Ben,

 Having just completed sea trials on my kayak with a sail rig I have some advice about rudders for you.  As far as using a rudder for kayaking, I would go with the Smarttrack.  Having used and installed both the ST and Feathercraft, I must say that ST is much nicer, smoother, easier to use and install.  However, for sailing, neither rudder comes close to being big enough to steer the boat. The rudder I finally ended up having to use to steer my kayak while under sail is much larger than any kayak rudder.  In fact it is probaby about 75% of the size of the leeboard that goes with the sailrig.  I found that using cables and footbraces to steer a rudder this size does not work as there are enormous strains being put on the rudder and it is no fun to constantly be "standing" on one of the braces in order to maintain course.  

 I ended up going with a tiller set up, that surprisingly does not hamper my paddling when the wind dies.  

 Check out my website for some examples of my setup:  http://www.keakayaks.com/sailrig/sailrig.html 

Ben,

I will second what Casey said. Generally speaking the rudder needs to be at least 1.5% of the sail area and I don't think there are any this size available commericially. Besides you can build one that's better for sailing  than any of the commercially available rudders. None of the ones I've seen had a significant foiled cross section. This is not important when using a rudder for steering while paddling. However the physics of sailing require a foil shape. Foiled rudders produce lift while flat rudders produce drag. Also, you can easily build a "balanced" rudder. This will allow you to use footbraces for steering without constantly fighting rudder forces. A tiller is great too, and I would want have both options readily available, especially when sailing in remote areas.

I've recently completed a Triple w/Sailrig and thought that making a rudder system from scratch was going to be a pain. Turned out to be the most interesting part of the build. The boat is currently stored for the winter but I will try to take and post some pictures of the rudder. BTW, the Triple w/Sailrig is a rocket.

 Dusty

Just for the record :)

The statement "However the physics of sailing require a foil shape" is false.

The statement "[foil-shaped] rudders produce lift while flat rudders produce drag" is misleading. Every rudder produces both lift and drag.

Oy, nailed, what can I say!

The statement "However the physics of sailing require a foil shape" is false.

I should have said "practically require a foil shape for efficient sailing".

The statement "[foil-shaped] rudders produce lift while flat rudders produce drag" is misleading. Every rudder produces both lift and drag.

And several types of drag at that. Foil shapes are lifty, flat shapes are draggy.

Yes, of course, I s

The Professor has raised your grade back to an A, Dusty.  Heck, I knew you knew this stuff, and had probably sent the earlier note in haste because you'd heard the dinner bell.

Thanks for the feedback, Gentlemen.  Sounds like I should go with the SmartTrack for the initial build (which will be my first boat build).  That'll give me a year or so to ponder a retrofited sail-worthy rudder setup when I get around to the sailrig.

Ben 

I just need to weigh in on the rudder discussion and add what I hope is some clarification and additional guidance.  Basically, a rudder generates two force components:  lift and drag.  Drag is the component in the direction the boat is moving, tending to slow it down.  A rudder in the neutral position still produces some drag.  Lift is a force component at right angles to the direction of travel and is the steering force.  This is why you have the rudder in the first place.  Lift is 0 with the rudder in the neutral position and varies with rudder angle.  So, what you always want is a shape that gives a high "lift to drag ratio" or the most lift for the drag experienced.  Enter rudder shape:

Foil shaped rudders perform better than flat ones.  Flat ones can have a "dead zone" at very low rudder angles and they tend to "stall" due to flow breakdown at lower angles than foil ones.  This is all a matter of degrees however and if for your particular application you made the rudder out of 3/8" plywood say, with a taper on the leading and trailing edges, you may well not be able to detect the difference between that and a full foil shape.  In contrast, take a tugboat rudder that may be 3 - 4 feet from leading to trailing edge with a 8 - 10 inch diameter rudder stock.  It works much better if that stock is burried in a foil shape than if it is stuck in the middle of a flat plate with all the surface behind the stock in the disturbed flow comming off that stock.

