudinal center of gravity is too far forward for the shape of the wetted portion of the hull lines;
■the weight of the hull is greater than the wetted dimensions of the hull lines can consistently and adequately support; and/or
■the rudder or rudders are ventilating, which can lift the stern of the boat.”

Doubt the COG is too far forward going on TAD's drawing, but the weight is very substantial for a skinny 28 footer.

 

I love their advertising illustrations - do I sense a bit of ummm, exageration ?

 

I haven't actually used the Stingray ones before, I just used them as an example, but no the examples I was thinking of weren't quite that ... err... extreme...

 

planing hull problems

I have dealt with several 'chine walking'* boats (and even a catamaran) ranging from 25' to 50'. Sometimes they can be cured and sometimes not. (*Blount uses 'progressive heeling', which is probably a better description).
A few warning features I can see here are:
- narrow beam
-overweight.
-short running surface (ie: nothing to 'plane' on)

Showing the 1/4 beam buttock would provide some key information to help visualize the hull bottom. I have no idea what the cabin is like or the VCG but if high, this will just make it worse.
Possible cure:
-lighten the boat by several thousand pounds (dream on!)
-lengthen the hull to increase running surface.
-transverse step(s) might help but correct placement is critical.
-if the bottom strakes run parallel to the centerline (ie: along buttock lines), that could contribute to the problem.... especially if they stop short of the stem.
- remove any keels, skegs, etc.
Best solution could be your first - all these mods may be good money after bad .....a new hull is probably justified.

cheers
John

A good read is Dynamic Stability of Planing Boats -Blount/Codega SNAME MT Jan. 1992

note: many good planing hulls can chine walk if trimmed excessively (tabs, drives, etc) by the bow (ie: run too level). But, they will slow down, burn more fuel, and steer funny.. so why do it.

 

Tad's description of the boat's behavior is an almost perfect fit for the "non-oscillatory instability" which Lou Codega described in his October/November 1994 Professional Boatbuilder article. (The same article mentioned in the link MikeJohns posted above.) Codega ascribes the behavior to bow down trim at speed which results in negative dynamic pressures on the immersed portion of the bow due to the curvature of the surface.

Attached is a set of lines for the bottom less chine flats developed from the drawing Tad posted.

 

Do you see anything in the lines suggestive of curvature that would produce "negative pressure" ? ( the dreaded "suction"). Looks pretty innocuous to me.

 

A "crazy" idea which would be relatively inexpensive to try and may or may not cure the problem:

A series of shallow forward facing steps on the curved portion of the bottom as shown in red in the attachment. The height of each strake could or perhaps should taper from zero at the centerline to maximum at the chines. An initial test of the steps could be made with temporary wedges.

Rationale: The shallow forward facing steps when submerged would increase pressure on the surface ahead of each step and cause separation of the flow way from the curved portion of the bow. This would allow the boat to right itself and trim bow up.

As for the boat always heeling to starboard, my guess is the asymmetry causing the boat to always heel to starboard is small, and if eliminated the boat would not become stable because the fundamental cause of the behavior would still exist. Rather the behavior would still exist, but with the boat heeling to either starboard or port. External perturbations such as a wave or wind, or a turn to one side would trigger the behavior.