Stability under power

[erics]

This is my first post so I hope I have it in the right forum.

I have a 7m (24ft) aluminum "V" hulled boat (Australian Star Hull) that was recently re powered by removing the original Volvo Penta Inboard outboard and replacing it with a single 100hp 4 stroke Yamaha outboard.

To fit the outboard the hull was lengthened 60cm (approx 27 inches) by adding what I call a "pod" which was a continuation of the existing hull bottom shape for the entire width at the stern and making an outboard mounting on the rear.

The boat used to be very much "stern down" with the old engine in place until it was planing - now it is very much on an even keel which is good. However when I take the speed above about 12knots it starts to "rotate" port down ...and the faster I go the more it "rotates" until it feels almost dangerous.

I have no qualifications in design ...but it seems to me that the hull "V" section is not countering and supporting the thrust from the motor.

I have seen side "sponsons" (floats that were from about mid hull length to the transom end of the hull sides.) on another similar aluminum hull. They were aluminum construction and welded to the outer hull sides at the chines - tapering in width from "zero" at front end to about 15cms (6 inches) wide at the transom end. From memory they were about 15 cms (6 inches) "high" on the outside plus the 45degree angled volume on the top side. The lower side was "horizontal" from the chine outwards and the upper side was angled from the outer side upwards towards the hull side I would guess the angle was so that any "water" could not "push down" on the hull. (Hope that all makes some sense)

My questions are ...
Do you believe this may fix my "rotation" problem?
Is there any "calculations" to what size they should be?
Does anyone have experience with hulls fitted with sponsons?
Is there any "downside" to fitting these?
Is there any other alternatives to fix my problem?

Thank you in advance of any comments.

Eric S

[marshmat]

Hi Eric,

Some photos of the hull would be helpful. It is hard to make a guess as to what a modification will do without a good idea of what the running surface looks like.

If you're lucky, it'll turn out to be the simplest fix in all of boating: most outboards have a little trim fin sticking down just behind the prop, which you move a couple of degrees to correct any tendency to pull to one side at speed. If the boat pulls to port, you move the back of the tab slightly to port. Repeat until it tracks straight.

There is another, less pleasant possibility: your "pod" might be a bit distorted. Take a large framing square and a straightedge, and verify that every single line and angle on the pod is symmetrical between the port and starboard sides. Make sure there is no bend or kink in the keel, and that the engine is mounted vertically and on the centreline.

If the boat didn't do this with an I/O, I would be very reluctant to fault the base hull, as the outboard's torque and thrust lines will be pretty similar. The add-on pod seems a more likely culprit.

[erics]

Thank you for your comments. I will get some photos and post them. However I may not of explained my problem well. The boat is not "pulling" and turning to Port. (Usually can be corrected with the outboard trim tab)

If you take a horizontal line through the hull from the stem to the transom at the center of gravity ...the boat is "rotating" around this center line or "rolling" over to Port. (the Port side down - and starboard up) It does this whilst still tracking well and without pulling to Port.

The "pod" was constructed and fitted by a local boat builder who has built over 400 similar sized aluminum hulls. I intend to go talk to him also but when I found this website I thought I'd give it a try. I thought that maybe someone else might have had a similar experience.

It seems to me that maybe the beam with a "V" section at the transom end is perhaps not enough area to "support" the hull when at planing speed when the power is on. My hull is around 30 years old but in very good condition ...but the beam is considerably less that the modern similar sized hulls. Hence my thoughts that "sponsons" might give it extra stability.

I'll try and post some photo's in the next day or so.

[CDK]

The same force which makes your prop rotate to the right tries to rotate the hull to the left. The effect becomes stronger when the power to weight ratio is high.

For that reason, a hull designed for a single prop is not symmetrical but has a slightly larger port surface when planing. The pod you added is 2 ft long so represents the main part of the planing surface and is probably symmetrical.

The simplest solution could be to move the engine a bit to starboard and readjust the trim tab for neutral steering. Tank and battery should also be on the starboard side.

[erics]

Thank you

I actually wondered about shifting the motor to starboard but on examination it is as close to the center line as possible. It would mean having it starboard of the keel. The tank is "built into the hull" below the deck (100 ltrs or approx 60 gals) and is centered about 3 ft from the stern ...so not much can be done with that ...the batteries are on the Port side so I can change them.

I know that when I have a passenger move to starboard it "slows the roll" so having weight on the starboard side certainly will help.

I felt that by adding "flotation" by way of "sponsons" each side it might give extra buoyancy at the stern and hence make it harder for the Port side to "dip" ..but I haven't found anyone yet who has experience with them.

Once again ...thanks for your thoughts and comments.

Eric

[tom28571]

You have added a lot of buoyancy to the boat with the pod and reduced the weight of the engine. The combination on a deep V hull makes it ride higher than before. This makes it less stable, especially when riding high on plane. The problem is called chine walking and if the boat is well balanced laterally, it can flop from side to side. Can be dangerous at high speed.

In your case, the boat is not so well balanced and so it flops to only one side, the port one. Frankly, there may not be any simple cure that will take care of the problem at all speeds. Individually adjustable trim tabs might be the best solution. Don't discount the torque tab on the engine. Correcting an engine steering moment with the steering wheel can cause a boat to heel to one side but your problem sounds more serious than that. A cross wind can cause the same problem, especially on a deep V.

My analysis is based on having a deep V hull and is not so good if your boat is not a deep V. Of course, it is just an educated guess and may be as wrong as any of the others.

[erics]

Many thanks to everyone for their input. I also had discussions with the boat builder today who added the pod. He wants to alter the outboard trim tab first and try that as he experienced another boat with a very similar problem that was corrected by simply altering the outboard trim slightly to starboard.

Tom I agree with your analysis and description of what is happening. The hull sits higher than it was with the old engine fitted and comes to plane much quicker. To me it "feels" like it's riding high and only the lower part of the V in the water. Hence not enough "support" at water contact to be stable. We're going to take some photo's under way from another boat so we can better see how it rides.

However I have just completed 10 hours with the outboard so I will have them alter the tab and try it at the 10 hour service.

Once again thanks

Eric