Working out a decent laminate for a rudder

Thank you all for the input!

 

Thanks for the effort! I will take a closer look later in the day. First blush I am a little surprised @ the weight of the laminate, I had roughly estimated (with an online calculator) that 4/5mm of laminate would put the Paulownia core safely under 15kg.

Anyway, I'm tied up for now, so later.

 

You cannot use a 2 pound core unless it is doing no work. .. meaning no fastenings rely on the core

otherwise, you need a 20 pound

so 20 pound foam is 7kg

the benefit over Palowina is it won’t swell if ever wet

 

Palowina doesn't take on water, it's supposed to be one of its strengths for marine use. If I end up using it, I'd probably treat it with saturating epoxy to be sure.

 

Have you given any thought to the mechanics of covering the rudder with glass? Things such as which way up it will be and how you will wrap the top and leading edge?Much better to think it through now than when you have resin mixed and a corner to wrap round.

 

Every wood I’ve ever touched takes on water. Sure, the degree to which varies. But all wood is cellular. In between the cells, there is space and the spaces allow free water. A dense wood allows less water. In drying wood, the free spaces dry first, then the bound or cellular water dries until the wood reaches a final dryness. After removal from the process, the wood will take on moisture from the environment to equilibrium.

That same property (density) allows less intrusion of saturating epoxy. Also, Paulownia is an oily wood, which may mean you acetone wash it before you attempt any epoxy work.

The wood will almost certainly have higher flexural rigidity than coosa. This means resistance to bending. But there are caveats to this statement. All wood has the inherent ability to bend or follow its grains in bending. To mitigate this; choose wood that is vertically grained which means more and shorter grains in a single board. This means a quartersawn piece.

Choosing the correct type of board will help.

I repaired a pine rudder maybe 8 years ago. The person wanted the old rudder used, but it had curved and started to crack. I broke it the rest of the way or overstressed it a bit and then neat-coated the crack and applied thickened resin in the margin. After that, I applied 6 oz cloth to it. I wanted 2 layers, but the guy wanted only one. Then I rolled a few neat coats to hopefully fill the weave enough to make it waterproof. Pine is a terrible choice for rudder material, but the guy wanted it original.

 

I would automatically laminate a blank.It would be a huge challenge to find a single piece of wood big enough for a rudder blade that didn't have flaws and a laminated blank can incorporate harder varieties where they contribute to durability or strength,which is why I have advocated a tougher wood for the leading edge and trailing edge.If it was my rudder,there would be a harder piece down the thickest section of the foil too as it will almost certainly weigh less than the water it displaces.You just need to take a little care about grain direction if you might need to plane it.A stronger wood imparts a bit more strength and allows for less sheathing.

 

Yes, absolutely. I have a plan, how practical it turns out to be is another thing! I expect to have to revise it, at least a little, when I have the material in hand. I have a few options in mind... but yes, it is being thought through.

 

It definitely will be laminated, 150x50 0r 100x50 planks. The trailing edge will most probably end up being vertically laminated with say 4mm of glass down the centre. When shaped, this will give an area of epoxy/glass for the trailing edge laminate to bond too. Maybe the leading edge as well.

Something like this...




I take your point, but I'm not so concerned about the leading edge given the boat's purpose, where it sails and conditions it sails in. It's highly unlikely to take a hit to the leading edge.

I have considered running some carbon down the length of the foil at the max foil depth. I'm not sure the extra stiffness will actually be beneficial, but I can't see it hurting.

 

You are probably right about a strip of carbon not hurting,but equally,I would be surprised if it actually contributed anything.The surface area of the rudder is large enough that just the glass will make it enormously stiff.I like the sketch of the assembly process and would make a suggestion;the final shape will not be rectangular in planform-which you show.You could save a bit of wood by shortening the strips on the trailing edge and at the head.the shorter strips would still leave a parallel section for the cramps to grip. I use marker pen and numbers on the strips,combined with alignment marks to locate the pieces.It also saves glue and glue spreading time.I will once more emphasise the importance of an accurate centreline once you have cut the blank to the desired outline,it is you only reference.If you take the purist approach to the section,the thickness needs to be adjusted proportionately to the chord before shaping commences and for optimal performance you need to offset for the predicted sheathing thickness.Any idea what that might be?

 

I don’t see much call for glass on the ends.

The strips need to be alternated off the saw so you don’t develop an angle. I mark things I/O or U/D on each edge to avoid a 0.1 degree error driving a curve into a desired flat product.

I think a 4mm layup sounds about good. I’d use 4 layers of db1200 in two gos or maybe 4 depending on how well it wraps, as it conforms nicely and you get variation on the tows. A uni will end up useless on the edges unless big enough to make it into a biax.

 

On the trailing edge, it allows you to join glass to glass without having to wrap the edge and increase its thickness. Once the foil is shaped, you expose an area of flat glass that you can bond too. Doing it this way gives you a contiguous external shell without having to allow for overlapping material, which would require a thicker trailing edge and fairing. The leading edge can be wrapped if you are laying both sides in one operation, so there it isn't required so much.

That drawing is lifted from an article discussing the creation of dinghy foils, it's a method they use. Typically, they layup one side @ a time. I want to wrap the foil in one operation. I will have to work out some sort of supporting jig to do it, but I think it is achievable.

 

That will only work under vac. Hand laminating this thing in a single go is impossible. You will introduce air somewhere. Even using vac, a double go is best unless you tried to infuse it all. Wet bagging it would work in two goes and any imperfection on the edges could be repaired after each pull.

You won’t leave 4 layers of laminate ending at that wrong way glass; they all get staggered past the middle. This will create a 4mm lump and that is dealt with by creating a recess or tape relief on each side of each end. Overlapping the laminate creates a ton of strength on the end. That middle glass shearweb is nonsense, imo.

 

finger sketch, 4 layer laminate, relieve core for overlapping section all the way around the end, stagger each layer 20mm min, one corner drawn, build 4, build upside down against drawing, set part on jig to allow wrap. If the laminate misbehaves doing it by hand, plastic wrap over peelply or laminate and wrap with masking tape and secure plastic to ‘bottom’, with 8 layers of glass on the end, you may decide 3x db1200 is enough and then the ends are still 6 …

wrapping the section will be a nightmare, as the glass will distort too much

 

A couple of points to ponder; 4 plies of cloth will give you an implement you can fell trees with,its a bit heavy for a sheathing layer on a structurally adequate core.Second,given the curved leading edge,it won't be possible to wrap the plies as they will need darts cut to conform to the shape.Wrestling with slippery,resin coated cloth and manouvering the sticky whole over to work on the other side is no fun.Something so critical to the operation of the boat isn't an ideal candidate for a learning exercise in infusing but it might ultimately be a good choice of method.

Before reaching that stage I might suggest using biscuits to align the strips when gluing the blank together.Also keep in mind that adding half a dozen or so sash cramps may impose enough of a load to lock in a downward set if the support isn't carefully arranged.