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Old 21-01-2021, 09:13   #1
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Swap my old foam rudder for a INOX steel rudder

Hi all:

This is Miguel, (proud) owner of a SO 51, 1990

I'm currently cruising around the world with her and I damaged (again per 3º time) the rudder. It always broke the down tale part, below the steel squeleton structure.

If I replace my rudder from one made of inox316 (same dimensions than original but built in steel) and this new rudder is built as follows:
* Use the original shaft of the former, so it is extracted to the former one by cleaning it up and remove all the oringal foam
* Use the original internal structure and enlarge to provide some kind of rib to the coming envelope material
* And wrap the thing with steel plates
* Soldering all, and getting the rudder hollow and waterproof, so there will be some kind of bouyancy to compensate the bigger specific weight of the steel vs foam
* Finally, a thin external cover of fiberglass , to reinforce the waterproofing, protect steel, and allows the antifouling paint to stick there

Does my boat have any problem with this modification?

Please do not refer to the issue that in an event of a stroke I will have worst implications in the hull or the rudder control mechanism. I dont need it thanks. I'm looking for facts and maths, not opinions or third part experiencies

Mainly this question is focused to know if there is a standard formula to know what would be new weight , being hollow so balance the difference between that and the former weight. Also if there is any other constrain, regarding the new weight I'm going to set in the stern to unbalance anyhow to sail.
Even if the forces to the sketch where is going to hinge the new and maybe weighed rudder, although I guess it is block,dense and reinforced GPB

Thanks!!
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Old 21-01-2021, 15:07   #2
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Re: Swap my old foam rudder for a INOX steel rudder

I can't help you but have you looked at all the threads on Jeanneau problems?

Rudder Fallen Off
Jeanneau in Pacific | Jeanneau Owners ...

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Old 21-01-2021, 19:16   #3
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Re: Swap my old foam rudder for a INOX steel rudder

Quote:
Originally Posted by coopec43 View Post
I can't help you but have you looked at all the threads on Jeanneau problems?

Rudder Fallen Off Jeanneau in Pacific | Jeanneau Owners ...

jeanneau.proboards.com › General Jeanneau Topics

We follow this couple on YouTube sailing a Jeanneau 45 called Hilma. ... As Malcolm wrote in this thread, the rudder was damaged by ...Dec 22, 2018 · 18 posts · ‎11 authors
RUDDER PROBLEMS | Jeanneau Owners Forum

16 posts 11 May 2013


broken rudder repair | Jeanneau Owners Forum

6 posts 17 Sept 2008


Rudder S.O. 52.2 damaged | Jeanneau Owners Forum

3 posts 21 May 2015


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Hi. Thanks a lot, but as I reviewed all, there is nothing refering Im asking for

BR//
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Old 21-01-2021, 20:39   #4
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Re: Swap my old foam rudder for a INOX steel rudder

Yes, you will have problems...

Your current rudder is designed basically to be (about) neutrally buoyant. Your proposed metal rudder will be very heavy, and when the boat is heeled will constantly be tying to sink, resulting in significant helm force to avoid the boat falling off. It will feel like a serious case of weather helm, but not one that sail trim can fix.

You are replacing a significant volume of foam and fiberglass with metal that is significantly denser. There is nothing you can put back that will weigh less than the foam and restore proper buoyancy. No math is needed to see this. That is not an opinion, it's just a fact.

Your new rudder should weigh the same as your old one, and ideally have its mass distributed in the same fashion. I just don't see anyway for this to happen given the design changes you are suggesting.
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Old 22-01-2021, 01:53   #5
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Re: Swap my old foam rudder for a INOX steel rudder

If you build it using SS tube for the stock with light steel formers and say 2 mm SS outer cladding and seal it it should be sufficiently buoyant to not weigh much when immersed.
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Old 22-01-2021, 08:30   #6
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Re: Swap my old foam rudder for a INOX steel rudder

Hi Combat Diver,

I'm in the middle of a similar project but I'm replacing a steel rudder with another steel rudder. I'm using the existing SS shaft as well. Everything else will be new. I was going to do it in 316 SS but the price was prohibitive, so I'm going with regular carbon steel, properly protected with 2-part epoxies of course. It will last longer than I will as long as I inspect it every few years and touch up as req'd.

I get the impression that you're thinking to design and build it yourself so my advice is based on that.

The earlier comment about weight is very important. Weigh your existing rudder and that's your design goal. If the new rudder is a bit heavier (say 10%) it's no big deal, considering the overall weight of the vessel. I question the earlier comment that a steel rudder is automatically heavier than a GRP rudder. Read on about that.

