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Old 20-02-2021, 18:28   #1
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Need Advice to Complete My Steel Rudder

Hi Everyone.

I'm building a new rudder for my 36' steel sailboat. I've got the ribs all welded up (photos) and now I'm ready to skin it with 1/8" thick plate.
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Problem is, I'm not sure how to form the skin to the foil shape. So I'm seeking advice on how to proceed.

I'm thinking of the following options:

1.) Pre-bend the plate to the approximate shape, then clamp it at the leading edge, weld , re-clamp further back along the foil, weld, repeat, until I finish at the trailing edge. This was my original intent but 1/8" plate is much tougher to bend than I thought it would be, especially as I don't have a bending brake (though I could make one on the front edge of my welding table if this is the best way to go). It's not helping that I don't have an oxy-acetylene torch. I do have a "high temp" soldering torch but it's not powerful enough to really soften the plate.

Another option to pre-bend the plate is to re-use the 3 jigs I made of the foil shape in negative (see pic) which I originally used to keep the rudder straight and aligned while I was assembling the ribs. The idea is to re-mount these jigs to the table, place the plate on top, then squeeze the plate down to the jig profile with something like a 4" dia pipe mounted on threaded rod (see sketch).
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2.) Forget about pre-bending and just tack weld the plate to the leading edge and then use heat and hammering to form the plate down onto the leading edge curve, tack, progressively as the plate forms to the curve, repeat until I finish at the trailing edge.

3.) Similar to (2) but instead tack the plate to the flat, thickest part of the rudder, then heat and hammer the plate down to the leading edge, tack weld, then do same towards the trailing edge.

5.) YOUR PROVEN METHOD! I'm absolutely ready to be man-splained how something like this is normally done, hopefully without expensive tools like a hydraulic press or such, as I don't have access to that kind of stuff.

It's gone so well so far. I don't want to screw it up at this point.
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Old 20-02-2021, 18:41   #2
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Re: Need Advice to Complete My Steel Rudder

Do you have access to a torch with a rosebud?
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Old 21-02-2021, 01:39   #3
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Re: Need Advice to Complete My Steel Rudder

Maybe you do not need 1/8 plate. What about something a little thinner that will just bend around your framework? You have quite a lot of strength of form there.


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Old 21-02-2021, 01:50   #4
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Re: Need Advice to Complete My Steel Rudder

If you want to do it with heat just set the plate up on bricks and light fires under it.

You might try tacking both sides along the front then use threaded rods along the back edges of the plates to pull the sides inward together by tightening the nuts.

Short pieces of flat bar on edge tacked to the plates with the rod in between through washers tacked to the outside edges of the flat bars will hold the rod in place whilst you tighten the nuts. I'd start with about six x 5/8 or 3/4 threaded rods.

Brute strength and ignorance has got a lot going for it when you're working with steel.
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Old 21-02-2021, 06:37   #5
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Re: Need Advice to Complete My Steel Rudder

Nice work on the frame.

As much as I love the whole 'beat to fit and weld air-tight' thing, without a torch, other tools, and a helper or two I could see it being difficult with 1/8" skins.

-I would have preferred to do this with a thinner skin that is pre-formed, either rolled or press-broke, to contain most of the curve. I would have maybe used some self drilling and tapping screws to hold stuff in place while welding.
-One could invent a procedure for scoring the outside vertically every inch or so and bending the 1/8" in the field to better conform to the foil, especially the leading edges and back-filling the scores with soft rod.

All the best, keep us posted.
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Old 21-02-2021, 06:57   #6
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Re: Need Advice to Complete My Steel Rudder

Thanks all.


RaymondR: I can get access to a torch and rosebud with some difficulty by moving all my s--t to my sailing club. I would prefer not to have to do that, but it's a fall-back option.

Boden36: I already have the 1/8" plate cut. Also I want some thickness, not just for strength, but as insurance against loss of material due to corrosion.

AKA-None: I'm visualizing your suggestion. The problem is I'd be welding both sides at once which prevents me from completing the welding inside the frame. My skins would then only be welded along the perimeter which will lead to a much weaker structure. From my research, it seems that rudders are welded together one side at a time. Once the first side is completely welded into place, the other side is put into position and welded on entirely from the outside by adding plug holes in the skin, which will align over strips added to the rudder frame's horizontal ribs. These plug holes are then filled in with weld. This ensures the rudder is welded in place throughout the surface areas of both sides of the rudder.

