Falling over?

I have recently acquired molds of a 31' sport flybridge fishing boat which is powered by twin centrally-mounted 230cv inboard diesels, driving through shafts to 4 bladed props. The boat having a hull length of 8m50, uses two tunnels.
The boat has a top speed of 30knts.
However on reaching approx 25knts in a straight line it has a tendancy to lean to one side (starboard) and stay there! untill speed drops to below 15knts.
It also turns well to port but not so well to starboard.
I have been told that this is due to a hull which is overpowered and is called "Falling Over".
I would appreciate any advise from the forum

Geoff Chapman

 

It sounds like its a trim angle problem more than a power problem. Try shifting weights aft in order to pick the bow up a bit more. The fwd part of the hull probably has quite a bit more curve to it than the aft part does. This curved surface creates a lot more downward lift that the relatively planar aft surfaces do. As long as the boats stays perfectly level this is no problem. As soon as that balance is upset the lift on the immersed side goes up drastically while the lift on the exposed side goes down. This pulls the boat over on its side where it wants to stay. If you can pick the bow up a bit and get the curved part of the bottom out of the water it should run alot more level.

 

Sounds interesting,
The hull is very tulipped with large curved areas in the bows, the engines are placed well forward under the cabine sole, and it does seem to have the bows quite low.
I'll try some extra weight in the cockpit area.

Thanks for the help, I'll let you know how it goes

 

It is common with tunnels for the hulls to ventilate. The aft end of the tunnel probably lifts out of the water allowing air to enter under the hull. Trim tabs or other appendages may help. This is a guess, other underhull features can cause it too. For example, rudders, rudder posts and lifting strakes.

 

Professional BoatBuilder magazine #84 featured an article on planing hull dynamic instability that addressed the issue of roll instability, and SNAME (Society of Naval Architects and Marine Engineers) published a couple of papers a few years ago on the same topic:

Prediction of the Dynamic Roll Stability of Hard-Chine Planing Craft
Authors: Edward M. Lewandowski
Journal of Ship Research June 1996 Vol. 40, No. 2 Pages 144-148

Abstract:
A method is presented to predict the roll restoring moments acting on hard-chine craft operating at planing speeds, which can be used to evaluate the dynamic roll stability of these vessels. Expression are developed for the static and dynamic contributions to the restoring moment. Tests were conducted to measure the restoring moment on five different planing hull models. The predictions for these hulls compare well with the experimental results.



Transverse Dynamic Stability of Planing Craft
Authors: Edward M. Lewandowski
Marine Technology April 1997 Vol. 34, No. 2 Pages 109-118

Abstract
A method has been developed to evaluate the coupled roll-yaw-sway dynamic stability of hard-chine hulls in the planing regime. Expressions for the linear stability derivatives are presented as functions of geometry and loading, speed, trim angle, and wetted keel and chine lengths. A stability criterion is derived, and the effects of length/beam ratio, loading, LCG location, deadrise, and appendages on stability are examined. The importance of coupling among roll, yaw and sway is demonstrated. It is also shown that appendages tend to reduce the roll stability although this effect is mitigated by coupling effects. A simple method to check the transverse dynamic stability of a proposed design is presented.

 

Look also for papers by Donald Blount & Lou Codega on this issue (including the Professional BoatBuilder article mentioned above).

 

Can anybody provide PDFs of these two papers?

 

are the props counter rotating or turning the same way. just a thought.

 

Apart the explanations given above, which concern the dynamic hull stability given by the hullform and weight distribution, I'd also check the propeller installation. Do you perhaps have two left-hand props installed below the hull?
Next, I'd check if there's something that creates an asymmetric aerodynamic force high above the deck.

Anyways lowering down your boat's center of gravity by moving the unecessary weights from the flybridge will be beneficial. Too many persons dwelling up on the flybridge during the high-speed navigation can also create a significant heeling moment. Tell them to go have some beer in the cabin.

P.S.
The point about props was also noted by Whitepointer23 before me, as I see now.