After many years “thinking about it”, I am converting my 17ft Mad River Freedom Royalex canoe into a sailing canoe. I am hand fabricating virtually all the accessories from hardwood (rudder controls, decks, outrigger arms, etc) because the canoe itself has hardwood gunwales, seats and trim. The 45 ft2 sail is from SailboatstoGo and is a lateen type with equal spars of 10ft length. I am presently about to position the leeboard mount and mast step. I believe the “center of effort” for the sail will be somewhat forward of the center of the canoe in order to keep the lower spar (boom) from interfering with headspace. My question is,… Does the position of the leeboard HAVE to be in alignment with the sail or can it be a little aft? If so, what are the consequences or affect on sailing that I should expect? Thanks to any that can be of help.
In theory…
... you want the leeboard to be at the same position, fore and aft, as the effective centerpoint of the sail. However, keep in mind that the effective direction of force applied to the sail is at a right angle the sail (even though the wind may be glancing off the fabric at an acute angle, the direction of applied force is at a right angle), so the effective centerpoint of effort relative to the boat will shift forward as the sail swings to a wider and wider angle. However, at such times, the need for the leeboard becomes progressively less, so I bet you can ignore that effect. See where the sail's effective center is when it's drawn up tight (nearly parallel to the boat) and use that as your guide.
Anyway, based on this quick, seat-of-the-pants, not-on-paper analysis of forces, I'd say you want the leeboard and effective center of the sail at the same location along the length of the boat. There must be directions online about how to do this, and I'd do some checking there as well.
Oh, you might be okay having it a little aft, now that I think about this more. The faster the boat goes, the looser the stern will become, causing the stern to slip sideways more than the front, and causing the need to correct with the rudder. Having the leeboard a little aft of the effective center of the sail should counteract that effect. You'll never get it exactly right because the perfect location will still change with sail position and speed of travel.
Maybe
I’m not sure what effect moving it aft would have. I read the instructions here though http://www.instructables.com/id/Sailing-Rig-for-a-Fiberglass-Canoe/ And the author used straps so that he could adjust the location as needed.
That…
...is a very good idea - nothing like a field test to get the position right. Or leave it permanently adjustable, for that matter.
Couple of ideas
I built this for my MR Freedom. http://www.duckworksbbs.com/plans/storer/sail/index.htm Most of the time the leeboard(s) need to be further back. This book is excellent and has all the Center of effort/resistance stuff in it. https://www.amazon.com/Canoe-Rig-Essence-Sailpower-Traditional/dp/0937822574
Personally I think the Freedom is kind of a pain for flatwater paddling but it is a good hull for sailing due to the rocker.
leeboard
Actually you want the leeboard center to be slightly aft of the center of effort of the sail. If you make a mistake and get it in front of the CofE you can have dangerous handling, better to err with it a bit aft.
Bill H.
DIY plans show leeboard aft of mast
The DIY plans from Sailboats to Go show the leeboards mounted approximately 1 foot aft of the mast. We have the commercially produced kit from that company mounted on our IK and have the leeboards mounted 18"-24" behind mast.
Leeboard Position
As you pivot the leeboard back the center of it already moves to the stern. My experience is that having it at the center when verticle really helps windward performance.
Also having it in the ideal position allows you to steer the boat by adjusting the leeboard and thereby keeps pressure off the rudder.
I would set it up so that the center of the leeboard and the center of the sail are at the center of lateral resistance of the hull. This is usually a tiny bit aft of the true center in a normally stern heavy canoe.
On my best sailing canoe the Lee board was dead center and typically raked back a bit but for upwind I sometimes raked it forward a bit. The sail was centered just a few inches aft of the center.
I often broke rudders sailing fast in shallow water and later just used the leeboard and a bit of paddle to steer.
Thanks a-bunch! Bamboo?
Thanks to all your responses! I feel pretty good about the information y’all gave me and with my planned leeboard position. I will be eventually fabricating my mast and spars from locally grown and personally harvested bamboo. We have stuff down here in SC that I can access which varies from 3/4" diameter up to 6" diameter. That stuff is really cool to work with! There is a serious art to drying and preserving it and not allowing it to split,… something I will be learning as-I-go!. I will be coating it all with a light layer of fiberglass for strength and preservation. Great winter project. Now I am thinking about using the 6" stuff to make my flotation outriggers with! HMMM!
grumman rig
75 sq ft sail rig on 15’ Grumman
the mast step is something like halfway from the front bulkhead to the front seat
we have always put the leeboards approx at the stern end of the front seat, which is maybe 1.5 to 2’ behind the mast
that’s off the top of my head, don’t have access to the boat or sail rig right now.