Plywood vs laminated strips for rudders or center boards?

[Tom Lathrop]

No good reason I can think of to use T&G or V grooved laminates.  More difficult and wastes a lot of wood, especially on the narrow laminates that we use on centerboards and rudders.  It has been proved over and over that in most cases a plain flat glue joint is just as strong as a T&G or biscuits or dowels.  There are cases where the mechanical properties of a dowel or biscuit makes a stronger joint but if you can get  well machined surfaces mated together with a good glue, it's very difficult to improve on that. 

I used a ton of biscuits in joining trim work in my house and for helping with alignment of awkward pieces.  Biscuits and dowels are very good for joints involving end grain.  I'd never do that with laminates on a CB or rudder.

[Charlie Jones]

I'm very much with Tom here. You are complicating something that isn't that complicated. A simple well machined flat joint will hold extremely well. The well machined surface can easily come from a table saw with a sharp blade for this purpose.

I might also point out that biscuits WILL NOT swell and lock a joint when using epoxy. The epoxy won't work the biscuit like white or yellow glue does- no moisture. Trust me- I tried it- with a 12 foot long, 4 foot wide, 2 inch thick walnut table top.

Just machine the surfaces, put them on a flat surface, clamp them up, hammer it flat and then plane it down. Make the strips a tad over thickness to allow some wiggle room.

This is a glue up for the board for  CS 20

[Apteryx]

I gave in to curiosity and called Graham, both to call and check on my forthcoming kit and to ask about the centerboard lamination.

Quoting Graham is obviously heresay on my part, and I may have misunderstood.  However, he cited some tests that somebody had done and said that he believed a tongue and groove joint in this grain orientation would not be as strong as well-fit flat surfaces glued with epoxy.

He also did not have confidence biscuits improved the strength.

On my centerboard, I'm not going to use full length strips for each lamination, for the simple reason I don't have enough clear grained fir, and being on call for work over the weekend, I can't make an out-of-town trip to fetch more.  Yellow pine would work for the centerboard, but I'd rather not mix wood.

So, to make up the laminations from shorter pieces I'm going to do scarf joints.  First I'll build a scarfing sled for my table saw - Graham's article is here - http://www.messing-about.com/forums/index.php?topic=6123.0 .

Tomorrow should be fun!

Incidentally, the delay in getting my kit is at my request.  I asked Graham to schedule me in when convenient, since between garage cleaning and centerboard work, I don't need the kit any quicker.  Particularly since I already have the plans - no need to put a rush on when it's just not needed.

[Apteryx]

Almost forgot - I've been daydreaming about how to get symmetric curves shaped on both sides of my centerboard, and I've hit on a method I hope might work.  Or, I might just be illustrating how little I know about boat building, but either way perhaps this will provide some amusement.    ;D

Once I've got a centerboard blank laminated up and planed true, I'll trace the profile I want on the end of the centerboard laminate.

Then I'll measure how much I need to remove at various stations along the curve.  Using test pieces of wood and depth indicating calipers I'll adjust the depth of a circular saw to match.  Then, using a batten nailed in place as a guide for the saw I'll make cuts at each station on the curve.  Each cut will be duplicated on each side of the centerboard.

Then I'll plane down to the bottom of the cuts, working to create a smooth curve between the stations.

Does that sound like a workable method?

[Norman Colter]

If I understand what you're proposing, to cut "contour lines" on both sides of the blade. Cool idea. I think a router would be a more practical tool. Much easier to steer against a batten. My circular saw would not have liked trying to make the profile of my rudder. FWIW

Norm

[Greg Luckett]

Yes, it is an interesting idea.  I have a Dremmil tool with the router attachement that would work for the detailing accuracy, much better than a circ saw.  It is a little light in the bearings for hardwood but should work fine for fir or pine.

[Tom Lathrop]

On my centerboard, I'm not going to use full length strips for each lamination, for the simple reason I don't have enough clear grained fir, and being on call for work over the weekend, I can't make an out-of-town trip to fetch more.  Yellow pine would work for the centerboard, but I'd rather not mix wood.

There is no logical reason not to use different species of wood on a CB or Rudder.  Just use the common precautions for straight laminates.  The CB below for Lapwing has pine, fir and cypress.  If I'm going to finish a foil bright, there is always some mahogany and/or cherry in the mix.  This CB got the rope leading edge treatment.  Otherwise it would have an oak or ash leading edge.

[Hirilonde]

Once I've got a centerboard blank laminated up and planed true, I'll trace the profile I want on the end of the centerboard laminate.

