View Full Version : A plywood centreboard and rudder for PDRs

26th June 2008, 05:22 AM
OK .. let me say from the start that this is not my idea (I'll let you guess who wants to try this!). However I want some feedback on it nevertheless.

For some PDRs we are planning to build, we are starting with the centreboards and rudders (easy to store away if the boats are being made over time whereas a boat takes up the garage till its finished). Now the suggestion was to make and shape these out of plywood, most likely from gluing two panels together first. Obviously, with the glass and epoxy coatings, we're assuming a 22mm plywood centreboard ought to do the same job as a normal wood one.

The pros we could think of are:
1. Reduced stress and work load compared to gluing a dozen pieces together which can warp and twist and result in pulling your hair out.
2. Interesting look along the front and rear from different levels of ply being exposed
3. Reduced cost (the heart of PDR ideology)

The cons are:
1. I haven't heard of anyone doing this
2. Shaping the different layers of ply can be a nightmare (due to being laid in different directions)
3. Some joker might quote some standard PDR setup/rule as to the construction and performance of the PDR (but we are honestly not too concerned for this)

I'm sure there is more to say here, other than please critique this idea to death. Oh .. and please no photos of fantastic looking rudders and centreboards made from lovely, contrasting colours of wood...


26th June 2008, 10:32 AM
The daggerboard for Redback is just a Pinus Radiata plank that was shaped and varnished. It hasn't given me any grief at all. Mind you, it's not 22mm thick either (can't remember the actual dimension).

Many boats have ply centreboards.

I don't think you'll haven any problems with laminating ply - been there, done that, not a problem. Shaping it will actually be easier than timber as you have the layers of the plywood to give you a visual key to how even it all is - you'll see a series of parallel lines that are straight if the shaping is even and wavy if not. However, have you priced plywood :oo:

I think you'll find that one of the reasons for the strip plank dagger board on the PDR was cost - lots of cheap, pine strips glued together as opposed to starting with $100 worth of plywood . You can essensially build it out of scrap.


26th June 2008, 11:29 AM
Actually, I have been thinking about this lately, 5 minutes ago...
The cost factor would be the key.

My idea was slightly different, not sure of the consequences of it either, being lighter and strength wise...

I was thinking a hollow core , basically 2 pieces of ply (4mm) bent around a former in the middle and then with a nice rounded cap on the front,top,and bottom to get the same profile as the solid ones... See attachment napkin sketch...

Maybe more work to do, but a lot less planeing, and more accurate at getting the curve..

Its just the weight, or lack thereof that may be an issue in the centreboard?

26th June 2008, 12:44 PM
I dislike wooden appendages and much prefer inert materials, which don't react with moisture, change shape or deteriorate over time.

The PDR is a low budget boat, so wooden appendages are the usual course.

A strip planked centerboard will be stronger and lighter then a plywood board of the same dimensions. This is because plywood has 90 degree alternating veneers in it's construction and several glue lines. The cross grain veneers add little to the strength that a centerboard needs, but does add to it's weight as does the glue.

If a strip planked centerboard or rudder is built, with opposing grains in every other strip, it will not warp, unless moisture has it way (it always seems too). A "balanced" laminate of plywood (equal number of veneers on each side of the center line) will resist warping too, but not as well as a well built stripped version, again because of the cross grain not doing it's job in the big picture of appendage strain.

The hollow appendage Nick has suggested is a reasonably common way to make a board too, though I'm not sure if any PDR's have one. I wouldn't leave it hollow, but would fill it with epoxy encapsulated foam and a weight to permit it to sink. Maybe leave it hollow, but have a few small holes in it so it can fill with water and act as a ballast when it fills up. You can still weight 65 pounds, but have maybe a 30 pound ballast bulb hanging below the boat. Naturally, don't let them weight the boat after a race until all the water has drained out.

Of course all these wooden boards need to be sheathed or at least have their leading edges protected (trailing edge on daggers) against strikes. You'll use an inert (hint, hint), probably hard material for this like metal or plastic. My general feeling is why stop there, just make the whole thing plastic and kiss rot good bye in this element of the boat.

26th June 2008, 02:45 PM
My 2 cents worth.

If you enjoy woodworking go the extra mile and use strip plank. It's not that hard and the result will provide a great deal of satisfaction. Miks detailed intructions along with the pattern will give you a high performance good looking foil.

