Stainless rudder stock needs replacing in foam core rudder?

With Hunter's insistence on these shoal contrivances in their shorter fins, you can expect a bit more leeward slip, compared to a clean, deeper and higher aspect fin. The rudder shapes Hunter used in this general era are pretty consistent with the thinking at the time. Have a look at the Hunter 41 (2006) and the 39 (2009) and you'll see the latest generation of their rudder and what I'm talking about in regard to changes to it's plan form. You'll also note they've changed their approach to bulbs and winglets. These are Henderson's versions and I think much better and your plan form would do well if shaped like the 36-2 (2008) rudder.

As to sectional shapes, you can drive yourself quite nuts with all the decisions and foil offerings. Because the top of your rudder is exposed, I'd be inclined to use a laminar section in that area, but the bulk of the foil can be a 00 series, with the tip again (whee the leading edge curve begins) employing a laminar section to control eddie making.

 

Thanks again PAR.

I know the profile of the boat shows the rudder slightly aerated, but in practice I don't think any part of it comes out of the water often. Even at rest it is fully under, but as soon as the boat gets any sort of speed up the stern squats quite a bit.

Is this what you are thinking of? Looks to be a nice shape.



Its hard to find a good pic but the 36 looks to be more tapered?

 

The 36 (shoal) is more tapered, but the leading edge radius is what I wanted you to focus on. Your trailing edge is fine. That second photo clearly shows a 00 series foil all the way down. Yes, I assume the top of the blade will be immersed, but it will be in turbulent water, so the laminar sections (there) will reduce drag.

 

I appreciate the input from everyone in this thread. It was good chatting to you today abcdefg. I am viewing this as a learning experience and the info on foils is very interesting to me. I used to fly RC gliders so this is right up my alley.

PAR, you mentioned that I should use the laminar flow foil at the top because the flow will be turbulent there? This seems to go against what I thought I knew regarding laminar flow foils. I thought that they only worked in very smooth flows and had to be very fair and clean to work and once the flow was turbulent the performance was worse than a non laminar flow section? Do you have any suggestions for articles that I might search to learn more about this?

 

Oh my, this gets complex really quick. I've recommended the H-105 for the top of the foil, because it operates at relatively low Reynolds numbers, while other types of "laminar" flow sections, such as an Eppler E817 or NACA 63-209, which are a typical laminar flow shapes, need considerably higher Reynolds numbers to be effective. In fact, H-105 was designed to offer low drag and minimal cavitation characteristics, while still showing better lift then the previously mentioned sections. There are other choices and a Reynolds number study would be wise before selecting a section in this area.

 

It's a bit of a wonder that the only problem with the rudder develops right at the weakest point, and develops on it's own, i.e. not because it's under stress, but for some other reason. It makes me slightly wonder if there might be a Cosmic Joker.

What's the possibility of rigging a fix, such as putting a short, inside sleeve at that most vulnerable point?

Maybe running a hone up and down the inside of the shaft and getting it cleaned up to a uniform ID, then have a stainless pipe insert maybe 12" long of the correct OD. Heat up the rudder shaft, cool down the sleeve insert and then drive or press it into place.

Before you honed up the shaft, you could drill a few 1/4" holes a few inches above the corroded area. Then after the insert is in place you could weld up those holes, which would secure the insert in place. At the same time you would fill in the crevice corrosion with weld and then smooth it back down to the original diameter.

 

I will be in need of new rudder. No fix will be employed.

We spent a full day trying to separate the halves without destroying the thing. But it was bomb proof. This rudder was ridiculously strong. Seems it was made by inserting the shaft and tabbing into a mould then filling it with a dense pour foam. After being released from the mould the foam was sheathed by a thin layer of mostly CSM and one light layer of roving.

The original 87mm shaft joins to a non stainless smaller shaft and the tabbing was a V shape which is why we could not separate it. The V filled with foam making it impossible. You can see how well we sawed it by how flat the remaining half is. Even though the secondary shaft and tabbing was not stainless steel it was not rusted at all due to the pour foam. There is rust in the pics but the pic was taken a few days later after it had rained on it. I am assuming I will need to make my new tabbing all out of stainless as I will not be using the pour foam method. The other half which is missing is smashed into small peices.

 

Here is the shape I traced out of the foil. I thought it was a NACA 0014. Both the root and tip calculate out to be very close to 14%. Now was 14% chosen because 14% is the best thickness or because the usual rudder stock on these boats was made of fiberglass and they needed it to be this thick so the shaft could be made strong enough? Seems like 14% is much thicker than what is usual on a boat like this no?

Root 840mm thickness 120mm = 14.28%
Tip 615mm thickness 88 = 14.3%

But when I put a 0014 section up against the outline, my foil appears to be narrower at the leading edge, so I am not sure if it is a 0014 after all. Seems it might be a 63-014

From my limited understanding of airfoils, the laminar flow 63 series have much less drag than the 00 series when in the "bucket". But they are more prone to stall probs etc, which can be lessened by making them thicker. Now by attempting to understand the engineering design thoughts of my rudder, it seems logical that if you are forced to design a thick foil due to using a fiberglass shaft maybe you may be able use a 63 series which may not have been such a good idea on a thinner foil. Since it is thicker it should not stall as easy, but it should still have low drag due to its nature and offset the thickness required by a glass shaft?

 

So, except for that small patch of crevice corrosion, the rudder was OK?

******* Cosmic Joker.

 

Yeah, well I never thought much else was wrong with it. The thing is would you put the rudder back on with that sort of damage then take your girlfriend around the pacific with it? One Hunter and 2 Catalinas apparently lost their rudders recently. My friend had similar corrosion and when he cut the shaft apart it was much worse inside.

Anyhow now I need to decide on the design of the shaft and tabbing system since I can not use the old stuff.

Do you think staying with the 63-14.3% section is a good idea? If I go to a smaller solid shaft can I use a thinner 00 section? Is there even much point to that. Can I just get bigger rudder bearings to make up the difference? (they are just plastic bush type so should be cheap). But then there will be an issue with the quadrant and I will need a bush made up so it fits, but I guess that can just be made up along with the new bearings.

abcdefg I took measurements of the rudder but I think I am missing a few vital ones in there. I will post a pic. Let me know if you need anything else to run the shaft size calc.

 

At what point of the rudder's span was this obvious 63 series section pulled? Can you take off a template of the section, at midway down the blade?

 

I take it you agree it is a 63? It was at the root (hull). Hence the hole I made in the fiber cement sheet to slide it over the shaft

All I noted was the section was 14% all the way down and the foils did not appear to change. I did not make any other templates as the tip seemed too curved off to get an accurate template by simple tracing. However I do have half an intact blade and I should be able to come up with something. I think it will be a 63.

 

Here are some measurements. I may be missing one or 2. Let me know if I need to make any more. I drew it in sketch up and it said the area was 1.07m2

 

Here is a good thread on rudder foils where Tom Speer says that there are much better options than the 63 series.

http://www.boatdesign.net/forums/sailboats/rudder-daggerboard-profiles-937.html

 

Well, no, I wouldn't put it back on like that. There were some girlfriends that I wouldn't have minded sending out into the Pacific with a rudder like that, but I wouldn't have wanted to go with them.

And now that I think about it, my idea probably isn't a very good one. If the shaft weakened, cracked and separated around that area, it would just slip off the insert and leave a useless stub in the water.

As far as the shape, will it make much difference on a stock boat like that? I'd worry more about upsetting the balance if what you had was good. Does changing the shape change the balance?

If you change to a solid shaft and add weight, make sure the rudder supports can handle the extra weight.