Building a new rudder

[lituya1617]

You have put a lot of thought into your design for a new rudder. I suggest you do a stress analysis of the connections between the rods and the shaft. The stresses on a rudder a very large and the joint between these two metal pieces will eventually fatigue and fail. My father had a wooden rudder on his 38' commercial fishing boat in Alaska and it worked for over 45 years. But it was made from cherry (Ironbark) and was hung between two iron straps that were in turn welded to the upper rudder shaft and the lower pivot bearing atop the skeg. The thick ironbark was held together with biscuits and galvanized all thread and simple nuts. Holes were plugged with ironbark dowels. The straps on the sides were through bolted to clamp each plank with galvanized bolts. Ironbark is heavier than water and so dense that worms cannot penetrate it. The whole thing was copper painted each year, otherwise, no maintenance ever. Fir is too weak and easily split for my taste in such an application. Also, rot and swelling in a light wood are real problems in an enclosed rudder. The swelling will pop the fiberglass off the wood. A very dense wood like ironbark is stronger and will not swell much or rot.

[BBill]

Not to rain on your parade but you asked...not many seem to know this because they don't own boats decades after applying 5200. It gets hard like a bowling ball, shrinks and gets brittle in 15+yrs. It remains tenacious but stops flexing and gets hairline cracks. I'm talking from my own experience on 2 vintage glass boats where i applied 5200 on 20 & 35 yrs ago...still have these boats and both did the same thing. The 5200 was leaking at the hairline cracks and took a chisel to remove it. You mileage may vary but I doubt it as I'm not the only one who has had this experience.

You are going from mild steel to 316? Chances are the original was actually 302 or 304 and looks like mild steel due to the exterior surface corrosion. My boat lift bolts looked like this after 15 yrs and you would swear they are regular steel rusted to hell. But have you considered how much weaker 316 is to regular steel?

If I was doing a new rudder today I'd probably follow what the original glass boats did...exposed wood rudder with bronze shaft and exterior straps mortised into edge bolted 2x pine or mahogany lumber. No glass and always visible for inspections or survey. Not fancy or pretty but no crevis corrosion, no waterlogging or hidden problems.

[LeighWebber]

Tx - I'll add the G-Flex to my ingredients list. The only problem with all of this is that the rudder is going to last way longer than my remaining lifetime, so I won't get to see how it eventually fails. :-)

For those following along at home, here's a short video that shows how flexible G-Flex is: https://www.westsystem.com/specialty...ughened-epoxy/. I sure wish they had this stuff when my original rudder was built 45 years ago.

[Bill O]

Originally Posted by lituya1617

You have put a lot of thought into your design for a new rudder. I suggest you do a stress analysis of the connections between the rods and the shaft. The stresses on a rudder a very large and the joint between these two metal pieces will eventually fatigue and fail. My father had a wooden rudder on his 38' commercial fishing boat in Alaska and it worked for over 45 years. But it was made from cherry (Ironbark) and was hung between two iron straps that were in turn welded to the upper rudder shaft and the lower pivot bearing atop the skeg. The thick ironbark was held together with biscuits and galvanized all thread and simple nuts. Holes were plugged with ironbark dowels. The straps on the sides were through bolted to clamp each plank with galvanized bolts. Ironbark is heavier than water and so dense that worms cannot penetrate it. The whole thing was copper painted each year, otherwise, no maintenance ever. Fir is too weak and easily split for my taste in such an application. Also, rot and swelling in a light wood are real problems in an enclosed rudder. The swelling will pop the fiberglass off the wood. A very dense wood like ironbark is stronger and will not swell much or rot.

I'll agree that fir isn't an applicable wood for a rudder and a rot resistant wood is would be better choice.

The wood referred to the common name of "ironbark" is a red colored wood, but is eucalyptus, not cherry. It was used for skeg shoes in ship building. Using common names (vs. true scientific names) to describe wood helps avoid miscommunication of the type of wood actually being used in projects and the proper selection of wood types for their intended purpose.

That said, you would like to keep the rudder (w/post) weight more towards a neutral buoyancy than a negative buoyancy for a sailboat, so a highly dense wood will make it too heavy.

Our old rudder was made of ply and apparently lasted for a long time. When we took it off we noticed it had some water penetration on the bottom edge of the rudder and overall the rudder blade was extremely heavy. We thought of making the new rudder out of ply again but since we've previously worked w/Coosa (it is ~1/3rd the weight of ply and doesn't absorb water), we thought this was a better choice.

