Replacing a plywood deck on a steel boat.

fanakaman

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Has anyone experience of replacing a ply deck with steel? Phanakapan and I have a 36ft steel Van de Stadt sloop. The deck hull join has failed and the deck ply is rotten. We can replace the deck with ply but we are considering steel.
We are thinking ... take a template from the existing ply sections.... have a steel fabricators make similar in steel... 2.5mm thickness then employ a welder to weld for us or buy welding gear and learn the welding art.
Any thoughts, ideas or pointers to the literature?
cheers
 

William_H

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I have no experience of steel boats. However one alternative might be to GRP over the rotten ply wood. Now this can be a terrible bodge or a good fix. It all depends on the thickness of GRP. I would advocate GRP of something like 6mm thick. This should be strong enough and stiff enough to be a deck in itself even if the ply wood disappears or is removed. Much of course depends on the suppoprt structure under neath. Like ply wood GRP is not stiff in large areas. But preumably there are deck beams underneath.
GRP could be polyester resin although epoxy woulld make a better seal to the steel around the edges. Polyester is cheaper and easier to accelerate hardening in cold weather with more hardener.
If you are considering the stiffness as a problem You might consider foam core to increase the thickness dimension. Or if thickness is a problem you might consider and price carbon fibre in lieu of glas. Having much greater inherent stiffness.
As said this might end up being a terible bodge however the advantage is that you could deal with the worst areas of damage first and keep sailing and of course easy diy. Certainly get more advice. good luck olewill
 

bristoljim

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The deluxe solution would be Airex or similar foam & Polyester resin for economy & the outer laminate must be reasonably thick.Lay up sheets roughly cut to shape off the boat then tape in position.Gives insulation & few stringers required.Steel is a bit heavy for your size of boat & 2.5mm will require stringers at frequent intervals.Economy system is epoxy sheathed ply,should be ok but requires great care with penitrations for fittings etc to keep water out of the ply.
Jim
 

KellysEye

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Steel would best but the problem is immediately after fabrication the top and bottom of the deck need to be painted with Zinc rich epoxy. Check whether the fabricator can do that or if you need to do it. Ideally it should be painted when the humidity is low otherwise flash rusting can occur. With all that you might consider GRP a better bet.
 

jerrytug

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Steel would be the way to go, but: work out the weight of the plywood which was there, and compare it to the weight of all the steel you intend to install. You don't want to be making her tender by adding lots of weight high up.

If you can get away with 3 mil sheet, weightwise, it will be noticeably more resistant to dents etc.

Don't forget the steel deck fittings which should be planned at the design stage if possible.

There are plenty of books about steel boatbuilding on Amazon and ebay.

If you want to learn to weld there is a forum, Mig welding forum, with lots of advice.

In my opinion, the idea of putting GRP over damp rotten plywood is ludicrous, and would make the boat worthless.

Let us know how you get on! cheers Jerry
 

pmagowan

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Personally I would not contemplate steel on a boat that has been designed for ply when there are plenty of methods with much better weight to strength ratios.
 

AndrewB

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A 3mm steel deck in place of ply? Ask yourself, with a steel hull, why was a ply deck used originally? In the long term these materials very often cause problems for one another where they come into contact. (If water gets trapped between them electrolysis induced rot will occur in the ply, and subsequent rusting in the steel.)

It is usually done to save weight high up, the real bane of steel yacht design under about 45' LOA. Look VERY closely at the implications for the yacht's stability before going down this route.

You might find it helpful to look at Ian Nicholson's "Small Steel Craft" (Adlard Coles, 1983), where he discusses at some length the use of ply for decking steel yachts.
 
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jerrytug

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A 3mm steel deck in place of ply? Ask yourself, with a steel hull, why was a ply deck used originally, since the materials are not compatible? (Each eventually creates problems for the other where they come into contact - electrolysis induced rot in the ply, subsequent rusting in the steel.)

Probably to save weight high up, the real bane of steel yacht design under about 45' LOA. Look VERY closely at the implications for the yacht's stability before going down this route.
Well we can work it out, right here right now, to get a vague idea at least..
The specific gravity of steel, 7.8
The SG of plywood, 0.6 at a guess? It must vary a lot perhaps the OP knows what type of plywood it is?

Need to know: Original plywood thickness and deck area.

That's enough for a good estimate of the weight difference.

So a rough estimate would be: since the steel weighs about 10 times as much as the ply, per unit volume, the steel would have to be one tenth the thickness of the ply to weigh the same.
So 2.5 mil steel would weigh the same as one inch ply.
Or 3mil the same as 30 ply.
It's looking good for steel decks...on the back of my envelope anyway!
 
