Leeboard direction

[canoe9girl]

In his book, "CANOE RIG", Todd Bradshaw states that the rounded edge of leeboards should be forward, and the tapered edge to the rear.

Some have seen mine mounted that way and said they were backwards, and that the tapered edge should be forward.

The question is:

Which way is correct? Does anyone have a picture of an older sailing canoe that proves this, or is it mainly users opinion?

C9G

 

[Dave Osborn]

I'm not much of a sailor, but I believe Todd is correct. Think of how an airplane wing is designed......the tapered edge trails the rounded edge for (in this case) hydrodynamic efficiency, stability, and speed. Makes sense to me anyway.
Dave

 

[Todd Bradshaw]

Unlike knives (which cut with the sharp edge forward) airfoils and/or "waterfoils" (which is what leeboards are) cut with the blunt edge going first and the sharp edge trailing. To some extent, they create lift to weather this way (this helps keep your airplane flying or your sailboat from just blowing downwind). More importantly though from a sailing perspective, foils with sharpened leading edges can make steering and boat control a nightmare. Every time the knife edge gets slightly out of line with the flow of water over it, it wants to veer-off in that direction. Your leeboards will be working properly part of the time (when aligned with the flow) and trying to tear themselves off the hull or change the boat's direction the rest of the time. On a rudder with a leading edge that's too sharp, steering is often really touchy and very annoying.

Thin, metal plate centerboards and rudder blades can tend to do the same thing. Luckily, on a canoe you can usually muscle the boat into position and correct for it without too mich trouble. If you do want to build airfoils though, for maximum performance and efficiency, build them with the blunt edge leading.

 

[Pernicious Atavist]

Todd, he has his blades with the convex curve facing forward. It seems [to me] they would be made so the leading [blunter] edge is straighter than the trailing [tapered, sharp] edge....that being said, they could have made made so the leading edge is convex, which makes little sense. Canoe9---my first impression from the photos you sent me is that your blades were made ass-backwards. That curved front edge will play hell with you, blunt or otherwise....

 

[Treewater]

Bernoulli's Theorem

Blunt/round edge forward, sharp edge to the rear. The forward edge divides the fluid flow, the rear is where it rejoins. Fluid flow will divide smoothly with only a modest roundness to the leading edge, but even a knife trailing edge will have some turbulence behind it which in turn is drag. Smooth fluid flow is the desired effect. I have identical canoes one with a square back and the other the normal tapered. The square back "pulls water" and is harder to paddle because the fluid flow is rough. Motorized craft they mostly don't care, 1/10th HP means little to them. Todd is correct, with a knife leading edge there will be a stalling effect at even a "low angle of attack." This will cause instability. All of this is according to Bernoulli's Theorem which says that a fluid moving across a surface exerts less pressure than a fluid at rest. Engineering school is 40 years behind me and someone else can prove this point more eloquently than I.
I see Old Town solved this by giving both edges the same shape. I have only one other set of leeboards and they did taper one edge, obviously the rear. If there is talk about convex or concave on the long dimension of the leading edge I doubt there is much difference so long as there is a taper toward the end to eliminate a small vortex (drag) there.
Tim

 

[Pernicious Atavist]

Canoe9...I went ahead and posted a pic of your blade....

Okay--note the bow is to the right in this image. Now, doesn't it look like the blade is backwards? If the blunt edge is that convex curve, then the blade is sure to have issues with turning, pointing and vibrating, or at least it seems that way to me.

 

[Treewater]

which edge

Which edge is tapered and which is round, Photo left or photo right?
Tim

 

[Pernicious Atavist]

The RIGHT side in this photo, if i understand Canoe9 right, is the blunt side...I'm SURE it was made wrong, all bass-akwards. I think he bought it.

 

[Todd Bradshaw]

I would agree that compared to what is "normal" (if there is actually anything about canoe sailing that's normal) these boards were "made backwards" or at least their edges were finished backwards. However, as long as they are installed so that the water flowing over those edges is moving the right direction (blunt edge leading, sharp edge trailing) they should work just fine. That's a far more important criteria than their profile shape. There are wide variations in the profile shapes of the centerboards, daggerboards, leeboards and rudder blades on small sailboats out there and a heck of a lot of them work just fine. Some have raked leading edges (Sunfish rudder for example) some have plumb leading edges (Hobie Cat rudder, for example) and there are a lot of other shapes. The mose efficient are probably long vertical slabs with both edges plumb, but they're fairly uncommon.

Keep in mind that the water is flowing over the blade from front to back, not across its profile from side to side. Orient the blade for the most efficient front to back flow - in this case, that means curved blunt side forward, straight, tapered side aft.

 

[Pernicious Atavist]

Well, he told me he was gong to try to sail it this weekend, so we'll see how he does!

 

[canoe9girl]

lee boards

Shhhhh...don't tell anyone, but tomorrow morning I am going for sailing trials, and the weather report calls for 10 mph breezes, and 37 degrees.

IF all goes well, I will send pics to Canoe Sailing Magazine. IF not, then I can blame the lee boards being made wrong (not my sailing ability!). Nothing like a built in excuse!

I built everything in my shop from pictures, except the lee boards which were from EBay.

If there are no further C9G posts, you will know I went for a swim and hypothermia set in...brrrrr.

C9G

 

[Pernicious Atavist]

okay...mum's the word....

It's friggin cold here {on the coast, just by Morehead City}, at least for this recent Florida transplant. You chose tomorrow to go? That's right, I already accused you of being a knucklehead! Go, Mike, go!

 

[canoe9girl]

test sailing today

Welp...it was 38 degrees here in the North Carolina mountains at 9 am this morning, and quite a breeze on the local lake.

I did have a small problem with the lee board bracket sliding some on the plastic gunwales. There are also two pulleys (blocks) mounted to this board, so I had to be a little tender with it.

The rudder worked fine, the sail filled good, and the pontoons worked fine, and I could "hike" up over the gunwales with security.

There was a crew working on a new dock, and they stopped working and watched. If the lee board bracket had been more secure, I would have sailed more, but as it was, the 'shake down' tests worked OK, and with some minor adaptations will be fun to sail.

The rig got a lot of looks as I was on the way to the lake, as I had the lee boards mounted, and the sail furled and fastened down. It is surprising how many people say they have never seen a sailing canoe!

C9G