Steel boat new roof ?

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Johnny5

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Hi all
I planning on extending my wheelhouse roof to extend over my open rear deck
It's a small all steel cabin cruser about 7m long the roof will be just under 3m supported half way by the existing wheelhouse with a frame and rear supports so I'm adding approximately 1.5m to the length
I'm just a little lost on what thickness steel to use I was thinking about 2mm but I wonder if anyone has any thoughts or experience with this type of work ?
It will obviously have box sections to support the edges and some sections under the main roof
Also the boat currently has no ballast after adding the extra weight Should I add some ?
Any advice would be appreciated
 
Johnny, any chance of a picture please ?

It's a bit difficult to imagine where the extra length is going, and without seeing the hull form, to guess at any extra ballast that might be needed.

Have you considered alternatives to steel ?
 
IMG_1238.jpg
I have considered a timber roof but I feel as it's a steel boat it would be more in keeping and I wouldn't really save much it terms of weight
 
So you are doubling the length of the present roof ? Is that where the extra 1.5m comes in ?

There will be some extra weight (aluminium sheet and poles will be a lot lighter) but a quick estimate - nothing more - is that 2mm sheet steel of 1800 by 1500mm will be about 40kg, and likely to double that with the box sections, so 80Kg.

Think of a medium-sized person sitting on the roof. I'd look at putting at least 80Kg in the bottom of the bilges.

Those masses above and below the metacentric height will make the boat behave differently, a longer roll period for a start. I'd also be concerned about the effect of the extra area on windage, with the boat being more susceptible to wind when heeled.

What about a canvas bimini ? It would be much lighter and could be folded/rolled in fine or foul weather.
 
Thank you for your reply
Yes I am doubling the existing roof length I will be cutting the old one off and making a complete new one over the full length
It has got a canvas cover at the moment but it's seen better days and with it up I don't have the headroom I would like so with a flat roof and then get three curtains made for the sides will enable me to make better use of the space without having to fold the canopy down every time
For the extra ballast I was thinking 3 to 4 bags of pea shingle (in the bags) in the bilges ? (25kg each)
I will take a look at using aluminium sheet etc but I can see this being much more expensive
If I do end up going for steel would the 2mm sheet be enough not to distort or dent if I need to get up there ?
Many thanks
 
Johnny5 said:
Thank you for your reply
Yes I am doubling the existing roof length I will be cutting the old one off and making a complete new one over the full length
It has got a canvas cover at the moment but it's seen better days and with it up I don't have the headroom I would like so with a flat roof and then get three curtains made for the sides will enable me to make better use of the space without having to fold the canopy down every time
For the extra ballast I was thinking 3 to 4 bags of pea shingle (in the bags) in the bilges ? (25kg each)
I will take a look at using aluminium sheet etc but I can see this being much more expensive
If I do end up going for steel would the 2mm sheet be enough not to distort or dent if I need to get up there ?
Many thanks
Click to expand...

Have a look into a commercial vehicle scrapyard for aluminium sheet from lorry sides. Cheaper than steel, easier to 'drape' over a framework, and can be rivetted with a simple hand press.

Any metal 'roof' supported by framework as you describe will deform if stood on. I'd be tempted to consider GRP - again, you might get a large strong sheet from a scrappy, and cut it to shape.

For the ballast, gravel has a density of around 1.6, steel around 7.8.


Try and stay away from the steel solution. wood or GRP is much more boat-friendly.
 
1.5m well supported steel could easily manage at less than 1mm thick, especially if you put a nice bend for a slightly arched roof. The problem is corrosion, thin plate just evaporates as you watch.
Your best bet is to consider the weight, even try the boat with some extra ballast where the new roof will be and see how much you can carry before it goes daft.
 
I've just re-read the OP's replacing all the roof not just adding 1.5m. That makes it double the mass (now 160-ish kg) at the highest point.

That's going to make some interesting stability/behaviour issues. I like the idea of steel up top less and less.
 
Thank you for all your comments
I have considered marine ply for the roof and I regularly work with timber in my work but maintenance wise it would still be a pain more joints for the weather to get in etc as the boat is all steel I do think if possible steel would allow me to blend the new roof in rather than it looking like it's been added on ? and although heavy i was thinking timber would need to be much thicker to get the strength I would need as far as I know a sheet of 12mm marine ply would be around 18kg
So would there really be that much difference ?
I am only adding on another 1.5m although I'm replacing the whole section I will be removing the exsisting roof
I will look into a steel frame with aluminium sheets riveted on as that sounds like the best compromise

The existing roof is no more than 3mm thick and I had a look today and 2mm looks like it may be ok for what I need ?
So by removing the existing 3mm roof that as far as I can work out comes out at 123 pounds and replacing the whole thing with 2mm comes out at 164 pounds that's a difrence of 41 pounds (20.5 kg) I know I will be adding some extra steel for the supports but is it really that far out if I can get away by using 2mm sheet ?
 
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