Since you want math and science (always good) I refer you to the books by Dave Gerr. "Boat Strength for Builders, Designers and Owners" is an excellent resource to help you determine the structural layout and thicknesses of the rudder's ribs and skin. It was selected by the USCG as their primary text on boat construction so that speaks to its utility and veracity. Gerr's other book, "Boat Mechanical Systems Handbook: How to Design, Install and Recognize Proper systems in Boats" has a very complete section on steering systems and rudders that will give you an excellent understanding of the design and engineering options as pertains to your boat. This book is also just an awesome reference in general. Both books available on Amazon.

I used both books extensively to design my new rudder. Now that I know what I didn't know previously I thank the heavens I decided to purchase and read these books. BTW I've nothing to do with Gerr or his publisher, just a satisfied customer.

Rudder shapes done in GRP are not always easily translatable to steel. Be careful there. You may end up with a less effective rudder shape/profile if you don't do your homework. You could also end up with a MORE effective rudder if you read up on your options. Gerr's "Boat Mechanical Systems" will help you there.

As for rudder weight, in order to reduce the weight of my rudder I added as many lightening holes to the ribs as possible. See pic of my rudder design as an example. When done correctly, it does not affect the rudder's strength. This is used extensively in the aeronautical industry where strength-to-weight ratio is even more important. As a result, my new rudder will be considerably lighter than my existing and is a more effective shape and profile, so I'm expecting a marked improvement in performance (my reason for the effort).

I had all the pieces of the rudder cut with a waterjet. There are companies all over North America that can do this for you. You get high precision and huge savings in work. Properly designed, the pieces fit together with holes and tabs like a jigsaw, then just need to be welded together. Of course you can always do it the old fashioned way with drawing the parts out on the steel and cutting with acetylene torch or plasma.

My other advice to you is to have threaded holes installed at the top and bottom of your new steel rudder. This way you can fill it with vegetable oil to prevent corrosion from the inside. Also, once every 5 years, drain the oil and install a compressed air quick-connect to one of the holes - plugging the other - and blow air into it to check for leaks. Best way to ensure your rudder isn't getting compromised.

PM me if you're interested in progress on my rudder. I'm picking up the waterjet cut parts today
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Old 22-01-2021, 09:17   #7
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Re: Swap my old foam rudder for a INOX steel rudder

A problem you did not mention is that most "stainless" Requires oxygen to work. Unless you use a very expensive type you will need tp protect the inside.
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Old 22-01-2021, 10:11   #8
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Re: Swap my old foam rudder for a INOX steel rudder

Quote: "Mainly this question is focused to know if there is a standard formula to know what would be new weight..."

There is no standard FORMULA that I know of. There is, however, a standard PROCEDURE that you would be well advised to embrace.

1) You measure the dimensions of your existing rudder and commit those dimensions to a properly scaled 3-view drawing of the rudder.

2) You make copies of that drawing, but you do NOT draw upon it. You draw upon the copies so you can scrap them when you make mistakes or go up dead ends, as you will.

3) You design the structure of the new rudder and depict that design upon a copy of the 3-view.

4) You list all components of that new rudder as depicted in the 3-view.

5) On the list you note the scantlings (dimensions) of every component part, and calculate the volume of each component part.

6) You look up the "weight of material" (which you can find on line), and calculate the weight of each component part.

7) You total all the weights. That total is now the weight of your new rudder.

Whether your new rudder will be "mass balanced" or not is a separate question entirely.

Whether your new rudder will be dynamically balanced or not is a separate question entirely.

The anticipated "time till failure" of your new rudder is a separate question entirely.

As raahell has pointed out, there are books - dozens of them - dealing with ship design and with mechanical engineering generally. I recommend that you get some of them under your belt before you undertake this project. One important matter when discussing such things a this is to use the relevant jargon with precision. You will be better able to do that after reading some of these books.

May you have success in your endeavours :-)!

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Old 22-01-2021, 10:17   #9
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Re: Swap my old foam rudder for a INOX steel rudder

316 Stainless Steel is widely used for marine applications including encapsulated steel assemblies. A SS rudder is hollow and will be left full of air so it is protected. I would add oil in the case of a carbon steel rudder. sorry for not being clear on that.

In my own rudder project I compared the pros, cons and prices of using 316SS, W44 Carbon Steel and doing one in GRP. In the end, despite my perfectionist tendencies, I couldn't justify the 316SS version as it will considerably outlast the vessel itself, which doesn't make sense. Carbon steel, well protected and inspected at every haul-out, made the most sense for someone with limited funds.

To be precise, the costs of the rudder parts in 316SS were 4x that of W44 carbon steel. Add to that the additional costs and complexities of welding 316SS. In my case I'm using MIG so that means more expensive 316LSi wire and tri-gas mix. The welding environment and tools used also have to be cleaner/better to prevent contamination of the SS weld pools. It just wasn't worth it.
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Old 22-01-2021, 12:37   #10
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Re: Swap my old foam rudder for a INOX steel rudder

Personally I would be concerned about making a rudder that is significantly stronger than the original.

