Building the Rushton Catboat

Steam bending

I've never met a more inflexible, opinionated, self-righteous collection of people than those that inhabit my chosen craft. Boat builders are insistent, positively jihadist, about the way that THEY do things. I think that every classic boat show should have a tent manned by psychotherapists who could offer free counseling to the frustrated, those who have had to mingle among their peers and be confronted by radical ideologies, apostates, faith healers and transcendentalists, in other words, people who do things differently than you do. We all need help.

Of all the Books of the Bible of Boat Building, none excites more interpretation than steam bending. There are many sects, each insisting that they have found The Way. I realize that in casting here my way that it is inevitable that I shall be called out. The server that supports this website will no doubt crash under the weight of incantation, opprobrium, name-calling, etc. I can only bow my head, close my eyes and say, softly, this is just my way.

First, the wood. For years I have used tried and true white oak for steam bent ribs. It does bend well, but what is crucial is that it have a high moisture content and not be kiln dried. In this day and age it is increasingly difficult to find lumberyards stacked with anything but kiln dried material, so for bending stock I've taken to prowling back yards and farm fields, and trucking a small log to a local fellow who operates a portable bandmill. These guys can mill up a lot of material for very little money. For the catboat, I decided to use some locust that I had stored for some three years now out of doors under my porch. The moisture content was around 16% but locust is a wonderfully stringy, tough wood that bends beautifully and really fights breaking. Where I live in Vermont it was planted around farmhouses as a wind break and grows like a weed, so it is readily available. The old trees are often rotten, so large clear pieces can be hard to find, but for small boat frames it is easy to find a straight six foot limb or sound section of trunk. If there is waste, so what, you shouldn't have had to pay for it anyway.

So I will start the sermon with a discussion of my steambox and steam generator. I've seen many different contraptions for generating steam for the process, and most of them would have been outlawed by the local fire marshall, especially given their location in scrap wood-strewn, sawdust-choked boatshops. I've seen industrial propane torches firing under metal gas cans (not a reassuring type of vessel, even if it is filled with water), devices made from hot water heater elements (fine in theory as long as they are not starved of water). An itinerant boat builder I knew used to rent an electric wallpaper steamer and that, my friends, is the bees knees. I've rented one ever since, and for fifteen years I inquired as I rented, "Would you be selling a unit?" The answer, until two years ago when my local hardware store went out of the rental business, has always been no. I wrote a check for $150 (just $10 a year for my fifteen years of waiting) and now I own one myself, nearly brand new. These units run on 110 volt power, have a sight glass to watch water level, a pressure relief valve and a power cutoff and, oh yes, they generate a lot of steam. If there is one thing that the different faiths can agree on, it's the more steam the merrier.

In 1992 I was restoring a Coast Guard motor lifeboat for the National Park Service in California. Park Service rules required that whatever species of wood came out of the boat the same species had to go back in. The boat was planked topsides with Philippine mahogany, an amazing dense, blood red meranti in the 1950's, that was now a light, brittle kiln dried wood forty years later. I could only get kiln dried material and all my boat building books said bending kiln dried wood was impossible. I decided to try a test and my first plank snapped. Feeling desperate I lashed my mahogany together in a raft and put it in Drake's Bay, tying it to the ramp where I was working. I wouldn't actually need it for planking for several weeks and I hoped that it might actually gain some moisture content (I have always wanted to ask a wood technolgist if this is feasible, whether soaking kiln dried wood can return some of its moisture and elasticity).

To maximize my chances I decided to upgrade my steambox. I had always used the tried-and-true plywood box. I made ones with tall, skinny openings for planks, keeping the amount of air volume around the plank to a minimum. I had seen some alternatives, such as PVC pipe (which melted) and galvanized stovepipe (which struck me as a condenser simply by dint of its material, and I knew that I didn't want a device designed to turn steam into water). While I was thinking about condensers I realized that the insulating properties of my 1/2" plywood were only marginally better than that of stovepipe, so I bought some fiberglass batt insulation and duct tape and wrapped my plywood box in a blanket. The effect was tremendous. Not only did it hold clouds of steam but it was HOT steam. So this is my first sermon: insulate thy steambox. Today I use a ten-foot piece of galvanized metal ductwork I was given that I have wrapped with 2" thick rigid foam insulation with all the seams taped.