Making a rudder is a decent amount of work and something you only want to do once. If you are on a budget ply would work, but if it will be your boat for a long time to come, then Coosa as the core would be a better choice.

[RaymondR]

Rather than rod use flat bar bent around a firmer to get your foil profile, then weld them to the SS rudder stock, fill the spaces with closed cell polyurethane foam, shape it with a hot wire and glass over the lot perhaps.

[LeighWebber]

Originally Posted by BBill

Not to rain on your parade but you asked...not many seem to know this because they don't own boats decades after applying 5200. It gets hard like a bowling ball, shrinks and gets brittle in 15+yrs. It remains tenacious but stops flexing and gets hairline cracks. I'm talking from my own experience on 2 vintage glass boats where i applied 5200 on 20 & 35 yrs ago...still have these boats and both did the same thing. The 5200 was leaking at the hairline cracks and took a chisel to remove it. You mileage may vary but I doubt it as I'm not the only one who has had this experience.

You are going from mild steel to 316? Chances are the original was actually 302 or 304 and looks like mild steel due to the exterior surface corrosion. My boat lift bolts looked like this after 15 yrs and you would swear they are regular steel rusted to hell. But have you considered how much weaker 316 is to regular steel?

If I was doing a new rudder today I'd probably follow what the original glass boats did...exposed wood rudder with bronze shaft and exterior straps mortised into edge bolted 2x pine or mahogany lumber. No glass and always visible for inspections or survey. Not fancy or pretty but no crevis corrosion, no waterlogging or hidden problems.

Another respondent suggested G-Flex, which may be a better choice than 5200. As for the flavors of steel, my new rudder post is an exact replacement of the old one: 316 stainless. I'm only replacing the post because the cost to salvage the old one exceeded the cost of a new one. The old one, you see, had mild steel ribs and a full-sized plate welded to it. And it was mild steel -- not surface corrosion, but full, eat-it-all-the-way-through corrosion. To grind off the old mild steel and prep the old post for renewal was just not worth it.

As for strength, the rudder has a full skeg, and the lower support is very robust: 3/8 steel flange bolted to the keg's internal steel frame. If some external force were strong enough to bend that post, it would have to first rip the skeg off. I think this highly unlikely, short of a Mark 46 torpedo . In any case, the new post matches the old one.

I considered the external strap approach, but discarded it for two reasons. First, straps would make it more difficult to create a foil shape for the rudder. Second, the bolt holes create more opportunities for water incursion.

I'm pleasantly surprised by how much activity this thread has generated. I do, of course, have to actually build the thing, so many decisions are already made, and construction is in progress.

[Bill O]

Originally Posted by LeighWebber

I considered the external strap approach, but discarded it for two reasons. First, straps would make it more difficult to create a foil shape for the rudder. Second, the bolt holes create more opportunities for water incursion.

I'm pleasantly surprised by how much activity this thread has generated. I do, of course, have to actually build the thing, so many decisions are already made, and construction is in progress.

A good choice not to make it w/external straps. We eliminated ours due to excessive marine growth on the straps and drag created by the fouling, bolt heads and nuts that stuck out. While you are correct the bolts do create more opportunity for water intrusion, we didn't see it on our rudder that we replaced and there was plenty of bolts for it to occur.

There are many of other foam materials to use as core for your rudder.
As Raymond mentioned, closed cell foams (PE, urethane, etc.) could be used but then you'll need a decent layer of glass to protect the relatively lighter density foams. Another reason we chose Coosa was it was a structural foam w/integrated glass stands and sheets. Very tough, but easy to work w/normal tools.

Once we determined what foil we wanted (NACA 0010), it was fairly easy to make the rudder using the router. You may want a NACA 0012 (Bob Perry's favorite foil). Since you are making a skeg held rudder an exact foil isn't as critical as w/a spade rudder, but I always attempt to do the best precision possible.

If you didn't read the link I provided previously, take a look at it. Literally we pulled the boat, assessed the rudder and started building it. We also did the bottom, installed new electronics/thru hulls and other work while making the rudder. We were back in the water in 3 weeks. Once you get going on it you will see it's not that hard.

Been 2 yrs w/the new rudder and have pushed the boat fairly hard. Works well and when we pulled the boat a few weeks ago to change anodes, we inspected the rudder. It still looked like we just installed it and expect it to last for a long time.

