You’d think a rudder was quite a simple thing, wouldn’t you? Just a plank of wood that directs the flow of water one way or another and invokes a turning moment in the boat, but no. It turns out, there’s science behind these planks of wood.

Here is the rudder that was on Kudu when I bought her, and the rudder than got me up the East coast of Britain.

Please excuse my crude drawing.

My calculations so far as follows. I’m new to this, and haven’t needed to do this sort of maths since I was in school, so I’m rather rusty. Please do feel free to correct me if you see an error.

According to the formula for working out the surface area of a trapezoid ( A = 1/2(42+54) x 49 ), this entire rudder has a surface area of 2352 cm2.

The area forward of the rudder line of the rudder shaft, or the balance are, is 588cm2, or 25%.

Now, for the proposed new rudder I’m follow the advice of the good people on the YBW forums, who suggest that the balance area should be between 18 and 15%, the latter offering more feedback to the helm.

I want (I think) to keep the surface area of the rudder the same, but increase depth where possible, so a rudder shape more like the following seems reasonable.

53cm is the maximum depth of the rudder, which is limited by the draft of the keels, so based on a = h x w (the surface area calculation for a rectangle), I can work out the width, or chord. 2352 = 53 x W. Oh gosh, I feel clumsy doing this.

Ok, W is, by my reckoning, 44cm, which doesn’t give me the shape I was after really, since it’s closer to a square, and doesn’t allow for a hydrofoil profile. At least not NACA12, since the rudder would have to be incredibly thick.

What to do?

When I bought Kudu, the rudder was in a sorry state. Parts of it were softening, and the whole affair had developed a slight twist, and the design itself left much to be desired. It was simply a flat sheet of ply, circa 18mm thick.

Since I was all but clueless about such things back in 2008, I “fixed” the small rotten patches by cutting them out and filling it with wood filler, then laminating the entire rudder with polyester resin and a layer of roving. It wasn’t a neat job, but it strengthened the rudder and was, in my opinion, sea worthy.

When I actually got to sailing Kudu in fresh conditions, I found she had a lot of weather helm above 25knots (of, wind, of course). I now believe that was mainly down to sail trim, in particular the main needs a third reed adding to it, but the rudder certainly had some helping hand in the problem.

I am no hydrodynamicist, but I have done enough research in this area to know that a flat board of plywood makes for a very inefficient rudder. Introducing an angle of attack in relation to the water flow (i.e. steering the boat, or countering weather helm) creates a lot of turbulence on the downstream side of the rudder. If you increase the angle of attack sufficiently the rudder will stall, and then become a brake rather and a steering device.

The solution is to create a hydrofoil profile to the rudder, which will not only stop the rudder stalling (at least anywhere near as soon), but it will increase it’s efficiency since it works along the same principle as an aircraft wing. The hydrofoil shape causes water to flow more quickly over the downstream side of the rudder, which according to Bernoulli’s principle induces a lower pressure relative to the upstream side, and thus lift, which pulls at the rudder and makes the whole affair of turning a much more efficient one.

The trouble is, I have no idea what dimensions this new rudder should have. If I get it wrong, it will really make a mess of the way Kudu sails, and use up a lot of expensive wood and epoxy to boot.

Luckily, there’s a MK1 Corribee on brokerage at Preston at the moment, so I popped along with a big sheet of card and cut myself a template of its rudder. Alas, the Corribee on brokerage is fin keel, and it seems that the rudder is somewhat deeper than whatever should be on the bilge keel version, because it’s too deep to fit on Kudu.

The other potential issue with it, is that it’s entirely unbalanced. This might be ok on the fin keel boat, but I suspect it will be far too heavy with my bilge keels.

I’m now back to square one. I need to make a new rudder for Kudu, but I have no idea where to start.

Any help, of the non guessing sort please, would be very, very much appreciated.

NB: Here the rudder as it was in 2008 before I’d moved aboard.

Following from my previous post regarding the quality of workmanship at the Newbridge yard (here), I’ve now sorted the problem.

Kudu now has new floors, cut from 18mm marine ply (doubled up on the forward floor to match the previous piece).

The doubled piece was bonded together using some magic 5 minute glue. It’s “safe for immersion in saltwater”, and dead strong. Within 5 minutes it sets, and after about 20 minutes, or a brew in the cafe, it’s ready for using.

Job done. Well, almost. Epoxy takes a while longer to “go off” than polyester, but forms a much better bond, especially for areas that likely to be wet. The downside is I’m stuck near the companionway until it does harden (24 hours in theory, but it should be strong enough to withstand accidental taps in a few hours). The good news about West Systems epoxy is that it doesn’t smell anywhere near as bad as polyester resin.

I prepared the area with course sandpaper (and a sander), then cleaned it with acetone which most certainly does smell (quite nice in my opinion, but perhaps that’s echoes of a misspent youth). The floors have been bonded with two layers of 50mm tape in four places. This is twice the amount used by Newbridge, and epoxy is stronger to boot, so it should be ok, although I might mix up another pot and add a couple more layers depending on how it feels tomorrow. Epoxy is far too expensive to waste willy nilly, you know!

Anyway, that’s it. Job done, but although I’ve spent a day on it, I’ve got no further in overall progress. I suppose I have now got a much better “canvass” for installing the bilge pump in to though.

It is a very handy thing to have a good relationship with your local chandlers.

I’ve just borrowed various bronze fittings to see if I can make up a solution to my bilge pump issue. If it’s workable, I buy what I need.

So, here it is…

So, in theory it’s ok but there are three problems.

• I don’t like all that heavy metal clinging on to the seacock. It just seems like a poor job.
• It creates a loop, so I’d need to fit non-return valves on each outlet to prevent pump straight back in to the bilges.
• To get the various fittings lines up, I can’t tighten them fully. I’m not plumber, but I’m guessing PTFE tape will sort this problem out?

What’s the verdict? I’m not ever so convinced. I might have a go at arc welding two bits of pipe together to make a much lighter t-piece out of stainless. I’ve still got the loop issue though, and no matter what solution I go for I think I’m going to need the non return valves.

The standard gimbals for the Origo Stoves are a bit naff and despite costing over £60, are not much good for mounting the stove in a small boat. They seem to have been designed to mount the Origo 3000 in place of  a full size oven. Since I imagine people buying the Origo 1500, or 3000 will not have the space for a full size oven, I really think Origo should offer a more suitable option to their customers. Perhaps I’m wrong; feedback appreciate.

Anyway, I decided I could do a better job, so set about butchering the £60 gimbals with an angle grinder, and making my own with the addition of a bit of Sapele.

Here you can see the two gimbals.

One side mounted in place using five screws.

And the finished job.

I’ve also started making the new companionway step, which also serves as a cover for the sink. It was cut out of 18mm marine ply.

And I came up with a nifty little design for it. The mixer tap you can see in the picture above, tends to drip a little. It’s not because of any leak per se, but as the water that remains in the tap neck dribbles out after switching the tap off. Also, I wanted to have some protection just in case it it got kicked on by accident, so I came up with this…

This is obviously not finished, but it will be smoothed off, and a hole drilled in the bottom of the “pit”, which then allows any stray water to run along a small routed conduit underneath the step, and drip harmlessly in to the sink.

It’s just a small touch, but worth doing I felt.