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Homemade Trimarans

 

Presented by "Dr. Frank" Smoot, and dedicated

to DIY boat builders everywhere!

 

How To Make GOOD Leeboards That Will Really

Work Well For You And Be A Pleasure To Use

 

By Frank B. Smoot (AKA "The DIY-Tri Guy") 

 

To paraphrase Mark Twain, I'm writing a long article here because I don't have time to write a short one. But why am I even writing this article? Good question.

 

Aside from a few "adventurous" designers like Phil Bolger, nobody seems to like leeboards very much. And I do understand why. Leeboards, especially if poorly done, can look like weird appendages or odd little "wings" tacked onto the side of an otherwise nice looking boat.

 

But properly done, leeboards look great. Even better, they work great -- especially in our skinny local waters here in Florida's "Sunshine Coast" (the south central Gulf coast).

 

[Please see the photos of two of my leeboards here]

 

Now, I won't bore you here with the reasons why I think leeboards are so great, because I already did that in my previous article :) So let's get to the point of this article: How to build a leeboard that works great and looks great.

 

We need to start with an understanding of what a leeboard actually does. And yes, I said "a" leeboard, because on a trimaran you only need one. Trimarans heel so little that the amount of the leeboard that's in the water varies only about 7 percent from one tack to the other.

 

Of course, I should note that my first "real" trimaran, No Commotion, did have twin leeboards, and still does. But that was because the highly curved and flared hull sides made it challenging to install a single leeboard. Twin leeboards also add weight -- not just the added board, but the mechanism that connects them so they are sturdy and work in tandem

 

The good news is that twin leeboards are even more effective than single ones, and have the added advantage of allowing you to get the leeboard away from the hull. This cuts down significantly on noise, turbulence, and splashing -- three things that are potentially annoying about leeboards.

 

But despite what I just said about the advantages of twin leeboards, single leeboards are all I now use. Why? Because having built and sailed four boats with single leeboards, I know that's all they need to be great performers!

 

How Leeboards Function

 

So what does a leeboard actually do? Why do we need to have one in the first place? The primary purpose of the leeboard is to help the boat go upwind, by keeping it from sliding sideways excessively when the quartering wind is trying to make it to so by pushing the sail downwind.

 

The leeboard adds a big chunk of lateral resistance (ideally, right where it's most needed) while adding only a minimum amount of resistance to forward movement. More importantly, you can actually move this lateral resistance fore and aft a couple of feet, which allows you to fine-tune your helm balance in a way that a leeboard's two main rivals -- centerboards and daggerboards -- can only dream about.

 

And a single leeboard mounted directly to the hull is going to weigh a lot less than any centerboard or daggerboard plus their attendant waterproof trunks and necessary structural bracing. And of course, there's no need to "hole your hull" when you build you boat with a leeboard.

 

Then there's the simplicity factor. It's a complex and fussy procedure to build a trunk for your centerboard or daggerboard, hope that it's watertight, hope that it stays that way, hope the leeboard or daggerboard actually fits the trunk so that it's not too lose or too tight, and hope the whole mess doesn't get clogged or jammed with shells, gravel, sand, or any other seaside junk.

 

Leeboards, on the other hand, are entirely outside the boat. The only hull penetration is a 3/8" hole about 6" above the waterline. Now, you do need to reinforce the interior of the hull for about 8" in all direction around that hole. But all you'll need to do that is a bit of plywood (details later).

 

Leeboards involve a few critical design considerations, so let's talk about them now. First, your leeboard needs to be strong enough not to break under even the most sever lateral strain it will ever experience. Fortunately, this is easy to accomplish. I have found that the best material to build your leeboard from is a "select" grade of pine, available at most all box stores.

 

As for the foil shape (cross section), I like an extended "teardrop" shape -- one where the forward half of the board is a bit thicker than the aft half. The maximum thickness is perhaps 1/3 of the way back. The leading edge is more rounded than the trailing edge, but both are pretty sharp. Is that the ideal shape" Who knows? But it works fine for me, so I'm sticking with it.

 

I use a 1" x 12" board, and if I want it a bit wider on the bottom end, I edge-glue on a 1" x 3" board to extend the trailing edge. The leeboard on my current favorite boat (the16' okoume-hulled "folder") is 14" wide at its maximum.

