Hull Shape as Keel

Hi tspeer,
Could you give us laymen any hints as to a fundamental crossing or turning point, where as a function of some type of hydrodynamic calculus the penalty of induced drag becomes the most significant negative contributing factor for an assymetric hull, and where foils start to gain the upper hand at efficiently countering leeway?

Many thanks,
Bob

 

I'll try and make this as simple as I can.

Early multihull design flowed thru from monohull hull shapes of the 19th century. These relied on the shape of the HULL to resist leeway.

Trial and error and test tank work by people like the late Edmund Bruce, showed that long slim hulls with symetrical and semicircular sections had the least drag.

WHY.? Because these sections had the LEAST form drag--the LEAST wetted surface drag--and the LEAST resistance to leeway.

They played a minimum part in the form drag resulting from the fact that when sailing on any course other than dead downwind the hull is travelling at an angle to the water flow. A semi circular section has less surface area than any other form, and thus has the least skin friction drag.

This then left it to the underwater "wings"---(the dagger board/centreboard /fixed keel ) to do their exclusive job of resisting sideways drift. ---Leeway.

On trimarans a single symetrical section board has proven to be the most efficient "Leeway Resister" for boats where speed is the prime requirement. Although it is not quite as efficient, a fixed low aspect ratio fin keel may be more convenient and in fact, if properly designed, works very well.

Catamarans on the other hand need two similar "Leeway resisters", in (or on) each hull. A single, surface piercing board, situated between the hulls for convenience, has been tried by many designers, but is now rarely used due to structural problems and the loss of efficiency due to air ventilation.

A board with a suitable symetrical section and an immersed area of 2% of the projected sail area, is most efficient at an angle of attack (leeway angle) of approx 4 deg. The hull is then working at a minimum drag angle and the board at its most efficient lift/drag ratio. This happy situation will depend on the efficiency of the sails and the skill of the helmsman. A GPS helps too.

This of course is why a properly designed (and sailed) multihull is so much faster than an equivalent monohull, particularly to windward.

 

Hi oldsailor7,

Great background behind multihull developement. Thank you for that.

I've been browsing the yacht design books on Amazon Reader (limited to excerpts, and unfortunately scant info on multihulls in particular). So I'm trying to decide which in that section to put in my proverbial "Desert Island Collection", and Gueogeon Brothers is on route as my construction reference. Starting from the top shelf, do you have any recommended titles on multihull yacht design? Especially with rules of thumb like the points you mentioned. Heavy on math is not my forte , but I don't mind that either cause there's always the chance for me to try out coding the formulas in Excel, TKSolver or AutoLisp.

Thanks so much.

Bob

 

Some very nice input here, thank you everyone ! I'll have to think about all this a bit.

Please don't hold back if you know more... let rip. He-he, I need some more confusion

 

Hey thanks to all you guys from me as well.

So for assy hulls, are they wobbly? Has anyone been on one and experienced this for themselves? I'm not particularly worried over a little wobble as long as it intermittent and never gets out of hand. I wouldn't be racing them, but I sure would like to know if they're a danger if you find yourself careening down the face of a wave before having a chance to drop a warp.

Bob

 

Ok Oldsailor and Tom,

What difference would there be in two boats - assuming the hulls are shaped vertical down to act like a keel - one without daggerboards, the other without.

The one with the daggerboards would be able to sail a bit more to windward and would experience less leeward slip than the one without daggerboards ?

The next question would be how much less to windward if you're without daggerboards ?

 

"do you have any recommended titles on multihull yacht design?"

I wish someone would write a multihull specific one There are bits scattered everywhere.
'Multihulls for Cruising and Racing' by Derek Harvey offers good discussion of multihull design characteristics. Thomas Firth Jones talks about aspects of this in MultiHull Voyaging as does Rob James in Multihulls Offshore. Jim Browns books etc. etc. talk about various design aspects. Some designers such as Richard Woods post design related articles on their websites.

 

Thanks Bob for your kind comment. As Jamez said, I don't think there are any comprehensive books on multihull design, but rather articles written by individual designers which express the ideas inherent in their own designs.

Assymetric Cat hulls with one side vertical were a feature of early Cat designs. The most famous of which were the CSK catamarans on the West coast of the USA. These of course were very successful by virtue of them being Catamarans. They showed up the Monohulls in the Trans Pac races so badly that (of course) they were banned from further mixed racing.

The first multihull to demonstrate the efficiency of rounded hull sections and effective daggerboards, was Derek Kellsals " Toria", which blitzed the fleet by a day and a half in the 1966 Round the British Isles race.

Todays modern racing cats and tris stick to these basic principles without question.

Other hull sections are still used for different purposes. For instance ease and simplicity of construction, where the expected use if the boats is going to be in protected waters, or basically downwind cruising.

 

Thanks, I just read an article on the dis-and advantages on keels and daggerboards... it seems the daggerboard comes out tops, this hoever brings up another one of my stupid questions

If the sole purpose of the daggerboard is to 'fly' the catamaran to windward or more to compensate for leeward drif then why can't one have a fore rudder also to act like a fin. This together with an aft rudder could have the same offect... or am I dreaming of easter eggs pecking ?

 

FANI.
It has already been done ---and Patented.

It is currently used on very big and expensive racing monohulls, where it is called CBTF. (Canting Ballast, Twin Foils.)

The jury is still out on that one.

It was tried some 30 years ago on catamarans, and worked ok in smooth waters. However the forward fin (rudder) proved vunerable to damage, and the vortex from the forward position disturbed the laminar flow over the forward part of the hull which affected the performance of both the hull and the centreboard.

In multihulls simplicity is best.

Compare with the complications of the canting keel monos where an engine has to be running all the time to power the hydraulic system needed to operate the canting keel. The wracking strains on the hull are enormous, requiring extra structure, which equates to more weight.
IMHO this is the last gasp in monohull development. So much complication and cost, for such a small increase in performance. The Law of Diminishing Returns in action.

 

Cheesssss... can't I come up with something new that could actually work eh !
Well, the best way to beat all this would probably be to have a small reactor aboard that have a half-life of say 30 years to provide power to the vessel as well as heat for shower, heaters in the hulls and cabin. No sails, no daggerboards, 24 hour power on demand. And all very possible.

 

This is a good topic discussion. Foils are inarguably the top performers. I do see the logic that makes for optimizing performance on individual aspects with each factor in the leeway equation refined to thier utmost.

In my case where on a relatively narrower hull of a design like the K-Design Duo 900, the floor space is essentially a foot path and so its reliance on an assymetric hullform is quite sensible in its approach. Long and thin, yet frugal usage of space and materials. At 27 feet (or 29, I forget) it'll make a fine coastal cruiser.

Cheers,
Bob

 

Last question -

At what angle to the centre line of a hull should the daggerboard sit ?

 

LOL, no need for nav lights either Fanie, you'd glow

 

Vertical or horizontal?