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ianj99

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Has anyone experience of replacing a ply deck with steel? Phanakapan and I have a 36ft steel Van de Stadt sloop. The deck hull join has failed and the deck ply is rotten. We can replace the deck with ply but we are considering steel.
We are thinking ... take a template from the existing ply sections.... have a steel fabricators make similar in steel... 2.5mm thickness then employ a welder to weld for us or buy welding gear and learn the welding art.
Any thoughts, ideas or pointers to the literature?
cheers

My 36' ketch motorsailer is 100% steel and 4mm throughout apart from the keel. Using anything thinner would result in both the location of every frame and stringer eventually showing, but also sagging between them over time where walked upon regularly. In other words it would look horrible.
You will also need to insulate the underside to avoid condensation: mine was foam sprayed throughout apart from the bilges and is warm in winter and cool in summer (when we have one).
Ply or a laminate will certainly be an easier diy job if you can get a good seal.
 

jerrytug

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My 36' ketch motorsailer is 100% steel and 4mm throughout apart from the keel. Using anything thinner would result in both the location of every frame and stringer eventually showing, but also sagging between them over time where walked upon regularly. In other words it would look horrible.
You will also need to insulate the underside to avoid condensation: mine was foam sprayed throughout apart from the bilges and is warm in winter and cool in summer (when we have one).
Ply or a laminate will certainly be an easier diy job if you can get a good seal.

Very true.
 

John the kiwi

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As a qualified welder, i would caution against the DIY welding route. There is more to welding than reading a book and putting a welding mask on.
There is also no substitute for experience when it comes to planning and executing weld schemes to minimise thermal distortion.
If you go the steel route you need to know the material the original steel hull was made of. Electrolysis will occur between different types/grades of steel.
You need a plan to shot abrade the new steel and prime it. Not easy on a new construction - hugely difficult on a existing craft i would have thought unless you are going to strip out the whole boat.
Steel is not as stiff as ply so will need extra beams as would any just glass alternative.
Few construction materials are as light and stiff as glass over ply.
Others have identified the weight issues with possible alternatives.....
All in all I would suggest replacing with the best marine ply you can get and sheathing it in a glass/epoxy system with the best edge sealing techniques you can manage.
I suspect the failure of the current deck system is down to poor materials or poor execution or lack of maintenance.
If you redo it it in plywood, then you don't have to worry about strength, stiffness, corrosion or weight and stability issues and your boat will still be within the original design parameters.
Do it right and it will last a long time.
Good luck.
 

ianj99

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Well we can work it out, right here right now, to get a vague idea at least..
The specific gravity of steel, 7.8
The SG of plywood, 0.6 at a guess? It must vary a lot perhaps the OP knows what type of plywood it is?

Need to know: Original plywood thickness and deck area.

That's enough for a good estimate of the weight difference.

So a rough estimate would be: since the steel weighs about 10 times as much as the ply, per unit volume, the steel would have to be one tenth the thickness of the ply to weigh the same.
So 2.5 mil steel would weigh the same as one inch ply.
Or 3mil the same as 30 ply.
It's looking good for steel decks...on the back of my envelope anyway!

Not when you take into account the weight of the frames and stringers, which would be numerous with 2.5mm plate.

If the overall weight using steel is going to be significantly more than the original ply then, as has been said, this will affect the stability, as well as how low the boat sits and the sailing & motoring performance.

You need to do some more work on the weight aspect and try not to increase it significantly.

Also heed the advice re diy welding - you need a skilled welder. I hired one to replace some rotten sections under the chain locker and head and was highly impressed. He cut perfect semi circles (2-4" radius ) with a 5" angle grinder & 1mm cutting disc, (square corners are a no no he told me) , his welding (with a stick welder) was beyond reproach, done on both sides and didn't leak a drop.

There is one other point. Once you start cutting and grinding steel, the dust and swarf will get everywhere resulting in rusty spots which will keep appearing for months. Also, you will not be popular (to put it mildly) with any neighbours unless you take stringent precautions to prevent them being contaminated as well.
 
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PlankWalker

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If its the boat I think it is, then Iam acquainted with the steel decked version of the mark. This of course has steel frames onto which the deck is welded and timber bolted for the internal structure to be fixed.
To weld onto this retrospectively would be a nightmare though not impossible, with the heat build-up causing scorching or minor fires to the internal structure, it doesn't bear thinking about.
I presume the boat must be well over 20 years old by now so my advice would be to rebuild another ply deck,with modern epoxy sheathing techniques it should last much longerthan the original.
I believe the deck has a considerable camber, the steel version does. So I would opt for 2 layers of 8mm perhaps 10mm or 3 layers of 6mm ply epoxied and stapled together,overlapping the joints.

Plank
 
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fanakaman

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I have no experience of steel boats. However one alternative might be to GRP over the rotten ply wood. Now this can be a terrible bodge or a good fix. It all depends on the thickness of GRP. I would advocate GRP of something like 6mm thick. This should be strong enough and stiff enough to be a deck in itself even if the ply wood disappears or is removed. Much of course depends on the suppoprt structure under neath. Like ply wood GRP is not stiff in large areas. But preumably there are deck beams underneath.
GRP could be polyester resin although epoxy woulld make a better seal to the steel around the edges. Polyester is cheaper and easier to accelerate hardening in cold weather with more hardener.
If you are considering the stiffness as a problem You might consider foam core to increase the thickness dimension. Or if thickness is a problem you might consider and price carbon fibre in lieu of glas. Having much greater inherent stiffness.
As said this might end up being a terible bodge however the advantage is that you could deal with the worst areas of damage first and keep sailing and of course easy diy. Certainly get more advice. good luck olewill

thanks for that ..... the main problem is there is rust under the join now so wood needs to be removed to address this. I guess therefore this counts against the GRP option.
 

