Galvanic protection?

[kbowen]

I am considering a 1" steel keel shoe because it should be cheaper and tougher than lead, but also imagined an aluminum centerboard maybe 3/4" or so because it should be easier to shape the edges than steel and might provide more stability than wood. If I epoxy / fiberglass either or both items would galvanic action penetrate the plastic? fwiw, the project at hand is a Mackinaw which has a long straight keel which could come from a local (Chicago) steel yard, and if this seems workable I would prowl the scrap yards for the aluminum. Is this "smart but not wise?"

[Ian McColgin]

I would not use aluminum for a centerboard because it's too bendable. Nothing worse than a bent board and aluminum could bend after the boat drops off a wave on a broad reach.

Wood's actually great but the liability is that it's fatter.

If you use plate steel, just round the front a little to relieve the corners. You might or might not grind down the trailing inches but really the plate is thin enough that you won't have a turbulence problem. A low angle board, such as most front pivot boards are, that makes a delta in the water really will generate lift once you're moving even though flat, and as you gather way foil matters far less than just wedge resistance which is all a matter of area, not shape.

G'luck

[wizbang 13]

I think both are bad ideas. Steel and aluminum.
Does the boat call for a lead keel?
Epoxy will slow down the corrosion, for about a week.

[Ian McColgin]

On rereading, I see that the steel shoe might be in place of a lead keel. This is not likely to be such a good idea.

A massive cast iron keel such as Goblin had (Alden 43' schooner, 12Tons) makes some sense if you're taking advantage of the casting's stability to ensure the long life of a centerboard trunk. A flat iron shoe has none of those advantages and casting something big enough to take the place of lead ballast is a pain that also requires a bit of re-engineering as the density is so very different.

Comparing an iron keel to a lead keel is interesting. The iron is certainly less likely to take a dent and it will slip off a rock more readily. On the other hand, the clang you get when you smack your iron keel on a rock will start all the seams and shake out your kidneys. Lead is a bit more absorbtive.

It would help to know what boat this is for before venturing too much further into a discourse that could easily have no point.

Epoxy is not a lasting barrior against electrolysis, especially not for parts that will be rubbed, and it's really hard to get it to stick to aluminum anyway.

G'luck

[Peerie Maa]

If your version of the Mackinaw boat was designed with external ballast, then swapping the lead to steel will not give you the stability that the designer intended. As Ian says the density is different so you would require lots more steel. If there was no intention to fit external ballast then a keel shoe would be OK. If you use an aluminium cb plate, the keel shoe will last for ever as the cb will act as a sacrificial anode in a cathodic protection system.

[Canoeyawl]

This is freshwater - No?
Electrolysis shouldn't be much of a problem.

Either way the weight of the centerboard will not effect the righting moment enough to bother with.
A thick wooden board with some foil shape is a good way to go.

[kbowen]

On rereading, I see that the steel shoe might be in place of a lead keel. This is not likely to be such a good idea.

Comparing an iron keel to a lead keel is interesting. The iron is certainly less likely to take a dent and it will slip off a rock more readily.

G'luck

The boat is the "Western Lakes Mackinaw" per Chapelle ASSC. It is projected to be about #6000 at the waterline drawn by Chapelle. The originals were all internal ballast, and would presumably go down if one filled it with water. Historically, they often used boiler-plate for a centerboard. There are no extant construction specs of this particular design that I am aware of but I have studied the construction drawings of other Mackinaw types from the Smithsonian. Actual specs would be moot anyway because I am planning for strip / composite construction. I am working with a NA on strength, ballast, and buoyancy but was also interested in inviting random feedback. The analysis by the NA showed only about 275# in a 1" steel shoe by the time one cuts a CB slot and keeps the weight longitudinally centered to reduce pitching moment, our last conversation left it as not worthwhile, but then I got to thinking that #275 at the shoe was comperable my #200 standing on a centerboard if the thing was flat in the water, hence my question. Re: the earlier post about soft aluminum: I have purchased scrap aluminum of various tempers for theatrical uses (my other life) which is seriously hard and tough, and any force that could bend a 3/4" plate of the stuff could also probably snap the boat in half. Thanks for the warning about GRP & aluminum. And thanks to all for your other responses.

[kbowen]

This is freshwater - No?
Electrolysis shouldn't be much of a problem.

Either way the weight of the centerboard will not effect the righting moment enough to bother with.
A thick wooden board with some foil shape is a good way to go.

I had a horrible time repairing electrolysis damage in a S&S Pilot that had been in fresh water it's whole life. There were bronze straps on the inside of the ribs fastened with brass screws. The brass screws evaporated and took a lot of oak ribs with them.

Your judgement on the board corroborates the original decision, thanks!

[MN Dave]

The steel shoe does need a good coat of paint. A cold galvanizing paint like the Devcon stuff on a recent thread is one. Those 90% Zn paints need a topcoat. Petit Trailercoat is an Al filled paint with a good reputation. A zinc anode on the keel can't hurt when the paint gets scratched. External steel ballast is more effective for righting moment than any kind of internal ballast. If the moment of inertia is that critical, when the crew weighs more than the keel...

Wiz has a point about plain old epoxy resin, but a good epoxy primer with the right inhibitors is pretty darned effective. Surface prep is the key to painting any metal.

[ramillett]

I had a horrible time repairing electrolysis damage in a S&S Pilot that had been in fresh water it's whole life. There were bronze straps on the inside of the ribs fastened with brass screws. The brass screws evaporated and took a lot of oak ribs with them.

Your judgement on the board corroborates the original decision, thanks!

I'm guessing the acid in the oak is what evaporated your brass screws . Wet oak next to any metal is going to dissolve things ( add salt and things go south even faster ) . A good coat of paint well protect the metal and the wood . Bob