Rubbing strake/wood bending advice

skodster

skodster

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Hello All,

I want to replace the strake on my 23' Jaguar (Inerga Puma). The strake will be 1" x 2" with a scarf or step joint.

Question - Would a piece of iroko of these dimensions bend around without having to be steamed?

The boat looks like this....



As you can see, the fiercest bend is midships.
 
skodster said:
Hello All,

I want to replace the strake on my 23' Jaguar (Inerga Puma). The strake will be 1" x 2" with a scarf or step joint.

Question - Would a piece of iroko of these dimensions bend around without having to be steamed?

The boat looks like this....



As you can see, the fiercest bend is midships.
Click to expand...

bend quite easily start @ the transom each side
 
I did my Vivacity 650 with iroko which has a similar hull shape without having to do anything special. The scarf was at the point of least bend radius and I had bolts through close on both sides. It does take a bit of effort and I had a bit of a double bend to cope with. The strake profile I had was an L shape half moon(shaped by Robbins to match the original) to go over the hull deck join so was about 1.5 by 1 inch. I worked from the middle to each end. I don't know if that is the best way but meant I could make it overlength and then cut and shape the ends after it was fitted. Being a L shape it made it easy to get the strake in the correct place. There can be a bit of a variation in the stiffness of a strip depending exactly where is was cut from tree. I had one bit stiffer than the rest so made sure I used it where it did not have to bend so much.

Regards
Neil
 
Depending on the length of timber you can get will influence where you place the scarph.
try to keep scarphs to bow or stern areas where the run offs are reasonably straight and use the length of the timber as a lever to assist in the bending process round the maximum beam area
 
I steamed iroko for a West11 dinghy. It certainly wouldn't have bent dry on that but I suspect your boat may have much more gentle curves.
When I was showing a friend (who has been boating on the lake for almost 70 years) he suggested doing what the boatbuilders used to do in Bowness which was to put the wood, weighed down, in the lake for a few days.
 
Lakesailor said:
I steamed iroko for a West11 dinghy. It certainly wouldn't have bent dry on that but I suspect your boat may have much more gentle curves.
When I was showing a friend (who has been boating on the lake for almost 70 years) he suggested doing what the boatbuilders used to do in Bowness which was to put the wood, weighed down, in the lake for a few days.
Click to expand...

Despite your friend's experience - which I don't doubt but may use a different mechanism - the purpose of steaming is not to get the wood wet but to get it hot. The aim is to soften (melt, effectively) the lignin matrix through which the cellulose fibres run. From memory, that happens at about 80C, so anything which gets wood that hot will soften it nicely. Steaming is simply a convenient way of taking it to the required temperature without any risk of burning, but hot sand or hot air would do the job just as well.

A boat builder pal of mine recommends wrapping thinner sections in towels and pouring kettles full of hot water over them. Useful if you have one tightish curve in something otherwise straightish ... might work for the OP?
 
UBK is right about temperature being the important factor and for that reason it is important that the wood is steamed for long enough to be heated right through. But the moisture also plays some part in the process. I don't believe dry heat will be as effective as steaming although of course you will be heating the moisture already in the wood .
It is also important to get the wood bent to shape quickly before it has cooled.
 
Many years ago I had to bend a piece of 2" x 1" mahogany (edgewise) to follow the curve of the deck and I used a length of cast iron drain pipe with one end sealed with cement and set at an angle with water in the bottom which was heated up with a butane torch. The upper end was plugged with rag.

As far as I remember I steamed it for about an hour and then it bent easily round a former. You have to be fairly quick putting the full bend in it before it cools because once it's bent you can't put it back in the pipe. Have all your clamps/fastenings ready in the right place. Also attach a piece of string to the timber so that you can pull it out of the pipe.
 
A scarf joint can produce two feather edges one of which is very vulnerable to breakage. Consider making the joint in the vertical plane with stopped ends. I am sure someone on the forum will know the technical name for this but it avoids the feathered edges. Little bit fiddly on such a small section.
 
