A crosshead engine, as shown in a British engineering text of the late 1830s.
The crosshead engine is a very old type of marine engine, having been used on the first commercially-successful steamboat, Robert Fulton’s North River Steamboat (or Clermont). The crosshead engine peaked in popularity in the early 19th century, but by the 1840s was gradually being supplanted by the vertical beam (or “walking beam”) engine that is much more familiar today.

Fulton’s North River Steamboat, from an old text.

Crosshead engines were used in many of the early coastal steamers that plied the Gulf of Mexico, running between New Orleans, Mobile and the Texas coast during the period of the Texas Revolution and Republic. The steamers included several of the better-known vessels of the period, including Columbia, New York and Neptune.

Like all early steam engines, the crosshead type had some significant limitations. There was no gearing at this point, meaning the engine ran at the same speed as the sidewheels. With a single engine – i.e., a single cylinder – running both wheels, it was impossible to run the wheels in opposite directions, in the way a tank or caterpillar tractor can do on land. This made these steamers somewhat less maneuverable than the sidewheelers that came later, although some ships fitted with crosshead engines were likely fitted with disengaging mechanisms that could disconnect one or the other wheel when needed.

Crosshead engine steamer Express, illustration by Samuel Ward Stanton.

Above all, these engines were (as my daughter says) ginormous. They operated under “low pressure,” usually not more than one or two atmospheres above ambient air pressure. This meant that to do a given amount of work – in this case, to turn large, heavy paddlewheels through a dense medium like water – that low-pressure steam had to act, push, against a very large piston surface. This resulted in very large but relatively weak seagoing steam engines; the trade-offs power vs. size in reciprocating engines was not fully resolved until the latter half of the 19th century with the development of compounded steam engines with multiple cylinders.

(By contrast, Western Rivers steamboats by this time were employing high-pressure boilers engines with operating pressures of 120 psi [8-10 atmospheres] or more; these allowed for much smaller, lighter engines, but with the much-increased danger of a catastrophic explosion.)

This is a neat video of the triple-expansion engine icebreaker Sankt Erik, a museum ship in Sweden in action. It’s not especially relevant to crosshead engines, but I like it anyhow, especially near the end when the camera focuses on the engineer on watch “answering bells” from the bridge, and adjusting the reversing valve and steam to the engine accordingly. The orders come down fast and furious — it’s clear that Sankt Erik was being maneuvered in tight waters at the time.

The other part of the power equation in these early that is sometimes overlooked is that while “live steam” from the boiler is the role of the condenser. Once the piston had completed its stroke in one direction, the “spent steam” on the back side of the piston was vented off to a condenser that rapidly cooled it to the point that it became liquid water again. This was often accomplished with a spray of cold water drawn from outside the hull. When the spent steam flashed back into liquid water, of course, it left a partial vacuum behind it; this actually helped pull the piston through the cylinder on its return stroke. When the piston reached the other end of the cylinder, the valves on either end switched again, reversing the process, with live steam pressing on one end and a partial vacuum pulling on the other. Indeed, the role of this patial vacuum was so central to early steam engines that the earliest engines designed by Newcomen were known as “atmospheric engines’ or “vacuum engines.” (Western Rivers steamboats had generally done away with condensers, due to their additional weight and mechanical complexity; on those boats, spent steam was vented straight into the atmosphere, giving them a distinctive chuff-chuff-chuff exhaust as the paddlewheels turned.)

If the role of the vacuum in running early steam engines seems a little hard to follow, think about the last time you drank through a straw. We would normally say that we “suck” through the straw, but what we’re actually doing is creating a lower pressure within the straw (and mouth and lungs) by using our diaphragm to inhale, as in breathing; in this closed system, larger volume = lower pressure. In response to this drop in pressure, the liquid in the glass actually gets pushed up the straw by the higher atmospheric pressure around us. Got it? Now imagine a tiny piston in the straw, and you’ve got a minuscule, working atmospheric engine.

But I digress.

Anyway, this is the start of a new thread that will follow the construction of a 3D model of a crosshead marine engine, like that used on many coastal steamships that traversed the Gulf of Mexico in the 1830s and 40s. Starting with heavy timber A-frame bracing, here are some initial images:

More to come. . . .

– Andy Hall

Tags: crosshead, engines, technology