In the OP pics, you can see the strips that I've added to the STBD side of the rudder frame. I also attach a screenshot from the CAD file which shows it very well. Sorry, I should've described this in my OP.
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Looking forward to other suggestions.
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Old 21-02-2021, 07:15   #7
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Re: Need Advice to Complete My Steel Rudder

Thanks Spot. I've considered scoring but am hoping to avoid that. If I were to score it would be on the inward side of the curve. Bending the plate then closes up the scores instead of opening them, which I believe is the better direction to go. Then I would fill in with soft weld, as you say.

There's another detail worth noting here. The rudder profile is not a typical NACA "teardrop" shape. It also has a bit of a fishtail at the trailing edge (see CAD screenshot in earlier post). This means there are two inward bending curves and one outward bending curve. So if I were to score I'd have two sets of them on the inside and one set on the outside.

No matter how I decide to do this, this S curve complicates the shaping a bit more and needs to be taken into account. BTW, this is called a "Thistle Rudder", promoted and heavily used by the naval architect and author, Dave Gerr. It helps especially with low speed manoeuvring, which is where my current rudder sucks.

I'd not thought of having the skin formed by someone else with the right equipment. I could give them one of my profile jigs and they could use that to confirm the shape. Extra cost of course. I'll look into this.

In the meantime if others have suggestions that would allow me to do this on my own, I'm all ears and eyes.
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Old 21-02-2021, 07:27   #8
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Re: Need Advice to Complete My Steel Rudder

like the pic above.....that's how I built the steel rudder on my boat..I drilled some holes in the plate where the plate fell over a frame, so the plate could be welded to the frame...spot welded the plate to the front of the rudder and used clamps to draw them together at the trailing edge.....was easy as pie....

don't forget the rudder post.....on my rudder post, I drilled a small indentation on the bottom of the rudder post and another small indentation in the rudder shoe...and placed a steel ball bearing between the indentations. The shaft was the same thickness as the prop shaft, so I used the same size cutlass bearing, and filled the shoe with grease....

on the top bearing, also the same same cutlass bearing and I had fashioned a packing gland type fitting to fit over the bearing with some flax packing....

finally, I was concerned about the paint wearing off the upper and lower posts where it rubbed against the cutlass bearing, so I coated these with West System epoxy to provide a smooth surface.

it all worked like a charm. about 10 years later, I pulled the rudder, to inspect it, and was surprised to see very little wear and tear...

I've seen some steel rudders with a small zinc attached to it, but I never did that. I had one sacrificial zinc plate, 12 x6 x 1 attached to the hull, near the rudder, and this lasted for about 5 years, before I had to replace it...

trust this provides some guidance
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Old 21-02-2021, 07:51   #9
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Re: Need Advice to Complete My Steel Rudder

Hi MicVughV.

Great to get confirmation of your method to weld together a rudder. I can see the advantage of welding from the leading edge, then working backwards, as it gives the greatest leverage when clamping down the skin.

I'll be re-using the existing shaft from my existing rudder (my project today is to cut it out of the existing rudder ) and I will weld it fully in place with SS309 rod before I attach the skins.

My rudder has a bearing on the bottom made from HMHDPE, as well as another up inside the rudder tube. There are no ball or needle type bearings. The bearings are held in place by SS316 fittings so there's no risk of abrasion causing corrosion of mild steel.
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Old 21-02-2021, 09:03   #10
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Re: Need Advice to Complete My Steel Rudder

my rudder plate was actually 3/16" as that is what the whole boat was made from.....but I didn't have any problems fitting it to the rudder frame....never had any problems skinning the hull either....just started at one end and pulled the plate in as I went along....it all went surprisingly smooth...

same went for my keel....which was 1/4" plate, started at the front and used clamps, come alongs, etc, to fold it back along the keel frames...

I built my boat solo, so often had to come up with innovative ways to fabricate something..

I had my boat sand blasted inside and out, in this regard, I did have some help....the inside of the boat was painted with a two part coal tar epoxy...the outside, a zinc primer, some base coats and two finish coats.

I never had a lick of problems with rust on the inside. Not once. The outside would get nicked on occasion, so I'd have to fix that.

That boat is 40 years old now and is still around..now on it's third owner...

here's something else I did.....I welded two eyes, one each side, on the top trailing edge of the rudder...about 1" inside diameter, 1/4" bar dia...in case I ever broke a steering cable, I figured I could tie some rope to the top of the rudder to steer the boat.

