Stainless rudder stock needs replacing in foam core rudder?

According to the way I read his post, some where telling him solid was the only way and this is just a lack of engineering understanding, which is why, paying attention to these folks isn't a good idea. You would be incorrect about 99.99% of shafts BTW, by a significant margin. Weight is also a constant consideration, which is why composites and tubular sections have seen such a dramatic rise in recent decades.

 

abcdefg, you say you are unaware of my rudder sizes, perhaps you should read my first post again? Thanks for the tips on building though.

The issue is not the material but the fact it is not solid.

Obviously a solid stock is stronger when you compare stocks of the same diameter. If the tube is made with a much larger diameter as mine is, the size makes up for the fact it is not solid does it not? As far as I am aware, a tube is stronger than a solid round bar of the same weight, so a tube can therefore be made lighter at the same strength if it has a larger diameter. My rudder is NOT high aspect, which means it can have a 3.5" hollow shaft. Obviously if my rudder only allowed a 2" shaft it would have to be solid.

So are you saying I can not get 2205 in a tube? How many 20 year old 40 foot boats had a solid 3.5" 2205 rudder stock?

 

Yes you have given some rough dimensions, but not enough for a detailed engineering analysis.....I am sorry I only skimmed across them, but I did have an idea of the size based on you bunnings sawhorses which I also own. As I said, sticking to dimensions and materials you already have will give you a new version of what you have.

I agree. You have a short stubby rudder that has a 3.5" hollow shaft. Off the bat I can see that it can be hollow, but is the wall thickness adequate? See if you can figure your way through the ORY rule to find out....remember this one has failed...

FYI, I recently saw a rudder I designed get fitted to a larger custom cruising yacht than yours. It is an 88mm solid 2205 stock for all the reasons I listed above...fitted into a cnc machined foam core and glassed up. A hollow stock did not fit the constraints of the job (which I don't plan to discuss). Not a cheap process at the end of the day, but as part of this new build its small peanuts.

Not saying you cannot get it, But I am say you won't get it easily whereas 316/304 will be pretty easy to find. 2205 in a solid bar is pretty easy to get, since it is used for prop shafts all the time. If I had to guess, Id say you are looking at 316...





Moving on, you would be best to start thinking about types of foams to use, laminate overlaps and skin ties etc if you plan to build new.

 

Sometimes tube stocks have an insert in way of the lower bearing areas.... missing the correct term here but makes stronger where it needs, cats we used to build had that, generally around a 60mm x 6mm Sandvic tube in 316, the armature was about 1/2 the depth of the blade in plate with webs including at the base of the tube, the "insert" went from above the bearing by maybe 250-300mm, tillers had bolts through the tube.
Using a cut off disc is low cost to buy & when you find some kind of metal inside, any kind of abrasive cutting implement will convert the kerf into dust.
Jeff.

 

abcdefg, do your saw horses also fall over at the worst time because you lost the wingnuts?

I have no idea on how to go through those rules, or even where to find them. I am relying on the fact that my boat has a CE A rating which means that the designers have already done the calculations and have used the correct size shaft. Just because it has corrosion does not mean it was badly designed.

I sailed this boat from the USA to Brisbane last year with this rudder shaft (passed survey but how much can you really tell without pulling the rudder). And even with this corrosion it did not break. So going by that an exact replica will last another 20 years. This one has NOT failed. It may last another 20 years for all we know, but I am not going to try and prove a point as to how much structural integrity it has left. I will be miles ahead of all the other boats out there with 20 year old corroded shafts whose owners are oblivious to the damage.

Waikikin. I will do some destructive testing of the old shaft to see if it has any solid inserts at the lower bearing. The shaft has been filled with foam so its hard to see whats inside.

 

I agree, there appears nothing wrong with the scantlings for this assembly, shaft included, though it would be helpful to have the make, model and year of this yacht. It's very probable the rudder specs and drawings are still available. All I see is what you see Dennis, a corroded shaft that needs replacing and while you're there, maybe some sectional and plan form updates.

 

I did lose the wingnuts, but they don't fall over any more. I put screws through the plate from the underside... Painfull things.

I thought I might get a reaction with the failure comment. It did fail but not for structural reasons. I have already said build it as it was if you like and you will get a new version of the old. Honestly, I didn't expect you to figure out one of the rules, but I thought you might at least google it.

http://www.eagle.org/eagleExternalP...&Guides/Current/37_OffshoreRacingYachts/Guide


Dennis, I know you know who I am from previous correspondence (please keep that offline - not that it needs to be said) so hopefully you might understand where I am coming from. Not trying to put a spanner in the works, just opening your eyes a little maybe.

 

Thanks everyone.

This is my boat. As you can see the keel is so small, the rudder is very important for performance.



http://sailboatdata.com/viewrecord.asp?class_id=4350

PAR you are right that if I build a new rudder, I want it to have the best possible planform and sections. If I am going to the trouble of building a new one, it should not cost more to build one that has the best shapes possible.

