Dissimilar Metals?

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walterv

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I am extending my trim tabs. Friend who has the same boat is also doing the same and had some "extended" tabs made. The tabs are made of stainless, same material for the extension tabs as the tabs it will bolt to. I will be using SS bolts to bolt the tabs together.
I need to put a piece of angle iron on the new tab to support where the actuator bolts to. I want to use aluminum. I will be using SS bolts to do this as well.

Anything I should be aware about using two dissimilar metals?
I also thought about painting the tabs with that Rustoleum Zinc paint I used on my props last season.

Comments?

Thanks
Walter
 
Walter- Couldn't you continue to use the existing mount on the existing tab and add the larger tab plate to the bottom for better support against water pressure?
I wouldn't use alu as you propose in SW. I don't know whether it would be necessary to move the mounting point farther aft.

And I suggest you ensure all the SS is 316 in SW .

I haven't tried the galvan paint . Still using 1/2 pancakes(on top) and Micron CSC on the tabs.

Could SS crevice corrosion become an issue with your sandwich plan if not tightly sealed together?
 
quote:
Originally posted by Sandy

Walter- Couldn't you continue to use the existing mount on the existing tab and add the larger tab plate to the bottom for better support against water pressure?
I wouldn't use alu as you propose in SW. I don't know whether it would be necessary to move the mounting point farther aft.

And I suggest you ensure all the SS is 316 in SW .

I haven't tried the galvan paint . Still using 1/2 pancakes(on top) and Micron CSC on the tabs.

Could SS crevice corrosion become an issue with your sandwich plan if not tightly sealed together?

Click to expand...


Using the existing set and having everything welded to the underside is how I extended my tabs. All was done in stainless.
 
quote:
Originally posted by Sandy

Walter- Couldn't you continue to use the existing mount on the existing tab and add the larger tab plate to the bottom for better support against water pressure?
I wouldn't use alu as you propose in SW. I don't know whether it would be necessary to move the mounting point farther aft.

And I suggest you ensure all the SS is 316 in SW .

I haven't tried the galvan paint . Still using 1/2 pancakes(on top) and Micron CSC on the tabs.

Could SS crevice corrosion become an issue with your sandwich plan if not tightly sealed together?

Click to expand...


Using the existing set and having everything welded to the underside is how I extended my tabs. All was done in stainless.
 
Sandy,

Still going to use a zinc tab, paint was just cosmetic.
Others on another site spoke with Bennett, the tabs on my boat go into a pocket, the next larger tab (still according to Bennett is too small). I could not make it wider, but had the fabricated bolt on ones stick out 3" further with dog legs.
David, good call on welding, never thought about that, but it is firmly pain stakingly bolted.
 
I have this expensive two coat aircraft stuff I bought when I had my bravo 3. It basically etched bare aluminum and stainless then seals it. Then you out a base coat down like th zinc or a primer. It helps the paint adhere

Let me know if you want to try it. It's in garage. Haven't used it in a while.
 
Not sure if you said it was done. My suggestion would be to weld the extension to the rear edge with the edge turned up to stiffen it. Add stiffeners to the underside outer edge kinda like darts with the small end forward. If it's 2' wide or more, you can add a 3rd stiffener in the middle. welding temps can degrade the stainless and non magnetic properties to stainless so paint it.
 
You will have problems with the aluminum corroding if you use it.

Not a good idea IMHO.
 
Agree with Charlie regarding going aft. Here is the sketch I provided my welder. Mine are the same span (I was prohibited by prop tunnels and the chines) as before but about twice as long and as stated, were welded to the underside of my existing tabs. If welding wont work for you could your drop the tab from the ram and bolt through? Just a thought....

My mod changed the entire attitude of the boat and decreased my fuel consumption considerably.

I know what works on little boats doesn't necessarily translate to big boats but thought I'd offer as food for thought.

Tab%20Sketch%201_zps9iogf1b7.jpg~original


Good luck.
 