 

I do the rough shaping with a hand-held electric plane (which is BTW the most dangerous tool I know of. 30 years in the construction biz and my only injury -- ever -- was from one of these buggers). Then I do the next stage of shaping with a belt sander with 40-grit paper. Final shaping is dome with sanding blocks and 60 then 100-grit paper.

 

I also cover all my leeboards with 4 oz. fiberglass and 3 coats of epoxy, which not only stiffens and strengthens them, but also makes them much tougher-skinned while allowing me to get a very sharp and durable leading and trailing edge. And because of this covering, I also don't have to have a super-perfect finish on the wood.

 

How Big Does Your Leeboard Need To Be?

 

That can vary quite a bit, depending on your situation. Here are some of the important design variables:

 

- Number and type of hulls your boat has: It's really important to understand that multihulls and monohulls have different leeboard requirements. That's not only because of the difference in heeling, but also in inherent lateral resistance.

 

A word about lateral resistance: We need to understand that the lateral resistance of a boat comes from many places other than just the "board," and this is especially true in multihulls. Depending on the shape of your main hull(s) and amas (if any), your leeboard will play a more or less significant role in helping you go upwind.

 

A perfect example is my wife, Laura's, boat. The original set of amas I built for her boat were "tortured plywood" and had a very sharp V bottom. I didn't understand at the time just how much these sharp V amas were contributing to her boat's excellent upwind capabilities. But now I have a better idea.

 

Laura's boat has what I call a 5-piece hull, with vertical sides, 45-degree bilge panels, and a flat bottom with just 2" of overall rocker. A hull like that, while it will turn on a dime, also is in desperate need of something to make it go straight. And to have any hope of going upwind well, it needs a serious dose of lateral resistance added into the mix.

 

When I built her boat about 2 ˝ years ago, I had originally assumed that the leeboard would provide all that lateral resistance, even though it was only 11 ˝ " wide. And it did go upwind beautifully -- until I swapped out her original deep V amas for some "better" one with rounded bottoms.

 

On her very first trip out with those new amas (which happened to be at Cedar Key, 2011) she started complaining her boat wouldn't point upwind like it used to. Since the only change had been the amas, I reinstalled the old ones. Problem solved! We found that her boat not only points better, but goes faster and rides smoother with the deep V amas. But that's another whole article…

 

The point here is that your own boat is going to have some built-in lateral resistance that exists before you ever factor in what you'll get from a leeboard. So your leeboard will end up playing somewhere between a medium role and a major role in determining your boat's upwind performance.

 

The good news is that your leeboard gives you a lot of flexibility in fine-tuning where your boat's CLR (center of lateral resistance) is -- much more so than any centerboard or daggerboard ever could. But even so, you will need to have it located in approximately the right place to balance out your sail plan, or your boat will want to turn upwind or downwind on it own, and require a heavy rudder to keep it going straight.

 

- Total square footage (or square meterage?) of your sails: This is a pretty important factor. If your leeboard doesn't have enough surface area (square footage), your boat won't be able to effectively resist slipping sideways as it tries to go upwind.

 

For reasons already noted, a monohull will nee a proportionately bigger leeboard -- the general guideline being at least 3 percent of the upwind sail area. But my trimarans seem to do just great with 2 percent. So if you're planning on 100 sq ft of sail (about 9 sq meters), then a leeboard that has in the neighborhood of 2 sq ft below the hull will do the job just fine.

For the sails I use on my boats (typically a single sail of 85-105 sq ft), I find that having as little as 1 1/2 sq ft of leeboard in the water gives excellent upwind performance.

 

 - Overall leeboard length: You want the top end of your leeboard to be just even with your sheer, and the bottom end to be sticking as far below the hull as is necessary to give you enough lateral resistance (on both tacks) to do the job.

 

- Length below the bottom of the hull: How far does your leeboard need to stick down below that hull? That, of course, determines how long it will be. Conventional wisdom says that "long and narrow" foils (boards and rudders) are best upwind. Probably true, but maybe not all that big of a deal.

 

Conventional wisdom also says that long and narrow -- i.e. "high aspect" sails are best upwind. Also probably true. But in reality, that advantage disappears quickly as your course moves off the wind. And high aspect sails are easily outperformed by lower-aspect ones downwind.

