rudder size

On the question in the original post: Bill Roberts designed the Arc 21 catamaran with the daggerboards further forward and smaller than usual with larger rudders than "normal" at the time. He called his concept "shared lift" and you can read about it here: http://www.aquarius-sail.com/catamarans/arc21/index.htm
Martin Fischers "Mayfly" A Class cat uses a forward position for the daggerboards and the 30' monofoiler Mirabaud uses a daggerboard position ahead of the mast.

 

I am a great believer in the elements of a boat design doing their own job exclusively.
What I mean is that, for instance, the hull or hulls should provide the displacement which supports the boat, with minimum drag and minimum Leeway Resistance.
The leeway resistance should be exclusively provided by an appropriate foil, be it Dagger, Centreboard or Keel.
The rudder should be designed for one purpose only, to create an imbalance which turns the boat, whilst having minimum drag.
Making the rudder share the leeway prevention puts a dangerous load on it and creates unnecessary induced drag. This has lead to many a rudder failure.
A spade rudder must have an appropriate section to resist stalling at high angles of attack. If it is too small it will stall out in a big gust ---resulting in a wild broach. Too big and it will be unnecessarily draggy under most conditions.
Experienced designers have the task of designing the most suitable rudder for their boats requirements. I'ts not easy.

 

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I'm not sure I share your concerns, Paddy. Many fast multihulls have curved daggerboards that are designed to do two things: lift vertically and provide lateral resistance. Same with the first America's Cup Trimaran-USA 17. Same with the 59' Hydroptere, A Class cats, Nacra 20, and some C Class cats and others.
The fastest 60' monohull ocean racers use curved(and some straight angled) daggerboards for the same two purposes as do three of the VOR 70's. Many performance multihulls and monohulls use rudders equipped with t-foils.
If you read Bill Roberts stuff he thinks "shared lift" works real well.
And, if you think about it, any boat that develops leeway uses a form of "shared lift" where the rudder shares in the production of lateral resistance:
the boat is moving sideways and ,of course, that includes the rudder which ,along with the daggerboard, will be at an angle of attack even when centered. Balance is another story- where an additional couple of degrees of angle of incidence relative to the boats centerline gives the rudder a slightly larger angle of attack than the daggerboard when properly balanced.
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Just a bit from Princibles of Yacht Design(Larsson and Eliasson), page 157:
"From the discussion of Canard wings in Chapter 6 we know that the keel/rudder combination may be analysed using biplane theory, and this shows that, since the wakes of the keel and rudder are separated due to leeway, there is an advantage in distributing the load between the two lifting surfaces."

 

Yes Doug. I quite agree with you,
"Paddy. Many fast multihulls have curved daggerboards that are designed to do two things: lift vertically and provide lateral resistance."

I was not talking about boards which also provide a righting moment as well as leeway reduction.
Notice I say leeway reduction---not leeway prevention.
Leeway prevention is a possibility with foils set at an angle of attack, but since the optimum AOA is constantly changing it is too much bother given the calculation of the resulting induced drag. I suppose, in the ultimate scenario, the AOA of the board could be constantly computer controlled to it's aprox 4deg ideal.

Over the 55 years of modern multihull development, there has emerged a basic rule of thumb for efficient leeway reduction. It is round bottom hulls, a board with a suitable foil section, a board area of 2% of the projected sail area (ie:-mainsail and fore-triangle) and ideally an elliptical plan form. The "Marvellous Buccaneer 24" is a perfect example of this. Angled and curved boards outboard are a development of this basic premise, which are being used with great success, but which are perhaps a bit too much of a complication for the amateur multihull boatbuilder.

But--- back to the thread topic.
In 1981 we had a club cruise to Lord Howe Island.
It was a demolition derby, with 50kt winds and 40ft seas.
Dennis Armstrong, in his Seawind 33 "Bill Bailey" had rudders which shared the leeway load. They were both torn off by the wild seas, and they only made it back to safety by skillfully arranging their fore sails on a downwind course. John Hitch in his cruising Spindrift 45 "Hitch Hiker" was the only surviver of this trip, With outboard retractable rudders, on auto pilot all the way, he, with two crew, succeeded in beating the then racing record held by Sid Fischers "Helsal", by over an hour. Food for thought.

 

I'd suggest that rudders are highly loaded enough as it is on multihulls without asking them to do more, I've seen more retirements from races due to broken rudders than anything else. A daggerboard and a strong case rigidly installed in the hull or float is more capable of taking the loads imposed by leeway prevention and lifting foils.

 

I once came across a rudder that was like a daggerboard in a case, but the case was built into a cylinder, the bottom end of which was flush with the hull bottom. The whole cylinder could turn in large diameter ballraces to steer the boat. A bit complicated perhaps, but it allowed for a strong rudder. Alternatively, just have a transom mounted rudder. Lots of people say those are ineffient, but really, just how inefficient are they compared with a rudder under the hull - has anyone actually measured that?

 

G'day 'Samnz'. "So no one knows etc etc". Yes they do & did. Back as far as the mid-70's - Bob Millar (aka Ben Lexan), Lokie Crowther, Alam Payne, Peter Cole & several other noted designers - did a large controlled analysis into rudders & of course keels including Bob's 'wing-keels'. We used identical design - radio controlled modle yachts & swapped keels & rudders between 8 different 'model yachts'. In the end there were more than 20 different rudders & at least 15 different keels being interchanged between all the sailors & all the combinations of rudders & keels. There was much learned & I've always been amazed that otherrs did not follow on & use what was learnt from this extensive 6 year long experiment.

This whole subject is extremely interesting & very complex as others have mentioned however the 'end-product' in effeciency is well worth the efforts. IMHO - of course.