[Pietje]

I rebuild the rudder of my Sun Fizz in Dakar after a accident in Bissau .
I make it in marine plywood glued with epoxy and filled all the empty place between the plywood and the rods with a spécial foaming epoxy . the rudder it self was paint by 7 layers of epoxy resine .


https://www.cruisersforum.com/galler...?i=14313&c=517





2.12.0.0

[Spot]

Leigh,
If you vacuum bagged your composite layer with peel ply you'd end up with a consistent texture and thickness that will accept the next layer of goop easier.
I did the rudder pieces and leeboard of my small boat that way.
Feel free to let me know if this idea is appealing and/or if you want more details.

[Mike Banks]

I would have reduced the moment against the welds and ribs by making it a balanced rudder.

I would also have made it entirely of steel, NOT stainless, then had it hot dip galvanized--and antifouled accordingly NOT using a copper based antifouling.

But even if you stick to the idea of wood and resorcinol glue--make it a balanced rudder--so much EASIER on the autopilot, and the rudder blade itself. It almost HALVES the load because it puts it and its bending moment each side of the rudder post, not all on one side.

The strain on the rudder pintle and bearings of course is the same either way.

As an interesting alternative, just as something on which to ponder, once the steel balanced rudder is complete--instead of galvanizing, have it heavily electroplated with COPPER. Then you have no further need of antifouling on the rudder for some time----but you may need to use Propspeed to protect your propeller from the bare copper rudder--

[mitiempo]

It is on a full skeg with a bottom bearing. Hard to make it balanced without adding draft.

[sscoll]

Doesn't sound like you've put this together yet. I'm about forty miles east of Stillwater and built a new rudder for my Pearson Vanguard 32 this spring using marine ply and basically the approach you've outlined.

I used Seahawk primer from West Marine in Stillwater. A nice vibrant yellow that seems to bring out the worst in my friends in the Ashland Marina.

I bored holes through the rudder to coincide with the ends of the bolt threads for tightening purposes and filled them with thickened epoxy which is what Pearson did fifty years ago when mahogany was cheap.

I also learned to make a cove cut on a table saw to match the shaft radius.

Crafting a rudder can be such a confidence builder. Good for you.

[LeighWebber]

Originally Posted by sscoll

Doesn't sound like you've put this together yet. I'm about forty miles east of Stillwater and built a new rudder for my Pearson Vanguard 32 this spring using marine ply and basically the approach you've outlined.

I used Seahawk primer from West Marine in Stillwater. A nice vibrant yellow that seems to bring out the worst in my friends in the Ashland Marina.

I bored holes through the rudder to coincide with the ends of the bolt threads for tightening purposes and filled them with thickened epoxy which is what Pearson did fifty years ago when mahogany was cheap.

I also learned to make a cove cut on a table saw to match the shaft radius.

Crafting a rudder can be such a confidence builder. Good for you.

I did the cove on my table saw as well. I first traced a semi-circle onto the end of the wood piece, using the actual rudder post. I then made multiple passes, easing the blade up to just touch the traced arc. This left a nice cove, but with wee ridges 1/8" wide. To smooth these out, I had a 12" piece of the old rudder post. I wrapped a sheet of 40 grit sandpaper around it, and used it to sand out the little ridges. Result: nice smooth cove that fits the post perfectly.

For the rods within my rudder, I don't have to worry about filling the holes with epoxy, because the rods are shorter than the rudder width by about 6". This was necessary in order to shape the trailing edge of the rudder to conform to the 0012 foil shape I'm using without the rods, nuts, and washers emerging from the wood. There are 3 or 4 laminations of wood aft of the rod ends, sealing them off. I'll wrap the whole thing with fg as suggested above.

Also -- someone else was concerned about the bending forces on the rudder eventually causing the joints between the rods and the posts to fail. If you look at the dwg I attached to my original post, you'll see that the rods are actually inserted into holes drilled right through the post, and welded on each side. For the rods to break off, they will have to rip the post apart as well.

[RaymondR]

You should not weld the rods on both sides, only the stubs on the front side of the rudder post. Welding on the back side where the bending moment is greatest risks the rods cracking there.

[Jim Cate]

Originally Posted by RaymondR

You should not weld the rods on both sides, only the stubs on the front side of the rudder post. Welding on the back side where the bending moment is greatest risks the rods cracking there.

Agreed... and those welds don't even add strength to the joint.

Jim