Hull Shape as Keel

Why can't the below water hull shape of a sailing catamaran not act as a keel ?

 

It does. The magic question is how much and how efficient is it?

 

Hobie 16

 

Wharram designs

 

KDesigns P95

http://www.ikarus342000.com/P95page.htm

 

I recently bought plans for a K-Designs Duo 900 and it's an assymetric hull, no keel. Other than not having a daggerboard to snap or contribute to drag, what are the pros and cons between daggerboard keels and keel-less asymetric hulls in various sea states? Do assymetric keels tack or gybe better or worse than having foils?

 

Assy hulls don't work terribly well because they are effectively very low aspect ration foils. As such they suffer huge amounts of induced drag and don't point well. Also, of course, you need to fly the windward hull, otherwise what lift there is from the leeward assy hull is cancelled out.
However, the hulls will still work (to some extent) as skegs, but that effect would still be the same if the hulls were symetric. Take a look at the Dart 18 for symetric hulls that act like skegs.

 

Hi PI,

So what you're saying is that a-symmetric hulls are only partly sucsessfull - symmetric hulls with a-symmetric daggerboards are more efficient. One also has a choice on how much of the daggerboard you want to fly against the wind ?

The Q that crossed my mind was that if it is so good, why doesn't everyone use them ?

 

Use what? Asym dagger baords? They require that you used them, in small cats, tacking the boards and rudders is not always worth the time and effort, so just leave the silly things down. On a cruising cat do you really want to be fussing with the boards all the time? Because they produce lift at zero leeway, they also produce drag. For straight line sailing downwind, they fight each other and slow the boat.

Simple is usually better, a properly sized dagger board does not require a large angle to work, the extra work may not not be worth the performance gain unless you are racing.

Don't know that these are the 'right' answers, just the ones that first popped into my head ... (I'm trying to decide if custom asym boards or tacking boards are worth it for a catamaran project myself)

 

Hi Fanie

My understanding is that two asymmetrical hulls in a cat will "fight" against each other (Chris White - The Cruising Multihull)

Now, in a pacific proa....

 

Ok, now say the under water part of the hull, ie the draft is vertical down on a symetrical hull, in other words like a keel, surely with a rudder you can steer to windward to compensate for the drift there may be ?

The hull surface is much larger than a daggerboard, and you have to have the hull in any case.

Since I won't be racing with it, should I still have daggerboards or can I go without them or do I miss something ?

 

I can picture a deep leeway preventing skeg or foil far enough down in undisturbed viscous flow, doing its intended job oblivious (for the most part) to the turbulance towards the surface. Proly the assymetric (skeg-less) hull's curved inside shape adequately provides the leeway preventing lift, but does so dithering between clear water, aerated water, and air. So, all things being equal (when are they ever? ) the assy hull's underwater center of lateral resistance is contantly "moving" fore and aft as a function of the ocean's sinosoidal area of wetted flow around the hull.

Could this be a significant contributing factor of assy hull's "fighting each other" for directional stability? And is this instability a white knuckle ride?

Bob

 

If you were sailing close hauled and tried to "steer to windward to compensate for the drift" you sails would begin to flap and you will stop. There is a minimum angle that the boat can sail relative to the wind so that the sails will fill.

A daggerboard is not simply a surface which the sails pull against. A boat does not actually sail through the water in the direction of the fore aft axis of the boat. It crabs slightly to the side as a result of the sail forces. This crabbing is called leeway and results in the foil moving through the water at a slight angle to its fore aft axis. This produces the angle of attack necessary for a symmetrical foil to produce lift. The lift counters leeway.

The hulls are very low aspect ratio foils and other factors control their section shape so they do not generate much lift for their area. It must be very difficult to design a hull which will perform so many functions well(produce lift, achive required buoyancy distribution, appropriate CLR, easily driven, resist pitching, interior space, ect). It will be simpler to design your boat if each component must be optimised to perform fewer functions.

There are advantages and disadvantages to high and low(skeg) aspect foils, you must decide which is more appropriate for your use. Windward ability is important in all sailing boats, dont dismiss this as trivial.

This is my opinion and I am not a qualified NA, so take it for what its worth. I hope those more qualified will correct me if I am wrong.

 

Leeway preventing is not the best choice of words. Leeway countering or mitigating is a better descriptor.


Bob

 

It can. Lift is not the problem.

The question to ask is, "What's the drag?"

The lift on the hull/board/keel is determined by the side force from the rig. At the same speed and point of sail, this will be the same, regardless of the design of the hull or keel. The leeway angle will be affected by the design, but this mainly affects the heading. It's drag that determines the course (Vmg). For example, a hull without a board will operate at a higher leeway angle to create the lift required to oppose the sail, but that will result in the bow pointing more to weather. It will turn through a smaller angle when tacking, but if the drag is the same as a comparable boat, the course through the water will be the same.

The lift-induced drag for a given amount of lift is inversely proportional to the square of the speed and inversely proportional to the depth. A lifting surface or hull can get away with being shallow if it's sailed fast. But deeper will be better to when sailing to windward.