Then I'll measure how much I need to remove at various stations along the curve.  Using test pieces of wood and depth indicating calipers I'll adjust the depth of a circular saw to match.  Then, using a batten nailed in place as a guide for the saw I'll make cuts at each station on the curve.  Each cut will be duplicated on each side of the centerboard.

Then I'll plane down to the bottom of the cuts, working to create a smooth curve between the stations.

Does that sound like a workable method?

I use variatons on this with a circular saw and a router for 3-D stuff at work

[tom151]

Then I'll plane down to the bottom of the cuts, working to create a smooth curve between the stations.

Haven't tried it but a tip I recently saw (maybe even on this forum ) that sounded useful...

when you're planing/grinding, but before you get too close to the bottom of your sawcuts, spraypaint the piece so that you get color down into the bottom of the cuts. Then as you plane/grind further it becomes easier to "see" when you are very close to the bottom and can see if you are touching  the painted depth areas.

If you try it tell us how it works (if it does)

YMMV

[Peter HK]

[quoteThen I'll plane down to the bottom of the cuts, working to create a smooth curve between the stations.

Does that sound like a workable method?]

I've used this method for the last few foils I've made. Works well. I usually use a router. You don't need as many cuts as you think.

Peter HK

[Howard]

At the risk of dragging this thread off topic, I'll bring this up for consideration. The following is a sketch I drew up a few years ago:

Basic comparison here is to a new, modern CB or dagger board design and old time CB that you would have seen on boats 100 years ago, and still might on some designs. My existing boat is a pie shaped CB like the old style, dropping from a CB trunk built into a stub keel.

The reason for bringing it up here is to look at the problems associated with this style of high aspect board, vs the older low aspect style. Plywood or laminated strips, if the board is down and you start drifting sideways, there is the potential for a lot of leverage to be exerted on the board, potentially snapping it right where it enters the trunk. With the old style, not as much leverage, plus the entire length of the board is working on the side of the trunk. Can't say that it wouldn't break, but it doesn't look like it would be easy.

Operating in shallow water and in constant contact with the bottom, the old style boards would seem to have a lot going for them. But in open water, the newer, high aspect boards generate more lift and no doubt allow you to point higher. The old style must have worked (along with barn door rudders), but the new style has won out and that can't all be attributed to fad. On balance, they must be better or the entire industry wouldn't have gone to them. But I think the potential of snapping them off if you start drifting sideways in shallow water is one of their drawbacks. If you find yourself in that predicament, remember the board and get it up. And even before then, if you angle it back 60 degrees by hiking it up, it will look and perform like the low aspect board.

[Greg Luckett]

Howard,

It is only slightly offtopic, but right on the money otherwise.  Good points to consider and learn about.  Thanks.

[Maria CS20 104]

Just a coment regarding biscuits and epoxy. Charlie and Graham are absolutey correct when they note that the can only swell in a water based glue.  When we used them on the centerboard blank we used very thickened epoxy and squeezed the living heck out of it when we pulled and hammered it together to drive the excess out and I suspect there were a few air pockets, not many I am sure based on what we did.  I think it helps and I think that when the pockets cured they provided an epoxy wedge that along with the biscuit helps stability and increased conectiveness to the edges.  Only my perception, but I got a biscuit cutter out of it anyway.

Jim A

[Greg Luckett]

About 6 or 7 years ago I built a cradle from red oak for one of my grand children.  It has since been used for 2 more, and another is due soon, and he will also use it.  I built the end panels by gluing boards together on edge without biscuits or dowels, using Gorilla glue.  It is still in great shape.  Based on that experience I will not be using dowels or biscuits on the dagger board, but darn it, I want a bisquit cutter too. ;D

[Greg Luckett]

Here is a picture of the dagger board that I built, sans the holding cleats and epoxy finish.

[John Burritt]

Greg

Looks like you did a great job on the board.  Much prettier than plywood, and a bunch stronger too.

John

[Greg Luckett]

Thank you, John.

This is a virgin effort for me, as have never made one like this.  It has some flaws, made while shaping, but it will do the job.

Now I am pondering the best way to finish the wood.

[John Burritt]

Greg

I'd do several coats of clear epoxy, making sure that I tipped it out well each coat so there aren't any runs.  Probably don't need varnish.  It'd just get scratched up anyway.  Only the top grip on the board will have any significant sun exposure (this could be varnished), so you shouldn't have to worry too much about UV degredation.  Main thing is to keep water out of the wood, as always.

John

[Greg Luckett]

Here is a shot with the handles on and one coating of epoxy.  I plan to shape the handles some more and do another coat of epoxy this afternoon.

[John Burritt]

Looking betterer and betterer - or whatever.  Very nice.