Monohull vs Catamaran Heel during turn

Hi all,

I'm trying to investigate the difference between the inward roll of a high-speed planing monohull against the outward roll of a high-speed cat during a turn.

I've tried to search online and in this forum but haven't had much luck.

I understand the principles around a monohull during a turn fairly well but get a bit fuzzy for catamarans, see attached image from Principles of Yacht Design.

For example, a monohull banks inwards (assuming low cg), a split monohull (small tunnel separation) banks inwards, but at some point the wider the tunnel, the vessel will bank outwards? I've attached a crude sketch of the section shapes in question.

If anyone can shed some light on this, or point me in the direction of some resources it would be much appreciated!


Kind regards,
Ant

 

The type of cats i design are probably different fro your "subject" area. However, we experience little if any roll during high speed turn. Principally because the hulls are far apart.

 

I'm not clear about what you want to find out, or why. As a general rule, for fast planing boats with outboards or sterndrives, where the change in the line of thrust steers the boat, if the boat has low initial stability, it will bank into the turn, if the boat is initially stiff, it will turn flatter or lean out a little. A boat like a sea sled would likely have the greatest tendency to lean outward.

 

What the OP mentions is common on many wide beam power cats. I haven't studied this but suspect it is at least partly a result of what Richard said. That is, a wide beam cat has very high lateral stability and resists the initial moment from the prop that heels the boat inward on a turn. Most monohull bottoms also accentuate the heel by force on the outside half of the hull bottom that lifts it as a result of the sideways motion of the stern. Powerboats like dories with their narrow bottom, low lateral stability and flared sides can heel inward drastically in a turn.

Neither of these are effective to produce an inward heel on a wide beam cat. Many fast power cats use active foils between the hulls and below the waterline to prevent the outward heel. It has been described in a past issue of ProBoat.

Some round bottom powerboats can also heel outward in a turn. This can be the result of the suction on the rounded outboard side of the bottom exceeding the moment of vectored thrust of the prop.

Top heavy powerboats like many of the recent industry offerings have an outward heeling tendency also. It has resulted in passengers being thrown overboard, injury and lawsuits.

 

We did a lot of tests measuring turning parameters of cats; some results are published. So, heel at turn, for high speed craft:

Monohulls - inside, angle is in proportion to deadrise angle.

Catamarans with symmetrical hull - usually heel outside;
Catamarans with split hulls - usually inside;

'Usually' depends on VCG and some other factors such as thrust line.

Catamaran heeling angles are very small, say 2-5 degree for normal maneuvers.

Beam of cat has significant effect on heel angle in two was:
a) increase of beam increases the stability;
b) increase of beam increases turning radius and thus heeling force.

In coming issue of International Journal Small Craft Technology there supposed to be my paper on cat design, these issues are also covered.

 

Cats heel so little in most cases, it is virtually imperceptible to outside observers, standing on one yourself it seems rather different, that is because centrifugal force is wanting to throw you out of the boat. Sit down and you don't notice it, just like a commuter bus going round a bend, it is the strap-hangers who think the bus is being driven too hard, not the seated passengers.

 

I think you're missing the influence of leeway. I think it's really the crabbing of the monohull toward the inside of the turn that makes it heel to the inside. The cross flow isn't shown in your diagram. It's responsible for the difference in pressure between the two sides.

I think you need to work out the side force equilibrium as well as the rolling moment equilibrium. You'll need to know what the side force, rolling moment, and yawing moments about the c.g. are as a function of speed and leeway angle. Then you'll be able to solve for the leeway and heel angle that produces the desired acceleration at zero net rolling moment.

 

Firstly thanks very much for all the replies, I'm humbled to see such a variety of inputs!

I think we're all on the same page Ad Hoc, Mr E, Alik - it's the high lateral stability of a cat that resists the rolling.

I guess what I am aiming for is something more quantifiable or measurable in predicting the turning behaviour between the three types of craft such as transverse metacentric height (GMt).

For example lets say a monohull has GMt = 2m, and a catamaran GMt = 10m (just random numbers). A split monohull would lie somewhere between the two, so at what value would it stop heeling in and start heeling out with all other factors being equal. Or is GMt an irrelevant measuring stick?

I wasn't sure if there was some more published data on this, perhaps someone has experimented with this before but it being a complex situation involving a variety of parameters such as lateral stability, hullform, vcg, speed perhaps it cannot be simplified so easily?

Tspeer - the sketch is from Principles of Yacht design, but I think the pressure force shown on the left hand side of the hull has incorporated a significant horizontal component developed from the cross flow.

And from a cross-flow perspective, would the outboard hull of a symmetric catamaran develop a similar pressure distribution to a monohull, but having less surface area to act on this heeling moment isn't enough? Or perhaps the inboard hull of the catamaran develops another pressure distribution to offset the outboard hull?

It's late and I'm going in circles! Hope some of that makes sense.

Cheers,
A

 

this shall help