CBTF(Canting Ballast Twin Foil)

any idea about common value of CL for CBTF rudders?

 

Collisions and broaches

I am interested to know if there is worry or experience with canting keels hitting the hull of another boat in close maneuvers. Of course, this would not be an issue with Volvo or Open classes, as they are usually out in the middle of the ocean with no one in site, much less 10 ft away. However, this could become an issue in Americas Cup or other Match races during starting, tacking duels, or luffing matches.

I am also interested in seeing photos of boats with canting keels broaching with the bulb sticking out of the water!

 

Instead of canting weighted keels to windward, why not cant the keel to leeward and place a lifting foil on it.

This allows much lighter displacement, uses the same mechanism, the foil will provide lateral resistance so the steering surfaces can be smaller and all forces are lifting instead of using weighting for stability.

Sounds redical perhaps, but this is the latest in foiler dinghy and sailboard technology, why not apply it to yachts as well?

 

CBTF/Flap

Ian, what is being worked on right now is a cantng keel-canted to weather with minimum ballast on a boat with a retractable main foil and rudder T-foil. The canting keel fin MAY also have a flap on it so that when it is near max cant angle the flap causes the strut to develop downforce and increased righting moment.EXPECT to see a system like this on some large mono's before too long.

 

Interesting, Doug. I've also designed a keel with a foil that can be swung to windward and a flap that can be used to generate a downward force, though on mine the ballast is fixed. See my post at
http://boatdesign.net/forums/showthread.php?threadid=2425&pagenumber=2

 

CBTF plus

Stephen, I have a copy of your patent; excellent work! Has the system been incorporated in a boat so far?

 

Surely to lift from leeward is far more efficient than to suck from windward!?? Sucking must be slower!

 

Lift or suck

Lifting to leeward sounds good if you don't have ballast out at the end of the keel/strut But then where do you have ballast? And frankly I don't see a particular difference in "efficiency" regardless of whether the foil is lifting up or down except for that part of the downforce that might be imparted to the boat as extra "weight"( wetted surface). Thats worth looking at.
But I don't get how you would get stability to help you thru the times where the strut does not lift optimally; can you enlighten me?
UPDATE: I can see lift and some RM with the keel canted to leeward with a T or Y foil on the bottom of it-but I still don't see stability on a 40 footer in slow(rough) conditions or in starting up just after a tack...

 

Thanks for the compliment, Doug. I lost steam after getting the patent and now the guy who was going to build a prototype with me has moved to Ireland. I still think it's right for some applications, esp. mega-sailers. If a partner/promoter dropped from the sky (or if someone wanted to build a boat as a prototype and perhaps write it off) I'd be ammenable, since nothing's happening with it at the moment.

I see using the downward force option only going to windward in a blow, and I think under those conditions it would help a lightish, shoal draft monohull sail like a heavier, deeper draft monohull. In the case of a multihull I agree that a lifting foil to leeward is preferable, but applying a downward force to windward in select conditions is consistent with the overall logic of ballasted monohulls.

 

The problem, as far as I can see, with using lifting (or "sucking") foils is that the force must by its nature be aimed somewhat to leeward, thus reducing it's effectiveness.
Using a canting fin with ballast, and a toed-in asymmetrical daggerboard, is really no different, in that the fin will provide RM, while the daggerboard reduces it while providing resistance to leeway.
I like the ballasted fin better for true offshore applications since it can be used to "overbalance" the boat in the event of a capsize/turtling and thus enable it to right faster. A non-ballasted fin in that situation is just something to hold while you wait for rescue....
Steve "too old and set in his ways to change now..."

 

CBTF Patent

What does it covers? and where?



Nico

 

CBTF patent

CBTFco patents are:5,163,377 and 5,622,130 . If you contact a reputable patent attorney they can provide you a copy at a small cost(don't pay much!) or there are places on the internet where you can download a patent if you have the number.

 

CBTF patents

If anyone interested, you can read them at:

http://patft.uspto.gov/netacgi/nph-...,163,377.WKU.&OS=PN/5,163,377&RS=PN/5,163,377


http://patft.uspto.gov/netacgi/nph-...,622,130.WKU.&OS=PN/5,622,130&RS=PN/5,622,130


Nico

 

I think one other good reason that you havn't seen any lifting type canting foils on monohulls is the likelyhood of ventelation. This same problem is a concern with the 'downward' lifting foil. Imagine your sportboat getting into a nice puff, and heeling before accelerating, the foil sucks a little air, and loses it's bite. Now you are powered up for X effective ballast and you've only got your keel bulb, which is .75X or so. Big wipeout, and until you get flow re-attached on your foil, you are grossly overpowered.

Ought to be awesome when it is hooked up though.

 

That's why I like that mine is well underwater and intended for use to windward. A problem might result in a knockdown going to windward, but probably not a wipeout.