I used a former to get the bend about right. In my case I had to bond the capping strip into place over a lip so trying to bend and secure in one process would have been too difficult.

finished.jpg

I used a plastic drain pipe initially (which drooped with the heat) and a kettle.

insulation.jpg

Both poor ideas.

MkII had the hard plastic centres from spools of bale wrapping and I got a wallpaper stripper for about £20 from Focus which supplied loads of steam for a considerable period.

That worked.

Westrepair05.jpg
 
parsifal said:
Many years ago I had to bend a piece of 2" x 1" mahogany (edgewise) to follow the curve of the deck and I used a length of cast iron drain pipe with one end sealed with cement and set at an angle with water in the bottom which was heated up with a butane torch. The upper end was plugged with rag.
Click to expand...
Some years back I tried the same thing with a length of PVC drainpipe and an electric kettle. The PVC melted.

Cast Iron sounds good.
 
Did a tubby little 19 footer with a much sharper bend than yours - no problem doing it cold. I agree with the previous comment about a stepped scarph - more fiddly but safer.

My boat I was lucky that it had a lip on the deck edge which was deep enough to be able to set up plenty of clamps, so that I could pull it in properly before placing each fastening, rather than using the fastening to pull it in.

I started with the end that had the maximum curvature - in my case the bows, so that I had plenty of length to apply leverage to get the timber in to the curve. it is much easier if you have supports to hold the timber at the right height while you pull it in to the hull, particularly at the early stages. Looking at yours, I agree it will be better to start from the stern, if the drawing is accurate.
 
When the temperature in a water bag is 40*C you know it's going to a tough day playing with steam.

The best alternative I have used is to scoop out a cup shape on the back of the strake, I just used a router, effectively making the timber thinner so easy to bend.

Good luck.:)
 
skodster said:
Hello All,

I want to replace the strake on my 23' Jaguar (Inerga Puma). The strake will be 1" x 2" with a scarf or step joint.

Question - Would a piece of iroko of these dimensions bend around without having to be steamed?

As you can see, the fiercest bend is midships.
Click to expand...

I think you'll manage without steaming it especially as there appears to be no significant vertical curve in the sheer line. If there is then you should really spile and cut it not bend it.

Have you considered laminating it out of two half inch thick bits?
 
Steaming in a bag is the easy way.
1) Fasten the wood at one end, in situ.
2) Slide a length of polythene tubing up to the point where the bend starts (Lengths cut from plastic sacks and doubled over and stapled work just as well)
3) Poke the pipe from a DIY wallpaper steamer into the end, and seal with rags or staples
4) Turn the steam on, and after a few minutes, gently try bending. Give it a bit longer if it isn't ready.
5) Move the bag in stages along the wood, fastening it behind as you go

The beauty is you only heat the bit you are bending, so there are no problems holding an entire hot length, and you are not racing against time to get the whole 20 feet in position before it cools.

For more complicated bending, eg internal ribs, or hull planking, bend leaving the bag on and then rip it off once in position. Once you have used steaming-in-a-bag you will wonder why anyone ever bothered with steam boxes.
 
I've fashioned my section of rubbing strake from teak (it cost), I'm told it cannot be steamed, but its dimensions will allow it to be bent without too much difficulty - we shall see.
 
cliffordpope said:
Once you have used steaming-in-a-bag you will wonder why anyone ever bothered with steam boxes.
Click to expand...

Probably because when they built wooden boats they didn't have plastic bags. (Although I suppose a length of fireman's canvas hose would do just as well)

But it's a brilliant idea. If I ever have to steam bend any more timber I'll certainly try it before I go hunting down the garden for my old bit of cast iron pipe.
 
I did exactly the same on a Vivacity 20 several years ago. I also had the section machined up locally and also was scarfed. Once scarfed, the overall length was about 2' longer than the actual length required. Start at the bows through bolting with mastic. The strake was held close to the hull with a ratchet strap around the hull, bolted through and then restrapped with a tourniquet at the aft end to pull the end in. The surplus was then cut off after the last bolt was in. It worked well.
 
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