Fenced surface piercing V foil sail inflatable.

[RogerO]

Inspired by the AC75 racing I am toying with the idea of fitting foils to my Caspian inflatable I converted to sail with an Optimus sail rig (YouTube slide show). Since trial Mk2 with increased sail area by 50% or so with sail and mast from a windsurfer (5m2 or so) currently untested (YouTube slide video).

I do not know speed at the moment craft has a displacement speed of 4 Knots but is a planing hull form and Im guessing speed with 4hp about 7 knots probably similar with sail maybe. So I want to aim for a take of at 5 knots and guess if successful 10 knots maybe (If mast survives). I want to keep it simple as possible and as simple construction so thinking surface piercing V foil attached to centreboard at the bottom. A convenient chord for attachment to centreboard would be 270 and the on line windknife calculator seems to show with a projected length of 1300mm of the two foils with 8 deg attack would seem to give some feasibility.

The all up weight would be 150kg or so and comparison with broomstick at higher lift off speed similar weight seems a reasonable comparison (lot less sail area).

I am thinking a cambered plate (8%) of similar size easy to construct would perhaps work OK at the relatively slow speeds envisaged. The NASA foil sim would seem to show similar lift drag possible at speeds envisaged to an aerofoil. I was thinking of fitting fences shown on line to work but see that broomstick removed them I actually think the AC75s could benefit reducing spray if allowed in rules. Not sure what to do at stern and keeping it simple. Do not think rudder attachment is feasible.

I am thinking of testing with outboard but rear foil would need to have additional 15kg lift.

Any comments or advice please and on balance between front rear foils. I have ordered Ray Vellinger book.

Thanks

RogerO

 

[Doug Halsey]

Sorry, but I don't think this is very promising.

Compared to a Moth: Your boat is a lot heavier, with more wetted-surface area, and a fraction of the sail area, and you're hoping for a lower take-off speed?

 

[RogerO]

Hi thanks you could be correct one reason I wanted to make construction simple as possible.
I think the moth can do 15 knots plus and has 50% more sail thanI have I was not looking for a similar speed just to get up
Do you happen to know how to find what equivelant thrust can be obtined from sails e.g. 5m2 maybe a bit more.
I have an idea of current drag from speed with engine and have a bit spare I think at 5 knots.
With Covid and in flood anyway I am unable to try my newsail set up but think in a decent wind might get it on the plane.
I think I have similar sail area to a laser and I have seen somewhere foils on a laser but not sure if thats with a standard rig or not.
So maybe not impossible ?
A challenge as they say.

 

[RogerO]

Sorry, but I don't think this is very promising.

Compared to a Moth: Your boat is a lot heavier, with more wetted-surface area, and a fraction of the sail area, and you're hoping for a lower take-off speed?

 

[RogerO]

Looking in a bit more detail at Radial laser 5.76m2 sail and 60kg hull. Inflatable is 50kg plus mods plus rig around 65kg plus me allowed 150kg.
Quoted speeds on forums up to 13knots (normal Laser 17knots)
Not sure if foiling laser is just standard sail or includes radial set up.
So maybe long way from Moth but close to a laser maybe ?

 

[patzefran]

Agree with Doug, your Caspian looks very draggy compared to a Laser. However it is possible to lift anything provided you have enough foil area/relative to weight and you
have enough power (sail area) to reach corresponding take off velocity with this foil area ! As Ray Velingha writed a rough estimate of the maximum velocity will be twice the take off velocity.
But remember if your max boat velocity w/o foils is only 5 kt you may not be able to reach it with the same sail area taking into account the very high initial drag of two foils at 8° angle of attack.

 

[RogerO]

Hi thanks for your input. The hull is a planing hull and I think with my 4hp outboard would do around 7 knots guessing 150 lbs thrust.
Believe based on other sail boats think in a good blow should do similar speed. On a forum I have seen radial laser speed quoted as up to 13 knots on plane at 6-7knots ( I think similar to inflatable) . Seen quoted laser foil speed of 23-25 knots think with 7m2 sail rig.
So I figured need to take off at lower than 7 knots hence why decided 5 knots.
If I chose 6 knots then could choose smaller chord lower drag and a little spare angle of attack left perhaps.
The sensible route might be to wait until try new sail set up and measure performance but under Covid lockdown probably make up something before that.
It would be usefull to know what equivelant thrust from 5-5.5m2 sail and if in range of 150lbs plus think would be feasible maybe.

 

[patzefran]

To be the most simple, with 5.5 m² with Cd = 1 sail square to the wind you would be able to get 150 lbs thrust dead downwind in approx 15 kt true wind, vb =7kt boatspeed , 8kt apparent wind
Any other course assessment would need a more accurate analysis including righting moment and other data on boat rig.

 

[RogerO]

Thanks for that.
So I think idea might be feasible just.
Assuming take off the AC75s seem to get faster speed than wind speed due to higher aparent wind speed by a large margin so tacking down wind might get more thrust perhaps which seems to improve chances of success maybe. ?
The wind speed you give is the speed of highest laser speeds reported I think.
I thought that choosing v surface piercing would give best chance of control in a heeling/high wind situation.

 

[patzefran]

Of course if you tack downwind you will increase apparent wind and thrust, but you must manage the heeling moment with adequate righting moment, the larger mean beam of your foil system
will gives you the higher righting moment (distance, between the middle of wetted span of windward and leeward foil). surface piercing foils are also the best to increase maximum velocity as their wetted area decrase with velocity after take off.

 

[RogerO]

I figured that a heel to port for example the projected area of port foil would increase giving increased lift and at moment of heel added angle of attack.
The projected area of starboard foil would reduce with reduced lift so together I thought would be quite a helpful righting moment.
I am considering another V foil at stern but unsure at the moment one problem is rear V foil for convenience would be smaller so would give a nose up attitude when foiling.

 

[patzefran]

I am sorry I made a big under assessment of apparent wind to get 150lbs thrust in the assumed downwind conditions : the dynamic pressure with air is
0.5*ro*v², with ro =1.3 kg/m^3, 150 lbs gives with MKS system 600 Newton so with 5.5 m² of sail you need v²=600/(0.5*1.3*5.5)=168 wich gives v=13.m/s
13 m/s corresponds to 25.5 kt apparent wind, 32.5 kt true wind with 7 kt boatspeed

 

[RogerO]

Hi thanks for your efforts.
So are your conclusions would need 32 knot wind speed for 150lbs thrust with 5.5m2 sail rather than 15kn? in which case less sure of outcome.
What thrust do you think in 12 knot wind. ?
I have seen on line stated a laser will foil in 10-12knot wind at 23 knot with a 4.7m2 sail.
Laser beam is similar to inflatable but is at least 2 feet longer and therefore possibly higher wetted surface and similar weight.
The caspian has an inflatable keel which does give a reasonable planing hull form and will plane with a small engine.

 

[Doug Halsey]

Is this the boat you're referring to?

 

[RogerO]

Hi yes.
The Optimus rig is I think 3.3m2 Since this picture increased rig with a sail from a windsurfer of around 5.5m2 and a lot higher mast of around 5m but untested at the moment.
The wind at ths time was light and shielded mostly by tree from the right hand side.
The overal length is 2.8m but the transom is well forward abou .7m and most of the bow is up at a relatively low speed.
Beam at waterline around 1m.