The issue is a good update would be a rudder of higher aspect, but as you see my boat is very shoal draft and I might get 100-200mm before it becomes lower than the keel. So it seems my rudder is pretty good already given the draft limitation. Well from a layman's perspective.

I would love to hear some suggestions on how the planform can be improved upon and if section profiles have had much innovation, but as far as I know everyone still loves the old NACA sections which were already pretty old when my boat was built weren't they?

Thanks for the link Abcdefg, I have it and will try to understand the rulings when I have time.

 

Dennis,

The rudder for your boat is most likely a symmetrical NACA section. You can determine which NACA series by the chord to thickness ratio.

NACA 0012 would be a good place to start. So if you make an outline of the planform of the rudder on a sheet of cheap plywood, then draw yourself some some horizontal lines at an even vertical intervals across the chord of the outline. This gives you the chord lengths for which you can resolve the tapered thickness for using the NACA profile with 12% chord thickness.

The easiest way to resolve the chord thicknesses is to first go here -> http://airfoiltools.com/airfoil/details?airfoil=n0012-il

click on, "send to airfoil plotter"

Then input your chord lengths and it will generate a full size profile for you of the NACA 0012 section.

Once it has generated the foil section of the correct size based upon your chord lengths, click on "CSV file of coordinates" and it will download an excel spreadsheet to your computer which is a simple table of offsets in the X and Y plane from which you can use to mark out and cut a section template (from plywood or MDF etc) for your new rudder.

Repeat this process for each chord length station down the rudder blade. Then you will have a set of section templates ready.

Use these templates to periodically check the section shape as you plane down your foam stock to arrive at a perfectly tapered NACA 0012 section rudder. Do the final shaping with a longboard before glassing.

 

I thought the section would probably be a NACA 0012. Looks about right for it, but I will confirm. Is this still the best section to use? I heard of some new fancy sections which are a compromise of laminar and turbulent, so work in OK in both modes but not 100% in either. Does anyone know much about this? I read it on here somewhere. I know that generally you should not use a laminar flow section for a rudder, or probably laminar flow sections at all on a cruising boat that may have some slime on the rudder/keel.

Here is a good paper by Eric S I have read. Makes me think my keel design is pretty bad with the bulb taking up about 30% of the lateral area.

http://www.sponbergyachtdesign.com/Keel and Rudder Engineering.pdf

 

In that era, designers started to veer away from using 00 series sections in rudders, though admittedly, most still did. Pull a template from the blade and have a look. A 00 series will be quite distinctive, compared to a 63A (for example). Actually, just a picture of it's leading edge will probably tell all.

 

PAR, what would be your section choice and why?

Also, these super stainless alloys like A22 and 2205. Can they be welded to? My friend is a stainless steel worker who I would order the stock from. He would then fabricate my tabbing etc. I would probably try to reuse the old tabbing if it has not been corroding. I wonder about welding issues?

 

What would you hope to gain with a higher aspect ratio? Would you increase the depth of the rudder, decrease the chord, or both?

 

I could live with the plan form and aspect ratio, mostly because of the performance envelop of the Legend 40.5. I would add more radius to the leading edge at the tip, but it's aspect ratio is limited by the fin. In the upper 1/5th of the blade I'd use a lower Reynolds number surface piercing section, such as H-105, then transition to various 00 series sections as the chord changed, going down the blade and lastly I'd consider a 63A, maybe a 64A in the tip, to control eddie making. Before making this decision, I'd want to crunch some numbers, but this is the direction I'd consider.

All this said and again considering the performance of the 40.5, you wouldn't go wrong with an all 00 series section, based on chord. I know a number of 40.5 owners and they all like the boat and it's performance.

 

DCockey. Higher aspect is always going to be more efficient, but as noted I am draft limited. To do it right the rudder would have to be longer to make up for the fact it has a narrower chord, which I can not really do. So going by the draft limitation, the rudder seems to have a pretty decent planform. I am sure Warren Luhrs knew what he was doing, but 20 years have passed.

Thanks PAR, it appears you agree the rudder is pretty spot on in its draft dependant overall shape and just some minor changes in plan form and section might be worth it. Thanks for your letting me know your thoughts on the matter. I will start cutting up the old rudder this week to determine if a new one has to be built.

We are also happy with the performance of the boat besides hard on the wind where we make a lot of leeway. We hope mods like the new rudder and folding prop may help us upwind somewhat. On our cruise from CA to Brisbane, we were always amongst the fastest passage times and our light air performance with our screecher was brilliant. We hardly added any engines hours. Most 40.5 owners love their boat and some people claim its the best hunter ever made. Anyway, its what I have for the moment, and even if I sell it I want it to be seaworthy.