Hull Shape as Keel

Books and more

On books - Jim Brown's Case for the Cruising Trimaran and Chris White - The cruising multihull are good and a must read for any multihuller. Rudy Choy of CSK fame also wrote a great book - Catamarans offshore - that sits proudly on my shelf. Arthur Piver wrote three books you can find on Amazon - very interesting about the development of early boats.

As to boards an analogy that may be useful when it comes to making hulls provide lift is found in the early days of aircraft.

In the 20's and 30's some planes had any piece of exposed metal shaped to provide lift. Door handles, wheel supports etc were made with airfoils shapes to help lift. The thought was "if they are there then make them do something useful".

In fact they increased drag faster than they increased lift and it was better to just increase the size of the wing a tad as this was very efficient compared to getting inefficient shapes to try to lift. The same has been found with boats. Reduce drag of hull and make foils do what they do best - generate lift.

I say this knowing that Rudy Choy sailed the 46ft cat Aikane incredibly quickly about 50 years ago. His cats didn't have boards and were fast even by today's standards.

As to your query Fanie - boards have to work on either tack so you need the case set straight in the hulls. But you can use a gybing board - they have been used for over 40 years in dinghies and the jury is very much out on their use. The Gougeons liked them on the 32 ft water ballasted trailer cat but I remember reading they were not always easy to trim. The there is the problem of them lifting you to windward when heading downwind - which is not what you want.

cheers

Phil

 

Looks like Wharram borrowed a bit from Aikane.

 

Up to 2 degrees is mentioned in the literature I've read

 

Right. Now I know why you're Mean Bean

Thanks Phil.

Ok, here's what I expected - the daggerboards have to be angled towards midship so that when you sail, the leeward daggerboard only, is down and the 5 or so degrees would steer the boat to windward. By trimming the daggerboard up or down you control the lift and drag on it.


So now I have 'boards have to work on either tack so you need the case set straight in the hulls' .

Great, the only way any object can provide lift is when it has a shape that allows it to fly. Roughly termed, do I kind of stick an aeroplane wing in the water below the hul to make it want to fly, ie a-symetrical board shapes

Or are you going to tell me they are just torpedo shaped ? Where is the lift created from what ?

 

It really amazes me how some of you guys are trying to re-invent the wheel.

These arguments were laid to rest by 1966, when Derek Kellsal built Toria and showed the world what the AYRS had been advocating for several years.

Round section hulls with MINIMUM resistance to leeway and SYMMETRICAL section boards set parallel to the centreline of the hull. KISS.

 

Fanie doesnt know the wheel has been invented

 

I saw this -

http://www.aeroyacht.com/NEWS/Articles/daggerboards.htm

It shows the advantage one will have with a daggerboard.

and

http://www.multihullcompany.com/Article/Daggerboards_vs._Keels

 

Butch is almost right. I know of the wheel but has never seen it

I found an article that showed a symmetrical daggerboard build.

Thanks you all for your patience so far... getting there slowly.

 

at 90 degrees.

Fannie, thats what I mean't. You can toe the boards in or out and take them off the vertical. My SW24 has them set parallel to the centerline and at quite a steep angle. Not the best setup really...

 

LOL Fanie. Good ONYA Mate. We all had to start somewhere.

 

I raced a Giesler designed beach cat called wouldn't ya know it, the G Cat for several years and to my delight was instantly able to outpoint a thirty boat mixed fleet and sail a little faster. This cat had extremely narrow, deep V hulls with no daggerboards.



beam reaching, we were able to hold our own, but down wind legs were disastrous whether we jibed or wing and winged it I always attributed this to all that wetted surface, but it was an extremely simple boat to operate and we put a lot of heavy air sailing behind us for that reason.

The problem with narrow hulls on cruising boats is that as you add weight the wetted surface area goes up astronomically compared to the rounded hulls which equals lots of drag and submarining in square seaways, pitching usually suffers because of those inherent fine ends don't dampen seaways very well.

Come to think of it, the sterling upwind work might be more aptly attributed to deck sweeping main and jenny, plus mast over rotation, all very hot stuff for the day.

 

It's ok oldsailor, glad you're having your moment on me. I have never been on a sailing cat, simply because it seems these sh2theads that have them think every one else is some kind of a retard that will break something, I don't know. I asked a few guys (nicely too) and they all seem to have this stiff upper lip (I love that term ) - probably from English origin he he

Then, when I explain they can relax, I have family members too that also have stiff upper lip syndrome they get all excited and even nasty... and that's on the phone... imagine a one on one

 

Now then what is considered a steep angle... and what is considered a good angle ? Surely there must be a rough middle of the road angle that would be ok... for better or worse ?

 

It depends on what you want the board to do.

If you only want it to reduce (notice I don't say prevent) leeway then straight up and down is most efficient. Most small catamarans have the dagger case over to one side in the hull to give more room, and as a result--tilted. If it is on the inside it will become more vertical as the boat heels and therefore becomes more efficient at reducing leeway.

If it is on the outside it will be more canted when the boat is heeling---and some want that,-- because then the vertical component of the boards force is helping to keep the Cat upright.

This has been carried to the extreeme on some of the very big ocean racing catamarans and as a result there have been some catastrophic structural failures. The worlds best designers have not been able to accurately calulate the complex forces on such boards in rough seas and high speeds and fatigue is an unknown factor, particularly in Trimarans with boards in the floats.
So right now these big boats are going thru a period of "Suck it and See".

Trimarans which have a single board upright in the main hull, don't have these problems.
Vertical boards in the float hulls have not proven popular, though some designers use them. They are dangerous to operate in rough conditions, and can cause "Wiggles" due to the changing positions of their thrust lines, relative to the centre of effort of the rig, in wave conditions.

Hope this is not all as clear as Mud.

 

The boards are superior immediately.

The drag of the board can be approximated by

D_board = CDp * (1/2 * density * speed^2) * area + Ki * lift^2 / (1/2 * density * speed^2) / depth^2

CDp is the profile drag coefficient of the board
Ki ~= 1.2/pi is a constant that accounts for the planform shape of the board and interference with the hull and water surface.

For a given amount of lift, the minimum drag of the board occurs when the profile drag, given by the first term, is equal to the induced drag given by the second term. However, that is not the optimum lift/drag ratio for the boat. That would occur at the speed when the induced drag equaled all other sources of drag from the hull and foils.

You could think of the speed for best L/D as a cross-over point. Above that speed, skin friction dominates and reducing wetted area is the most effective thing do do. Below that speed, the induced drag dominates, and making the foils deeper is the most effective thing to do.

The best indicator of a boat's L/D potential is the wetted aspect ratio. This is the depth-squared divided by the total wetted area of everything in the water.

Note that LAR keels only make sense if there's a huge reduction in wetted area. The squaring of depth in the wetted AR gives depth a huge amount of "leverage" on the lift/drag ratio.