On the lifeboat, either the soaking of the material or the improved steambox, or both, in the end I got the planking to bend. It may have been around this time that I tried something I had read: adding diesel fuel to the steam water. I've also read stories that adding soap to the water can help planks bend. Let me just say, after my diesel experiment, that your time is far better spent getting your steambox right, and a good steam generator, using good air-dried materials. The memories of the stink and feel of diesel steamed planks is too awful to repeat.

Before I bent frames on my catboat I had to laminate my outer stem. The locust would not bend enough cold, even planed down to 1/4" thickness, so I steamed and pre-bent them, and glued them over the same form a day or two later. You can see how much they sprung back, and I have found over the years that an extra couple of days left on the form makes a big difference in the amount of springback. One boat shop I worked in I was in the habit that if I bent anything on a Friday I just left it clamped through the weekend. One note: In that shop once I had a plank crack about ten days after I had hung it. When I unfastened it I found that it would not spring back at all, it had completely taken the shape. Some books of mine say that planks have "tension." In fact they have nothing of the sort. Within just a few days they are frozen to shape. Anyone who has repaired boats knows that.

For most of my boats I have bent frames off the boat around a mould. Usually this boils down to whether the boat builder sets up their moulds with heavy ribbands that allow in-the-boat bending, or lighter ribbands that force them to be bent elsewhere. The hull of the glued lapstrake catboat is simply too fragile at this stage to go bending hardwoods inside her. I would risk deforming her shape. Bending in the boat, even a small one like the catboat, also requires a contortionist, or an assistant, or both. I was able to bend all the frames I need for my boat in a couple of hours, with no stress. Experience gives me a feel for the springback, and my form was made to the tightest curve in the boat with a little more for overbend.

Ye Olde Rule of Thumb is to steam material an hour for every inch of thickness. I agree. It may seem counterintutive, but going beyond that formula one finds that the wood comes out of the box strangely dry. That is a bad sign. I suspect that going too long raises the core temperature of the wood and the water flashes off the surface. What is best is a nice, hot frame soaking wet. You have to work fast, less than thirty seconds, to get the wood bent either in the boat or on a form. Those quarter inch laminations were in for just fifteen minutes and they didn't get very supple so perhaps I've learned something about the lower limit of that formula.

If you are going to round over your frames, do it before bending. Every boat builder I know cuts their material so that the grain is flat to the curve of the timber (think the pages in a paperback book as you bend it). However, I have also read that theoretically this should not matter. All I know is that once when I was bending white oak it got overcooked and the fibers separated along the grain line, as though the grain represented layers that were letting go. Where the grain ran vertically through the bend I could see the added stress. I just cut my frames with that flat grain to the curve . I can live with a few superstitions. Having the grain run straight through the length of the frame is something we can all agree on. If the grain runs out, and runs out quickly, that is where a piece of wood will break. I was impressed with this locust because I had a fair number of pieces where the grain did run out. I stuffed them in the steambox anyway and many of them bent just fine.

Here's a digression: when I worked on the west coast of the US I planked most of my boats with Port Orford cedar. It's an incredible planking wood, easy to work, with an unforgettable smell. One thing, however, was that while it steam bends fine when it did fail it was catastrophic. Normally oak will slowly let go, fighting on your behalf for all its worth, but Port Orford just goes, and does so with a bang. On a trip east I was talking to the elderly son of the former naval architect at ELCO (the Electric Launch Company). His father had worked there through the 1920's, 30's and 40's and had a prominent role designing the famous PT boat in World War Two. My friend essentially grew up in the yard in Bayonne, New Jersey in the 1920's. ELCO planked many of their boats in Port Orford and when I told him about using it he said to me, "That stuff is brash." I asked him what that meant and he replied, "I don't know. I just remember that the yard foreman, who practically raised me, used to always say that Port Orford was brash." My friend has now passed away but I came across an old dictionary and stumbled on the word brash: "brittle, as in wood (of unknown origin)."

You may be wondering how I am going to use a batch of frames all bent to a single curvature. Well, frames are actually superflous to this catboat, since the hull is glued-lapstrake, but I've decided to put in about half as many frames as I would were I building it traditionally, in part for looks, in part to add weight, and because frames provide a convenient place to fasten things like thwart risers and deckbeams. But back to the single curvature: the fact is that a steam bent piece of wood, once cooled off, cannot be given any more bend, but it can be straightened. Therefore I have the tightest curves I need and for the rest I will just carefully put pressure on them over my knee until they straighten out to the gentler curve that I want. You just have to work slowly, maybe taking half a minute or a minute on each one.

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