Besides they look really cool if you use alternating contrasting wood. I'll show you mine if you like:D


26th June 2008, 08:30 PM
Terrible how the paperwork builds up when I am busy for a couple of days.

The point of the OZ was not just the lowest possible cost but the highest performance for close to the lowest cost.

Almost every performance boat in OZ will use a timber strip blank to make the centreboard and then glass (or carbon) it. This is still the standard way for fast high load boats like 505s, Fireballs, Australian Sharpies, 470s, 420s, FDs and transom hung rudders on sportboats.

Problem is that it is easy to damage the glass and if you use a non structural core the foil will fail in catastrophic way.

Foil blanks made of plywood are very much second class citizens - ply if it has thin outer veneers are similarly vulnerable to folding over if the outside veneer is creased by a hard edge of the centrecase or rudder box. They are usually considerably heavier. We used really cheap fingerjointed planks for our OZ PDRs and had no problem at all. IT is not quite so relevant for little boats like this, but there is a review by WEST of some of the boats that they built in the 70's and early '80 (Golden Dazy was one of them) where they found the only systemic failure had been plywood rudder blades - those guys are good engineers and workers so if their ply blades fail ...

But that is bigger boats - I'd say if you are saving HEAPS of money then consider it for little ones, but I really am sure that there will be no problems at all from following the plans with the timber strip ones.

I have built hollow box keels for bigger boats out of ply - it is just the BEST method in the world because the skins are thick and the overall thickness of the foil so large - so the "beam effect" gives the strength and stiffness and the material is not so important.
But for little boats it gets very fiddly and it is easy to glue up with a twist or some other sort of defect. Also we go back to the problem of the edge of the case crimping the ply so it is "out of column" to find that the whole thing breaks away after a few tacks. Some in the PDR world are playing with this, but not this little black duck.

You see .... as a boat builder and boat designer I stick closely to what gives the performance that I want and is completely reliable. You can't win races or even cruise with some sense of security - so despite the fact that my structures are quite light in one sense, they are actually quite conservative because I know what racing boats have gotten away with.

So if you want to try it - good - and if it works for you - great - but I won't recommend it because I need for things to be as close to 100% reliable as possible even if people stray a bit from the plan.

I won't stop you if you want to go with ply - but if they break it is your fault and if they don't break it is my brilliant engineering - this is what I said to midge when we used the really bad pine for the first PDR masts - as it turned out both the material and the engineering were a bit wrong!!!

Good question!!


26th June 2008, 10:18 PM
Thanks Mik...

I Hope we haven't offended you, or given the impression that we want to undermine or question you skills and experience... Its just human nature to want better things..

Actually, now that I have completed the Eureka(just varnish left to do!), and have had similar thoughts on "improving" it during the build, I realise that its IS best if we just follow the plan as per instructions... The designer knows best, sorry. I'll be building my PDR as per instructions...

Thanks again Mik for your patience..

Oh, one more question if I may, just out of interest, what's the advantage of the centre board over a side mounted one?


Cliff Rogers
26th June 2008, 10:50 PM
...Good question!!


Thanks Mik...

I Hope we haven't offended you,...
I doubt that he was offended, he thought it was a good question. :)

26th June 2008, 11:28 PM
Jim Michalak in his book "Boatbuilding for beginners (and beyond)" wrote: "The leeboard is made just like the rudder - always best to laminate it from thinner plywood to avoid warpage. So if the leeboard is to finish at 3/4" thick you should made it from two layers of 3/8" plywood or three layers of 1/4" plywood."

Whenever wooden centreboard and rudder more elegant are ...:;
... but they are not 'instant' as Jim Michalak's leeboard and rudder :B

27th June 2008, 12:42 AM
Howdy Robert,

No good centreboard or leeboard or rudder is instant.

It is one area where a few extra hours of effort makes a huge difference to the performance and handling of a boat by making an accurate wing shape.

That is why I provide templates in my plans. I found out in my youth that I could make a nice looking centreboard and rudder by eye because I understood the main ideas. I also was pretty good at getting smooth finishes.

I raced my boat for a year against the same boats each week - I was a bit above the middle of the group in the racing.

I then paid a fortune for accurately made foils the next year and suddenly I was near the top of the fleet in all wind conditions and miles ahead in the strong stuff even though we were the lightest crew. Also instead of losing control in strong winds and waves and capsizing sometimes we found that we could control the boat without any problems at all.

