Clark February 23rd, 2017
To be a true marine carpenter is to live in the high country of the craft, because boats are curved every which way. There is seldom a right angle, seldom even a simple beveled angle, because all those intersecting curves mean that every place two pieces of material join together is a compound angle. To put a finer point on it, terrestrial carpenters can frame a four-bedroom house in a day or two. A team of talented marine carpenters can frame a 40-foot wooden boat in a couple of months? A couple of years?
I am not a marine carpenter, but I often get into marine carpentry projects, or more accurately marine joinery, like building new electrical panels out of teak, rebuilding consoles, and of course endless work on my own boat. I have many more tricks to learn, but here are a few rules I’ve picked up:
Thou Shalt Measure Angles
To make a piece with a compound angle, a tape measure is only going to get you so far. You need some way of measuring angles accurately, then you later replicate these angles to your cuts. I haven’t found a great product for measuring angles in tight spaces, but this protractor is what I’ve got, and I can always make it work by turning it one way or another:
The important thing is to be able to measure accurately down to the degree, because if you’re off my more than a degree in finished joinery it’ll stand out like a sore thumb and reveal that you are a hack.
Thou Shalt Draw Diagrams
All the angles and measurements you take will only lead to confusion unless you document them and get them to the cutting area without reversing something. Overcommunicate with yourself: Note top, bottom, inside, outside, port, starboard, forward, aft, athwartships, all the lengths, and all the angles, and you’ll still manage to screw something up:
Thou Shalt Make Templates
There may be master marine carpenters out there who just go take their measurements, then cut perfect finished pieces, but I doubt it. Everyone I’ve seen makes lots of sacrificial templates, these templates go through multiple rounds of tweaks, and end up with hieroglyphics scrawled all over them.
Good quality teak runs $30 to $40 per board foot. A board foot is a foot wide by a foot long by one inch thick. A sheet of 3/4-inch teak-fronted plywood is $220 where I buy it. Ergo, mistakes become very expensive. This piece of trim I’ve installed in a head is over a board foot of teak, and it sure don’t look like much:
For trim pieces I make templates from cheap, usually scrap, plywood of the same thickness:
Simply mock up your measurements and diagrams in cheap plywood, then go try them out. Of these four templates, one ended up perfect and the other three needed tweaks. I purposely make the templates a little short, which means I accidentally made one too long and it ended up perfect…by mistake. By making them a little too short, say by half an inch, you can fit your template in place (if it’s too long it can’t even fit), and ensure that the angles are all right by sliding it from side to side. If not, you can measure much more accurately now that you’ve got something to go from, and make notes on your templates. You can also now get your final length measurements very accurately, since you’ll be increasing the length of your template by some fraction of an inch, rather than relying on overall measurements.
I guess this is just another way of saying I know I’m going to make mistakes, so if I make mistakes on purpose I feel better about myself. There are two other reasons for cutting your templates too short: First of all, those overall length measurements get very confusing with a piece that has compound angles on one or both ends. Do you mean the length at the inside corners, the outside corners, top, or bottom? These will all end up being different length measurements. Ideally you want your length measurements based on where the saw blade will enter the material, rather than where it will exit, as the entry point can be measured and marked very accurately, while the exit point is only known for sure when, well, the blade is finishing the cut.
Second, due to that pesky fact that boats are curved every which way, your piece will probably have some bend induced into it in final installation, and with a template that’s a little short you can bend it into place and see how this bend changes the angles. Unfortunately plywood is more flexible than hardwood, so there may be some surprises in the final installation due to the different properties of the two materials, but these surprises are usually minor.
For larger panels, to be made out of plywood, there are several 4 x 8 foot sheet products, such as hard board and utility panel, that cost less than ten bucks per sheet. Make templates, and your mistakes, on these. Once you’ve got a template for a larger panel, even if it’s way off, it can be made perfect by stapling, taping, or hot gluing extensions here and there, or cutting off excess material with a saw or utility knife. I discuss how to be gentle with veneered plywood here.
These cheap template materials are usually thinner than the final product, and this sometimes leads to some unpleasant surprises too. If you’re making a really complicated piece out of plywood, best to make the template out of the cheapest plywood you can find of the same thickness.
Thou Shalt Have a Good Way of Cutting Compound Angles
If you’re building something like the trim for this instrument cluster, count yourself lucky. It’s all right angles, and the pieces are so small that mistakes aren’t terribly expensive:
I’ve had this simple hand miter saw for years, which makes accurate angled cuts, and the blade has stayed sharp through countless pieces of teak, but it can’t cut compound angles unless I use some cockamamie method with shims:
To cut compound angles you need either a table saw with a crosscut sled (miter attachment), or what is called a compound miter saw or chop saw.
In other words, you need a way to accurately cut angles, and accurately angle the blade while cutting said angles. Of the two, a chop saw is way cheaper. I’ve seen them for as little as $100, but can’t vouch for quality at that price. A chop saw is the better machine for trim work, for pieces less than six or eight inches wide, but to make long, straight cuts a table saw is king. A chop saw can be stowed away on a shelf; a table saw needs its own room.
A handheld circular saw is great too, and can cut both compound angles and long cuts, but it will never be a precise as a chop or table saw.
When preparing to make your final cuts on your final material you can set the angles on both the miter and the blade angle, then cross-check these angles with your protractor and your template. Use a fine-toothed finishing saw blade to avoid splintering, and clamp your material where possible.
Thou Shalt Not Try to Correct Your Mistakes (Too Much) By Sanding
It just seems like you can nip off that extra material with your belt sander and it will fit perfectly, right? It won’t. It’s nearly impossible to square up an angle by sanding. Sanding always rounds off the edges and leaves the middle too proud. By all means clean up the splinters and rough edges by sanding, but you’ve got to get your angles right with the saw.
Split Your Angles
If you measure a 110-degree angle where you want a miter joint between two pieces of material, each piece needs to be cut at a 55-degree angle. If you make one 35-degrees and the other 75-degrees it won’t work and it’ll look funny.
Shape Your Pieces Before Cutting
Your pieces will undoubtedly be shaped, with rounded corners, or in the extreme quarter-round or half-round material. If you cut your pieces from square or rectangular material first, then shape them afterward, it will be hard to make the miters match. If you shape your whole piece of material first, then cut the miters, it will still be hard to match the miters sometimes, but you’ll be off to a better start.
It’s very satisfying when you get it just right: