Carbon Rudder Shaft Sizing

Hello, I'm in the process of building a new rudder for my older 1/4 Tonner, '74 Stoner 25, it weights #3000, draws 4' and is rather tender. The new rudders area will be 616 sq in. The blade's construction will be laminated western red cedar wrapped in fiber glass. The rudder post goes through the hull, not transom hung, and I would like to use a carbon rudder post to save weight. The OD needs to be ~1.5" (I have a little wiggle room), I will use a mostly Uni tube, with some +/- 45 fibers. The part I'm struggling with is wall thickness. I had a composite company that started giving some advice, but quickly backed away once they felt there might be some liability. Can anyone share what tube sizes the've used on similar (or not similar) sized boats? The current rudder is a bit of an gargantuan oddity, i.e. 22% foil thickness, 2" OD SS post with 3/16" wall thickness. I don't feel like I can use it as a reasonable go by.
Appreciate any help,
-Jason

 

How much weight will you actually save by using carbon rather than a conventional material such as stainless steel? One has to wonder why the composite company opted out, don't you think?

Two inch diameter stainless with three sixteenth walls would weigh in the region of four pounds per foot. I repeat, how much weight could you save? There are far more ways to reduce gross weight than modifying the rudder post. That part is the tiniest proportion of the total displacement of the boat.

 

It's tough to say exactly how much weight saving the carbon would bring without knowing the wall thickness I need for the Carbon, but lets say the Carbon ends up weigh .9 pounds/ foot. My rudder post needs to be 5.5' total. So that works out to a 4.95 # Carbon rudder post. 1.5"OD 3/16" Wall SS should be 2.65#/ft, so 14.57# total for SS, 9.6 # difference. While 9.6# is a tiny proportion of the 3050# displacement, it is more weight in the very end of the boat (weight at ends bad), and a lighter foil feels more responsive to the driver, IMO.
Another way to look at it, the cedar blank should weigh 10# once it's been milled out, and lets say 2# to skin in fiberglass. 12# + 4.95# for Carbon = 16.95# total rudder weight with Carbon shaft, 12# + 14.57# =26.57# total for weight for SS shaft. So a carbon shaft will produce a rudder 36% lighter than an SS shaft, that's not an insignificant number.

If I drop the wall thickness of the SS shaft it gets tighter. I can definitely find some similar boats around me with SS shafts to measure as a go by if we decide SS shaft. I probably should have mentioned this is a race boat.
-J

 

Consider that the rudder will have some of its' own buoyancy. If the area is 616 inches square, lets take a guess that its' average thickness is one inch. That will create a buoyancy of right at 22 pounds. Figure 0.o361 pounds of flotation per cubic inch........ (62.4 pounds/1728 cubic inches)......The boat will think that the rudder assembly with the stainless shaft will pretend to weigh only about 4.6 pounds.

All that is fine except when the boat pitches a lot.. The flotation will matter (assuming the rudder is fully immersed) but the inertial forces do not much care about flotation when the buoyant object is underwater.. The rotating moment will be distance from pivot point times the weight of 26.57 #. The heavier rudder assembly will affect the pitching recovery. The question now becomes how much will that affect the speed of the boat. The velocity of the pitching motion also plays into the equation. If your boat is skinny with fine ends that is one thing. If it has fat ends then the pitch will not amount to as much except in really rough water and how fast you are going.. There are way too many mysterious variables to lose sleep over the probable behavior pattern of the boat with lighter or heavier rudder assembly. Having been a competitive sailor, I can sympathize with your quest for better performance. Especially so when the fleet is closely matched. You miss one shift and the deal is off anyway, no matter the weight of the rudder shaft.

The structural part we can calculate with a modest degree of assurance. If the rudder is cantilevered, then the shaft had better be plenty stiff because of the long length. Think of it as a long lever. On the other had if there is a tang or bracket of some kind that reliably supports the bottom end of the shaft then we need not have so much shaft stiffness strength.
We can also speculate about what might be the result of a grounding or collision, of some sort, that involved the rudder. I am confident that you have no intention of grounding or colliding. But then "stuff" happens sure as god made little green apples.