So investment of time in making good foils (probably the difference is about 3 hours for the PDR) has a huge benefit.

Howdy Nick,

I used to think that "boats don't notice the difference" between a leeboard or centreboard. However this advice is wrong.

Some of the PDRacers have been fitted with a biplane rig. This is where there is a freestanding rig (mast sail boom) on each side of the boat. When the wind is light and medium both sails are used at the same time. But when the wind comes in strong the sensible thing is to drop one of the sails.

One of the PDRs had a leeboard.
1/ when both sails were set it steered and balanced fine.
2/ when the sail on the same side as the leeboard was the only sail set then the boat would sail fine.
3/ when the sail on the opposite side to the leeboard was the only sail set the boat was almost impossible to control. It had bad weather helm on one tack and bad lee helm on the other tack. Without being technical it meant the boat made up its mind were it was going without consulting the skipper.

I suspect now that this asymmetry results in a degree of drag as it has to be corrected with the rudder.

Also there are complicated flow patterns as the water moves around the hull and they are particularly confused at the side of the hull near the chine. Put a leeboard there and it creates a bit of confused movement in the water as well. Generally the more you confuse the action of the water, the more energy you are wasting - energy that should be moving the boat forward.

The centreboard as opposed to an offcentreboard (bolger's term) has been proven to be the most efficient method for all racing classes. Bilge keels have been tried in keelboats (by some heavy duty dudes too) but not with a good deal of success.

Catamarans do usually have their centreboards offset, but they take account of that by having them in a different position to a monohull in terms of fore and aft. And catamarans do have a symmetrical setup so when they tack there is the same relationship to the sail on both tacks.

However a compromise can be to make the leeboard offcentre but don't put it out at the side of the boat.

The PDR that is being built in the Philippines has just such a setup. But I really do prefer the centreboard.

Best wishes

27th June 2008, 05:30 PM
Foil shapes are one of my secret weapons. They can make huge gains in a boat's abilities over a much wider range of conditions. Knowing what shapes to use, is a bit of black magic, with few willing to give up their prized sections.

I've had more foils banned from race courses then you can shake a stick at. They usually weighed what they were suppose to weigh, but they were really light except at the bottom, especially stiff compared to others in the class and the first thing my sailing "friends" would point at come inspection time (the bums).

Leading edges (trailing edges on daggers) can be easily reinforced against impact damage with a length of line embalmed in epoxy. My method is simple enough. I prefer to use polyester single braid, but double braid works to. I saturate the line with epoxy. I use to do this by hand, but now use a plastic wheel bearing greaser. This is a tool I picked up about 12 years ago to ease packing wheel bearings, which I did by hand (lovely mess). It's a plastic tube with a plunger. I coil up the line and stuff it inside the tube, pour in epoxy, the smash down the plunger, which forces the epoxy through the line.

The board is prepped with a cove about the same diameter as the line, but cut along the centerline of the leading edge (about 1/2 the thickness of the line). I place the gooey, dripping line at the top of the board, in the shallow groove and tack it with a brad. Then I stretch it down, placing it in the groove and tack it at the bottom. Fair it into the leading edge with some thickened epoxy and you can hit it with a hammer with no damage. The line size is about 1/2 the radius of the finished leading edge, so if you have a 1" board, I'd use 1/2" line. It works on rudders and rub rails too.

Tests have clearly shown that centerline appendages have an advantage, but it's usually slight if an outset board is reasonably close to the centerline. If the "wrong" leeboard is down or you're sailing on the disadvantaged side, then you have a "turning couple" created, plus considerably less aspect area (if heeling much), which decreases it's effectiveness. The same is true of offset rigs (in relation to the boat's appendages), though in this vain there is less lose (check out Gary Hoyt's new square rig for example). Though he's typically pretty mum about design details, it clearly shows a "preferred" side underway.

28th June 2008, 12:49 AM
No good centreboard or leeboard or rudder is instant.

Exactly MIK!
The name of that boat is PDRacer
Hydronamically efficient profile of rudder and CB are basic for succes (as like as good sails).
And ordinary pleasure boat is better when with perfect windward ability.
To many ordinary pleasure boats have ordinary boards and steel plates as CB and rudder.

I intend to make Beth's centreboard and rudder as designed :)
(in future...:B)