scarphing-help for a novice.....

[yourmomm]

hi all. time has come to scarph new strake sections onto the old having repaired the rotten timbers in the gunwhales underneath. i had to remove a 17 foot section of a 33 foot strake (1/2" depth, 3.5" height). it is solid mahogany. it was impossible to save the removed strake, as some numpty had epoxied the whole thing on, and no amount of careful prying or fein power-tool work would release it in one piece. i am therefore saving the pieces from it to make plugs

my question is this: cutting the scarph into the new timber is no problem (i think, using a scarphing jig i can make up-although im feeling my way in the dark here too, truth be told....), but how on earth do i cut the complementary scarph into the strake that remains attached to the boat, without actually removing the strake first?! (which would destroy it). it obviously needs to be a precise cut to accept the join with the new timber being scarphed-in smoothly, and i am impeded from using larger power tools by the fact that the strake is still attached to the boat. the answer may be obvious but im stumped. any help appreciated...thanks ryan

 

[tillergirl]

Well you have to get to the back of the 'old' strake - of course - and you are going to want to fasten through the scarf anyway, so you are going to have to release some fastenings to the frames. You don't say how wide apart your frames are or exactly where the scarf is going - about mid-ships from your measurements I guess. Are you replacing the front bit or the after bit. I would expect if you are scarfing on a new aft strake, that as you release a couple of the fastenings at the end of the 'old' strake, that it will spring out a little. The more you release, the more the ability to get clearance behind the 'old' strake. I guess you are going to be cutting it at least 4" long (min 8 x thickness of plank). You may need to wedge the 'gap' open and you will need to get the area to be cut clear of the plank above. Then cut very carefully; probably you are going to be using the end of the saw to cut as you will have no or little clearance beyond the cut. Worth a new saw! If you can't do that, it's a chisel and slow careful work. But you still going to need to release a couple of fastenings.

On TG I was fortunate to be doing it on the beam shelf so I had clearance from the top but as I cut so the natural desire of the timber to spring outwards tended to trap the saw in the cut. On my previous boat, she was clinker and that required a mixture of key hole saw and chisel.

I think I would take the precaution of cutting the scarf on the old timber first in case of 'creep back'.

Bon chance. It's immensely satisfying when you've done it!

 

[old_salt]

Carefully with a mallet and chisels is most probably the only way.
Make them to spread over a least three ribs or frames longer if you can.
You could set up to do it with a router but that would be very tricky.
It would help if you could lift the end so as to put the scarph in the right way the leading edge of the NEW Wood is on the inside of the old and a step at the aft end of the scarph is done to make the outer face stronger.
If you can only make the scarph with the leading edge of the new wood on the outside of the hull and foremost then you must cut a rebate with a forward angle to it in the old wood (to prevent easy water ingress into the joint) this is the way I had to replace 6 planks on Mercia III. The engine had previously been bolted direct to the ships side planks and the holes had over sized and deteriorated to such an extent they required to be completely replaced. If you look you can just see the rebates or return to give a stronger leading edge to the new wood, on the first pic.
Here a 2 pics I hope will help.

 

[woolleyhatter]

i know it seems like a big ask, and i assume that the scarf is 1.2mtrs away from any other joint, or has 3 clear planks before the next joint. If so assuming that there is no way to prize off the plank and wedge it open (as has already been suggested), and you have a multi master, then the easiest way is to cut in situ. Don't worry about small in-accuracies at the moment, and remember to make the receiving feather scarf at least 6:1 ratio. By that i mean the length of scarf has to be at least 6 times the width of your plank (so 3.5"=21" scarf on each plank- Lloyds rules).
You can put the two scarfs together and by chalking on one face you will soon see high and low spots if you move them against each other slightly.
Slowly by taking off a little at a time, it will seat well.
Finally be sure to use resorcinol glue to secure joint (which will also take up small errors!), but don't clamp tight for about 30 mins, and even then only clamp to secure well, don't overtighten!!.
Furthermore clean any drips straight away as hardened resorcinol is a nightmare to chisel/cut off again.
Hope this helps

 

[tillergirl]

As I understand the scarfing 'rules', its dependent upon the thickness (or depth as its called here) of the plank not the width. I was always taught 8 to 1. Don't think anyone does a 21" scarf on a 1/2" plank.

 

[woolleyhatter]

sorry, just re-read my thread, and yes meant the thickness not the width, as was pointed out by tillergirl.
Sorry, and the ratios, as suggested are at least 6:1

 

[DownWest]

Resorcinol glue needs high clamping pressures and is not a gap filling adhesive. And while not a problem at the moment, it is sensitive to low temps. Below 10ºc you are risking a joint failure. If the fit is a bit off, epoxy might be a better bet.
Andrew

 

[2nd_apprentice]

Reminds me of Larry Pardey - Classic Boat Construction, The Hull.
Apart from all the epox/resorcinol stuff it's quite a good book though!