Ronin's Third Foil - the story so far

[PaulJ]

What's the story with Ronin's third foil

When Ronin was last in Weymouth for a bit of TLC, Sam showed us the new foil he had built for his 14 but hadn't had a chance to fit. He had all the parts made and suggested we give it a try on Ronin as it only needed a few hours work to fit. The horizontal foil is as near as identical to the current t-foil, vertical 14mm thick moth rudder.

Test day 1 - Weston. T-foil depth 350mm below hull, leading edge 75mm behind transom. Used one race only as it produced a very heavy helm. Lots of spray from canting mechanism as it was touching the water. In a straight line it seems to be no slower than before.

Modication A - canting cassette moved forward 75mm and up inside the gantry, vertical cut down so the foil was 200mm below hull as per the I14 at Weston

Test day 2 - Thornbury. Helm much lighter this time but still not as light as with the other t-foil, only one lap completed but that was due to other issues not associated with the foil. The t-foil tips do now break the surface when the boat isn't flat and suck air down which makes a great noise as its released behind the boat when the foil fully submerges again. Again it wasn't slow as we got round the first leeward mark first, not bad considering we didn't have any trapeze wires on both sides. I think the ventilation could be a huge problem on a windy day though

Modication B - the cassette is moving sideways a little, so this needs stiffening up. It is also over tall so will be shortened, this will allow the foil to have a max depth of 275mm. We think the vertical section is just too big and maybe causing the problems with the heavy helm, this will be replaced with a 9mm thick section shortly

Planned test - sail without the third foil to see if the rudder is the cause of the heavy helm, I doubt it but we need to eliminate that one. Hopefully we can get a rescue boat at Royal Vic to take it so we can test it between races.

Suggestions welcome

[Phil Alderson]

Paul,
It sounds like an interesting project.
Is this where you have a T-foil on its own vertical support just in front of a rudder with no lifting foil?
If so, what are the benefits of doing this, there will be some additional drag from the extra vertical foil, and potentially some interaction with the rudder.

[PaulJ]

Yes the foil has it's own vertical support located just behind the transom. The rudder is a conventional rudder.

Pros (in theory)
1. Moving the lifting foil 250mm forward allows the crew weight to either stay forward which makes sailing / ergomonics easier, or get back to where we are now which gets more weight over the foil.
2. The lifting foil does not get disturbed by the movement of the helm, i.e one wing tip going forward whilst the other goes backwards
3. Allows some energy recovery from the upward flow off the transom
4. The foil can be removed in light winds, saving on wetted surface area

Cons
1. Additional drag from vertical
2. Slightly heavier than a single t-foil system
3. More foils to deal with when launching and coming in

[john_hamilton]

pictures?

[Phil Alderson]

It will certainly be interesting to see how it goes.

I have to say that I have never quite got the energy recovery from the upwash thing, in my head I can't see it adding up.

[Clive Everest]

If the lift to drag ratio of the foil is better than the tan of the upwash angle then the force vector from the foil will be tilted forward, pushing the boat forward.
Even if it is not actually tilted forward the backwards component can still be less than the reduction in drag due to the reduced displacement.

I find it harder to believe that the winglets on commercial aircraft are actually pushing the plane forward.
One can only assume that they are or they would not be there.

I think that Kevin's article describes nicely how for upwind the foil wants to be as close to the transom as possible for max energy recovery and to allow the weight to be put above it whilst for downwind it wants to be as far back as possible for maximum pitch damping and deep enough to guarantee that it will not ventilate.

When we first put foils on a Moth (Skippy 1991) we put them on the transom - 2in below the surface.
When hard pushed you would sometimes hear them ventilate just before the pitch pole.

Andy P moved them to the bottom of the rudder. This solved the ventilation problem however the boats with the foils on the transom would pickup waves and surf earlier than those with the foil at the bottom of the rudder.

The gain did not make up for the pitch poles and the rudder tip position became universal.

