Dave
As we talked about earlier, the max loads on the rudder occur during a very
high wind bear away - given the flow pressures around the hull and the CB
the likelihood of hitting the rudder with debris and not the blade
(especially with that force) is very low.
The fact that the pin bent at the top and the top gudgeon pulled out tells
me that there was a loss of connection at the lower gudgeon - if the issue
was stress loading on the pin, it would have failed first at the lower
gudgeon since the loads are higher there.
The lateral force-couple analysis has the rudder actually pivoting on the
upper gudgeon with a lever arm half the length of the whole rudder - I can
attest to this having tried to re-insert more rudders underway than I care
to admit to. Invariably the trick is to get the lower gudgeon in first if
possible, then the upper one is trivial (or think about which arm carries
the load when steering with an oar or paddle).
Now to the force loads. Our rudders are roughly 1 meter long. So the
lateral force couple is 0.5 meters. Absent impact force (which we will
assume is not an issue here) - the amount of shear force you would need to
cause that sort of failure is very high. High enough that I would have
expected the rudder to fail first.
Why bolts instead of screws? More because of the fore-aft loading the
rudder is put under as well as the water-creep that you will get into the
joint from the continuous working.
As for why a transom bar? Let me take a guess at it before Bram actually
answers:
The ideal is a completely open transom so that the water can just drain
straight out of the boat - but this causes two design problems:
1) you need to deal with the compression force on the hull caused by the
shrouds - so you either make the hull-deck box stiffer and bigger - or you
install a stiffener across the hull - ala the transom bar
2) you have to mount the upper part of your rudder somewhere. 49ers use
an aluminum gantry that is just as susceptible to being fallen on. Some
I-14s use a transom bar, others build a box in the middle of the boat just
for the rudder. The latter is a much bigger pain to build I suspect - and
it looks ugly.
How'd I do bram?
-----Original Message-----
From: Dave Lindsey [mailto:lindsey@xxxxxxxxxxx]
Sent: Monday, September 10, 2007 10:33 AM
To: Robert Harper; Swiftsolo
Subject: Re: Rudder rip-off
Robert,
You are correct in assuming I used screws. As I said, this was done
purposely. I have screws in both gudgeons. I worried that some day I would
hit a piece of wood or the beach with the rudder down damaging either the
bar or the transom.
Our rudder boxes do not pivot and do not have a shear pin for safety.
During the construction I put a small wood knee in the hull between the
transom and the bottom of the hull. It is laminated with carbon/kevlar and
incorporates drain holes. Good thing.
I do have the option to remove the screws and use bolts backed as you
suggest. But as I said this does worry me.
I think the main reason for the failure is the bending moment on the
pin. The metal spacer must have caused a serious stress raiser on the pin at
the point between the rudder box pin bushing and the spacer. Note that the
pin bent forward i.e. toward the bow. This makes sense to me. Looks like a
pretty straight forward force couple. The rudder is being forced aft by the
water motion (relatively speaking).
What I don't understand is the screws in the lower gudgeon should have
pulled away first as they were under tension. I guess we will never know.
I am contemplating making the rudder box with a pivot mechanism.
Dave Lindsey
> Not knowing how you attached the bottom gudgeon, I can't say for sure but
> I'd agree with Cristian that it looks like you used screws to hold it in.
> There is a lot of force here and you'll want to do backing plates and
> washers. The 1/4" strips and some glass will not hold everyting under
load.
>
> The big problem now is getting the pin back out of the cassette without
> ruining the cassette. When you put the gudgeouns back on the boat, make
> sure that you bolt it all on with large washers, 5200, and locking nuts.
> You might have long enough arms to reach from the inspection ports. I lost
> my rudder in a similar way in Florida and had to borrow Mark White's arm.
> I can't get my forearms through the inspection ports. If you did use
> screws, there shouldn't be too much damage to the transom.
>
This is the Swift Solo mailing list. For unsubscribe instructions,
visit here: http://catzooks.com/swift-solo/
This is the Swift Solo mailing list. For unsubscribe instructions,
visit here: http://catzooks.com/swift-solo/
|