The Geometry of Rowing
Or: Why do I need a sliding seat for that nice Wherry?
By John C. Harris
This is an article I wrote for WoodenBoat Magazine in August 2014. The inspiration was years of frustration with oars being the wrong length. This article is concerned ENTIRELY with fixed-seat rowing boats. Sliding seat craft are another kettle of fish!
There are many sources offering a standard formula for oar length, but nearly all of them are based on a single variable: the boat's width at the oarlocks. I'd known since I was a kid, splashing around in oddball rowing craft, that the problem was a polynomial equation. Many factors are at play: the type of boat, the seat position, and especially the freeboard, or height of the oarlocks above the water. I spent weeks measuring traditional small craft that were known to be comfortable rowers. Many points of data were entered into a spreadsheet, and patterns began to emerge. I wrote and illustrated the article while on vacation with family, earning myself many demerits in the process.
There's a sentence in the concluding paragraphs that bears repeating: "The formula works well for the middling spectrum of traditional smallcraft." In other words, you're all set if it's a rowboat of normal proportions in the 8' to 18' range. If the formula spits out 14' oars or something similarly impractical, you've probably got a strange case on your hands, and you should pause to wonder why.
This boat would be faster and easier to row if the oars were about six inches longer; note the broad span between the hands halfway through the stroke.
My email software has a cunning, if slightly cynical, feature called “Canned Responses.” Faced with a common boatbuilding question, I can select a friendly and articulate response that I composed long ago, add my correspondent’s name to the top, and press “send.” If my canned response feels facile, I might add a line or two of exposition and a note of sympathy. Invariably, a canned response is explaining why someone can't do what they want with their boat.
When a canned response starts to feel worn from overuse, I take it as a signal that I ought to address the issue in public somehow. Which brings us to a sleek, low-slung 18-foot lapstrake wherry that’s among my stable of build-it-yourself designs. With a freeboard of scant inches, it depends upon a sliding seat and riggers to elevate the oarsman and the oarlocks so that the 9’6” sculls can clear the water—and the oarsman’s knees.
The wherry’s builders contemplate the thousand-dollar cost of the drop-in sliding seat unit and specialized sculls, then send me an email. “Why,” they ask ruefully, “can’t I mount ordinary oarlocks on the rails of that boat and use short, inexpensive oars?”
The glib answer is, “Because the oar handles will hit your knees every stroke,” though my canned response limns on for a few paragraphs, attempting to explain why exactly this is so. The three-dimensional interplay of a rowing boat’s width, freeboard, seat location and height, and oar length determine whether you’ll glide across the harbor like a swan or thrash the sea into foam.
Why a fixed seat and gunnel-mounted oarlocks don’t work on a 38-inch-wide wherry with six inches of freeboard.
Talking to other designers of pulling boats about the problem of getting all of the parameters in order on the drawing board, I learned that they had applied the same method I always had: they experimented until it felt right. Physical mock-ups of seat height and oar length will eventually untangle the multi-dimensional riddle of a comfortable rowing experience, but it doesn’t feel very scientific.
So, in hopes of retiring my canned response explaining why a skinny sliding seat wherry is a lousy fixed-seat boat, I set forth with ruler, calculator, and a big spreadsheet. I surveyed a dozen fixed- and sliding-seat boats, recording every dimension and noting how the boat really felt on the water in all conditions. I mounted many of the boats in cradles in the parking lot and had a photographer record the positions of the rower’s body at different stages of the stroke, which I could then digitize and study on the computer screen. I wanted to devise a simple algorithm for oar length, and a set of rules guiding the positioning of the seat in relation to the oarlocks.
The Characteristics of the Boat
Most of the formulae in circulation for calculating oar length use just a single parameter: the beam of the boat at the oarlocks. One of the essential findings of my study is that freeboard—for our purposes defined as the height of the oarlocks above the water—must be considered along with the beam of the boat. Relying only on the boat’s beam to calculate oar length can result, for example, in oars that are too short if the boat is narrow but has relatively high freeboard. The oarsman’s hands will swing high into the air during the stroke, which isn’t efficient and even looks sort of silly.
