Engineering question on replacing wood transome with Alum

[PKomrowski]

I have a 26 foot StarCraft that's been totally rebuilt, that is except for the stern. It has the normal 2X 3/4" plywood, but its a high and wide boat so that may be slightly under sized for this boat. Its 9' -6" wide and at least 4' 6" from keel to rail.

It has been replaced 2x and this time Im going to replace it with aluminum.

The diamond plate is .14 and the diamond side of the plate will go to the inside of boat, with tube toward back. and I will slot the back boat sheet of alum and tig it right to the new stern tubing no bolts

No through bolts all welded, My question is what size should the tube stock be that's welded to my interior sheet plate and how close should I space them apart. Tube comes in several wall thickness and Im want to make it about 10 to 15% stronger than the original.

Any engineers have the numbers for this?

 

[metriccrescentwrench]

My suggestion would be to look at some of the composite alternatives to wood which are well tested for this type of use. I can't make any recommendations because I haven't used this material yet, but if not here then certainly someone on scremandfly.com can point you in that direction.

As far as using Aluminum plate - to do it right you're going to pay a structural engineer to totally redesign the transom for you. You're looking at a major change to a critical part of the boat's structure and getting it wrong can have disastrous consequences.

 

[bob johnson]

what are you calling the "tube" of this aluminum?????? is your starcraft aluminum or fiberglass? not only do they make several composites JUST FOR TRANSOMS, there is also a honeycombed aluminum sheet, that is ridged enough.... I priced out the composite and it was about $500 for my 22 ft starcraft. a 4x 8 sheet of plywood was large enough to cut my transom board. I used two sheets as well... if you bought exotic plywood and epoxied the wood after the layers were glues... youd be getting closer than you think to the $500... and like you have experience.... you might be looking at doing it again sometime... the composite wont rot!!!! its a good route to go....

bob

 

[PKomrowski]

I am not aware of any composite that would work, I don't like most because of weight and cost. I have the sheet metal/alum in stock only have to buy tube. The tube is nothing to do with the boat, its what fabricators refer to as the square tube used to make the transom.

As far as "to do it right you're going to pay a structural engineer to totally redesign the transom for you. You're looking at a major change to a critical part of the boat's structure and getting it wrong can have disastrous consequences." Thats not true at all for me in my case. I have been doing a lot more difficult things than this 30 years ago. You can tell by the specific questions that im asking. Im looking for a short cut of info from some one who knows.

Please if you only have an opinion I'm not looking for them, no offence ment to anyone. The deflection strenght of 3/4 marine ply, and the equal in alum tube, if you understand that statement they you are qualified to answer the question.

PK again no offence Just dont want dramma.

 

[Bondo]

Ayuh,.... Ya don't mention the drive, but if a Merc, make it 2" thick,....

The spec is 2" to 2, 1/4", true, 'n even where ever the housin's meet the transom,....

 

[Scott Danforth]

Which alloy are you thinking? Diamond plate is 3003 which has no strength. It is a decoration. Most marine aluminum is 5052 for non structural and 5083 or 5086 for structural.

If you are planning on this for an I/O application or outboard application you will have to take into consideration the thrust and moment loads in addition to the clamping/mounting loads

10 gauge aluminum sheer has almost no strength even if you use the right material.

The next hurdle is the weld prep and process. You can stitch the tube on one skin, however would be limited to plug welding the second skin.

The next hurdle is crevice corrosion. For this reason most welds are solid welded.

Any hull welding should be back gouged for marine buckling stress reasons. Its actually a requirement of the Jones act.

Now let's talk costs. If you have a steady supply of aluminum coming in, sheet will run on average $3.30 per #. Shapes are a bit more. Shapes are 6063 or 6061. So now you have 3 different alloys coming together, so you have to be mindful of your filler.

That is a broad brush review of the execution. The design still needs to be completed.

Can it be done, yes. Could I do it, Yes ( I have the naval architect connections, acess to lasers, breaks, and a large inventory of aluminum). However I would still most likely use wood.

 

[jbcurt00]

Just for reference, why has the transom ply had to be replaced twice?

Good luck w/ your project

 

[PKomrowski]

Thanks Bondo, I will make sure.

Scott
I know the sheet has no strength, it will rely on the tube tension. Yes it will be stitched welded from the out side, its in fresh water so the time it take for the welds to fail my kids will be dead, you worry about it im not going to foolishness to me. Wood fails with time now you act like this cant ever....

I was going to use 6061 it will work find in this app. It dose not need the 5083 thats BS. Sell it to someone thats doesn't work with it. Keep in mind this is only a 26 foot boat, not a cruise liner, car transport or some application that only exists in the mind of ... that has used plywood since 1983 and probably 8 of those years the wood was rotten past half strength and it was still no sign of failure.. And there are no "Standards as your spouting for what kind of wood and its shear load etc when guys use everything from CDX to pressure Treated. And although I don't recommend people to do it, I recognize those who do succeed and they work and last, although the longevity is some times short it has to be gauged with the all other matters.

Yes you can be over retentive and try to "figure" shear loads of thrust kind of bs, OR I can just make it stronger than 2X 3/4 plywood and not worry about it.... The difference between a guy who actually welds and makes things and someone who thinks about making them. I have replaced dozens of transoms, and repaired piles of boats. You have connections, I use them with my own hands on a regular basis.

