steam bending of strakers and chines

We are restoring a Randell crayboat and have had to remove strakers and chines in order to remove rotted plywood to hull. Now we want to renew these timbers with karri rails. How do we steam them, considering that they are over 5 m long?

 

You could try a Steam Box to which you would have to make yourself to suit the lenghts that you want.

 

Some species steam well, while others don't and as a rule you don't steam anything unless you have to.

 

I would have to agree with you their PAR. Janbo has to find out what timbers he can use in order to use the steam box for his project.

 

The original timber is actually karri, so I was going to use the same again and I can't think of another way of bending the timber, without breaking it?

 

Heat is what does it and you'd be surprised how much you can bend a piece, if the grain is oriented properly and hasn't any run out. Green lumber is also quite limber, but breakage is to be expected, particularly if you're a novice at bending. Most breaks will be obvious to an experienced bender, the usual suspects are grain run out and other defects, making the bending candidate inappropriate.

 

Janbo, you need a beer keg......a gas ring, there's some that are used for a prawn cooker or a 4 ring cast iron one(its good to have a sheet metal shroud around the ring for wind), with the keg you need a pipe spigot welded out the top for the steam, the best kegs have a hole in the side so you can fill on the go(with boiled water so you dont chill the water already in)- plug this hole loosely with a timber bung the rest of the time.
For the steam box some 3/4 radiata ply is all you need, make the boxes in 2.4 meter lengths & butt strap together on the outside to suit your length(I had 2 x 2.4 & 1 x 1.8) about 250mm x 200 high is pretty good although mine was wider & handy as a low scaffold on saw horses. You pretty much use the keg & burner as the support one end- the joins are supported on saw horses(mine I sleeved them together & "caulked" the joins with old towels/nappys- also wrap up in some old blankets) you need some small battens to the floor so your timber gets steam all around & doesn't block the steam spigot-make a donut of toweling for here too, the spigot goes in the "blind" end, you need a door on the opposite end...... of course.
This is pretty much the setup that has worked in a few yards I've worked in (& my own) "the rule of thumb" for cooking is an hour an inch thickness but will vary on timber species etc & more needs more. you will need gloves & clamping systems too, often we'll pre drill the starting end of the bend, you need to work quick & "melt" the bend in, clamping & winning the bend as you go, a team is usually required. Sometimes we use the boat as the form but sometimes will template & jig the bend. It's only a job of work, just have a go & be carefull as bending stuff esp big has stored energy safety issues.
Regards from Jeff

 

They dont steam in Thailand just heat with a blow torch that ive also seen being used to melt roads so yes im talking a huge blow lamp. They secure at the stem fitting then with many many g clamps then start to heat slowly bending in the 1inch and a half plank. They also use a engine hoist to chain pull the plank in, down or up.

They liberally paint on something that looks like grease. I think to stop scorching but I don't think it is. when it reaches home it is nailed with 8 inch galvanized nails with a sledge hammer or drilled through the ribs and bolted.

They make it look like childs play.

 

Thank you all for your very helpful advice, we are going to try using a 75 mm insulated PVC storm water pipe, with the steamsource fitted to the centre of it and with a vent in one end. We will cut and shape the karri to the right dimensions, drill a screw hole in each end and use the boat as the mould, when we have steamed it for a couple of hours. A bit scary, but it should be fun!!

 

Hi steam bending karri is very difficult tear your hair out frustrating timber its better boiled but we have found that 10mm x 70mm works quite well and stack them to required thickness clamping as you go when dried take them back off and laminate with epoxy back on and clean up.i have done this on quite a few sister frames in 85-90 foot fishing trawlers when i read the beer keg post i thought he was going to say drink it first before making any quick decisions i was disapointed when it was to be the steam heater.

 

A torch and a helper wetting the wood with a brush works very well. It is better if you submerge the wood to be bent for a week or so.

 

A PVC pipe will distort at steaming temperatures, so must be well supported.

Soaking the piece for some days prior to steaming is very helpful, but again stock selection and application are the real keys to breakage rates below 20%.

Of course you could skip all of this and just laminate the stringers and chine logs and eliminate all the fuss. Personally I don't think you'll need to steam as much as you think. Maybe you can post some pictures. Most look at a bent piece of wood and think, damn I'll have to steam that, but in reality, wood loves to bend to a degree and with some convincing, such as some heat at the tightest portion of the bend, you can get it to do all sorts of things, without a steam box.

 

I have done a lot of steam bending on smaller boats, and I think the traditional steam box is way too sensitive, too much trouble and results in lots of broken pieces, even with high quality (expensive) wood.

What I do now and works far better is to submerge the wood in water and heat up the water until the wood is limber. You can leave it soaking for hours, the wood is not nearly as sensitive to quality (I have bent successfully wood with grain run-out, knots, sap pockets, and other defects without splitting). I have even used very old, dry, poor qualtiy salvalved lumber without splits or breakage.

For your long piece get a length of metal pipe, put the wood in filled with water, prop it up at an angle, and start several fires under the pipe (or you can use several burners or camp stoves). You do not need to heat it until it boils, just hot enough to form the tiny bubbles (just below boiling), and leave it in long enough to get it limber. You can even take the wood out, test "fit", and if it will not bend all the way, put it back in the heated water pipe for another 10 min or so. I find the smaller kayak and sailboat ribs only need about 15-20 min, but they can stay in much longer without issues.

I have also used a large pot of heated water, and slowly fed in the lumber, bending it as it goes. Than feeding it though the water, bending it as you feed it through the heated water (wear heavy gloves!).

It is simple, does not require a lot fancy equipment.

Good luck.

 

Thanks again, Guys, now my head is REALLY spinning...!
I thought the idea was not to increase the moisture content to a degree where we cant epoxy/dynel the strakers without waiting for the timber to dry..? Am I wrong or..? I have read that steaming timber actually dries it , as opposed to soaking it. I will take some photos of our project so you can see what we are dealing with.

 

Steamed or boiled water leaves the wood very rapidly. You will not get any drier wood as a result. It will find and eventually settle at it's natural equilibrium point in regard to moisture content.