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 Post subject: Fixing a rudder
PostPosted: Tue Mar 30, 2004 10:00 pm 
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...or rather, making a new one. My friend and I are halfway through this project now. Anyone done this before? The goal is to make a suitable replacement for the one which is now apparently at the bottom of the lake.

Here you see my friend and I sailing.
Image



Here you see my friend and his boy.
Image



Here you see the cause for my concern.
Image



We didn't hit anything (that we're aware of) so it might be just that 30-year-old plastic decided to let go while under load? Dunno.



Here you see the new rudder approximately 30% completed below the other rudder we used for a template. That's fake-flower-mounting foam and oak.
Image



Since that last picture we've put fiberglass on half of it. Hopefully by the weekend we'll have glass on the other side and paint on the whole thing so that we can try it this coming weekend. Thoughts? Comments?

Thanks in advance!

Jim


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 Post subject:
PostPosted: Wed Mar 31, 2004 7:33 am 
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Wow! You've done a neat job of building up a new rudder from scratch.

The old rudders have a nasty habit of doing exactly what happened to yours - the sideways forces on the rudders are pretty extreme. I've had at least two or three of them go on me. Hobie "fixed" the problem in the late 70's by introducing Lexan rudders. They didn't break, but they flexed like crazy and you couldn't get a good, smooth finish on them.

Then came EPO rudders - foam core epoxy fiberglass. Far and away the best rudders ever made. Stiff, light and virtually unbreakable. They have a hard finish that would take a high polish. Even though they've been out of production for at least 15 years, they are still highly sought after.

The current racing rudders made by Hobie are nearly as good. My only complaint is that the gel coat finish is soft and scratches / dings easily.

I've seen several hand-built rudders over the years - from beautiful, all wood creations to high-tech all carbon fiber. Each have had their problems.

In looking at your photos, the biggest concern I would have about the way you've constructed your blank is the hard transition between the wood and the foam. That's going to create a shear line, which if it falls on the bias of the fiberglass cloth, will be a significant weak point. (The bias of the cloth is diagonal to the weave - it is the weakest part of fiberglass.)

Going back to my college statics engineering, the rudder can be simply modeled as a cantilever beam with a uniform load:
Image
Nomenclature:
R = Reaction force
V = Shear force
w = Distributed loading force
M = Moment
l = beam length
x = location
E = Modulus of elasticity
I = Moment if Inertia
y = deflection

Relationships:
--------------
R1 = w * l
M1 = -(w * l^2)/2
V = w * (l - x)
M = -(w / 2) * (l - x)^2
y = (w * x^2) / (24 * E * I) * (4 * l * x - x^2 - 6 * l^2)
ymax = -(w * l^4) / (8 * E * I) at x = l


As you can see from the graphs, the moment and shear are at their maximum at the base (left side) of the cantilever. That's why your old rudder broke off just below the casting.

In reality, it's more complicated. The load is not uniform; the beam is not uniform; and the three dimentional effect of twisting is not taken into account.

Good luck on your rudder! Post photos when you are done - let us know how much it weighs. Give us a performance review - who knows? You may have come up with the latest and greatest new design!

Matt Bounds
(one-time naval architect)


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 Post subject:
PostPosted: Wed Mar 31, 2004 5:05 pm 
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MBounds wrote:
The old rudders have a nasty habit of doing exactly what happened to yours - the sideways forces on the rudders are pretty extreme. I've had at least two or three of them go on me.

Well at least now I know it's not uncommon. 8)

Quote:
Then came EPO rudders - foam core epoxy fiberglass. Far and away the best rudders ever made. Stiff, light and virtually unbreakable. They have a hard finish that would take a high polish. Even though they've been out of production for at least 15 years, they are still highly sought after.

Sweet. Sounds like our design. Kinda.

Quote:
In looking at your photos, the biggest concern I would have about the way you've constructed your blank is the hard transition between the wood and the foam. That's going to create a shear line, which if it falls on the bias of the fiberglass cloth, will be a significant weak point. (The bias of the cloth is diagonal to the weave - it is the weakest part of fiberglass.)

That's kind of what we figured. We used the oak to make a stronger area underneath the rudder hardware, and the large-pore foam because the epoxy would be less likely to delaminate with the big pores. We're putting four layers of glass on either side of the rudder, plus a strip all the way down each side over the thickest part of the foil, and a piece the same shape as the oak but slightly larger over the oak/foam transition, alternating layers of bidirecitonal and unidirectional glass. Hopefully that'll make it sufficiently strong. 8)

Quote:
Good luck on your rudder! Post photos when you are done - let us know how much it weighs. Give us a performance review - who knows? You may have come up with the latest and greatest new design!