Charlie, I believe the issue with a new tab would be the hinge point. I looked at new custom tabs and the hinge was the long pole in the tent so to speak. That and $$$. Overall, the mod was cheaper and easier. I want to say $400 for both tabs out the door payable in cash. He had the stainless left over from a set of fuel tanks he had just completed for a customer. I've saved more in fuel costs than the mod costs and, knock on fiberglass, not had any problems with the hydraulics leaking etc. Bennett builds quite a system.
 
quote:
Originally posted by CurrentSea

I have this expensive two coat aircraft stuff I bought when I had my bravo 3. It basically etched bare aluminum and stainless then seals it. Then you out a base coat down like th zinc or a primer. It helps the paint adhere

Let me know if you want to try it. It's in garage. Haven't used it in a while.

Click to expand...


Is that Alodine & Aluma-Prep?( See BeeGee's ening forum sticky: http://www.boatered.com/forum/topic.asp?TOPIC_ID=97217 )
I used those last summer after I had stripped my tower's paint-blistered aluminum console box and preparing to paint. Easy to work with (taking care it is an acid etch) and presumably created excellent adherence.

Walter- Curious... why would you even want to use alu for the ram mount? Sealed or not, I believe it & its fasteners are going to corrode unless perhaps it is some very special alloy and isolated from the SS. I'd think SS would be about as easy(?) to fabricate and attach and be reliable if you don't use the original one.
 
Al is not a good idea Walter. Even if you insulate it with paint, one little scratch and boom, you got a galvanic cell because your "atmosphere" is water. What you need to do for the angles is have some SS sheet bent on a brake. If you cannot bend a thick enough material, double it or event triple it then bind them together with SS fasteners. Even if you could get material thick enough and bend it, I would still double it as it would make it load redundant, like a wing spar. There is a lot of pressure on those tabs (especially when you extend them out like that) so beefy is the word. You may want to run a couple angle pieces longitudinally to account for the extended chord of the tab as well.

As for welding, it depends on how it is done. Welding SS is not for wimps and a SS weld in water has to be done right or it will corrode and eventually fail. Not to mention the annealing that can occur to the temper of the material itself.
 
Thanks all
Glad I asked.

David,
The Silverton 45C is extremely under-tabbed according to Bennett. What was originally spec'd was too small then they put a smaller one on.








 
Walter - the stainless work you have posted here with the drop tabs looks great so far.
On certain Bayliners the tabs have been known undersized and there has been many posts with much information over on that forum. Here is just one of them where Tabman (Bennett marine) posts in with a few comments on extensions, there are many more as well.....

http://www.baylinerownersclub.org/i...l-discussion/42725-trim-tab-extension-on-3258
 
Looks great Walter. Even the stainless looks to be the same gauge as mine. The only differences I see are my tabs are welded vs. Bolted and I had the rear edge turned up to add stiffness to the span. My math was Rhode Island simple and probably laughable to the engineers here, but it worked for me.

Long ago and far away, I recall Bennett recommended 1" of span for every foot of boat. What I could never figure out is if the calculations were for a 9 or 12 inch chord. So for me I figured 33*9 = 297 square inches and 33*12 = 396 square inches. The originals were 22*9 = 198 square inches (actually less because they cut away part of the inboard edge on an angle for some reason...Sex appeal? Who knows) Anyhow, I'm at least 33% under-tabbed at minimum. Because I couldn't go wider, I too had to go longer. The original plans called for the flat stainless to start at 20 inches fore and aft and with the 2 inch bend up, end at 18 inches and provide me the 396 square inches I desired. Miscommunication and he started at 18 inches fore and aft and I'm at a 16 inch finished product for 352 square inches per side. Not what I wanted but still makes all the difference in the world.

Because you are doing this on the boat, I would recommend a 2x4 between the tab and the boat to preclude the drill from going into the hull.

Good luck!
 
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