Aluminum transom vs. wood???

[mcattac]

Anyone know someone who has done this?
I know that aluminum is stiffer than plywood when equal thicknesses are compared. But the exact ratio........I am not sure.

I had a 1.5" thick plywood transom in my 18' aluminum Starcraft.
(2 layers of 3/4" glued together)
I was all ready to replace with marine grade plywood.

Forget the cost of aluminum for a minute.
Because I get it cheaper than most.

How thick would I need to go with aluminum plate in order to match the strength and stiffness of the plywood?

Similar question with the floor.
Currently it is 3/4" thick.

How thick would I need to go in order to get the same strength and flexural strength if I used aluminum plate or sheet versus the 3/4" plywood.

I would love to eliminate all of the wood for obvious reasons.

Thanks.
Mac

 

[Bondo]

Re: Aluminum transom vs. wood???

Ayuh,.... I'd think 1/2" for the transom, With some structure... Maybe 3/8"... 'ell,.. .125 if Built Right....
And .060, or .090 for the decks,... With structure....

 

[ChriswithOsprey]

Re: Aluminum transom vs. wood???

Don't forget the vibration damping wood will give you over aluminum. I replaced 20-year-old 1.5 inch African Walnut transom on my pontoon boat with a similar piece of the same wood. When I cut into the old transom, it was still perfectly solid. It is a pretty impressive wood.

 

[Bob_VT]

Re: Aluminum transom vs. wood???

If you want to eliminate wood in the transom.... you could create a mold or pocket from thin aluminum stock and use seacast or nidabond. I also think you would have to go thicker than 1/2" and add some additional knee bracing.

The transom on a starcraft is fairly simple and can be replaced w/o much problem....... re-inventing the transom from all aluminum may give you more headaches.

 

[mattrogers5]

Re: Aluminum transom vs. wood???

I used 1/4 aluminum plate on the inside of mine ..thrubolted and glued to the wood transom...Mainly to seal and add support to the splash well side..following seas kept the splashwell wet all the time and I wanted to seal that area up. The end result was one very stiff transom!

 

[BWR1953]

Re: Aluminum transom vs. wood???

Anyone know someone who has done this?
I know that aluminum is stiffer than plywood when equal thicknesses are compared. But the exact ratio........I am not sure.

I had a 1.5" thick plywood transom in my 18' aluminum Starcraft.
(2 layers of 3/4" glued together)
I was all ready to replace with marine grade plywood.

Forget the cost of aluminum for a minute.
Because I get it cheaper than most.

How thick would I need to go with aluminum plate in order to match the strength and stiffness of the plywood?

Similar question with the floor.
Currently it is 3/4" thick.

How thick would I need to go in order to get the same strength and flexural strength if I used aluminum plate or sheet versus the 3/4" plywood.

I would love to eliminate all of the wood for obvious reasons.

Thanks.
Mac

Good questions! I'm thinking of eliminating wood on my little boat when the time comes.

 

[Stachi]

Re: Aluminum transom vs. wood???

if price is not a worry,...fill that pocket with 1.5 " aluminum...it won't ever rot out....I would put some rubber between outboard mounts and transom....

 

[mcattac]

Re: Aluminum transom vs. wood???

Here's what I decided to do.

The transom will be a constructed composite.
Two 3/8" thick plates separated by 3/4" solid square stock in the middle.
I will plug weld everything to bring it together.
In the middle, where the motor mounts and the bolts pass through, I will sandwich two 3/8" plates 18" wide by 12" high instead of using the square stock as spacers.
This will provide a very dense and solid area for the motor to mount and be bolted to.
With the additional 3/8" backing plate on the outside, and the hull skin of the transom, it will be darn near 2" thick of solid aluminum!!!
I could hang 150-200 HP off the back if I wanted to - but that's not the goal.

I will then MIG in the entire composite assembly into the transom.

This transom will be stronger than heck and bulletproof.
But best of all, if I want to mount anything on the outside, I can.
Stuff like trim tabs, transducers, eye bolts, etc.
And it won't rot the transom from the holes being drilled in and through it.