I certainly *will* post pictures when we're done, although as a still-inexperienced Hobie sailor, I'm afraid my "performance review" will be anticlimactic for you all.

Jim


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 Post subject:
PostPosted: Fri Apr 02, 2004 7:01 am 
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A little progress update:

The glass on the first side looks really good. Last night we gouged out foam around the edges and melted 3/4lb. of lead for the tip of the rudder so that it'll be easier to put down. After throroughly sanding the foam out of the gouges, we filled them with flox and glued in the lead piece. The flox around the edges is to give protection against hitting stuff since the flox will be much stronger than foam.


Image



Then we spent two hours laying up the glass on that side. For some reason we had almost no problem with bubbles on the first side but this side was a pain. Anyway finally we got them out to our satisfaction.

Image



Tonite we'll sand and fill the weave in the glass and tomorrow morning we'll paint it. My friend said "I'm gonna be really pissed if we go out sailing and right away it goes SNAP. Oops maybe we shoulda put two more layers of glass..." Image


Last edited by JaimeZXv.2 on Sun May 23, 2004 3:13 pm, edited 2 times in total.

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 Post subject:
PostPosted: Fri Apr 02, 2004 7:47 am 
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Putting lead in the tip is, at best, unusual.

Most of us hard-core racers try to avoid high density metals at all costs! :shock:

The EPO rudders I mentioned in my previous post only weigh 4.5 lb each. To the best of my knowledge, nobody has ever had a problem putting them down. They have a foam core along the entire length of the blade, including the head.

I think you've come up with a solution for a problem that doesn't exist, but hey - they're original!

I'm still waiting to see what happens at that foam / wood joint. Unidirectional carbon or glass running the length of the rudder would be most effective in controlling flex.

BTW, how did you produce your foil section? Are you using templates or just eyeballing it? Your rudder could have some nasty high-speed characteristics (ventilation, stalling) if you don't get the foil section right.


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 Post subject:
PostPosted: Fri Apr 02, 2004 4:44 pm 
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MBounds wrote:
Putting lead in the tip is, at best, unusual.
Most of us hard-core racers try to avoid high density metals at all costs! :shock:
The EPO rudders I mentioned in my previous post only weigh 4.5 lb each. To the best of my knowledge, nobody has ever had a problem putting them down. They have a foam core along the entire length of the blade, including the head.

Hm. Well... the PVC (or whatever it is) rudder weighed like, 6.5 lbs, and the foam/oak setup without glass only weighed 3 something. We were concerned without some weight at the bottom they wouldn't go down. lol

Quote:
I think you've come up with a solution for a problem that doesn't exist, but hey - they're original!
Alas! Oh well. As you say, I have a custom rudder. heheh

Quote:
I'm still waiting to see what happens at that foam / wood joint.
Unidirectional carbon or glass running the length of the rudder would be most effective in controlling flex.
Yeah... In order (inside to outside) our layers go
* Strip of uni down the thickest part of the foil
* Full coverage bid
* Full coverage uni
* Full coverage uni
* Full coverage bid
* Bid over oak/foam joint

Hopefully that'll be sufficient.

Quote:
BTW, how did you produce your foil section? Are you using templates or just eyeballing it? Your rudder could have some nasty high-speed characteristics (ventilation, stalling) if you don't get the foil section right.

Nope! Measured the other rudder VERY carefully in lots of places and reproduced it in AutoCAD. Then printed cross-sections, glued 'em to aluminum sheets, cut 'em out, tacked 'em to the foam, and hot-wired the foil. Pics:

Us hot-wiring the foam:
Image


Me holding the completed cut:
Image


Whatcha think?


Last edited by JaimeZXv.2 on Sun May 23, 2004 3:11 pm, edited 1 time in total.

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 Post subject:
PostPosted: Fri Apr 02, 2004 6:54 pm 
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Kewl! 8)

The only thing you didn't do was vacuum bag it!

I just weighed the EPO rudders off my 17 - 4 lb 5.25 oz

The racing rudders off my 16 pegged the scale at 5 lb 11oz (it's a 5 lb postage scale)

The EPO's feel light as a feather. They're staying behind when I sell my 17.

