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Cheechako · 08-05-2012, 09:45

They make beautiful arches and towers for powerboats out of Alum. , especially in FLorida. The one thing I will say about alum is it often turns powdery on the surface, and may even have sharp little corrosion points like a grain of sand. Messy, black on your hands and streaks the hull. With enough fresh water rinsing and washing you can avoid most of this, but for blue water cruising maybe not. $10k sounds ridiculous to me for SS. Have you looked at Kato Marine or Mystic Stainless? I know Kato's not cheap, but not sure they would be that much!

JayH · 08-05-2012, 10:30

I have an aluminum arch on the back of my boat. It is a very common material for fishing boats and catamarans. Possibly related to the comment above about Florida.

Aluminum is cheaper and lighter for the same strength.

Aluminum is weaker for the same size, so aluminum structures are built out of bigger stock to get equal strength. This is really an aesthetic issue. A small boat might look funny if the strength requirements demand 2" AL pipe. Of course you're not likely to hang a 14' RIB from the stern of a 35' mono.

SST is shinier than AL. Aesthetics again, but also a price factor since for many SST is more desirable.

Durability of the finish I think is equal. SST wants to be polished, anodized AL wants to be waxed. AL has to be isolated from SST fasteners to prevent corrosion. AL can be painted or powerdercoated, but neither will last. The comments about AL being messy must be referring to raw AL.

For a Niagara 35 I think you want SST, but totally my opinion based on aesthetics. $10K is absurd.

Cheechako · 08-05-2012, 10:40

Alum is usually built of much larger tubing/pipe. That's because although it has similar yeild strength as SS, It is much less rigid.... in fact about 1/3 as rigid as SS. Alot of the beautiful work they do in FL is 2-3" diameter! So , I guess, depending on how good the desgin is.... it might be just as heavy or heavier than a SS design!

Ram · 08-05-2012, 11:10

Quote:

Originally Posted by Cheechako

Alum is usually built of much larger tubing/pipe. That's because although it has similar yeild strength as SS, It is much less rigid.... in fact about 1/3 as rigid as SS. Alot of the beautiful work they do in FL is 2-3" diameter! So , I guess, depending on how good the desgin is.... it might be just as heavy or heavier than a SS design!

I think your mistaken Aluminum is more ridged, SS will flex some Aluminum much less-

Cheechako · 08-05-2012, 11:14

Modulus of elasticity for aluminum is about 1/3 that of steel or SS. I built alum boats for years and worked as an engineer in aerospace.... if both pipes are the same size the alum flexes a LOT more. If designed to avoid that it really doesnt matter though! A cage design like the Kato shown above will be rigid enough in alum I would think.

Ram · 08-05-2012, 11:19

Quote:

Originally Posted by Cheechako

Modulus of elasticity for aluminum is about 1/3 that of steel or SS. I built alum boats for years and worked as an engineer in aerospace.... if both pipes are the same size the alum flexes a LOT more. If designed to avoid that it really doesnt matter though! A cage design like the Kato shown above will be rigid enough in alum I would think.

My Mistake!!

Cheechako · 08-05-2012, 11:23

It's one of the challenges of going to alum on car frames.... weight = fuel. it could be done, but the costs increase due to the more elaborate frame. Audi came to us once discussing hydroforming of a frame for one of their high end cars.. we never did it, but I think eventually they made that car....

1affiah · 08-05-2012, 11:59

Riversand:

Check out the website for Atlantic Towers www.atlantictowers.com
located in New Jersey. The have an item called "Tower in a Box" made of 1.9" aluminum pipe, anodized, that you can cut and fit for under $2K., you install it yourself. Cut off the legs to adjust for height, and bolt them horozontally for dinghy davits. They are now starting to market these as Radar Arches for sailboats.

Good Luck!

Larry

riversandbar · 09-05-2012, 11:18

I have 2 different guys coming out this weekend to give me a quote... I'll let you know what they say.

This is the picture I sent the guy that I wanted the arch to look like... seems fairly simple to me... being that I already have the stern rails.

**Pblais** · 09-05-2012, 11:59

"Simple!" The good stuff looks simple but the welding is clearly world class when it looks simple. SS quality in 316 is not easy or cheap and the similar can be said for aluminum. Lots of Mainship power boats use Aluminum rails all around. They look sharp and are nicer to grab with the bigger diameter. I would go for marine grade aluminum without hesitation. Nice 316 stainless looks purdy too!

You need to look at the work they do and the detailed specs on the metal. Crappy 303 stainless is very very strong but will rust in a few years in salt water but isn't too bad in fresh water. Poor stainless welds will also rust but they would be cheap. Good marine aluminum does not require a coating! So why would anyone coat it? Loads of ways to get taken with poor grades of metal.

Cheechako · 09-05-2012, 12:04

I wouldnt call that one simple! is this a davit system also? How compicated does it really need to be?

CAELESTIS · 09-05-2012, 12:12

I've posted this photo before. I think it is the best looking arch I have ever seen. I will build one just like it when the time comes. Three inch diameter schedule 40 tube.

Cheechako · 10-05-2012, 09:27

That looks nice and simple.... I dont see any diagonal supports though...? is it just bolted to the transom? I would NEVER paint aluminum for salt water use though....