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Busted Rudder


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Hi All-

 

A quick PSA to report on the failure of a rudder blade on the CS20.3.  I was sailing Chessie (CS 20 Mk. 3 Sail # 4) this weekend in Raritan Bay, out of the Keyport Municipal Boat Ramp.  Tacking gently in about 5 kts wind, there was a soft snapping sound, steering wasn't so good, and we saw a little bit of the boat floating out backwards behind us.

 

We used the motor, picked up the part, and got back home no problem.  Took some pictures to illustrate the breaking point.

 

thanks,

Erik

 

 

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That is always alarming having pieces of boat float off. It sounds like you handled the situation well. To my eye it looks like the outer veneer of the plywood is very thin. It is that outer veneer running vertically that gives the rudder its strength. Some cheaper plys make the outer layer quite thin, or it may have been sanded thin. It should not have failed so readily. You could build a new blade out of solid wood as Steve suggests  or you could sand and add some glass.

 

Could you check to see how thick the outer veneer is?

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Erik, I am sorry to see that. All of our plywood rudders are meant to be fiberglassed with 8-10oz fiberglass cloth. Plywood is used for the rudders for the kit boats because it is dimensional stable and saves a lot of time in construction but half of the grain runs the wrong way as compared to a laminated solid wood rudder built in the same way as the centerboard. To compensate for this the plywood rudders should be glassed on both sides. 

 

I would not attempt to repair that rudder. Instead I would simply build a new one. One challenge may be that glassing the new rudder will make it too thick to fit into the cheeks unless there is sufficient gap already. We would be glad to send you a CNC cut rudder blank. Please feel free to contact me. bandbkitboats@gmail.com or 252-631-0015 if you need any help at all.

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Plywood is a wonderful material that is better than lumber in many applications.  A cantilever that is loaded across the thickness like a foil is not one of them.  I started sailing life wit both a plywood daggerboard and a plywood rudder.   The DB failed while righting a capsized boat and the rudder was replaces by a more stable solid wood one.  Many plywood foils do not fail but none are as stable as a laminated solid one would be.  I suggest you make a new one in teh manner that C&B recommends.

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Hi Alan,  experience is usually the best teacher.  Actually I've had no problem with well sheathed plywood rudders on small boats although Joe shows what can happen when an adequate sheath might have prevented it. 

 

We did have a glass reinforced foam rudder shear off approaching Ocracoke is a strong wind in a Grand Slam 26 footer though.  It was the same mode of failure as well.  The glass sheath was inadequately strong and was rebuilt much stronger than the original one by S2. 

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I chose 3/4" ply for the Core Sound rudders because being low aspect foils for shallow draft they had a pretty good cross section area. I did intend for them to be glassed.

 

I don't have the numbers in front of me but I recall that there are at least 500 of Core Sounds out there with ply rudders not to mention 1400 Spindrift's and quite a few Bay River Skiffs. My memory is slightly less than perfect but I recall that Chick built two Core Sounds with ply rudders and I recall that Tom used a ply rudder on loon.

 

Laminated solid rudders are clearly stronger than ply. Steel rudders are stronger still.

 

My solid wood rudder built in 1991 is still going strong in Skimmer but it is glassed. My glassed ply rudder is still fine on Carlita. 

 

I would not be offended if Erik replaces his rudder with a solid wood one but glass it.

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That rudder was on “Chessie,” which I sold to Erik on July 10.  Learning of its failure troubled me not just because I had sold a defective product, but because I wanted a better understanding of the cause of failure.  I just didn’t believe that failure to apply fiberglass and epoxy was the cause.  I kept thinking about it over a day or so — and then I remembered an event that may have been a probable cause.  I’ll describe it here along with my analysis.  First a diagram for reference:
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On Tuesday, June 9, with Chessie fully loaded and prepared for an overnight cruise on the West River — the pickup, trailer, and Chessie were parked at the curb in front of our house.  An Amazon delivery truck passed [heading opposite direction] and apparently [not seen by me] pulled in behind the trailer as shown in the sketch.  The driver approached me [sitting in the pickup] wanting to hand me a package.  I told him to place it up on the porch of our house — which he did.  Then, in order to pull from the curb, I needed to back up a few feet, and while slowly doing so [and not knowing he was parked at an angle closely behind the trailer] I heard soft sounds of collision.  Immediately I stopped and got out to inspect — finding the situation as shown in the sketch [except that I had pulled forward a few feet].
 
There was a deep scratch on the side of the Amazon truck caused by the aft end of the rubrail — and the rudder/tiller assembly was displaced a few inches (it had been set on the centerline with the tiller clutch engaged).  There was no damage to the rubrail but the boat’s keel had been moved an inch or so off the center of the most aft roller.  I removed the rudder cover, and there was no apparent damage to the rudder, although I did not make a close inspection.  I released the tiller clutch and re-centered the tiller/rudder assembly — and proceeded to the launch site on the West River.  The launch, sail and motoring (appx 10 nm), and recovery all occurred without notice of any problems with the rudder, its raising/lowering or effectiveness in operation.  Chessie was trailered and sailed on one other occasion in June as well as a demo-sail on July 10 — all without notice of any problems with the tiller/rudder assembly.
 
My analysis is that the rudder (held tightly in position by the tiller clutch) — was stressed to the point of significant damage, but not showing displacement.  Then, by ordinary usage, during the overnight cruise, a day-sail, the demo-sail on July 10, and the sail in New Jersey— the flexing of the rudder (side-to-side) in regular usage, further weakened the rudder to the point of complete failure.  If the tiller clutch had not been engaged, there would have been no damage.  If the rudder had been glazed, the rudder wouldn’t have been damaged, but the forces on it might well have transferred to the cheeks, tiller, and the gudgeons or pintles — damaging some or all.
 
