OPEN 60's: Lifting Foils (still) OK!

[Tanton]

Foils.

Certainly, the Monomaran is slim, while the Monohull is narrow at the waterline.

 

[Southern Cross]

This...

"Historically IMOCA 60s have taken this to the maximum, gaining extra righting moment through ultra-beamy hull shapes and canting keels."

And this...

"The end result could weigh approximately 6 tonnes, almost two tonnes less than the present lightest IMOCA 60s. And once again, a substantially narrower and lighter boat, with a fixed, rather than a canting, keel and with straight foils, as opposed to L-shaped ones, can all result in a less expensive boat."

Looking at two Maxi's for example, Wild Oats XI and Perpetual Loyal, Wild Oats (an older design) is narrower and has utilized DSS successfully and proven to be the faster boat.

WOXI/LWL = 30.48m Beam = 5.4m NB Transom tapers.

Loyal/LWL = 29.99m Beam = 7m NB that beam is carried through to the transom.

My question. What do they mean by "substantially more narrow" under the rules? How narrow? And if there were no rules, how narrow could you theoretically go? Wouldn't DSS at the transom of a narrow boat and DSS forward compensate for the RM of a wider boat and significantly reduce wetted surface and provide more lift? (Forget about where to put the chain plates for now)

I'm picturing a canoe outfitted with all sorts of retractable dagger boards and foils.

 

[Doug Lord]

Look at a "normal" Mini then look at this one which hasn't proved itself yet but is indicative of the direction DSS allowed Hugh Welbourn to go design wise. The "scow" Mini's have proved to be very fast despite their looks(pictures 2 and 4). :
Pictures of the DSS Mini sent to me by a friend of the owner, the other two from David Raison's website:

 

[Southern Cross]

Thanks Doug. The Mini is under rule constraints. Still wondering what the theoretical narrowest boat (taking displacement into account as well) DSS could permit.

The length:beam ratio on WOXI is closer to 6:1. This is a narrow boat. Loyal closer to 4:1. The new Comanche is closer to 3:1.

Using more than one DSS, could you theoretically go to higher number, say 7:1 to 8:1?

Think of an something like an ama with two sets of DSS, a canting or fixed keel and dagger boards or a 100ft Maxi with a 12ft beam. Would these dimensions provide enough displacement to carry the ballast needed to power it up?

Could DSS provide the same righting moment for a boat with a 7:1 or 8:1 ratio as a boat with NO DSS and a 4:1 or 3:1 ratio?

That's my question.

 

[LP]

This is all very interesting. I can foresee some extreme DSS design gyrations taking place in the future. Sort of "plank-on-edge" meets "the sandbagger". Boats with lots of sail area, little ballast and little initial stability. At the extreme anyways. I don't normally follow the competition side of sailing, but this thread caught my eye and reminded me of something I posted about eight years ago. Hopefully, common sense will prevail. These appendages could promote mono hull performance enhancements like we have not seen before.

Maybe I was on to something and didn't know it.



The fins in this configuration are all positively loaded so there is no high pressure/low pressure conflict. The orientation of the fin is probably not the most condusive lift production. The horizontal, sliding DSS seems to correct a lot of inefficiencies of my quick model from a few years ago.

The rest of the thread.

http://www.boatdesign.net/forums/boat-design/sailing-sci-fi-16139.html

 

[Doug Lord]

Dss

The fins in this configuration are all positively loaded so there is no high pressure/low pressure conflict.

==================
I'm curious what you mean by this?

 

[LP]

==================
I'm curious what you mean by this?

I see an upward (positive pressure) loading along the entire length.

In post #10, you had evaluated the foil there with a pressure conflict; positive pressure and negative pressure on the same side of the foil. I am not an expert, but I think that a proper transition at the juncture would calm the turbulence there. I envisioned singular (all on one side) pressure distribution along the foil I drew up.

I'm not trying to say my foil is better or worse. it was just a quickie something I drew up years ago. My foil probably has too much non-horizontal lifting surface to be effective.

 

[Doug Lord]

Thanks. It's important to realize that any foil that lifts has high pressure on one side and low pressure on the other side. You also have to take into account the effect of leeway on the foil. Seems to me like the foil you drew would tend to have high pressure on the low pressure side with any heel on the lee foil. I'm not sure you would want a lifting foil on the windward side?

 

[LP]

Please understand that the model was rough and drawn symmetrically. The intent would have been for these foils to retract independently and as such only the leeward foil would be submerged in a normal sailing regime. I've managed to dredge up the file that I last modified in 2007. I've included a bodyplan view so you can better see how the foil enters the water. I see very little ability to control leeway on that portion of the foil. I believe my intent would have been the winglet portion handle leeway issues along with a more standard keel or center board. I understand lifting theory. There is ALWAYS pressure differential when a surface if producing lift.

"They speak of the possibility of using an "L" foil with the tip pointing outboard to combine a daggerboard and a DSS foil. Not a good idea......

Two renders from the article and my sketch that explains why its not a good idea(leeway arrow shows the direction the boat and foil move due to leeway) "


These are the comments that I am referencing when I'm talking about conflicting pressure differentials. There is no pressure reversal at the "L" in my foil. That's really all I was saying. I think these vessels will sail much flatter than traditional mono-hulls, especially at speed when the foils are working best. I think that leeway at higher speeds will be reduced so less of a factor unless the more highly powered rig drives in more leeway. The best option is probably to continue to handle leeway with the keel proper and to save the DSS for handling heel and keeping the rig powered.

"the foil you drew would tend to have high pressure on the low pressure side with any heel on the lee foil."

If my foil was more vertical at water's surface, I think I would tend to agree here. Even if it was the case, the pressure reversal would be a smooth transition rather than abrupt one caused by a surface discontinuity ("L).

Best Regards.

Guess I should upload the bodyplan. Oops.