Next comes aspect ratio:  The relationship between the vertical dimension of the rudder and the fore and aft dimension.  For any given rudder area, high aspect ration rudders, such as the Smart Track, work better than low aspect ratio rudders (such as what you might see on a Cat Boat.  This is why airplate wings are shaped as they are and are not short stubby things extending the full length of the fusaloge.  So, the best way to configure your rudder is to make it as deep as you are comfortable with in your application and then determine the fore and aft length by your area requirements.

Finally there's lead.  How much of the rudder blade goes ahead of the stock.  For power steered systems, this is generally 20% to 25% of the total area.  This balances the rudder forces back into the system pretty well.  For sailing however, this is probably to much as at low rudder angles the torque is negative which means if you let go of the tiller the rudder angle will increase which is not desirable.  So, you should strive for the most lead without getting into this negative or lee helm.

Just one additional point regarding shape details.  Assume using a chunk of plywood for a moment.  Most folks know that eliminating the abrupt corners at the teading and trailing edges is a good idea.  However this leads to many 45 degree bevels.  Water does not deal well with 45 degrees.  The upper limit for an angles that water can manage to flow around is about 20 degrees.  Above that and flow seperation sets in.  Again, not a huge deal with the application that started this whole discussion, but someting to keep in mind.

That's all I'll say on the issue on this Christmas Eve.  I have been wordy enough.  Merry Christmas to all.

Paul,  P.E.

Naval Architect & Marine Engineer

Good summary, Paul. Nice to have a professional handy on this forum to keep us straight on the technical stuff.  I think it's worth the words you spent, as some of us are making little design decisions as we build, fix, or modify.  It can't hurt to have some of these engineering principles in mind.

Ben,

I am looking for someone close to Texas in order to see a SailRig project - would you be local to me?

Marc

Marc,

I ended up totally changing direction, postponing the triple & sailrig, & building a Kaholo SUP as my intro. to boatbuilding.  Hopefully the triple will happen next winter.

I'm located in western NC, so I'm probably no help on that front either, he he.

Ben

This material is solid gold.

I'm a sailor getting ready to build my first kayak this winter, and hopefully a sailing rig thereafter, and looking at these systems, I've found things which (appropriate disclaimer: from a purely theoretical armchair builder perspective) appear incredibly neat from the perspective of getting a kayak to move better, but seemed a little lacking if viewed as a pure sailboat. I think things like having a properly ballanced rudder of sufficient size would be a huge step towards bridging that gap. In the same vein, I'm curious what can be done with the leaboard and perhaps a skeg to get tracking perfected, and get tacks as sharp as possible. I'm very eager to get my kits, and start experimenting to see how close to the ideal I can get. 

Thank you for your information, it's really encouraging to soon-to-be builders =) 

-- James

As a builder and sailer (racer, even) of small multihulls from way back, I have always been intrigued at the SailRig concept when added to an appropriate kayak.   As designed, the lee-board set-up has always seemed to me to be a weakness (both structurally and from a performance perspective), so I mulled over an improved concept.

It seemed to me that a conventional centreboard or daggerboard, operating in its own casing INSIDE THE COCKPIT, along the exact centreline of the kayak, and with the correct aerofoil shape to it (as on conventional sailing boats and multihulls) would be the way to proceed.   Certainly, this would require major reconstruction of the cockpit, thereby demanding in turn that the kayak would most likely only be useable in sailing format.   Weight would also be added (but not too much, with careful construction).  

However, this - with the proper rudder system discussed above - and a decent sail rig - should give a real flyer!

Food for thought maybe?

W

The way the sailrig is designed, a daggerboard would pretty much have to be mounted right between a kayaker's legs.  There just isn't that much room between the front of the coaming and where the crossbeams are mounted.  I have found I get very good performance from my leeboard.  This past weekend I took my kayak + sailrig on a camping trip and got great performance.  I estimate I was able to sail about 15 degrees off the wind with not a lot of leeway.  I have even noticed that with the leeboard up the floats work pretty good to keep the leeway down.  

 The most valuable thing I learned happened on the last morning getting back to my car.  There was no wind at all and I was able to keep up a steady 3 knot pace with the sails furled for about 4 miles.  It was surprisingly easy especially considering my rudder uses a hand operated tiller.  

BTW, the CLC plans/kit specify that the leeboard should be shaped as an airfoil.