It is my opinion that an unglazed rudder made of 18mm marine plywood would hold up well under all the limits of normal usage.
 
Pete McCrary
The embarrassed builder of the broken rudder.
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That explanation (about which you have nothing to apologize for, pete) makes a lot more sense than plywood snapping from sailing too much.  Like a lot of us, I suspect,  I went and looked over my rudder.  I'm pretty sure I didnt glass it, but there is absolutely no sign of stress.  Plus the pressure on the tiller is so light, I dont know how a "stress fracture" could arise anyway.  All very informative, tho.  And nicely handled by all involved. 

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On a recent trip with our CS20.3 we stopped at a interstate rest area,  walked the dogs, etc,, I then walked around the boat checking everything over, all fine.  I have quite restricted view to the rear with the truck camper loaded so I am double vigilant on the walk around. In the time I completed my walk around and got belted in a young man in a spiffy RED jeep pulled in as close as reasonable.  I had to reverse a foot or two to turn into the rest area exit lane,  well my rudder blade went between the plastic fender and the metal body of the Jeep. It unzipped about 15 of those plastic rivet looking things!  Almost removed his left front fender.   I was soooo embarrassed,  fortunately he was a nice guy and admitted partial fault which helped my bruised ego...   $20 bucks to the Jeeper and a good inspection of Southern Express and back on the road.
 

Think I will go look it over again.....

 

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Thanks to all for the suggestions.  I'm going to construct a new rudder blade and glass it, just to be sure.

 

@Alan Stewart, as you suggested, since the current rudder blade fits snugly in the rudder assembly, I think just glassing the blank as-is would make it too thick.  I wonder if the right thing to do would be to sand the rudder blade down (or possibly, run it through the thickness planer?) before applying the fiberglass.  Do you happen to know, offhand, how much thickness a layer of fiberglass cloth + epoxy would add?  I would guess somewhere between 1/16 and 1/8".

 

My thinking is that, if I want the rudder blade to be 3/4" thick, and each layer of fiberglass is X", I would plane or sand the blade down to 3/4-(2 * X) before glassing it.

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My guess is a lot less.  Take a scrap piece of ply, mic it, and do one side — mic it again.    Then you’ll know half the add’l thickness.  But I wouldn’t bother.  As Paul says, the helm is quite light and In normal usage the stresses are too.  And if a glazed and much stronger rudder is over stressed by encounters with the bottom, rocks, docks, other boats, or highway traffic — the unyielding rudder could over stress other parts of the rudder/tiller assembly.  Easier to replace just the damaged rudder.

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3 hours ago, NowWeTryItMyWay said:

Thanks to all for the suggestions.  I'm going to construct a new rudder blade and glass it, just to be sure.

 

@Alan Stewart, as you suggested, since the current rudder blade fits snugly in the rudder assembly, I think just glassing the blank as-is would make it too thick.  I wonder if the right thing to do would be to sand the rudder blade down (or possibly, run it through the thickness planer?) before applying the fiberglass.  Do you happen to know, offhand, how much thickness a layer of fiberglass cloth + epoxy would add?  I would guess somewhere between 1/16 and 1/8".

 

My thinking is that, if I want the rudder blade to be 3/4" thick, and each layer of fiberglass is X", I would plane or sand the blade down to 3/4-(2 * X) before glassing it.

 

A single layer of glass is no where near 1/16", here is a handy table from the WEST Systems epoxy folks, laminate thickness

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Q: Is the plywood rudder strong enough?

A: Yes* 


This morning we have some empirical test results to share. We just cut out Erik's new rudder today and we took the opportunity to look at the 18mm meranti plywood we use for the CNC cut rudder blades. Pictures attached show a test using our lovely assistant Nate who clocks in at 240lbs today. The test piece is a virgin piece of 18mm meranti marine plywood, 8 5/8" wide and the thickness is 0.707" (the closest handy piece of scrap). At 15" from the supported edge of our deck the ply showed roughly 2" deflection and no audible complaint. i.e. no cracking sounds. A slight amount of permanent deflection (about 1/16" was noticed after the test). Note that this width of full thickness ply (and then some) are present in the CNC cut rudder for exactly that reason. The outer veneer running vertical along the blade provides a lot of strength to the ply and I suspect that it would have broken during our test if we had scored through it with a razor blade. 

 

Ok, some calculations....Using the dimensions of the scrap I calculate the maximum bending stress to be approximately 5,000psi during the test. Meranti is listed at 12,700psi. I suspect the plywood however to be about half the strength of a piece of solid meranti for obvious reasons. 

 

*  While 300 ft*lbs is many times more bending moment than the rudder will ever see while sailing in normal conditions, it is nearly impossible to account for all unknowns and I have often said that if a boat will be damaged it will more that likely happen when the boat is sitting on the trailer which I have seen over and over again. 

 

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  • 3 weeks later...

Just wanted to close this thread out. . . Thanks to everyone for their suggestions and advice!

 

B&B sent me a new rudder blade, very quick and much appreciated.  I haven't finished finishing it, but I stole the already finished & glassed rudder from the other CS20.3 that I'm building and used it in Chessie.  Even though that one is glassed on both sides it fits just fine, @Mike Vacanti was totally right about the thinness of glass.  

 

Went out for a nice sail yesterday in NY Harbour with Chessie, took some nice shots of downtown Manhattan, Hoboken Lackawana Ferry Terminal, and the Statue of Liberty.  With the water ballast in, handled the wake from the ferries and pleasure boats no problem.

 

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