Hook in Hull?

[witchdoctor]

G’day Guys, I have a 19 foot, Long Boat and it has a hook in it's hull. The boat gets on the plane very well but the motor has to be trimmed in all the way to stop it from porpoiseing. The boat broaches in a following sea and we are trying to work out the best way to fix it. I think that trim tabs would fix the problem but I’m keen to get some opinions on the hull design because it is a real dog at the moment.

Would a hook in the hull be the reason why the boat porpoises so badly?

We have raised the motor and this made it a bit better. The motor has a foil on it and this helps to keep it on the plane, but it's all wrong the way the boat goes through the water at the moment.

Thanks for your help….Shawn.

[nevd]

Hello Shawn,

A hook in a hull will not normally cause porpoising. If you have not already done so, please do a search in this forum as there are many suggestions eg moving a large container of water to determine effects of changed CofG.

What type of boat do you have and in what speed range does it porpoise?

Regards,

nevd

[witchdoctor]

G'day Nevd,
I've done a couple of searches on the net (that’s how I came across this site, great site by the way) and haven't found too much in relation to boats with a hook in its hull.

I'm looking to see if a hook would maybe cause the water going under the boat to not support the hull weight properly and cause the boat to perform badly.

We have done some experimenting with the weight distribution around the boat. It does perform better with more weight up the bow but it makes the boat want to nose steer very badly.

The boat is a 5.8-meter Stinger Long Boat with a 100hp 4-stroke Yamaha outboard.

I have been told by a Naval Architect that the porpoising and broaching are two separate issues and should be treated as such.

As far as I'm concerned a 19-foot boat with only a 100hp on the back should not need trim tabs just to get on the plane.

The boat will porpoise at all speeds from just on the plane through to flat out, and I can only trim the motor out a fraction before it starts to porpoise. The transom already has a wedge on it to take the angle out to 19 degrees.

We have tried two separate foils on the motor and the boat is still a pig, any ideas?

Thanks……Shawn.

[nevd]

Hello Shawn,

This sounds nasty to have porpoising over entire planing speed range with motor trimmed in and foil on cav plate - wow. Have you looked at the other thread "Broaching in powerboats" - some very useful comments there?

I have seen a 5.8 Stinger, but can't remember it well, but seem to remember the hull was not very straight in the planing surface. I suspect you may have the boat overloaded and therefore have curved sections of the bow in the water - determine the boat contact with the water (both at chine and Keel) at planing speed and check that area with a straight edge. A combination of hook and rocker could explain your problem and a straight edge check should show if that is the cause - ideally the boat should be straight to better than 3 mm in the planing surface. Some designs have rocker to keep bow up at high speeds and some others have a hook (wedge near the transom) to stop porpoising at high trim angle.

Checking trim angle (both of keel and chine) with a spirit level at different speeds could also give useful info. Let us know how you go.

Regards,

nevd

[witchdoctor]

G'day Nevd,
You won't believe this but the other post about "Broaching in Powerboats" was posted by the boat builder who built MY boat, and that’s MY boat he's talking about.

I did notice a hump towards the front of the hull, when I was running the string line over the hull. I will try to measure the size of it in the next few days (it’s a bit hard to get an accurate measurement of the under side of the hull when it's on the trailer). If there is a hump as well as a hook, what is the best way to fix it?.......Is it fixable?

Thanks for your help, I appreciate it.......Shawn.

[David Robb]

I can't comment on the hook and hump without seeing it but I did some rather technical studies on porpoising of hulls with Weston Farmers genius engineer son David in the early 90s and came up with a solid conclusion that porpoising is caused by prop pitch and diameter. Porpoising occurs when the prop bites solid water causing the hull to lunge forward until the prop cavitates at which point it falls off speed until the prop can get another bite. We proved this conclusively using an analog computer and then took his own 22 foot boat out and swapped props around and created porpoising by using a prop that had too much pitch and not enough diameter. Westy Farmer designed several boats back in the 40s that had a reverve curve in the keel called decollage that helped a planing boat get up on her plane faster. No conclusions here but try the prop thing and see if that eliminates one of your problems first. Good luck.

[nevd]

Originally Posted by witchdoctor

G'day Nevd,
You won't believe this but the other post about "Broaching in Powerboats" was posted by the boat builder who built MY boat, and that’s MY boat he's talking about.

I did notice a hump towards the front of the hull, when I was running the string line over the hull. I will try to measure the size of it in the next few days (it’s a bit hard to get an accurate measurement of the under side of the hull when it's on the trailer). If there is a hump as well as a hook, what is the best way to fix it?.......Is it fixable?

Thanks for your help, I appreciate it.......Shawn.

Hello Shawn,

Have you made any progress since your last post? Yes hooks and rocker are fixable by filling and sanding if not too severe. I had a look at a longboat yesterday which was produced at the old Yamaha longboat factory in PNG.

The factory is now manufacturing hulls for a number of "boat manufacturers" who finish their boats in Australia and I suspect Stinger maybe one of those "manufacturers". So please confirm your hull was manufactured in PNG and if you want to contact me as a private email, I can tell you of the other "manufacturers" I am aware of. This will allow you to have a larger collection of similar boat owners so you can contact others to learn from their experience. These long boats incidentally have a good reputation in NQ.

The boat I looked at had some curvature in the planing surface, but much less than the average Australian pressed tinnie, so I doubt it would have caused the problems you describe.

Hope this helps.

nevd