Clive

[Torchy]

It will certainly be interesting to see how it goes.

I have to say that I have never quite got the energy recovery from the upwash thing, in my head I can't see it adding up.

...a bit too like perpetual motion? I suppose energy is stored and then released but...

[phil_kirk]

With the modern boats with 05 rules rigs I think there is only a speed band of 1-2 knots in wind above the nationals 5 knot minimum starting wind speed when the foil would be a hinderance. In 5 knots upwind you would have the crew weight forward unloading the foil and at this point drag from the foil would be a problem.  A little more wind speed and you would have 1 person wiring and starting to move back to load up the foil a bit. Downwind you should be able to generate enough apparent wind in 5 knots to get some benefit from the foil. 
On the weekend at Thornbury we didn't adjust our t-foil around the course and used boat trim to load or unload it as well as get the transom clear in the low speed conditions. Our T-foil is a bit smaller than others so may have less drag and help us in these lighter winds.

My point being that the benefit gained from being able to remove the lifting foil may not be practically gained on a championship racing day where in one race it's a benefit and in the next its not.

I'm also not sure that a Cherub generates the same level of up wash in it's wake as an int 14 once it's moving.

The most interesting benefit in my mind is moving the point of lift further forward.

I look forward to hearing about further testing though.

[john_hamilton]

ahh, but if they could find somewhere to put it in the boat when not in use....... wouldnt be to much of a problem, when the foil isnt in the water, the crew will be on the foredeck anyway!!!

[Torchy]

Surely it's easy to simply raise it out of the water like you do launching and recovering...as long as it runs freely (unlike Loco's currently) 

[john_hamilton]

what happens with the tiller then nigel...

[phil_kirk]

Surely that's ok if all the marks are to port?

[Torchy]

what happens with the tiller then nigel...

Doh! 

[kevin_ellway]

It will certainly be interesting to see how it goes.

I have to say that I have never quite got the energy recovery from the upwash thing, in my head I can't see it adding up.

Paul Bieker originally suggested that the T foil near the surface could recover some of the bow wave energy. The wavelength of the bow wave is related to speed. The bow wave will have a peak at the bow. At 9kts, the trough of the bow wave will have moved to the location of the transom. So provided you are doing less than about 9kts, the water will be flowing upwards at the transom. The lift from a foil is notionally at 90 degrees to the local flow. So if the local flow has a component up towards the water surface, then the lift force is rotated forward and is providing an element of forwards drive. This is energy recovery.

The bow wave is a transverse wave, so I do not subscribe to the idea of bow wave recovery. The bottom of the boat , however,  behave a bit like the underside of a foil. If the boat is trimmed bow down so that an extension of the rocker line from the transom is pointing up towards the water surface, then the flow close to the transom will have an upward component. A foil placed in the downwash from the transom will thus be able to recover some of the transom wake energy. Energy recovery will only occur when the bow is trimmed down. Once the bow is up such that the aft rocker is pointing down from the surface, the local flow will also be down.

Waves have an orbital local flow within them. The orbital velocity decreases exponentially with depth. A foil placed close to the surface can also recover some of this orbital energy.

Placing the foil close to the surface basically means that wake energy and wave energy can better be recovered than with a foil at depth. These are low speed effects.

Placing the foil farther from the surface reduces its wave drag and gets it out of the downwash from the hull when the boat is trimmed bow up (i.e. planning) and this also reduces drag.

Placing the foil farther forwards means you can put more load on it. Above about 6kts, the foil can carry load with less drag than the hull. Offloading the hull to put weight on the foil thus reduces overall drag.

In summary, put foil as far forwards as practicable. Whether you put it close the the surface or not depends on what speed range you're trying to optimise.

I hope this helps

[Torchy]

Does this thread solve the problem with having 3rd foil up in light weather but still available if needed?


http://www.sailingsource.com/cherub/forum/index.php/topic,1199.45/topicseen.html

...or am I still Homer Simpson?