It is one thing to damage the rudder. Not good but usually not a disaster.

If the same impact does not damage the rudder (because it is so much stronger) but, instead, tears out the stern of your boat the situation might be catastrophic.

I know of a Moody 54 that this happened to when it got entangled with a drifting meteorological buoy. The crew survived but the boat was lost.
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Old 22-01-2021, 17:09   #11
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Re: Swap my old foam rudder for a INOX steel rudder

Please keep in mind that one of the things a foamed rudder supplies a a place where water can’t fill. A hollow rudder if punctured will get heavier as it fills with water. Just something to consider
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Old 22-01-2021, 17:18   #12
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Re: Swap my old foam rudder for a INOX steel rudder

There is a variety of incorrect information in some of the above posts. Points to be aware of in designing your metal rudder:

316 (or other common stainless steels) is in many ways an inferior material for this application. Firstly is strength. 316 has a yield strength of 30K PSI. The weakest carbon steel, A36, has a yield strength of 36K PSI. You can use one of the HSLA steels (high strength low alloy) which will have about double the yield strength of 316.

Stainless is a poor choice for welded structures in that it has a thermal expansion about double that of carbon steel. The larger thermal expansion of the stainless means that weld distortion with stainless is significantly greater.

When making your rudder, weld in a small threaded port in the bottom. When welding is complete, connect this port to a compressed air source and pressurize the inside of the rudder. Bubble check to find any leaks in the welds and weld them closed. When the rudder is sealed it is impossible for it to rust internally. The oxygen trapped inside the rudder will allow a very small amount of rusting, after which it will be consumed. From then on there will be no remaining oxygen inside the rudder and it cannot rust any more. When you haul out you can remove the threaded plug in the bottom port to check rudder integrity. If no water comes out the rudder is still sound and safe. Water means that a crack has formed somewhere. Pressurize, weld shut and carry on.

For welding the 316 (or whatever) rudder shaft to the carbon steel rudder use 309L. That is what that alloy is made for.

You can make a steel rudder that will perform very well. This is old, fully developed technology.

Be sure to install mounts for a small zinc.

Good luck on your project.
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Old 22-01-2021, 17:23   #13
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Re: Swap my old foam rudder for a INOX steel rudder

Quote:
Originally Posted by raahell View Post
316 Stainless Steel is widely used for marine applications including encapsulated steel assemblies. A SS rudder is hollow and will be left full of air so it is protected. I would add oil in the case of a carbon steel rudder. sorry for not being clear on that.

In my own rudder project I compared the pros, cons and prices of using 316SS, W44 Carbon Steel and doing one in GRP. In the end, despite my perfectionist tendencies, I couldn't justify the 316SS version as it will considerably outlast the vessel itself, which doesn't make sense. Carbon steel, well protected and inspected at every haul-out, made the most sense for someone with limited funds.

To be precise, the costs of the rudder parts in 316SS were 4x that of W44 carbon steel. Add to that the additional costs and complexities of welding 316SS. In my case I'm using MIG so that means more expensive 316LSi wire and tri-gas mix. The welding environment and tools used also have to be cleaner/better to prevent contamination of the SS weld pools. It just wasn't worth it.

Too late for you but you can get 316L ss flux cored mig wire that only needs CO2 shielding gas. Cheap as chips CO2 gas. You can get as small as 0.8mm dia wire.

Seems to work well but forgotten the name of the Japanese manufacturer
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Old 22-01-2021, 22:00   #14
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Re: Swap my old foam rudder for a INOX steel rudder

The rudder on an SO51 will be about 7 feet long, 2 feet wide, not quite square, but close enuff. It will be about 6 inches thick on the average. Thus the interior volume sans structure will be 7 x 2 x .5 = 7 CuFt. A cubic foot of seawater is said to weigh, as we all know, 64 lbs. Thus an SO51's rudder full of seawater will weigh somewhat less than 7 x 64 = 448 lbs more than the rudder full of air, let's call it 400 lbs. That is the weight of two men my size. A fifteen ton boat doesn't even lift an eyebrow, let along her bows, when two guys move from the forward stateroom to the stern-sheets :-)

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Old 23-01-2021, 00:10   #15
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Re: Swap my old foam rudder for a INOX steel rudder

I am not familiar with your yacht. 2 points to consider:

If you strengthen your rudder to much next time the supporting ship structure might break unless strengthened too

For light weight and strength I built rudders, daggerboards and even the rudder stocks for my cat in carbon & foam
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