Outward facing J/foils on a cat

[mij]

Very nice Gary. How did the performance in the two configurations compare?

 

[Doug Lord]

Jim, nice find on the video! I posted it in the Foiling Week thread as well.

I'm not sure whose boat this is-was on SA a while ago:

 

[Gary Baigent]

Very nice Gary. How did the performance in the two configurations compare?

I've tried over the years a number of different foil configurations; they all work fine imo. Here's a shot with inverted V's; they were very good too. Right now on Groucho I have L's with a slight uptip, Sid has J's. They are excellent too.

 

[Doug Lord]

Gary, do the foils on your boats allow the whole boat to fly or are they primarily for keeping the lee ama up? Do you use a rudder T-foil?

 

[Gary Baigent]

They lift clear in 15 knots apparent wind. You can see the T rudder on Sid if you hit enlargement. Also can see the rudder tips with the high view shot of Groucho.
By the way, Groucho is 35 years old now.

 

[mij]

rc cat with outward facing foils

I have been looking at this type of foil on rc cats. As predicted by others, altitude control is an issue. However, the boat seems really stable and the foils seem to produce a lot of RM:



Any thoughts on a solution for better altitude control?

 

[Doug Lord]

Do you have a way to control the windward foil so that it develops downforce or is neutral--no lift?
If not the foils aren't developing Righting Moment(RM), in fact when both foils are lifting vertically the boat has much less RM than it would have if the windward foil was retracted.
That's part of the reason all the AC boats and other cats using uptip foils retract the windward foil.
Same story on Q23-though it can sail with both foils down, for max RM and minimum drag they retract the windward foil. The Q 23 has very similar foils to the ones you are using-they don't work like uptip foils but they work more like "V" foils and high aspect planing surfaces.
The video seems to show pitch instability which could be too high an angle of incidence on the main foils or not enough area on the rudder T-foil,or both.

 

[mij]

Doug,

WRT RM, my comment was based on observation. The boat seemed to be much more upright than I would have expected with a more conventional setup. In fact, I don't think that I would have been able to sail that boat in those conditions with a conventional board, or dual, fixed inward facing C-foils.

Jim.

 

[Doug Lord]

I think what you are seeing is that the foils are working and when there is not a
requirement for a lot of RM the boat is steady in roll but not in pitch. When a lot of RM is needed the windward hull pops up.

 

[mij]

Less windy conditions:

 

[Stumble]

Thinking about this I think the foils not only need to be mirrored, but the angle of the foil needs to reverse. So instead of an Uptip foil they need to be downtip. This way as the boat heels it looses lift. Otherwise as the boat heels the end leg of the foil generates a huge amount of extra lift pushing the boat out of the water just as the boat is loosing control anyway.

 

[Doug Halsey]

Do you have a way to control the windward foil so that it develops downforce or is neutral--no lift?
If not the foils aren't developing Righting Moment(RM), in fact when both foils are lifting vertically the boat has much less RM than it would have if the windward foil was retracted.

It looks to me that, when the boat heels, part of the windward foil comes out of the water. And the part that remains underwater is very close to the surface. Both factors reduce the lift on the windward foil.

The boat has a lot of righting moment because the lift on the leeward foil is so far to leeward.

 

[mij]

It looks to me that, when the boat heels, part of the windward foil comes out of the water. And the part that remains underwater is very close to the surface. Both factors reduce the lift on the windward foil.

The boat has a lot of righting moment because the lift on the leeward foil is so far to leeward.

I think that this might be right.

Doug L's point that this is something like the Quant Scowl arrangement is also interesting. It makes me wonder if the foils could be lifted close to the hull for very shallow foiling and a conventional centreboard added? In this arrangement a small amount of heel would lift the windward foil out of the water, and the central board could improve upwind performance?

 

[Doug Lord]

The Q23 is a good model for an outward pointing, shallow foil/high aspect planing surface-go here and watch the videos and look at the two pdf's carefully. I'm certain a version of the system would work on a cat or tri.
http://www.boatdesign.net/forums/sailboats/quant-23-foiler-scow-53468.html

 

[mij]

I'm impressed by the stability these foils provide. I cut down the width of the cat considerably, and it is still very stable: