Increasing Centerboard lift

[Doug Halsey]

Hi Randy,

I was away for the weekend, so I'm just now replying.

Originally Posted by RHough>>
Is your foiler a single track boat like a Moth?

No, my boat is a trimaran currently using high-dihedral surface-piercing foils, quite different from a Moth. I'm attaching a photo, below. Also you can read more about it in the following threads:

http://www.boatdesign.net/forums/sho...167#post202167
http://www.boatdesign.net/forums/sho...442#post214442

I think you are equating the immersed span of the daggerboard while foiling to raising the board in a displacement hull. I'm not sure if the two are alike.

This probably makes more sense now that you realize my boat is not like a Moth.

Foil wake and rudder interaction is a whole other topic ... Rudders always operate in downwash from the foil. Wake implies turbulence to me, if a canted board causes turbulence under the hull ... it would be a pretty poor design.

Actually, I was just passing along Bethwaite's caution, before really thinking about it much. It seems to me that the trailing vortex sheet behind the daggerboard would deflect itself & any viscous wake sideways enough to miss the rudder in most cases.

Regards,
Doug

[RHough]

Originally Posted by Doug Halsey

Hi Randy,

No, my boat is a trimaran currently using high-dihedral surface-piercing foils, quite different from a Moth. I'm attaching a photo, below. Also you can read more about it in the following threads:

This probably makes more sense now that you realize my boat is not like a Moth.

Actually, I was just passing along Bethwaite's caution, before really thinking about it much. It seems to me that the trailing vortex sheet behind the daggerboard would deflect itself & any viscous wake sideways enough to miss the rudder in most cases.

Regards,
Doug

I am very impressed with your boat and your approach!

Not how I would have done it, but I understand your stated reasons, and the results are very good.

I agree that the tip vortex will probably miss the rudder, however the rudder does operate in the downwash field of the foil (assuming foil and rudder on centreline). When I was designing RC sailplanes, I had a reference that gave an estimate of downwash angle at the horizontal stabilizer so the rigging angles could be designed properly ... alas I donated the book to a library and it has since gone out of print ... drat.

How much do you think the dihedral angle of the foils costs you? I would imagine that the acute angle at the tip causes quite a bit of interference drag? Adding a spanwise extension was a very clever move.

Again, great work there. A 'just do it' attitude is something that many of us could use more of!

You inspire me to dust off some old sketches and apply what I've learned since I drew them ... my wife is gonna kill me ...

[Doug Halsey]

Originally Posted by RHough>>
I am very impressed with your boat and your approach!

Thank you. I've had a lot of fun building & sailing this boat.

Not how I would have done it, but I understand your stated reasons, and the results are very good.

This boat is meant to be a highly modular platform for testing & learning as much as I can before attempting something more serious. It's not necessarily what I think is the best either!

I agree that the tip vortex will probably miss the rudder, however the rudder does operate in the downwash field of the foil (assuming foil and rudder on centreline). When I was designing RC sailplanes, I had a reference that gave an estimate of downwash angle at the horizontal stabilizer so the rigging angles could be designed properly ... alas I donated the book to a library and it has since gone out of print ... drat

I'm sure I could get a pretty good estimate of the sidewash at the rudder by simply:
1- Use lifting-line theory to estimate the sidewash at the daggerboard & far downstream
2- Choose a suitable function to blend between the 2 values

Or, just as easy (for me, since I have the codes) : Modify either a lifting-line code or a lifting-surface code to calculate it.

However, this will have to get in line behind a bunch of other calculations that I've been intending to get back to.

How much do you think the dihedral angle of the foils costs you?

It's a compromise, of course, but I don't think it's costing me as much as you might think. I started writing some of the pros & cons, but decided it was too much for this thread & I'm almost out of the time I've alloted for this post.

Again, great work there. A 'just do it' attitude is something that many of us could use more of!

You inspire me to dust off some old sketches and apply what I've learned since I drew them ... my wife is gonna kill me ...

Great! I hope you do.

Doug Halsey

[lunatic]

once built a rigid leading edge flexiable double skin Sunfish CB with concave trailing edge and variable tensioning cable to change camber. Hard to see effect visually or in racing standard Sunfish, but forgetting to flatten camber on a dead run, capsized, showing some unintensional effect