Stainless rudder stock needs replacing in foam core rudder?

True however, the chance of that happening would be much lessened, which is the whole point of this exercise.

I think the shape will make a difference. Probably not a lot, possibly more than we might think depending on how bad the old rudder was. But since I am at it I may as well do a good job and learn what makes a good rudder and why for my own benefit. As I said the boat has a small keel and the rudder is a major leeway preventer on this design, and since upwind performance one thing I would like a bit more of, I may as well do what I can to make it better. I have already spent a lot of time and effort in other areas and it makes sense to go for a good rudder design.

Apparently the 63 series does not like a lot of angle of attack, and rudder angles are often over 5% plus the boat makes considerable leeway thanks to the shoal draft. I think I will have to learn a bit about programs like xfoil and see what I can figure out.

 

This has been an interesting thread, I've followed for some time. My own inclination is that PAR has hit the nail on the head for section shaping, on this rudder. You need the angle of attack tolerance, especially high up and a little less so low down. Follow up his suggestion of blending the profiles slightly and I'm sure you will end up with the best possible blade for this type of craft.

Looking forward to seeing the result. I suspect it won't be a dramatic difference but the blade will quietly go about its business with no drama. If it behaves well on knock down type gusts and awkward turns, with a fraction less leeway or weight of helm it is a winner.

Quite how to inhibit the original corrosion may be more tricky. Must be down to being free oxygen (ie air) right on the waterline and max effect. If possible coat the shaft with epoxy or epoxy paint right up to the part in the bearing. Hopefully that is just that trace higher and wil give less opportunity for trouble.

 

To make the shaft last longer eliminate earth fault leaks on the boat and insure that the rudder shaft is bonded to the vessels ground plane.

Bonding spreads the load between all bonded , wet, metal.

 

Agreed Michael. I maybe wrong but I thought the bearings were of polymer type so you should not get an electrolytic connection. I stand to be corrected, but totally agree if there is any possibility of electrolosis it should be eliminated.

Normally the (waste) anode is somewhere on the hull rather than the rudder in my experience. So your suggestion of bonding say the upper portion of the rudder shaft to the earth makes good sense.. Still worth giving the shaft maximum protection where it lies at that water/air interface.

 

My whole boat is isolated from earth anyway. I do not run shore power so there is no voltage reference to the grounded center tapping of the terrestrial power transmission network. And when I am not there everything is off besides the bilge pump and solar system. The quadrant is made from aluminum alloy which is highly galvanic to stainless steel and will act like a sacrificial anode, if anything a zinc will prevent further cosmetic corrosion of the quadrant. The quadrant has quite a bit of corrosion so it has worked. 316 is not infallible to corrosion and this sort of corrosion could probably be expected after 20 years of being in an anoxic saltwater environment around a plastic bush. Anyway, even though "earthing" (a redundant term when it comes to boats) is an interesting topic I would like to keep this thread to rudder design and construction.

http://www.corrosionist.com/Corros1.gif

 

I can get plywood laser cut cheaply as opposed to a 3d foam core which is much too expensive. I was going to cut say 6 profiles out of plywood then seal them with epoxy then align them over the shaft and tabbing. They would act as a fairing guide and would remain in the rudder.

Does it really matter if I leave epoxy sealed plywood ribs in the rudder?

Or If I use enough plywood ribs (say 10) and a thick enough glass skin, then glue and fasten each rib to stainless tabbing, can I use very light weight foam which is cheap and easy to fair? The foam would just allow something to lay the glass up on and the ribs would transfer loads?

Anyway. I just payed for profili pro.

 

I'm sure you can do either, we used just ply to laminate over the armature, going too light with the foam would need some compensation in thickness of glass skins. Sometimes builders have stacks of H80 offcuts.... save them till they pile up to get chucked later........... as I have, once rats nested in the offcut box Stunk like *i$$.
Jeff.

 

I wouldn't get too hung up on paying to get your inserts profiled, by the time you sent the files & got them cut & picked up/delivered you could have lofted & cut & installed & made any allowance for any "wobble" in the armature as welded, the rudder as one off will be hand faired to a finish, I'd also consider marking out both sides of your foil profiles but only partial cutting of one side with a datum margin left on to assist in set up in reference to the stock centerline on/over the bench/strongback.
Jeff.

 

Thanks. I guess you are right if I only use a few sections. I think if I go for the lightweight foam with many rib sections the laser cutting may be worth it. I am liking the idea of this method the best as I will be able to get a very accurate compound section shape even at the rounded tip of the blade, plus it has a kind of nostalgia with me and my RC gliders. I can use cheap heavy duty styrofoam which is easy to cut (I can hot wire it using the 10 plywood ribs as a guide. The foam would not be structural in this case. The 10mm plywood ribs would be fastened and glued to the armature along with a thicker skin. 10 station sections would give 160mm spacing. What sort of laminate schedule would complement this type of construction?

I think I will get the rudder infused so a thicker laminate will not be much harder with this method. Will atmospheric pressure distort styrofoam (is there another cheap light foam I should use)? I guess it does not really matter as after the resin has flowed in pressure has equalised anyway?

 

http://www.boatdesign.net/forums/at...building/4679d1133258488-anodes-galvanics.doc


"The propeller, the rudder and their shafts

The ideal solution is to have a good electrical bond of the prop-shaft and rudder shaft to the hull, these will then be protected by the main hull anodes. "

 

I don't think I'd go down the route of styrofoam, in the grand scheme of your rudder with armature etc. cheap shouldn't be in the material selection, I EPS has some gaps between the ganules & can absorb water, I'd just epoxy glue some Klegecel or other foam between the formers & just carve it down with an electric planer, soft pad with 40-60grit, a skim of micro ballons, fair, glass & tune up with a light screed of microballons, sand prime & paint...............
Jeff

 

Ok. I did not know styrofoam could absorb water.

 

The pressure wouldn't equalize. I'm thinking your rudder would look like a starved animal and require a bunch of filler, hand fairing and then some more glass.

 

Final nail for EPS

 

I made a thread in the hydrodynamic section regarding foil shapes.

http://www.boatdesign.net/forums/hy...design-40-foot-sailboat-49138.html#post667808