 

So if the leeboard that meets your needs turns out to be completely square in its underwater section, no worries! My best rudder is also virtually square in its underwater section, and all of my sails are low- to medium aspect. And you know what? They work great!

 

- Leeboard pivot point: This is also important. I'll cover below the importance of the fore-and-aft location of your leeboard's location, but it's also important where your leeboard pivots in the vertical dimension.

 

Your leeboard with experience its greatest stress when you are close-hauled and trying to go upwind in heavy air. Ideally, your leeboard will be vertical or very close to it in this situation. As such, you want support both above and below your pivot point.

 

But you also want to be able to get your leeboard entirely out of the water when going downwind. (Not that it will always be so, but in light air this can make a difference) So the pivot point needs to be high enough on your hull to get the leeboard completely out of the water when it's in the horizontal position, but low enough to give it great support when close hauled.

 

This is something you'll have to determine for yourself, based on the boat you have or build, and the leeboard you make for it. This is why your leeboard should not be "tacked on as an afterthought," but designed in from the start as the rest of the boat is created.

 

 

A Huge Factor: Aligning Your Leeboard

Parallel With The Centerline Of The Boat

 

I can't overstress the importance of making sure your leeboard's fore-and-aft alignment is perfectly parallel to your boat's keel line. If it's not, you have just added a new "rudder" amidships, which is going to make its handling weird, unpredictable, and sub-optimal.

 

The simplest solution is to locate your leeboard at exactly the widest point of your hull, which will guarantee parallel alignment. This is great if that widest point happens to be in the right location to balance out your sail plan. (Yet another article…) But if it's not, you'll have to shim out the exterior of your hull just a bit to make sure this alignment end up being correct. Fortunately, that's easy to do.

 

You also may want your leeboard to be as vertical as possible, though that's not as important as you might think. I have gotten great results from leeboards that were as much as 15 degrees off vertical, so don't worry too much about this. It's the fore-and-aft alignment that's much more critical.

 

Major Key To Success: The All-Important "Friction Washer"

 

How do you install your leeboard so that it pivots smoothly, yet stays where you put it? The secret is in the "friction washer." Now, I could see early on that just bolting your leeboard directly to the side of your hull wasn't going to work very well. I knew there would be wear and tear and well as friction I couldn't predict or control very well.

 

The solution came in the form of a cheap but very tough translucent plastic cutting board, available for $2 at my local flea market, and probably at your local dollar store or equivalent. Once I know exactly where the leeboard's going to mount, I cut a "washer" from this material that's rectangular with rounded edges, just a bit smaller than the area covered by the leeboard when it's vertical, and not extending down as far as the waterline.

 

Then I drill a 3/8" hole in it (because I use that size stainless steel bolts to mount the board). Then I drill two holes about 3/16" near the upper corners of the washer. I countersink these holes and run two 3/16" SS bolts through the hull and plywood interior backing board. A couple of 3/16" flathead bolts keeps the friction washer from pivoting along with the leeboard, and the washer itself provides the perfect amount of adjustable friction to keep the leeboard right where you want it.

 

Secret Tip #1:

 

To help distribute the bolt pressure over a wider area (so everything works better and longer without adjustment), make some 1/2" or 3/4" thick plywood "washers" either 2 ˝" or 3" dia., or however big your biggest hole saw is. Then get some stainless steel fender washers, as big and as thick as you can find them.

 

All together, the wood and metal washers will make for a much superior finished product vs. not using them. They will also allow you to use a longer pivot bolt, which is inherently good because it makes for a more torque-resistant setup. Use a locknut with a captive nylon piece in it, and then add another locknut to keep the whole thing from loosening up.

 

Secret Tip #2:

 

Once you have everything drilled and installed exactly where you want it, take it all apart. Then drill out the hole in your plastic friction washer to at least 2 ˝". Why" because now, instead of having all your pivot bolt tension near the dead center of the hole, the friction point moves out to the edge of the bigger hole you just drilled.

 

How does that help? The larger friction surface now permits you to have a lot of friction with less locknut tension, and also stays tight much longer without adjustment because the pressure is no longer concentrated in such a small area (which compresses / wears more quickly).

 

OK, I know I haven't answered all of the questions you might have. So please do feel free to contact me here with any additional questions you may have, and I'll post my responses here in the article as an ongoing supplement.

 

Happy sailing!

 

 

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