A rule-of-thumb is to double the span (the distance between the oar locks). However, this may not be the best length for the oars. For example, if the freeboard is greater than normal, then longer oars may be needed.
Having struggled with deciding what length of oars to make for my Ross Lillistone Flint, I created this worksheet.
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| Rowing Geometry Worksheet (Click on image to enlarge) |
(You can download or print a full-sized JPG version of this worksheet from Dropbox.)
An Example:
For this Oar Cruiser, the span is 52" (1321 mm). The distance from the oar lock to boat's centerline is 26". The distance from waterline to bottom of the oar lock is 20 " (508 mm). I placed a dot at the intersection of these two dimensions on the worksheet above.
Using an 8' 6" (scale) oar with a pencil mark at 7/25ths from the end of the handle. I placed that pencil mark on top of the dot and the blade just under the waterline, as in the photo below.
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| Rowing Geometry During the Power Stroke |
This shows that when I'm rowing, the handles will almost touch (about 1" (25 mm) apart.
I then changed the oar to horizontal for the recovery stroke. Now the handles overlap about 1" (25 mm) which means on the recovery stroke, I'll have to cross my hands.
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| Rowing Geometry During the Recovery Stroke |
Using this Rowing Geometry Worksheet, I've been able to confirm that 8' 6" oars will work.
See this post for additional information on rowing geometry.
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