Alu or stainless steel outboard plate (for bolts)?

[bossee]

Hi,<br /><br />1) Is aluminium plate enough or is stainless steel plate better for this plate that is behind the upper bolts? (se picture below)<br />2) Is 3-4 millimeter thinkness for the plate enough? (it is a 115 hp 4-stroke, 402 lbs)<br /><br />I'm not sure aluminium plate is good because of risk for galvanic problems (different metals involved)? The bolt, washer and nut is stainless steel and outboard itself is aluminium.<br /><br />Thanks for any suggestion on this topic.<br /><br />

 

[quantumleap]

Re: Alu or stainless steel outboard plate (for bolts)?

All of the aftermarket plates are usually made of cast or billet aluminum.

 

[bossee]

Re: Alu or stainless steel outboard plate (for bolts)?

Thanks quantumleap,<br />Yes I have also noticed that aftermarket metal plates are usually aluminium.<br /><br />In my case it is like this:<br /><br />Around one of the bolts it has developed cracks (circular) in the gelcoat. The boat is basically new (2003) and it a warranty case. The dealer that is fixing the problem has fixed the cracks last week but now the repair must be covered by someting, prefearably a metal plate (that may as well cover both bolts). So they suggested an aluminium plate. Since I know it is in general not a good idea to mix different metals (that touch each other) in boats because risk of galvanic currents I'm unsure if I should accept an aluminium plate.<br />Since I'm not going to pay for the repair maybe I should demand a stainless steel plate?

 

[Spidybot]

Re: Alu or stainless steel outboard plate (for bolts)?

The risk of picking an aluminum plate not strong enough (many sorts of alu, different thicknesses) makes it obvious to ask for stainless of a proper dimension. Remember, that stainless is not by default polished, so it may look dull.

 

[Mettaree]

Re: Alu or stainless steel outboard plate (for bolts)?

Bo: Most of the motor is aluminum - Alum. plate OK. Think would be better if use 8 to 10 mm thickness - spreads the load of the motor over more of the transom.<br /><br />

 

[kd6nem]

Re: Alu or stainless steel outboard plate (for bolts)?

Any way you look at it you already have two different metals there. It is above the water line anyway, probably less critical as a result. Use whichever you can find easier.

 

[cobra 3.0]

Re: Alu or stainless steel outboard plate (for bolts)?

A thick piece of aluminum is just fine. A thick piece of stainless will cost you a bundle!

 

[Bass Runner]

Re: Alu or stainless steel outboard plate (for bolts)?

I use 1/4" alum plate with stainless bolts and washers with no problems for many years, You get into 1/4" stainless and you are talking bucks. Unless you have plenty to toss around.

 

[Forktail]

Re: Alu or stainless steel outboard plate (for bolts)?

Most of my boats are custom made aluminum boats. All have 100% aluminum transoms. And all my outboards are attached with stainless hardware. I have never had any problems. <br /><br />I don't believe dissimilar metals would be a major concern. The real issue would be strength, as the stainless would be stronger...if strength is what you're after. <br /><br />It looks like that plate is acting as a big washer. IMO, a better solution would be to run the aluminum over the lip (where the outboard rests), lengthen the plate as much as possible (width), and bring it down to the well as much as possible. But it's probably fine.

 

[quantumleap]

Re: Alu or stainless steel outboard plate (for bolts)?

You can get a transom support plate from a couple different manufacturers. These are specifically designed for this purpose and cost less then thirty bucks. If they didn't work, or were breaking, I don't think they would stay in business. Good luck.

 

[mred 2436]

Re: Alu or stainless steel outboard plate (for bolts)?

bo : you state that the dealer is covering the cost of these repairs, therefore i would demand the stainless. also, the aluminum has a tendency towards stretching and cracking due to its low tensile strength, the stainless is better. as far as a galvanic reaction, you would only worry about that in the case of steel or iron washers and bolts, which you will not get with the stainless, but you would with the aluminum.the coating on zinc or galvanized washers enhances the oxidation of aluminum, which results in the two metals bonding.

 

[Hooty]

Re: Alu or stainless steel outboard plate (for bolts)?

The last s/s sheet I bought (2-3 wks. ago) was cheaper then aluminum.<br /><br />c/6<br />Hooty

 

[bossee]

Re: Alu or stainless steel outboard plate (for bolts)?

Hi,<br />I visited the dealer today where my boat is fixed (under warranty) and they have now made a stainless steel plate (3 millimeter thick) that is to be mounted under both upper bolts for the outboard. They showed me the plate and they made a good job as far as I can understand, so I think this will be fine now.<br /><br />Thanks to all who read my post and gave valuable information on this topic. Hope it helps others that may want to use a metal plate this way.<br /><br />/Bo