For the floors I have decided on 1/4" aluminum plate.
The largest span is only about 27".
So I placed a 4' x 4' plate of 1/4" aluminum on top of two 2x4's spaced 27" apart and I stood my fat butt on it.
It did flex a wee bit if I jumped up and down a little.
But the wing-ends that extended outside the 2x4's were not anchored down!
So I figured that when they are anchored down the flex in the floor will be minimal, if any at all.


The 1/4" plate for the floor will add about 80 pounds to the boat as compared to a 3/4" plywood floor.
The transom will add about 50 pounds.
BUT, I feel the rod lockers (that will double as a step when boarding) will be less in weight being fabricated out of light gauge aluminum, as compared to plywood and 2x2's. So the end result I think will be 100 pounds or less in added weight over using the plywood.
And that was compared to standard plywood, not pressure treated or marine grade, which does weigh a little more.
Also, I was going to coat all the plywood with epoxy resin, and that adds a little more.
The end result might even be a wash, but not much to be concerned about.

BTW, a long time back I had considered that plastic composite decking fake wood junk and it is a lot heavier to use than plywood.
Even the light weight stuff is quite a bit heavier than plywood if you do an honest comparison.

After I am all done I will have all of it coated with Rhino Liner.
It will act as sound deadening, provide a lighter color so as to reflect heat, and be a nonskid surface.

Mac

 

[Stachi]

Re: Aluminum transom vs. wood???

not to mention that the fake wood decking material has NO structural strength , and has an expansion/contraction rate that is unacceptable for this use....tight lines bro !

 

[BWR1953]

Re: Aluminum transom vs. wood???

Here's what I decided to do.

The transom will be a constructed composite.
Two 3/8" thick plates separated by 3/4" solid square stock in the middle.
I will plug weld everything to bring it together.
In the middle, where the motor mounts and the bolts pass through, I will sandwich two 3/8" plates 18" wide by 12" high instead of using the square stock as spacers.
This will provide a very dense and solid area for the motor to mount and be bolted to.
With the additional 3/8" backing plate on the outside, and the hull skin of the transom, it will be darn near 2" thick of solid aluminum!!!
I could hang 150-200 HP off the back if I wanted to - but that's not the goal.

I will then MIG in the entire composite assembly into the transom.

This transom will be stronger than heck and bulletproof.
But best of all, if I want to mount anything on the outside, I can.
Stuff like trim tabs, transducers, eye bolts, etc.
And it won't rot the transom from the holes being drilled in and through it.


For the floors I have decided on 1/4" aluminum plate.
The largest span is only about 27".
So I placed a 4' x 4' plate of 1/4" aluminum on top of two 2x4's spaced 27" apart and I stood my fat butt on it.
It did flex a wee bit if I jumped up and down a little.
But the wing-ends that extended outside the 2x4's were not anchored down!
So I figured that when they are anchored down the flex in the floor will be minimal, if any at all.


The 1/4" plate for the floor will add about 80 pounds to the boat as compared to a 3/4" plywood floor.
The transom will add about 50 pounds.
BUT, I feel the rod lockers (that will double as a step when boarding) will be less in weight being fabricated out of light gauge aluminum, as compared to plywood and 2x2's. So the end result I think will be 100 pounds or less in added weight over using the plywood.
And that was compared to standard plywood, not pressure treated or marine grade, which does weigh a little more.
Also, I was going to coat all the plywood with epoxy resin, and that adds a little more.
The end result might even be a wash, but not much to be concerned about.

BTW, a long time back I had considered that plastic composite decking fake wood junk and it is a lot heavier to use than plywood.
Even the light weight stuff is quite a bit heavier than plywood if you do an honest comparison.

After I am all done I will have all of it coated with Rhino Liner.
It will act as sound deadening, provide a lighter color so as to reflect heat, and be a nonskid surface.

Mac

Sounds really great! Looking forward to seeing pix of the project.

Have you considered adding longitudinal box stringers under the floor to act as stiffeners? Maybe a matrix? Not much extra weight for a great deal more strength.

Keep us posted!

 

[mcattac]

Re: Aluminum transom vs. wood???

Sounds really great! Looking forward to seeing pix of the project.

