9m Steel Catamaran; thoughts?

The Skoota 32 built in infused foam sandwich costs GBP150,000 complete with engines, interior etc and including 20% VAT

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Advantages of steel.

1.It will be quicker to make. You can get 3mm plates lasercut to an accuracy of about 0.1 of a mm if the designer supplies you with the computer information so there should be no lack of fit. Properly done you can even have all the holes you need for bolting and through fixings, so for instance you wont have to drill for fixing winches, etc. We even have holes cut to tapping drill diameter so all we have to do is run the tap through.
You can get the supplier to engrave the surface with part numbers and setting out locations ['x' marks the spot] but this needs to be on the computer files, they won't respond to a separate set of information supplied freehand on a sketch or verbally like 'can you put an extra hole there?'
You can have the scantlings notched so you weld continuously where they touch and don't leave a water trap where they are not welded.
3mm steel laser cuts wonderfully without distortion but a good quality steel is required and it won't be available at a rock bottom price.It can be had blasted and primed so the boat wont go rusty while you're making it apart from the welds, which you can paint straight away. This way the final blasting prior to surface treatment will be less arduous and time consuming.
Laser cutting is very common, any small town will have several places that can handle 3m long sheets but you may have to look further afield if you want the whole sides cut from one piece since apart from a large laser they will need de-coiling facilities. Of course these long sheets may be too heavy for you to handle. If you decide to cut the stuff yourself with an angle grinder you may save money but the job will be vastly more arduous and it will be difficult to get as good a finish and you'll spend all day looking like a coal miner. You really should find your nearest laser profiler and pop down to see what they can offer. You wont get this for £300 a tonne.
2. Mig welding 3mm mild steel is dead easy and a few days practice after being shown how should be enough to get you started unless you're ham fisted. You wont need a big expensive welder either, a good quality hobby one will will be OK, get one that takes a decent sized coil of wire and a decent sized argon bottle and you'll whistle trough it.You will need a sheltered wind free environment so the gas doesn't blow away. Automatic welding masks that shade the instant an arc strikes up help no end and are cheap. Avoid stick welding like the plague, its all stop and start, wire brushing and much more skill is required.
3.The steel hulls will be stronger than other alternatives and are unlikely to get holed, but if they do the boat may sink, whereas a wooden one won't. This is why fishermen loved their old wooden trawlers, possibly erroneously.
4.If it's heavier then it will need a bigger engine.Only you know what sort of a hurry you are in.
5. Properly made [no spot welding please]; properly treated and properly looked after there is absolutely no reason why a 3mm steel boat shouldn't last 50 years but maintenance is the key. Of course you can cut corners, lash it up and watch it rust away in a few years.
6. Steel boats are easily reparable but of course you may have a lot of stripping out to do so you don't set the lining on fire.
7.It will be difficult to get as fair a finish as with a plywood boat, it can be as shiny but internal welds will show their presence on the outside as a general rule.
8.Get genned up on galvanic corrosion which can be disastrous on a steel boat if good practice is not followed, but need not be a problem to the wise.

 

That's a very competitive price, especially for a UK boat . I like your designs very much, but unfortunately on my pension income I can't afford capital payments, so home building is my only option.

Foam sandwich would make a very sensible option for good resale value

 

Many thanks for all that info; I think you're right, steel makes the best home build material choice .

Every engineering choice comes with a compromise , with steel its the weight; not insurmountable with thin 3mm plate

 

I believe modern ply and epoxy methods are by far the best for the home builder. I know your set on metal steel has too much ongoing maintenance issues for me. Encapsulated ply or ally are the way to go.

 

I don't like the OP's design choice that much, and really when you add all the real costs of a complete boat, steel would not be an ideal choice.

 

Maybe a less slender "stabilized monohull" design would be better for steel construction?

Something like this: http://www.kastenmarine.com/PowerTrimaran.htm

Or possibly a proa configuration, a much smaller cousin to the gawkish Asean Lady mega-yacht?

 

Steel fails for small recreational boats because the guage is too thin to withstand rust pitting, unless you are very vigilant. OK to say that improved coatings have altered the equation, but you still have a maintenance issue that other materials do not. Even alloy has corrosion issues to be aware of, but steel in a salt environment particularly, absolutely must be isolated from O2.

 

Now they got carbide tooth saw blades that work on steel. UniStrut Inc uses carbides to cut struts out of about 3mm all day long. Very loud.

For more calm cutting, regular fine tooth metal cutting jigsaw blades work better than abrasives even on stainless, you just gotta go slow, and change the blades often. But it can be done without even a hint of discoloration and without blowing sparks or lots of noise and smoke...so you don't set off alarms etc.

IMO the only time to use abrasives is if you can't get a jig or circular blade to cut that deep, or the abrasive is cheap for one time cut.

Worked with guy who had a special heirloom short stroke jigsaw for metal.

 

I found that epoxy on a small ply boat kept on getting chipped and deeply scratched when i launched the dinghy. Even small bangs cracked the surface coat.

IMO epoxy and gel coat have approx the same durability and toughness which could let moisture into ply esp under the water line where you're likely to get scrapes in normal day to day use.

Of course you could say steel coated with two pack epoxy has exactly the same problems.

 

I agree with you, I don't like the superstructure of the original design. My plans are to replace it with a trawler wheel house like the Coastal cat photo in post #23 built in ply to save weight.

My test is ' could the boat be tied up with commercial boats without looking out of place'

 

I love the idea of a tri, but how do you step off the damn thing to tie it up esp if you are short handed? Next year I hit the big six zero ...


At least a Cat operates like a normal boat , you can even have large side decks if you wish.

 

In remember looking at George beuhler's diesel duck designs in the 1990's when he first started, and back then everyone thought he was crazy advocating steel in the age of GRP. Seems to have a big following now.Glen L designs have several thin plate boats including the lovely union jack 30' mini steel pilot house in 3mm. Bruce Roberts also has some very nice micro sailing boats in 3mm that have crossed oceans etc etc

you have a valid point re corrosion ; but bearing in mind that an alloy hull with damage below the waterline can dissolve in front of your eyes if its tied up to a steel piling in a salty environment or if there's a problem with stray currents from the shore power connection .

Not forgetting that ply is laminated wood !

 

What about small hand held profile cutters?

Has anyone had experience of these, and what amperage would be required for thin 3mm plate?

 

Homebuilders can also build the Skoota 32 in foam sandwich from plans. But it sounds like you are determined to build in steel. In which case I do not recommend any power catamaran under about 15m, preferably bigger

Richard Woods