sailboat centerboard fabrication

[cebump]

The project of the week in my shop is to replace a centerboard in a 27" sailboat. the board will be approx 15 by 40" by 3/4 thick. the original was cast iron, but it rusted away. the question I pose to you guys is What to make the replacement from. It will always be in salt water. Steel, stainless or aluminum? The board pivots up into the keel when needed on a 1" shaft. any input would be appreciated.
thanks
Brian

[SGW]

Whatever you choose, investigate the properties of the particular alloy you plan to use. There's a gazillion kinds of "aluminum" and a gazillion kinds of "stainless steel," and some of them will like being in salt water a lot better than others.

Wouldn't you want to more or less duplicate the weight of the cast iron, thereby ruling out aluminum, anyway?

[Koot]

Don't forget electrolysis is a real concern in salt water. Maybe the cast iron center board was designed to be sacrificial, although I think zinc blocks are usually used for that purpose. Stick to what the designer specified, if you can, for stability and durability.

[WJHartson]

If I remember correctly 5086 aluminum will hold up in a salt water environment. The center board would be a lot lighter and would have to have something added for ballast. You could also install zinc plugs that were replaceable to act as anodes to offset the effects of electrolysis. Power coating the center board would also increase the life and cut down on the drag.

You could form and weld aluminum sheets to the desired shape and fill the void with lead or something else to get the weight right. Round sections could be cut into the centerboard that would allow you to bolt in the zinc anodes.

Hope this helps.

Joe

[cebump]

good ideas on the zincs. weight is not really an issue as the boat has a substantial cast iron keel. the aluminium has one major thing going for it being lighter. the board is retractable via a 1/8 cable. a few less pounds would shure cut down on strain. As for making another one in cast, as much as I want to learn that skill, the boat needs to be in the water this month, not this decade. http://bbs.homeshopmachinist.net//smile.gif Would anyone think a brass, bronze or nylon of somesort bushing pressed in the pivot be a good idea?
Brian

[JCHannum]

How much money is allotted for the project?
Flame cut steel would possibly be most economical, and would approach weight of cast iron, eliminating any potential ballast problems. A good epoxy finish would hold up well in salt water.
Aluminum would probably be next, but not all alloys are any better than steel in salt water. Reduced weight may be a problem.
Stainless would probably be most expensive, and again, not all alloys are suitable for salt water.
Nylon or teflon would probably be best material for a pivot bearing as brass or bronze would be attacked by electrolysis depending on materials selected for the keel or the pivot pin.
How long did the cast iron last? Steel will not last that long, but if cared for properly will approach it. I think I would use steel with a zinc anode.

[WJHartson]

Not knowing the complete design you are working with put me at a disadvantage. The use of a bushing is probably a good idea but remember that dissimilar metals in an electrolyte will cause a galvanic reaction unless they are insolated. Salt water is a great electrolyte.

If weight is not a consideration for the centerboard you might consider fiberglass. Shape a piece of marine plywood and cover it with fiberglass. That would be very quick to make and it is easy to work. Mold the bushing into the board. No corrosion, lighter than the metal ones and relatively cheap to make.

Joe

[cebump]

the original lasted 20 years. the only problem with it was the 1" pivot hole rotted out to a 3" crater. Cost is a factor but no the most important one. The worst part of this whole project is getting the old shaft out! I think they are tapered in two both directions http://bbs.homeshopmachinist.net//smile.gif

Brian

[JCHannum]

If the only problem is the pivot hole, why not just install a smaller one?

[Evan]

Been there, done that. I looked after the maintenance of two 27' boston whalers for the local Sea Cadet corps for about 5 years. These are centerboard boats, both with aluminum centerboards. One was run aground and bent the centerboard. I had to remove it, cut off a couple of inches and reinstall with new pivot hole drilled. Aluminum is not a problem, even in salt water as long as it is not in electrical contact with other metals. It depends on the boat, the whalers are all wood and fiberglass construction, the centerboard is the only metal component in the water. This means that there is no battery possible so corrosion doesn't happen. Corrosion induced by salt water is electrolytic corrosion and it requires two different metals as well as the electrolyte (the salt water)

Use aluminum and bolt a small zinc block to the bottom leading edge of the centerboard as a sacrificial anode. Use stainless steel bolts. Replace the zinc anode every few years and you will have absolutely no problems. Aluminum weighs 3 times less than cast iron for the same volume and in this case strength is not an issue. A lighter board makes it much easier to hoist. In fact, if run aground it is better for the board to bend than to break. Even if bent the boat is still seaworthy.

You probably will only be able to find 6061 in 3/4" plate, that is no problem as long as you use a zinc anode. Make sure the pin for the board is stainless steel.

BTW, a glassed plywood centerboard isn't strong enough. On a boat that size the forces can be very high in rough seas.

Make sure that when the board is in the down position that a good portion of it is still in the trunk so to give good support. The pivot pin is not meant to withstand the side loads, the trunk does that.

[This message has been edited by Evan (edited 04-05-2004).]

[cebump]

Evan

thanks for the info. i am getting quotes on steel and aluminum today to make my decision. I will report later.

as to a smaller hole, there is nothing left of the entire area of the existing board.

[Evan]

Also,if you do decide to go with aluminum I wouldn't worry too much about compromising stability due to lighter weight. A centerboard produces most of the stability as dynamic stability rather than static stability as produced by a lead keel. It functions by simply being hard to move sideways throught the water and the weight is not really relevant to stability.

[wierdscience]

I would go for either aluminum or stainless,aluminum will work whatever alloy if you hard anodize it,plus if you install the bushing after the anodize there is no problem with electrolisis.

[wierdscience]

One other possibility would be hot dipped galvanized steel,if you have a galv. plant nearby,this would be the cheapest and easiest.If your near saltwater there should be a galv plant close by.