Broaching in powerboats

can anyone give me some info on causes and cures of broaching in powerboats.

Regards Murray Hallam Australia

 

The cause is, the back end wants to pass the front end. One form of prevention is to travel at the same speed as the waves, in a possition such that the bow is always trying to climb up the back of the wave in front.
Another likely behavior is that as a wave approaches from astern or quarter, there is a large volume in the big butted boat, this volume has great bouyancy and lifts up with the wave, chances are that at the opposite end of the big butted boat is a fine pointy end that does not develop bouyancy without a great amount of immersion, which causes lots of wetted surface and resistance as it is plunged downards. Then the back end wants to pass the front end and... broach. To prevent this the CLR can be moved aft with a skeg, larger rudder, or an additional rudder, etc... But as usual a compromise, needs to be reached- adding a skeg that works great in a following sea may make manuvouring in close quarters more difficult. Good luck, and may the seas be at your back, but not over taking.

 

Thanks Doug for your reply. This boat is carrying a bit too much weight in general and has a tendancy to porpose which has been corrected by adding a fin to the cavitation plate of the outboard. Perhaps with the tendancy to porpose we are applying a bit too much in trim thereby forcing the nose down ??? Would trim tabs be a better fix for the porposing ? Regards Murray

 

Murray, forgive me if I'm wrong, but I think you're mixing up your terms a little. Porpoising is a phenomenon found in planing craft - usually as a result of having the LCG located too far aft (try moving some of your on-board weight forward - like fuel, batteries, people etc). It may also be as a result of trimming the drive out too far. The addition of tabs I would expect to be better than a foil on the "cav" plate.
Broaching is usually confined to boats travelling in displacement mode and is as Dougfrolich described it - not very nice!

 

Planing craft can have a dynamic broaching problem. See Codega and Blount or Cohen and Codega in SNAME sources or Codega in Professional Boatbuilder. These articles also discuss the criteria to develop this problem and some fixes for it. It is not a wave problem in powerboats, and can happen in relatively flat water.

 

That's true - I hadn't considered broaching as a result of dynamic instability.... though, if the boat is porpoising, then I think I'd try to fix that 1st.

 

They may have overcorrected the porpoising. Bow down = broach, bow up = porpoise. It is possible to come up with a configuration with a very narrow band or acceptable trim between the two problems.

 

Thanks for your input gentlemen - CDBarry , I am interested in your comments as I am inclined to believe that If we were to remove the fin from the cavitation plate of the outboard and then fit some Bennett Trim Tabs to take care of porposing. Having done that then we can apply extra out trim on the motor to lift the nose. - Any comments ? The weight distribution in the 19 foot power boat is fairly even, but in general the vessel is too heavy which I believe is causing the problem. Comments please. Regards Murray Hallam Australia.

 

Murray - can you clarify for us whether the boat is indeed broaching - and if so, whether it is doing it at planing speeds.
I believe the best way to address the porpoising issue is by moving onboard weight - at least in the 1st instance. Adding tabs may or may not be beneficial for a whole host of reasons, but adding them to overcome the propoising will simply hide the problem, rather than fixing it.
Incidentally, what kind of boat are we talking about? And at what sort of speed are you experiencing the problems?

 

Forgive me for butting in but I am concerned about the effect of foils on the ‘cav’ plate, I have heard stores that they can cause a ‘broach’ effect when the boat is turning at just sub planing speeds. What I imaging is happening is that while one foil (fin) is lifting from the horizontal the other is diving thus cause the boat to roll in X, what are your thoughts?

 

The SNAME "Small Craft CD" has some articles on this subject.

 

I have broached in sailboats a few times, and think the causes will be similar
What happens is that you have a spinnaker out too far and cant sheet it in to change the loading from front to side, the moment raises the stern, buries the bow and at some point the rudder cant hold it and around she goes almost laying the mast on the water. The low speed, loss of power as the wind is spilled from the sail, and the keel counterbalance saves the boat from disaster usually. In a big following sea you can roll it I have heard but not done. It is caused by loading up the boat ahaed of the cg such that the momentum of the cg pivots around the front loading.

 

Sorry Doug, you said the same thing, but I didn't read carefully till after my post