Any reason to get stainless 316 over galvanised mild steel for a rudder blade?

Laminar Flow said:
Another solution is to order the foam cut to profile from one of these companies that hot cut foam wings for the model airplane crowd. Not all that expensive either. Mount over a solid core and glass heavily.
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Since the OP has already ordered a new galvanised mild steel rudder, would it be a sin to use that as the solid core and simply adhere the shaped foam to it, fair it and glass over?
 
Rogershaw said:
On my 50ft steel boat I have a mild steel foil shaped hollow rudder. The shaft is solid stainless steel

The outside was grit blasted and painted with epoxy tar then antifouled. There are also a zinc anode on each side.

The hollow rudder was pressure tested to ensure no was water could leak in. I then filled the inside with old engine oil to protect the inside and if the rudder got damaged oil leaking would show up.

I also made a auxiliary rudder for my self steering gear from 316 stainless steel. It again is foil shaped and hollow but as this is hinged up and down and will only be used immersed in sea water when in use

Inside framework before external plating

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Very interesting pics - just to be sure I understand, I assume we're looking at (top to bottom):
1. Stainless core of the auxiliary rudder on the pontoon
2. Stainless core of the auxiliary rudder mounted on the transom; aft of that, the servo pendulum rudder
3. Main ship's rudder, fully plated, painted and fitted.
Lovely bits of work. More engineering than needed for Steve's 18 footer, but it shows the shape, and might be useful for those of us tinkering with larger projects...
 
HissyFit said:
Since the OP has already ordered a new galvanised mild steel rudder, would it be a sin to use that as the solid core and simply adhere the shaped foam to it, fair it and glass over?
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No sin at all. It's a good way to achieve what he wants while retaining the strength and weight of the steel (he needs the weight as it's a lifting rudder)
 
HissyFit said:
Since the OP has already ordered a new galvanised mild steel rudder, would it be a sin to use that as the solid core and simply adhere the shaped foam to it, fair it and glass over?
Click to expand...
That's what I would do. Buy a sheet of PU rigid insulation board, it will be 100% closed cell, shape it yourself, very easy to do, think surfboard. Then stick with epoxy and coat with epoxy/glass cloth.
 
Or, google foam cut model airplane wings. Give them your profile of choice, cord and span, tell them you want it in two halves; quick and easy. Glass the lot over and done. Perfect profile.
I know someone who did just that.

Just a note on resistance: for the same resistance as a flat plate you can make a profiled form that is 30 times thicker. i.e. 10mm versus 30cm.

36mm of ply is more than plenty strong for an 18 footer, steel core or not.
 
Some very ingenious solutions. Having done a similar job on a snapdragon 23, Oldwills suggestion is most appropriate for a Bradwell18. Its what I did, copying the leadinf and trailing eddge profile of my old solid hardwood rudder (which split) by eye. I left the middle flat.

A wooden rudder needs a downhaul as well as an uphaul. But floating when released is an advantage when taking the ground on a falling tide. Better not to get tipped forwards by the rudder digging in and jamming!
 
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