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Old 22-09-2015, 13:07   #1
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Changing Rudder Design

DON'T FLAME ME!

Have no plans to change....don't need to know how stupid the question is... don't need to be called crazy... just satisfying curiosity.

All Cabo Rico's are built with solid fiberglass rudders with 1.5" aquamet steel. They are extremely heavy barn door rudders.

Is there any need to stick with the same exact super heavy rudder? If we found ourselves needing to rebuild our rudder at some way off land in the future is there any advantage to building a lighter rudder or changing the dimensions? Basically modernizing the rudder.
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Old 23-09-2015, 07:18   #2
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Re: changing rudder design

Compare with other designs; do not make it so light that it will float when heeling.

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Old 23-09-2015, 10:24   #3
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Re: changing rudder design

Not a stupid question. Group of Cal40 owners had Schumaker (?) design them a new rudder. Better balanced and less drag. Fits on the Cal36's too.


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Old 23-09-2015, 10:44   #4
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Re: Changing Rudder Design

You've got a full keel boat with attached rudder. A barn door just about describes what the underwater hull design allows. You can make a lighter rudder out of foam and glass but it's not going to work any better than the current rudder in turning the boat unless you make it bigger in width. That may take more force to move it which could mean a total steering system redesign with more negatives than benefits.. Redoing the stock rudder may take some weight out of the stern but that's about the only advantage.

If you have a separate rudder then design can make a big difference. A spade rudder can benefit from a better air/water foil shape, moving the shaft along the cord of the rudder and making it narrower/deeper and/or fatter/ shallower. Lot's of spade rudder boats have benefited from rudder redesigns like Hobie 33, Yankee/Catatlina 38, etc. The original designer's rudder configuration can sometimes benefit from different/newer/better design.
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Old 23-09-2015, 10:46   #5
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Re: Changing Rudder Design

No advantage really, The solid rudder weighs less in water than out. You could fill a new one with epoxy and lightweight filler, but you are probably not talking a lot of weight change.
As far as shape, it depends on how the boat handles now and the current shape of the rudder. How much weather helm? etc. I upgraded a boat I had from the old style "crescent moon" rudder to the newer large rudder and shape once. (the manufacturer/Bill Garden had redesigned it over the years) I found minimal difference actually.
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Old 23-09-2015, 12:35   #6
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Re: Changing Rudder Design

A foam core rudder, even a barn door rudder has advantages over a solid glass one. 1. they're lighter which mean less stress on the rudder gear, should also be lighter to steer with. you still dont have the advantages of a semi-balanced rudder. but its less mass to move. but harder to retain waterproof.
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Old 23-09-2015, 21:38   #7
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Re: Changing Rudder Design

When heeled, the weight of an unbalanced rudder may actually be (through the benevolent forces of gravity) helping counteract weather helm. Making it lighter may make the helm "heavier" under those conditions!

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Old 23-09-2015, 22:11   #8
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Re: Changing Rudder Design

Technically speaking, you could put together a rudder which has a better foil shape. Which would yield you less drag. But in terms of quantifying such, it'd be fairly small, especially when put up against that of your given your hull & keel shape. AKA, you're not going to add 5 or 10 miles/day on a passage, or anything near there to. The gains at most, would be at the bottom of the single digit range, if that.

Such a foil shape would also be a touch more responsive, but as already stated, that could also translate into being more prone to weather helm.

By going with different materials, yes, you could lose a fair bit of weight. Which, in the ends of any boat, is a plus. As it helps to reduce pitching.