I did in fact break the steering chain where it went around the steering pedestal, but was able to repair that without resorting to the rope idea...
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Old 21-02-2021, 09:14   #11
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Re: Need Advice to Complete My Steel Rudder

For the sides you can just bend and tack readily with 1/8" can't you?. Is it just too long a piece to do that? Start at the center. I was a certified welder fitter for years but it's been a long time ago. Do you already have the plate? Seems it doesn't really need to be 1/8".

Weld a pivot tab after tacking the leading edge. Have a leverage bar bolted through that tab to push the plate down. Tack and move to adjacent weld slowly wrapping to the trailing edge. If that makes sense... See pic.

Make the side panels 2 piece to make bending easier?

If you are trying to wrap around the leading edge you will have to have it brake formed to match the radius.
You need a welder helper.
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Old 21-02-2021, 10:03   #12
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Re: Need Advice to Complete My Steel Rudder

Thanks MicHughV for the additional info.

Cheechako, nice lever solution! Thanks for adding the sketch. A pic is worth a thousand words. I could use tha method for sure. The length (top to bottom of rudder) of the sheet is just under 48". I think if I add 2-3 levers I should be able to get a pretty consistent curve.

The plates are not wrapping around the leading edge. The pipe is already the leading edge. So I'll be welding the front edge of the plate to the tangency point between the radius of the pipe and the foremost curve.

Yes I need a helper. My wife is apparently not a candidate, judging by her behavior whenever she comes into my makeshift shop.

Looks like the consensus is to tack to leading edge and then use creativity and brute strength to leverage the plate down onto the profile.

If there are other suggestions, please send them along! The next couple days will be spent getting the existing shaft prepared, so I've got time to ponder other solutions.
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Old 21-02-2021, 10:25   #13
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Re: Need Advice to Complete My Steel Rudder

Did this recently as well- here’s the set up we used. Slightly different design in that I wanted a partially balanced rudder, so the rudder shaft was set back 6” from the (sharp) leading edge. It was also 1/8” skin, but my welder pre brake- bent it to get the approximate shape first; then tacked and welded from there. Other thing I did with the design was use a tab-and-slot approach for the skins- each one of the baffles had tabs sticking off it on either side, so it all self-aligned and stayed square and true on assembly. The tabs were only 1/16” tall, so we were able to plug weld on top of them where they went partially thru the skins, locking the whole thing together beautifully. One of the real benefits of cad layout and cnc waterjet cutting!

I also had all plate blasted and coated with a weldable primer (Nippy Ceram) after cutting, so it arrived clean and ready to weld.

All turned out really well- and I can’t say enough about how much better the boat steers with this partially balanced rudder vs the barn door we had before. Oh, I also moved it aft 7’ too! Huge change. The boat actually balances now; and she’s significantly faster now that we’re not wandering all over the damn place and dragging that barn door sideways thru the water!Click image for larger version

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Old 21-02-2021, 11:00   #14
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Re: Need Advice to Complete My Steel Rudder

Moved the rudder back 7 ft?! Your boat is how long? Certainly longer than my 36 footer.

My rudder is semi-balanced as well. Shaft is roughly 4" back from leading edge. The cut out at bottom goes around a protective skeg, where the bottom bearing is attached. See CAD shot.
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Nice that you applied the ribs to the skin first to ensure the shape. Too late for me to try that.
I also had my parts cut with Waterjet from my CAD file. I did all the rib pieces with slots and tabs and had no problems as a result. I decided not to do same with the skins because I didn't feel I would be able to maintain the precision at this later stage. But I do have my skins cut by Waterjet also - with plug holes on the STBD skin - and have them slightly oversized to allow a bit of misalignment. I'll let you know how it turns out.

Good to know others have tackled this. Makes me feel a bit more confident that this will turn out.
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Old 21-02-2021, 13:10   #15
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Re: Need Advice to Complete My Steel Rudder

Following this thread is balm for my soul....

I had a blast building my boat...2.5 years of blood, sweat and tears...even wrote a book about it....PM me if you want to know about my trevails, my ups, my downs...my successes, my failures...

You don't see many people building their own boats much anymore...and if you do, it's usually a " steely".....

these days you can order a kit boat...all the bits and pieces already pre-cut, etc...but back in my day, this did not exist...you ordered a big wad of steel, then tried to make sense of it all...I had never welded prior to starting my build...for that matter, barely knew which end of a screwdriver to hold...but......I learned.....
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