fanakaman

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The deluxe solution would be Airex or similar foam & Polyester resin for economy & the outer laminate must be reasonably thick.Lay up sheets roughly cut to shape off the boat then tape in position.Gives insulation & few stringers required.Steel is a bit heavy for your size of boat & 2.5mm will require stringers at frequent intervals.Economy system is epoxy sheathed ply,should be ok but requires great care with penitrations for fittings etc to keep water out of the ply.
Jim[/QUOTE
thanks.. existing deck has 2 layers of approx 9mm ply epoxy sheathed has lasted 15 years. In fact the ply system has held up its the join which obviosly leaked causing rust which has ruined the bottom layer of ply
 

fanakaman

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A 3mm steel deck in place of ply? Ask yourself, with a steel hull, why was a ply deck used originally? In the long term these materials very often cause problems for one another where they come into contact. (If water gets trapped between them electrolysis induced rot will occur in the ply, and subsequent rusting in the steel.)

It is usually done to save weight high up, the real bane of steel yacht design under about 45' LOA. Look VERY closely at the implications for the yacht's stability before going down this route.

You might find it helpful to look at Ian Nicholson's "Small Steel Craft" (Adlard Coles, 1983), where he discusses at some length the use of ply for decking steel yachts.

thanks for this sounds correct .... and the pointer to Ian Nicholson..... I will get that.
 

fanakaman

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Well we can work it out, right here right now, to get a vague idea at least..
The specific gravity of steel, 7.8
The SG of plywood, 0.6 at a guess? It must vary a lot perhaps the OP knows what type of plywood it is?

Need to know: Original plywood thickness and deck area.

That's enough for a good estimate of the weight difference.

So a rough estimate would be: since the steel weighs about 10 times as much as the ply, per unit volume, the steel would have to be one tenth the thickness of the ply to weigh the same.
So 2.5 mil steel would weigh the same as one inch ply.
Or 3mil the same as 30 ply.
It's looking good for steel decks...on the back of my envelope anyway!
thanks
the ply is about 12kg per square meter, steel 2.5 ml is about 20 kg 3.0mm 24kg so about double 10 msquare deck about 100kg difference. without stability calcs ... is that a lot... i am 77kg on a good day.
 

fanakaman

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As a qualified welder, i would caution against the DIY welding route. There is more to welding than reading a book and putting a welding mask on.
There is also no substitute for experience when it comes to planning and executing weld schemes to minimise thermal distortion.
If you go the steel route you need to know the material the original steel hull was made of. Electrolysis will occur between different types/grades of steel.
You need a plan to shot abrade the new steel and prime it. Not easy on a new construction - hugely difficult on a existing craft i would have thought unless you are going to strip out the whole boat.
Steel is not as stiff as ply so will need extra beams as would any just glass alternative.
Few construction materials are as light and stiff as glass over ply.
Others have identified the weight issues with possible alternatives.....
All in all I would suggest replacing with the best marine ply you can get and sheathing it in a glass/epoxy system with the best edge sealing techniques you can manage.
I suspect the failure of the current deck system is down to poor materials or poor execution or lack of maintenance.
If you redo it it in plywood, then you don't have to worry about strength, stiffness, corrosion or weight and stability issues and your boat will still be within the original design parameters.
Do it right and it will last a long time.
Good luck.

thanks this seems like very sensible considerations ..... very persuasive points.....I dont aactualy think there was much wrong with original work..... perhaps inherent lifetime of a wood metal join in these conditions ( 15 years) so what would you consider the best join and edge sealing techniques or where do i look for that info?
 

fanakaman

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If its the boat I think it is, then Iam acquainted with the steel decked version of the mark. This of course has steel frames onto which the deck is welded and timber bolted for the internal structure to be fixed.
To weld onto this retrospectively would be a nightmare though not impossible, with the heat build-up causing scorching or minor fires to the internal structure, it doesn't bear thinking about.
I presume the boat must be well over 20 years old by now so my advice would be to rebuild another ply deck,with modern epoxy sheathing techniques it should last much longerthan the original.
I believe the deck has a considerable camber, the steel version does. So I would opt for 2 layers of 8mm perhaps 10mm or 3 layers of 6mm ply epoxied and stapled together,overlapping the joints.

Plank

Yes there is quite a big camber .... its a V de S Seal design. It has 2 layers of ply with epoxy that has all held up fine its only the join to the margin plates which must of leaked in time ...... expansion differences i guess and then rust started and has destroyed the bottom layer of ply which eventualy soaked up to the top layer and spreading towards the boat centre.... its now rotten about 10cm in from the plates round much of the edge ....time to do something .... like replace it....
actualy we circumnavigated in her and returned last year after a 3 year cruise so she has seen big temp changes and lots of water? I guess she deserves a new deck.
 
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