Foil Ratio

last but not least.This is for the rudder snatch/servo issue which was discussed on a previous thread.I would like to get back to that when the new build starts.
Vital Statistics-
Boom-136"
main sail -Tack to Clew (ie foot) 10'6".
Hoist,(ie luff,tack to head) 33'
Top of hoist in a straight line to the clew(leech with no accounting for roach) 34' 10"
Mast foot to fore 'inner'(??) forestay tang-11'3".(that's the original position when I got the boat)
The 'outer' fore stay tang is 6" ahead of that.
I apologise for not getting the rudder out to the boat to give a realistic comparison.
The jib is ashore.Luff-27'
Foot-11'6"
Leech-24'
Thanks for the compliments for the boat.I've seen photies of yours so it's praise indeed.I wish I could say that I built/designed Caiman.It was about 45 seconds after first seeing Her folded up on Her trailor that the decision was made,the only question was 'how much?'I paid a bit more than I wanted to,the Seller got a bit less than He was asking,and so we were both moderatly unhappy, but we both had had a good day anyway
Cheers

 

Caiman.
Gary is right.
I printed the plan view of your rudder in post#56. I laid a rule from the top front corner to a point close to the projected rudder post line. That is what I would cut off and then re-shape the leading edge. Would really help with the feel of the helm, and reduce the area of the rudder at the same time.
It's really a case of "Suck it and See".

 

It is ironic that we are on this subject, because I have just been trying to follow up on a New Years resolution to sort out all the junk from my archive files. I found a drawing of the "Lifting rudder/board combination" which I "invented" and which appeared as a paper in the AYRS magazine in (I think) 1966. It consisted of a small flap in a daggerboard which could be controlled by a tiller with selective friction. This sat between the regular dagger and
the transom hung rudder, enabling proper balance to be attained on all points of sail.
So much for the CBTF patents.

 

That's very interesting OS7 - because my sailing mate Jacques de Reuck competed in the 1979 Mini Transat in a Daniel Charles' design 6.50 named Vileda, a lightweight, narrow alloy boat with a bulb keel (Vileda came in fourth, won that year by Norton Smith in the breakthrough Tom Wylie designed American Express) - and Vileda had a balancing board, a dagger foil as deep as the rudder, set aft in the cockpit and just forward of the rudder ... and that board was used to keep the boat running straight when hard surfing in waves; it took a load off the rudder - said to work perfectly. You can see it in the lifted position in the haul out photograph.

 

Yes. There were many precedents.
I never did hear about how the CBTF people made out with their legal challenges. :?:

 

======================
Last major battle, as far as I know, was CBTF vs Maximus. Maximus had a "normal" rudder a canting,telescoping keel and a daggerboard forward of the mast with a flap. CBTF said it infringed, Maximus said no- CBTF won. Funny thing is Maximus has never beaten a "real" CBTF boat like Wild Oats or Alfa Romeo as far as I know. The real beauty of a CBTF boat was that with "collective" control of the twin foils(foils turned the same way) leeway was eliminated from the hull AND from the keel strut which could be made significantly thicker and smaller with less wetted surface than a conventional canting keel. The thick section was ok because it didn't have to lift... Also, the placement of the foils vs a vs the hull wave pattern was supposed to be advantageous with CBTF.

I would think twin foils w/o the canting keel would work real well on catamarans...

CBTFco on CBTF: (see fee schedule below-eye opening!)

"The CBTF patents cover sailing yachts using a canting keel or ballast with fore and aft controllable foils or rudders to affect changes in side force and/or maneuvering, including sections with trim tabs and jibing daggerboards."

click on image

 

I've done the maths.I've done everything in inches.The combined main sail area and the foretriangle amount to 52041 sq".The DB needs to be 2% of that which is 1040.82 sq".My present DB is 800sq".I've worked out that for every inch increase in span of DB I get a 20sq" increase in area.If I add 12 inches to the span,this will bring me to the 1040 area required.
Rudder.Using 3-1 DB/rudder ratio,With a 1040sq" DB,I require a 346sq" area rudder.The present rudder is 544sq".If only reducing the span,I would need to cut off 12 inches to get to the required area.With the other rudder problems,I can reduce some area before decreasing the span to get the required amount,and also hopefully get rid of the 'servo' action on the rudder at the same time.
Out of interest,I worked the equation the other way.Taking the present rudder area,I would have to increase the DB to 1632sq" to balance.By only increaseing the span,I would have to double it to get the immersed area to 80".This would make the board about 10 foot long overall.Please check my theory and conclusions.If anyone spots any errors, please let me know.I will gather the materials required before I do any cutting,so it'll be sometime before I will be able to report back to you.I wish to thank you all for your help and patience.Please don't be suprised if you see a thread asking for further advice.
Cheers

