Thread: Is there a way around needing 5.5 m gunwhales?

You can glue shorter lengths together using scarf joints to obtain the length you need.

The link is to Greg Rossel's Building Small Boats. Scarf joints are described and discussed at page 148 (in case the link doesn't take you directly to that page).

Building small boats - Google Books

Thanks BW.

So using a joint like this will i still get a uniform bend on the Gunwhales or will it bend either more or less at the joint?

Do I glue the joint with epoxy?

I glue mine with epoxy. I coat both mating surfaces with straight epoxy, then apply thickened epoxy to one surface, and clamp them together.

I clamp them "on edge" and use two pieces of ply (one on each side covered with packing tape) overlapping the join. This works for me to minimize the pieces slipping. If I can find a picture of this arrangement I will post it. Others glue them flat with heavy things and gravity to apply clamping pressure.

Spot on Bobwes.

There is a little bit about gluing endgrain that is useful here ...
FAQ - Boat Building and Repair Methods - Plywood Epoxy Fibreglass Cedar Strip - Michael Storer Wooden Boat Plans

Look down and you will see a section called gluing endgrain on scarfs and joints. The most usual risk with scarfs is that the endgrain absorbs too much of the glue. This shows a quick and easy way to avoid the problem.

MIK

Originally Posted by BobWes

I clamp them "on edge" and use two pieces of ply (one on each side covered with packing tape) overlapping the join. This works for me to minimize the pieces slipping. If I can find a picture of this arrangement I will post it. Others glue them flat with heavy things and gravity to apply clamping pressure.

Look at this post about my Little Black Dog. It discusses how and why ... and has the advantage that it actually works
(it's the same as Bob's method )



Richard

Does is matter which way the Scarfe joint goes? I.e across the thick side or the thin side of the timber.

Rossel seems to recommend a 12:1 ratio for scarf joins and says,

"...in other words, the scarf should run 12 inches for every 1 inch of stock thickness."

Rossel says shorter scarf ratios may not be a strong as 12:1, and may produce a "hard" spot when bent.

I'm using an 8:1 ratio for the chine-logs in my Goat Island Skiff but cannot comment on strength or whether that will create a hard spot as the chine-logs have not been installed yet. I think I have a source for long enough lumber for the gunwales and inwales to avoid the need for scarf joints in those pieces.

Howdy Bob,

The old standard used to be 6:1 except for spar building where I usually describe it as "8:1 or 10:1 depending on how paranoid you are feeling".

Each time someone goes a bit further, it forms a new standard for someone else to go further still. So if Mr Rossel is quoting 12:1 then there will be someone a year or two from now who will be touting 15:1. Much the same has happened to the amount of glass on plywood canoes - that's gone crazy as well.

Welcome to the web!

Which really sums it up. I have used 6:1 on spars using the info on prewetting the end grain to make sure the joint does not dry out. This will be more than adequate for gunwales and other bits that get glued to plywood.

The USA forest industries guidelines are a bit conflicted on this point. States in one place that for plywood and fingerjoints that

In plywood scarfs and fingerjoints, a slope of 1 in 8 is typical for structural products. For nonstructural, low-strength joints, these requirements need not be met.

but also setting off the problem of ever lengthening scarfs with comments like this on the next page

Slopes of 1 in 12 or flatter produce the highest strength.

Which opens up the way for the never ending tail chase to start. "or flatter" is a stupidly unscientific statement - of course anyone can say this. But what we need to know is how far is good enough. Lets look at older boatbuilding practice.

Most of the older material and plans from experienced builders seem to indicate that ply scarfs of 6:1 or 8:1 are entirely adequate.

I would be happy with this guideline for any piece of boat other than spars. With spars I would be thinking 8:1, or if paranoid 10:1. One thing is that if the spar is not made of one "stave" but is laminated, built in a box section or a birdsmouth, then if the scarfs are staggered then 6:1 appears adequate at least for small boats. Bigger expensive boats I would probably go up to 8:1 for similar masts.

See ... I am not immune to paranoia either!

It is one of those areas where people think they are doing a better job if they do more. I mention that in the second podcast of my US talks with reference to the specifics of the plywood Kayak example. We used to just glass tape them ... but now a couple of dudes in the field are saying you need to put 4oz (125gsm) glass over all surfaces inside and out of your 4mm ply kayak.

But the Australian Jarcats, which are a little bit heavier with higher loads than ply kayaks use 4mm ply with 2oz glass on the outside only - so someone is a bit screwy! And it ain't the Jarcats!

A 5 metre boat of 4mm ply and 125gsm (2oz) glass - compare that to a kayak!


Best wishes
MIK

I am still trying to figure out whether the diagonal line of the scarf joint runs across the width of the timber or the thickness of the timber.

I think I am explaining it poorly but Im not sure how to explain it better.

Sorry - your question kinda got lost...

The answer to your question (as I understand scarf joints) is, "thickness."

Using 16mm thick timbers, the joins will be 96mm (6:1), 128mm (8:1), 160mm (10:1) or 192mm (12:1).

Mik: Those comments are very helpful.

Richard: Thanks for the pic. Looks just mine... and for a second there I was concerned that I couldn't remember posting...

Originally Posted by hereselmo1

I am still trying to figure out whether the diagonal line of the scarf joint runs across the width of the timber or the thickness of the timber.

I think I am explaining it poorly but Im not sure how to explain it better.

Sorry ... either way is fine ... whichever is convenient for the situation - sometimes it makes a difference to the appearance of the finished product.

Best wishes
MIK

Thanks guys.

Also should I try and line up the scarf so that it is over the ribs or doesnt that matter?

My Dad has suggested we should try and line it up over the rib to avoid there being a weak point.

Originally Posted by hereselmo1

Thanks guys.

Also should I try and line up the scarf so that it is over the ribs or doesnt that matter?

My Dad has suggested we should try and line it up over the rib to avoid there being a weak point.

If you scarf it properly, there is not weak point

Don't sweat on it mate. If you were putting them together in some sort of structure such as a mast, you'd stagger the joints but otherwise, don't worry about it.

Richard
bin there, worried about that

Scarfs, regardless of ratio employed, do have a right or a wrong direction. On all scarfs that are to be exposed to the on coming rush of water, water flow, standing water or spray, the slope of the scarf should prevent water from migrating up the joint. This is especially true of rub rails and deadwood assemblies, where water pressure can be fairly high, driving water deep into a joint. It also looks better.

6:1 ratios will produce epoxy joints stronger then the wood fibers they bond on nearly all species. This is the point of a scarf, to transfer load through the transition of multiple pieces, without lose. 8:1 scarfs will usually show some "stiffness" at the scarf location, particularly with softwood (rails usually are hardwood). Hardwoods may get by with a 10:1 scarf for a smooth radius, though softwoods often need more if the radius is tight or compounded.

The orientation of the scarf can also affect the "flexibility" of the joint, while conforming to a radius. If the adhesive is thickened epoxy and perpendicular to the bend, you'll have a stiffer, less flexible joint, then if the scarf "flows" with the bend.

One last point about scarfs like this, they are to slope outboard. Not only is it better looking, but if water does manage to get wedged up inside the joint, when you park the boat the moisture can run down the slope of the scarf and out of the joint. The idea is you don't want to create a ramp that will lead moisture into the joint, either at rest or underway.