Design, goodness, how about the shape of the wood that comes out, not real difficult ... Yeh I know it needs reinforcement around the IO kind of obvious there. I didn't ask for warriors to come and share your concerns, I don't care about them. Im not asking for approval, or my qualifications to be approved by anyone on this list. And am not impressed by yours. You did exactly what I didn't want typed a bunch of crap about stuff I didn't ask.

 

[jigngrub]

I have replaced dozens of transoms, and repaired piles of boats.

So why did you ask the question if you already know it all?

 

[PKomrowski]

Jigngrub to get the answer of how close of spacing by some one who dose the work.

 

[dingbat]

Stop being so belligerent and lesson to what your being told.

I wouldn't consider do what your considering. To potential problems for what?

I'm a ME with multiple welding certifications in carbon steels and exotic alloys. Spent the past 20 years designing, building and selling equipment to determine and verify the phyiscal and mechanical properties of metals at the producers level. I've forgot more about fabrication and materials properties than most people know.

I could give you many reasons why I wouldnt consider doing what your planing on do, but you already know the falsies so I'll not bother.

 

[jbcurt00]

Guys, lets not do this

P, if you only want a qualified, experienced metal engineering replies and no other, you may not get any replies.

I'm on my phone, but I'll be back in a few.

For now, lets not do the back and forth that is developing.

Thank you

 

[jigngrub]

Jigngrub to get the answer of how close of spacing by some one who dose the work.

I'm not an engineer, but I am a master carpenter and I work with both metal and wood. I'll give you my suggestion and you can take it for what it's worth... nothing, because you're getting it free on the internet.

To start off with, your .14 plate is just a smidge over 1/8" thick (9/64"). This means you'll have to find and use 1-3/8" OD square tube to achieve your 1-1/2" transom thickness, I don't even know if they make a 1-3/8" OD square tube.

If I was building this for myself I'd use heavy 1-1/2" square tube (1/8" walls) I'd space them on 4" centers except around the keyhole where I'd bunch 3 or 4 together on each side and the same bunch top and bottom for a header and sill. You can then weld it into place.

Since I'd be using 1-1/2" tube, I would use .063 (1/16") sheet for the inner skin... not welded, but riveted to the framework with closed end blind rivets. But before installing the inner skin I'd fill all the voids between the tubes with 1-1/2" polystyrene sheet foam for added rigidity and sound deadening.

Like I already said, this is the way I would do it if I were building an all aluminum transom.

 

[Scott Danforth]

you wanted an answer from an engineer. you have recieved a few. HAZ, and anealing have nothing to do with the size of the boat. neither do proper weld techniques.

 

[Bondo]

To start off with, your .14 plate is just a smidge over 1/8" thick (9/64"). This means you'll have to find and use 1-3/8" OD square tube to achieve your 1-1/2" transom thickness, I don't even know if they make a 1-3/8" OD square tube.

Ayuh,.... It's a Merc. I/O,..... 2" to 2, 1/4" thickness,.....

 

[Woodonglass]

Here's my 2? worth. It seems to me that if you're a qualified aluminum welder/fabricator and you have a Free supply of the aluminum needed for the job an you want to attempt to create a transom for the boat then I don't see an issue. As long as you understand the specs and requirements then go for it. If you're asking for those specs and requirements then I'd recommend doing so in a much more respectful and humble manner and I KNOW the members here on the forum would be more than happy to oblige. I KNOW nothing about aluminum welding or fabrication but I DO enjoy research and knowledge gathering BUT I only do so when it appears it's for someone that would be appreciative!!!

My transom is made from wood. It's over 50 years old and still going strong. It's well maintained and cared for and I expect it will last another 50 years as long as it continues to have the same maintenance and care. I always recommend people fabricate their transoms from wood, but then again I AM just an Old Dumb Okie!!!

Again JMHO!!!!

 

[jigngrub]

Ayuh,.... It's a Merc. I/O,..... 2" to 2, 1/4" thickness,.....

He says it's double 3/4" plywood in the first line of post #1 in this thread and then says it again in post #8... but who knows for sure eh?!

Maybe PK has an 1-1/2" transom where a 2"- 2 1/4" is supposed to be and that's why it's had to be changed out twice already?:noidea:

My suggestion stays the same whether it's a 1-1/2" or a 2 to 2-1/4", just change the size of the square tube.

 

[64osby]

IIRC there were two projects here where a square tube transom was attempted.

Pretty sure weldzilla was one, don't recall the other. Also don't recall the end result or any follow up.

 

[jbcurt00]

62cruiser or 64cruiser was the other. I think his is an OB transom on a LoneStar Cruisemaster

 

[PKomrowski]

He says it's double 3/4" plywood in the first line of post #1 in this thread and then says it again in post #8... but who knows for sure eh?!

Maybe PK has an 1-1/2" transom where a 2"- 2 1/4" is supposed to be and that's why it's had to be changed out twice already?:noidea:

My suggestion stays the same whether it's a 1-1/2" or a 2 to 2-1/4", just change the size of the square tube.

No like almost all Merc cruser IO transoms, bondo is right, they make the stern at 1.50 ply then around the out drive they add another layer of wood, just around the oudrive where it bolts on. I know what your talking about bondo.

I have done a few, anyone that has done some of them would know as well.