 

Boards

Caiman, I think you about there Make the board as long(deep) as you can, and then wide enough. Almost any foil section will be better, as long as the edges are smooth, and "fix" the rudder. You boat will work much better. Your fore triangle C/E is pretty far forward, but a good board, even oversize, will really help. Good luck, and start sanding B

 

Don't reduce the rudder span before you're sure you need to. The "Servo Effect" that you noted is either due to an excessively aft stock, or too much forward sweep (or both, though the latter in your case). Fix that problem, then any change to the span will effectively change the operating drag of the rudder, when the boat is trimmed. To allow for out-of-trim conditions, a big rudder is a help, so be cautious when it comes to changes in span!

Cheers,

Tim B.

 

Rudder Design

Hi,

I'm a brand new poster, so if this is redundant, let me know, so I can read previous.

I am re-fitting a 1974 C&C 35 and want to rebuild the rudder which was poorly repaired at one stage in its past. I have 2 questions, related to one another.

Based on my measurements of the chord (21" or so) and the thickness at its widest point (3.75" or so) and reverse computed a NACA #0018. Is this correct?

Next, following the latest thread on the topic of sail area vs appendage area, I was wondering if I should change the original size/aspect ratio of the spade rudder and if I do so, would that also force a keel shape change? The latter would be a challenge.

Alex:

 

The CBTF case against Maximus was total BS, should never have been allowed, judge (and jury if there was one) was/were incompetent. Twin rudders/foils have been around for decades, generations even. What is the difference (save that the keel cants) between "their CBTF" concept and say, AC NZL20. The two rudders and twin keel on the kiwi boat were connected ... but the concept is/was the same. How about we go back to the NZ scow of 19th Century,; some had two or even three centreboards; they weren't rudders in the sense they could be turned but they acted the same as rudders: lift one, lower another and the boat could be turned, or run straight depending on the combination. How about South American "Thor Heyerdahl-type" rafts with their multiple boards, or Pacific catamarans with multiple deep paddles and steering oars, how many centuries ago?
That case was a disgrace and the Maximus crowd were shafted ... and a stupid precedent was set.
Correct me if I'm wrong ... but not with semantics, please.

 

Many people thought and still think that CBTF patents are too broad. The litigation did not sell more licenses AFAIK.

I also don't see 100's of CBTF boats being built by Catalina, Hunter, Beneteau etc ...

Nice thing about multi's ... no ballast ... so no frivolous lawsuits.

R

 

And how about this brilliant 1980 design from Van Allen Clark (with help from C Class catamaran guru David Hubbard), the first US canting keel boat, Red Herring. The twin boards may not be twin rudders but they can behave as if they were, same as a NZ scow ... or any other multiple foil sailing craft.

 

Unfortunately, under US Patent law ---no precedent can be claimed after the date on which the patent was awarded.

 

With ref to the mast position,I moved the whole rig back with a view to eliminating the lee helm ,and giving the boat weather helm.This was only partialy succesfull in removing some, but not all of the lee helm.After the corrections to the foils have been done,the intention is to move the mast forward to its original position,or as close as possible,while still retaining some of the new (yet to be experianced by me on Caiman) weather helm.
I was asked earlier,and have discussed on another thread, about water ingress from the DB case at speed.At the moment there is no water coming into the boat from here.Prehaps I've not been going fast enough I feel the answer to this would be to continue the DB case up to the deck head.The extension would not need to be structural.And as pointed out on the other thread,it would do away with the 'hatch' over the DB entry hole in the deck.By all means put rubber at the front of the case as a 'fairing'.Eliminating the cause is better than curing the effect.Also,some kind of uphaul/downhaul would have to be sussed out.A cheap and cheerfull way,shape some lipped moulding and mount on the deck head and DB case.Make a 'sock' out of WP material through which the DB slides.Have 'bungie cord' sewn into the hem at each end of the sock.Use the hem to stretch over the lip of the moulding at the deck head and DB case.Imagine a Spray Deck on a Kayak,the same idea.
Alex_.I work out 18% of 21" to be 3.78",so it's not far off.
Cheers