Advice on scarfing joint in dinghy mast

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mezereon

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I've acquired a wooden dinghy (Oughtred Guillemot) without a suitable mast but with 2 unsuitable ones (ex mirror dinghy). My cunning plan is to scarf an extra length (additional 60cm) onto one of the masts by canibalising the other.
The resultant mast will be slightly thin (50mm compared to specced 60mm), but my greatest concern is that the mast will be unstayed and this may apply more bending force to the join than a stayed version.
So, advice would be appreciated please:
1, is this a good idea.
2, should the extra portion be at the top or bottom of the mast.
3, Epoxy or PU glue or ...
I would use a 12:1 joint made with a router jig.
 
Hmm, need more info really for a proper opinion. What sort of rig is it? You are only talking 2' extra, on a bermudian that is probably safe enough at the top.

Gaff & Lug sails can add a high load at the point where the main halliard block is positioned. You could use the top half of the second mast as a "topmast" with two bands holding it in place, or if bermudian, you could re-rig as a Gunter using either the mirror gaff or the top of teh second mast.

Think carefully about where the stresses will be, if you choose to scarfe a section in, remember the minimum 12:1 angle & have the angle running so that it meets the stresses as strongly as possible - rather than simply peeled open when the stress comes on.

Good luck

Edit;
Old harry, Ian Oughtred's designs ARE absolute classics.
 
Thanks for the answers. I may try the Classic Boats section as well. I didnt want to be accused of multiple postings;)
The rig will be lug fashion. I hadn't thought of using the second mast as a topmast. I'll have to think that one through.
 
I had a Heron dinghy where the mast snapped at the gaff halliard fitting (PO had taken it off & put it back the other way up - leaving 4 screw holes in the same plane!)

I scarfed a 2' piece of oak in across the break & it lasted fine until I found a cheap replacement. So it can be done. But it was clearly a repair - mainly because I used oak to patch pine. But it was a handy piece of firewood & so free.
 
Either end will be fine, either glue will be fine though theoretically the epoxy should be well varnished to prevent uv degradation.
What concerns me is that in the laws of bending ( unstayed mast) the difference between 50mm dia and 60 is really quite significant. If it is going to snap I reckon it will do it where the spar exits the thwart or deck, if the edges are sharp and hard there, so maybe fit a rubber collar.
 
Searush, you got me thinking. If I change the sail plan only slightly, I can have a loose footed spritsail, which is close to what you were suggesting, almost as simple to operate and no spar cutting involved.
Lateral thinking :)
 
mezereon said:
Searush, you got me thinking. If I change the sail plan only slightly, I can have a loose footed spritsail, which is close to what you were suggesting, almost as simple to operate and no spar cutting involved.
Lateral thinking :)
Click to expand...

And a really elegant soloution for an Ian Oughtred design - nice one.

Just for testing, you could set up the rig using the sail unmodified (assuming you already have a sail). It will set with a crease of course, but you can thoroughly test the principle before making any cuts.

Kindly post a pic when it is done. BTW, you may enjoy this forum too.
 
Hi, I have an Oughtred design too.

The correct joint would be a 'V' 'clothes-peg' joint at 12:1. But I would worry about the reduction in diameter. Stiffness goes up by a cube of the diameter.
Your sprit idea is probably the best way or you could use the Mirror gunter rig with stays.
A
 
Hey!

12:1 is good. Use epoxy and make the scarf at the bottom of the mast if it's stayed, or at the top if unstayed. Yours is unstayed isn't it?

For some pictures have a look at our blog, adress in my signature. The early posts from 2009 shows us doing the same thing to a 50ft wooden mast. Good luck!

/Hampus
 
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