When I have the time, I'll pull some sections off both rudders to see what, if any difference in foil is between the two.


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 Post subject:
PostPosted: Fri Apr 02, 2004 11:23 pm 
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Hello, I'm JaimeZX's friend

We didn't vacuum bag it because:

A. It is a pain in the butt and is more likely to produce poor results if not done exactly right.

B. We weren't going for extreme light weight. We even put lead in it so vacuum bagging would be toatally counter productive. In fact, we intentionally built this rudder really heavy, i.e. lots of epoxy, flox leading and trailing edges, lots of glass. I'm used to building airplane parts and believe me, this thing is heavy.

Even with .75 lb of lead in it the new rudder weighs approx. 4.5 lbs with both sides glassed. I don't expect it to weigh much, if any more when complete. Despite this it is a lot stiffer than the old rudder.

If I were going for light weight, I could probably build one of these under 2.5 lbs. I didn't want to do this, however, because I was afraid the rudder would be too bouyant and hard to lock down. I've had problems with this with my monohull before. It sounds to me like Hobie sailors haven't had this problem before because even the EPO rudders are really heavy.

Structural integrity has yet to be proven but the rudder is pretty dang solid. I thought about the wood/foam transition problem when I designed it. That's why the wood ends in an angle instead of a perpendicular line. It tapers to nothing in a transition to the foam. At any rate the glass is providing the primary structural strength against bending loads, not the foam/wood. The foam and wood are simply providing a form to lay the glass on and strength against crushing, especially in the area where the rudder is attached to the boat. I am anxious to try this rudder out in some stiff wind. I hope we don't find out the hard way why the Hobie EPO rudders weight so much. I don't really think we will have a problem though.

The foil shape was an approximation of the one from the existing rudder. I just took a few measurements and fit a curve to it. It should be fairly close to the original and I doubt that it is that critical. I've seen lots of boat rudders that are nothing more than a board with rounded edges so this should be more than fine. I would be interested to see the foil secitons of your rudders though.


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 Post subject:
PostPosted: Sun Apr 04, 2004 6:13 pm 
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Update for y'all:

As of yesterday morning, the new rudder looked like this, next to the old one:
Image


Yesterday we used micro to fill the weave so that we could sand it smooth without going into the glass:

Filling the weave:
Image


That cured overnight and this morning my friend put another layer of micro on in places where it needed help. We sanded it down to 320-grit smoothness:

Image


After that we drilled the holes (one size larger than the bolts) and mixed up some more epoxy to swab inside the holes with a Q-tip to seal them. We made sure to swab a lot inside. Then when it was cured enough to be stiff (not gooey) I pushed the bolts in and out a few times to make the holes just the right size.



Finally, just about an hour ago, we sprayed one side with "Appliance epoxy" which is basically some kind of white spray paint that smells funky that I guess you use on washers & stuff. I chose it because it said "Resists moisture, scratches, and chips." It also said "not for continuous immersion" but I think it should be okay for a few hours on the weekend. We'll see. :)


The different colors to the white here are due to the sun and the halogen garage lights being different "whites."

Image


I was hoping to try it out today but we just didn't have time. Next week though... if the weather cooperates. 8)


Last edited by JaimeZXv.2 on Sun May 23, 2004 3:12 pm, edited 1 time in total.

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 Post subject: Wow
PostPosted: Wed Apr 07, 2004 1:32 am 
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What a huge amount of effort you guys have gone to to create this rudder. I'm impressed! I hope that after all of the hours that you must have spent on it that it works well and doesn't go the same way as the original one. Have you tried it yet?


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 Post subject: Re: Wow
PostPosted: Wed Apr 07, 2004 5:46 am 
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schobiedoo wrote:
What a huge amount of effort you guys have gone to to create this rudder. I'm impressed! I hope that after all of the hours that you must have spent on it that it works well and doesn't go the same way as the original one. Have you tried it yet?

Not yet. Hopefully this weekend, but weather.com is calling for rain. 8/
It's definately stronger than the other stock rudder. I just hope the *other* stock rudder doesn't let go now. heheh


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 Post subject: UPDATE
PostPosted: Sun May 23, 2004 3:17 pm 
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There was FINALLY enough wind to put the new rudder to a fair test. We flew the hull a bit, but unfortunately the wind was too gusty (10 gusting to 16) and the lake too small to hold it up for more than a couple seconds at a time. Still, The rudder performed like a champ! I'm thrilled. :D


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