For the sake of simplicity, I confine these remarks mostly to traditional fixed-seat rowing boats: Peapods, dories, Whitehalls, dinghies, and so on. High-performance rowing craft like shells break most of the rules in the interest of speed. A shell’s freeboard is very low yet the oars are very long. Racers accommodate this mismatch simply because they are highly skilled: they feather the oars precisely, barely skimming the surface on the catch. The best of them can make it look easy even in waves, but the rest of us find ourselves undone the first time our surface-skimming shells encounter a big motorboat wake. What I’m proposing here will give recreational rowing craft an agreeable feel in all conditions.
After 35 years of fixed-seat rowing—sometimes for days at a time—I know just how I like it. The blade of the oar should be immersed just to the throat, no more nor less, and I always row with about a hand’s width of overlap at the handles:
Typical overlap of the oar handles. The additional power is pronounced.
Rowing Posture
I still occasionally encounter someone who hates the idea of overlapping the hands while rowing, but the reason for doing it couldn’t be any simpler: An oar is a lever. The oarlock is the fulcrum, and the length of oar inboard of the lock determines your leverage. Every extra inch means more power. No one is making you row that way, but it’s worth getting used to. A hand’s width might give you 10% more power depending on the beam of the boat.
Oar Length Considerations
Oars that are too short for the boat will lack power and require an awkward windmilling stroke, and sometimes will actually chafe on the boat’s gunnels. (If the oarlocks are the enclosed round type, I’ve seen people rip the oarlock sockets off the rails when short oars bind in the locks.) Oars that are too long may prove impossible to lift clear of the water on the catch, and the inboard “lever” will feel too short for the oar. Long oars will also require feathering no matter what the conditions, and I try to avoid feathering unless I’m in a rowing shell or I’ve got a long way to go upwind. It’s hard on my old wrists.
With freeboard as part of the calculation, and cross-checking against my table of small boats that are known to have good proportions for rowing, I came to this formula for oar length:
((A+B)+9) x 1.34 = Oar Length in inches
where A = beam at oarlocks in inches and B = the distance from the water to the bottom surface of the oarlock in inches (freeboard)
You can plug your own numbers into my Oar Length Calculator
The small Whitehall shown in the drawings here is 39-1/2” wide and the freeboard (taken from the plans) is just under 12”. This gives us an oar length of 81”. Since oars come in 6-inch increments, we round up to 84”. The 7-footers feel natural; other calculations based only on the beam suggest 6-1/2 footers, which in trials feel stubby on the boat.
Characteristics of the Oarsman
What if you’re very tall or very short? With most of the test boats active members of my shop’s demo fleet, I’ve had the opportunity to observe that oar lengths based on beam and freeboard seem to fit the middle 75% of humanity, even for serious rowing. If you’re quite short, you are effectively sitting lower in the boat, which means you’ll be reaching “up” to the oar handles. If you're short, longer oars will lower the handles so that you’re pulling even with your rib cage.
The right oar length for you and the boat will keep the oar handles about mid-height on your rib cage through the stroke.
Especially tall rowers have a different problem: knee clearance. Shortening the oars makes it easier for the blades to clear the water without banging into your knees on the catch, but a taller oarsman is unlikely to want to resort to shorter oars. If there’s some way to lower the thwart an inch or two that will be a lot more helpful in creating knee clearance.
The Anatomy of a standard oar. Bottom–The oars' lengths are critical to rowing geometry. If they're too short, the arms will be too high during the stoke; too long, and they'll provide insufficient leverage.
Seating Considerations
Nothing turns a nice row into a forced march more than uncomfortable seating. Fortunately for designers and builders, my survey suggests that the geometry is very consistent, from small dinghies right on up to big dories.
The longitudinal and vertical placement of the thwart and oarlocks is the difference between a nice row and a painful slog.
"C", the height between the top of the seat and the bottom of the oarlock in inches equals the beam in inches multiplied by 0.17. Thus our Whitehall finds a height of 7 inches for the oarlocks above the thwart, and everyone from about 5’6” to 6’3” reports comfortable clearance all around.
Likewise, I found broad consistency in the longitudinal location of the oarlocks in relation to the aft edge of the seat. Multiply the beam in inches by 0.22 to get "D." This gives us just under 9” for the Whitehall. (Much more than 14 inches would be uncommon.) The seats themselves ought not to be narrower than about 9 inches. Making them as wide as possible allows rowers of differing heights to slide fore and aft for more comfort.