So to answer your question; yes it does make a difference & a rather significant one at that. Some of the gains are; less drag (not necessarily a smaller foil), better balanced vessel, less leway, more lift, easier helm & of course less (breaking) load & a quicker yacht through the water.

I feel that we (all of us) are no where near close to the - absolute ultimate design - as yet - not even the AC 45's. Whilst the likes of Nigel Irenes are leading the charge - I would think that he would agree - that we aren't - at the 'Holy-Grail' - yet.

Good luck with your investigations. It's a great subject - for sure. There can be many differences in profile, section, cord & drift all within this seemingly simple subject & millions of words can be written to explain just a small part of the answers to just some of the questions. Ciao, james

 

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Welcome to the forum, James! Sounds like you did some exhaustive testing!
Did you ever test a twin foil concept(like CBTF) on a multihull? Twin foils(one forward, one aft-replacing the "normal" rudder and daggerboard) that could be turned at the same time in opposite directions and/or at the same time in the same direction?

 

The subject is huge - but Sam, you're going to have to do the development of what is best for your new boat yourself, trial and error, suck and see, try this, try that, etc. (if you can be bothered)... because there are no rules, all that counts is that it works - and works is the definitive word - because to get the "apotheosis" of "works" in rudder and foil design for your specialist boat ... will take time. I still suggest your transom float rudders should be of equivalent size and area to that of the local 8.5 cats ... as a starting point.
One point about Silver Raven's comments about Lock Crowther's rudders; his were very small area for his trimarans, can't tell you figures, just look at his plans - and this size and profile shape would be a result of their elaborate testing. I remember Auckland multihull sailors questioning his rudders; always thought too small for control in fresh conditions. Be interesting to hear what the Aussies here have to say about this.

 

It isn't faster to have the smallest possible rudder if you can't tack reliably in big seas and wind. Getting caught in irons or having to wear ship can cost more than a few boat lengths. Sort of a case in favor of vertical retracting rudders. There are threads on the drum solution which can be adjustable. Of course you need a flat section which is easier to design in a monohull with those wide hulls.

 

Rudder design etc etc

G'day & tnx Doug. I've been 'in-here' for a while but have some handicaps (extreme dyslexia), etc etc, which even at 70 find a tad bothersome. Still trying to keep - foot out of mouth - so I just sit back & watch & always try to learn of course. I also belong to MH4US & crews.org.nz - both really great sites at least to me & for my interests. They're right grand people.

Think you, Doug are the first & only person that has asked a relevant question & do it in a polite manner without telling me I'm a jerk (that might also be correct - Ha ha) I'm also totally alergic to people being rude - it's just not worth the effort to be bothered with. I do thank you.

Simple answer is NO we didn't go into that area. Just stuck to the complexity of rudder & keel - foil shapes, sizes, angles & distances. Geeze - found that was more than enough for 'me-wee-grey-bits'. However 'canard' foils & canting or tacked foils are very closely related to all other foils & as such have many substiantial advantages to offer for the little effort of building, installing & making them work at the top-end of efficiency. Again IMHO

Haven't come across anyone who 'seems' to understand the importance of shape vs size vs foil vs 'trim-angle'. For instance there needs to be a very different sections used at the top (next to hull(s) than the section chosen at the bottom (deepest point) of both rudders & keels however not for the same reasons when total analysis is investigated in full. Very few understand the true importance that diverted distance out of normal flow direction effect has as a product of impedence to forward motion & that - that effect is far greater than the resistance offered by mere wetted surface area. When I've attempted to 'raise the subject' I've got - 'shot-down' - being told, in no uncertain terms - that it was not necessary to bother with all the 'gobbledegook' & I should just "sail the boat" - so I just shut-up & went away. I'm far to old to be bothered with negative people as I was only trying to be of some help. These 'experts' (X being the unknown quantity & 'spurt' being a drip under pressure) obviously thought they knew far more that I did & possibly so - so like I said - I just pulled my head in & shut-up. By the way - we were at athat time sailing the fastest 'B' class cat in OZ with a big wing mast (38.5' hi, 4' - ofre & aft - 2.5' at the top, 14'' thick @ 18'' back from leading edge & including all rigging - weighing in at 100 lbs gross) which - way back then - was no small achievement. At the same time we were able to reduce the area of both c/b & rudders by 2/3rds & have even more control. Must have been just a tad lucky but it did work. Along with sail shape & design modification we were able to do rather well, all considering.

Really interesting subject & without doubt - almost endless in detail & complexity.

Enjoy your sailing, the festive season, Christmas, New Year (do remember to work at having a grand one in '12), along with good health & big smiles - OH & a few (6 - pronounced the NZ way) 'sundowners' with close friends. Ciao, james

 

Thanks James! I just read-maybe in one of the links or in Martins paper in the "Mayfly thread here- about changing section shape over the span of a foil. Interesting that you mention it.

 

I'll restate the proa rudder/dagger combination foil as a good all in one route. They work -ask Newick and Russel Brown. The different section approach makes sense, is there software to explore this or would it be better to dig up the old AYRS testing tank plans?

 

In a 1966 edition of the AYRS I postulated the combined retractable rudder/board combination together with drawings. The idea was to take the sideways load off the rudder in straight line sailing and to make the board area variable to suit the particular course the boat was on. Thus the rudder area could be reduced to give the best operating range without making unnecessary induced drag. This idea may have been patentable, but I am not in favour of making money out of fellow sailors like the CBTF mob have.

 

I'm with you there Paddy, get the word out so development can happen instead of stagnate. Sure spreads out the R&D costs.