Have you considered adding longitudinal box stringers under the floor to act as stiffeners? Maybe a matrix? Not much extra weight for a great deal more strength.

Keep us posted!

Yes, but decided to simply brake some sheet strips to make angles and will MIG across from stringer to stringer. This will provide plenty of added strength.

I'm adding a belly fuel tank and will need to rais the floor about 3.5-4".
So we need to increase the stringers and thought about 2x4 box tubing.

But will probably just add to the height with angles fabbed from breaking sheet like I mentioned for the cross braces.

I will be taking plenty of pix.
I'll make sure they get posted.

I bought a new Yamaha 60 HP 4-stroke tiller for the boat. The boat was a bowrider and I'm making an open fishing boat out of it. I feel the 60 HP would be marginal on a heavier console type boat as it originally was. But I have removed a lot of weight and with relocating the fuel under the floor, this will help the boat to plane. Additionally, the batteries will all go forward to help keep the bow down. (You may have also noticed my mention of trim tabs in my earlier post - more to keep the bow down while planing).

But I won't take possesion of the motor till next April. So the boat is a project to complete over the winter. I fortunately have a heated and insulated garage and also have a buddy that owns a large machine shop, where it currently sits.

No rush!
Have all winter.

Mac

 

[BWR1953]

Re: Aluminum transom vs. wood???

Yes, but decided to simply brake some sheet strips to make angles and will MIG across from stringer to stringer. This will provide plenty of added strength.

I'm adding a belly fuel tank and will need to rais the floor about 3.5-4".
So we need to increase the stringers and thought about 2x4 box tubing.

But will probably just add to the height with angles fabbed from breaking sheet like I mentioned for the cross braces.

I will be taking plenty of pix.
I'll make sure they get posted.

I bought a new Yamaha 60 HP 4-stroke tiller for the boat. The boat was a bowrider and I'm making an open fishing boat out of it. I feel the 60 HP would be marginal on a heavier console type boat as it originally was. But I have removed a lot of weight and with relocating the fuel under the floor, this will help the boat to plane. Additionally, the batteries will all go forward to help keep the bow down. (You may have also noticed my mention of trim tabs in my earlier post - more to keep the bow down while planing).

But I won't take possesion of the motor till next April. So the boat is a project to complete over the winter. I fortunately have a heated and insulated garage and also have a buddy that owns a large machine shop, where it currently sits.

No rush!
Have all winter.

Mac

Way cool!

 

[mcattac]

Re: Aluminum transom vs. wood???

There's actually another thread elsewhere here in "Boat Restoration and Building" that I started a while back with my "original" plans.

I was originally going with a single console, increasing the height of the transom to accept a 25" shaft 90HP Yamaha 4-stroke and 8-9 hp 4-stroke kicker.

But since that original thread, I decided to change things the more I looked at everything.

My buddy bought a new 60 hp 4-stroke Yamaha for his 73 Whaler this past summer (on my recommendation). Anyway, I have been out in his boat a few dozen times since and I absolutely fell in love with that motor. It runs beautiful. Really stingy on fuel and trolls WAY down for walleye when we want to.

So I got to thinking........"Why not?".

It would really save me a lot of money.

So I got looking at everything on the boat to get rid of weight and going to a tiller has allowed me to really peal some away. Going to one motor on the back saves about 110 pounds alone. And the weight difference from the 90 to the 60 is huge too!

One of my targets when starting this boat project was to create a rig that was very light to tow long distances (Florida) and was also very fuel efficient.

Then I saw a very small article in In-Fisherman magazine a month or two back where they said they would go with a tiller type boat AND 60hp 4-stroke for their favorite multi-purpose rig.

Then I found a test report on the Yamaha website of just such a boat. One very similar to my old Starcraft here.

Here's a link to the test report -

Yamaha 60 HP Tiller

The link shows a boat that is 16' 11" and mine is about 17' 6".
But the boat in the test is 84" wide and mine is only 78".
So I think they are very close as to the amount of water they each "push".
Of course width is always a more contibuting factor than length typically.

When I am complete I will be weighing the rig too in order to see how close I have calculated everything.

Stay posted to this channel kids!