Also, FYI, there are other, better options than foam core. Especially as, over time, historically, most rudders built thusly, develop leaks, & then the degradation from within process begins.
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Old 27-09-2015, 16:35   #9
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Re: Changing Rudder Design

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Also, FYI, there are other, better options than foam core. Especially as, over time, historically, most rudders built thusly, develop leaks, & then the degradation from within process begins.
What do you have in mind?
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Old 27-09-2015, 16:58   #10
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Re: Changing Rudder Design

In my own case I have a Columbia 29 with a solid rudder, and the main competitor, back in the day, was the Triton with its wooden rudder. Old Tritons need new rudders, I don't. I agree with others that the weight is not necessarily a liability. Changing the shape may be tempting but adding area to the rudder (if that was your thought) will add more stress to all parts involved and strengthening will be needed. I don't think your question is crazy but I just happen to think your rudder is fine as heavy as it is.
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Old 28-09-2015, 10:04   #11
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Re: Changing Rudder Design

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What do you have in mind?
I suppose if you are worried about weight of solid glass or glass with epoxy lightweight slurry filling, you could build one with a western red cedar core. Cedar fenceposts in the ground up here in rainy wet Washington last 50 years or more unprotected. Super lightweight to.
You need old growth cedar though, todays small cedar is not as good.
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Old 28-09-2015, 10:48   #12
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Re: Changing Rudder Design

Not a stupid question at all! I had some water ingress into my rudder a couple of years ago. I'm not sure what the stuff is that it was made from but it was very solid and very heavy. I ended up building a new one out of closed-cell airex foam and glass and i'm very pleased with it. It's much lighter than the original and gives better feedback. Mine is skeg-hung. With a spade rudder many of them are built to be positively buoyant, which is kind of cool!
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Old 28-09-2015, 13:23   #13
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Re: Changing Rudder Design

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What do you have in mind?
A while back, in a couple of different threads, I think it was Nicholson58 who chimed in with some great thoughts on, & a recipe for fillers for rudders & other items/areas which have a realtively high volume of fill. But need it to be both high strength, as well as fully waterproof.
One or two of said threads was specifically on rebuilding rudders, & the other I believe, was on what to back fill the core of a deck with. When beefing up it's core, in order to mount highly loaded jib tracks.
That one, specifically, was when Dockhead was trying to figure out how to mount conventional, new tracks, for his new headsails.

IIRC, the mix was a very slow curing epoxy & microballoons. With a ratio of somewhere around 2 gallons of resin to 3-5 buckets of microballons. But it'd need looking up/contacting him in order to get it right. Especially as with the mixture & resin which he uses, it allows for a thick pour (several inches deep) without any exothermic problems. A key thing.
And it cures up, hard, & watertight.

One might, in addition to using a core such as this, bond glass cloth to as much of the surface area of the rudder shaft as is possible. Using standard practices for such. As when done right, not only is the glass, chemically bonded to the shaft, it's bonded to it in several mechanical ways also.
- Via the mechanical abrasions put onto the shaft, during sanding, as part of the standard bonding process. Which creates a "toothed" surface on the shaft for the glass to grip onto. No too much unlike Velcro.
- And via the contraction of the glass & epoxy, as the epoxy shrinks slightly when it cures. Thus locking it in place.

If this is done, then anything, subsequently attached to these skins on the shaft, have a far higher chance of staying well attached. In addition to being bonded on in a leak free manner.

One of the other critical things, which is (sadly) often missing in production rudders, is a structural frame work made out of materials which are more corrosion resistant than standard stainless.
And components that, structurally speaking, are mechanically fitted & locked together. So as to not be dependent upon welds in order to remain attached to the shaft. And to stay on there in the shape of a blade. Or a structure akin there to.

For example, taking the shaft, & machining slots into it for plates which fully protrude through it on both sides. And having the slots cut with a taper to them, so that the plates can only move in one direction. Regardless of how they're attached.
Also, in such a design, the ends of the slots in the shaft would be rounded, in order to avoid stress concentrations.
After which, the plates would be welded to the shaft, & the welds fully passivated.

Basically, I'm just advocating for having redundant systems in a rudder that, each, are designed to keep it together. Not too tough a thing, from an engineering perspective, but... in the real world, not necessarily all that common either.
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