The proportions of seat and oarlocks make this Skerry a very comfortable boat to row. However, the absence of footbraces throws away a lot of power and will result betimes in a sore butt.
Additional rowing stations, to maintain trim when carrying passengers in light boats , should be given careful thought so that comfort and efficiency are maintained.
Footbraces
Footbraces are absolutely essential in good rowing craft for transferring power. If you don’t have them, you’re actually transferring your forward thrust to the boat through the friction of your posterior on the thwart! It’s as uncomfortable and inefficient as it sounds.
The longitudinal placement of footbraces is obviously going to vary greatly with the oarsman’s height. Thus, some scheme that allows footbraces to shift fore and aft is worthwhile.
The vertical height of the oarsman’s heels will also vary greatly depending on the depth of the boat. A deep-bodied traditional rowing boat will have the thwart set high above the floorboards and the oarsman’s knees are bent comfortably. A shallow boat will require the legs to be straighter. If you find that you just can’t fit your knees under the oar handles during the catch, that’s a signal that you need to elevate the oarlocks with outriggers or taller mounting blocks. The engineering of oarlock lifts is not trivial; your leverage on the oars puts a great deal of torque on the oarlocks and I’ve torn off more of them than I care to admit.
Conclusions
I tried plugging boats with oddball dimensions into my spreadsheet to see whether garbage begat garbage. Reassuringly, these calcs spit out short oars for narrow, shallow boats, and long oars and logical seat heights for wide and deep boats. But this is the place where a warning for the boffins is in order: If you’re designing a boat with odd proportions, expect odd results. This table works well for the middling spectrum of traditional smallcraft.
My experience in issuing mathematical prescriptions of this sort also suggests that the math will bang up against common sense sometimes. I’ve sold about a thousand 8-foot dinghies whose proportions suggest 7-6” oars. But storing such long dinghy oars is a hassle quite out of proportion to any advantage you’ll gain paddling the thing across the harbor. We’ve always shipped 6’6” oars and the dinghy goes just fine. So don’t get too hung up on the numbers.
Likewise, most sliding seat boats are exempt. Every rowing shell or wherry of my acquaintance has a beam of 63” between the locks, uses 9’6” sculls, and the drop-in sliding seat units takes care of the seat height question. The sliding seat guys have been refining that geometry for 150 years and we’d best not monkey with it.
Why Adirondack Boats Break All of the Rules and Get Away With It
Adirondack Guideboats are a historic type of hunting and fishing skiff, evolved by 19th-century sportsmen for angling in the eponymous lakes. They are famously fast under oars, but compared to conventional skiffs and dories they have somewhat peculiar rowing geometry. Anticipating an avalanche of letters from Guideboat partisans, it's worth taking a look at why they work the way they do. Guideboats are low and narrow. 36 inches of beam is typical, far below the usual threshold for oarlocks mounted on the rails. The oarsman sits in a reclining caned seat, and the seating position most resembles that of a kayaker: the knees are bent and flattened out towards the rail to create clearance for the oars. Even so, the stroke tends to be shorter than in a conventionally proportioned rowing boat. The overlap of the oar handles is pushed to extremes. Something like 5-1/2" of overlap is typical, says Guideboat builder and designer Steve Kaulback. The oars are springy and light, and handling is simplified in some ways by having them pinned at the locks. With a lot of leverage thanks to the overlap, plus a narrow waterline and light hulls, Guideboats are capable of startling bursts of speed. It really works because Guideboaters are willing to have so much overlap of the oars at the handles. "It's a tough argument to win," says Kaulback, who's probably built and sold more Adirondack Guideboats than anyone in the last century. His 13-foot "pack boat" uses 7-foot oars, whereas my formula suggests 5-1/2-footers. The success of Guideboat oar geometry offers useful lessons for anyone contemplating mounting oars on low and narrow boats: You'll need to sit very low in the boat, you'll need a lot of overlap at the handles, and you'll be taking a shorter stroke. So maybe you can make fixed-seat oars work on that lowslung wherry after all. |
John C. Harris designs, builds, and writes about boats at Chesapeake Light Craft in Annapolis, Maryland.
