Minimising banking in a turn

What design elements help to minimise banking in a low to medium speed planing powerboat?

 

Full length, horizontal chine spray rails.

 

lower rudder aspect?

 

More beam, harder chines. Beware of a planing boat that remains flat when turning. Skidding and perhaps tripping is not good for ones health.

 

Lower CG.
Also w very soft chines the inbd chine will pull the inbd side down and cause excessive banking in sharp turns. I had a Mukilteo Boat (a long and narrow OB boat w canoe like bottom aft and very soft chines (softer than most canoes)) and while making sharp turns it felt like I was going to get thrown out of the boat. This boat lacked strakes aft and close to the WL.
Incidentally the "Muk" boat had (basically) an Axe bow.

Easy Rider

 

Chines and strakes have nothing to do with how much a boat banks in a turn. The reason a boat banks in a turn is due to the sum of dynamic forces during the turn. Obviously!

Several things affect this.

1) Height of the CG above (or below!) the other force vectors. CG will have an outward horizontal force vector (centripetal force). However, on most normal powerboats, this is actually minimal, because the CG is usually pretty low. Those hideous gin palaces with very high superstructures, and vessels like container ships, a very high CG results in an outward lean instead of an inward bank.

2) Vectored thrust below the other forces (outdrive, pods, outboard). This is often a major effect, because the thrust to push the boat is as much as all other forces acting on the boat.

3) Rudder depth: usually not a factor with powerboats, but a big factor with modern sailboats.

4) Perhaps surprisingly, Deadrise: there can be a huge rolling force due to water pressure being much higher on the outside of the turn compared to the inside of the turn. The deeper the V, the steeper the bank. The bow contributes to this too, so boats with lots of deadrise forward for a smoother ride exhibit more roll than otherwise one might expect.

But chines and strakes have essentially zero effect, as the roll axis force from pressure differentials very close together (opposite sides of a given chine or strake) is very, very small. Its all about the lever arm, and strakes and chines have none locally.

Broad beam -- or really, lots of stability -- of course reduces banking simply because so much more force is required to cause a roll.

The thing that causes the rolling force in hulls is the flow of water diagonally across the bottom.

--> A soft chine can, as Easy mentioned, allow this diagonal flow to remain attached, and therefore resulting in a large downward force on the inside of the turn -- lots of lift downward, just like putting the flaps down on an airplane wing causes a lot of lift upwards.

--> Deadrise causes high pressure on the outside, low pressure on the inside, so a big rolling force.

--> a keel causes lots of drag to this diagonal flow, and so results in an anti-roll force: the high pressure side is now pushing laterally on the keel, instead of only upward on the bottom.

I might be a rocket scientist, but this isn't rocket science. You just need to think about the flow going diagonally across the bottom during a turn.

 

Well said David. There are a lot of different things that can cause heeling one way or the other in a turn. A round bottom boat will often heel outward due to suction created on the outside turn in the bilge. Direction of the heel will depend on which force dominates at the start of the turn. A V hull will almost always heel into the turn and gets worse with more V as David said. A dory can heel inward an alarming amount as the entire bottom acts like the outside of a V in creating pressure and the inside flare does nothing to limit the heel.

not a rocket scientist but used to track Redstones in the Bahamas.

 

Thanks Gentlemen.....
So, a low deadrise, relatively beamy hull is a good place to start....(from tha point of view, at least)

 

Banking in the turn keeps the dishes from sliding off the table.

 

Might you think you would be less of an ******* to say something like, "IMO, Chines have less to do with how much a boat banks in a turn than the sum of dynamic forces during the turn" than the original "Chines and strakes have nothing to do with how much a boat banks in a turn. The reason a boat banks in a turn is due to the sum of dynamic forces during the turn" Obviously! Why does every thread have to have someone like you beating on his chest?

 

When a boat turns the stern moves outboard and so the water can be easily moving at 45 degrees to the inboard chine. The water is moving outboard as fast as it's moving aft. Water tends to follow a curved surface and has considerable mass so as the water is pulled up at the inboard chine the lee rail of the boat is pulled down and the outboard side of the boat is acting like a lateral plow causing the outboard side of the boat to rise ..big time ..as the kids say. Try those dynamic forces on for size .. and if you find something wrong w my theory ..show me. Otherwise don't call me an idiot ..again.

Easy Rider

 

You are more diplomatic than I, Easy. What you say makes sense to me.

 

Thanks .. I'm learning .. from my mistakes the other day.

Easy

 

Yes, you are correct. Sorry about that tone.

 

Banking on turns is one characteristic of well behaved boats. Why minimize it?