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Thread: Oar gearing thread

  1. #1
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    Jul 2013
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    Default Oar gearing thread

    I was wondering if there was anyone in this forum that has a good knowledge of oar gearing.

    I am currently building a 19’offshore dory and am trying to work out oar specs.
    Previously I have set up rowing boats with the same geometry and dimensions as a scull, E.g. Sliding seat,
    1600mm spread between gates (oarlocks), standard 9’6”sculls. Although this works fine it is far from
    optimal for a large heavy boat that will be rowed in all sorts of conditions all day long.

    Sculling oars are designed for an ultra-fit rower to propel a 14kg boat though the water for a 2000 meter
    sprint. They have a standard gear ratio of about 2.3. I would assume that all other oars for recreation rowing
    skiffs, dinghies, dories etc. would have a much lower gear than that of a scull, but from the oars I have
    measured it seems that most recreation oars are geared between 2.5 and 3.

    Can anyone explain why this is?

    Any input would be appreciated regarding best setup for a sliding seat dory. Where blade shape, oar
    length, gear ratio and gate spread are all adjustable.



    Cheers, Tom

  2. #2
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    Default Re: Oar gearing thread

    There was a year or so back a comprehensive article in WoodenBoat that really covered the issues, including gunnel height above the water line. However, for most normally shaped row boats, the Shawn & Tenney formula works just fine. You don't need to be exact. If the formula calls for oars 7'6" and you're buying oars, either 7' or 8' will be fine.

    Oar length drawing.
    Shaw & Tenney Oar Length Formula for Correctly Fitted Oars
    Inboard length of the loom equals ½ the span between the oarlocks + 2″
    Total length of oar equals 1/7 of inboard length multiplied by 25
    Distance from the center of leather to end of grip equals 7/25 of the total length of the oar
    Oars measured according to this formula will have a leverage ratio of 7:18.



    You might also search the back numbers for the article on the doryman's stroke. An efficient workboat open water stroke is quite different from a scull.

  3. #3
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    Default Re: Oar gearing thread

    Quote Originally Posted by Ian McColgin View Post
    There was a year or so back a comprehensive article in WoodenBoat that really covered the issues, including gunnel height above the water line. .
    That was WB #240, written by John Harris. The formula he used is on the CLC website https://www.clcboats.com/ext/screen-f781f4b64b.html

  4. #4
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    Default Re: Oar gearing thread

    Thank you Dusty.

  5. #5
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    Default Re: Oar gearing thread

    There are many other factors you need to consider. Racing sculls are set up for flat water, so much of the “oar gearing” is useless for offshore work in wind or swell. Since we don’t have the specifics of this 19’ offshore dory, we can’t give very accurate recommendations.

    I’d say build the boat and sliding seat, then borrow some oars and test in real conditions. That will not only help you pick a length but also blade shape, assuming you can borrow some different designs. For outrigger dimensions you might consider fabbing up some temp ones and testing those to dial in the very best setup for your style of rowing and waters.

    If your gunwales aren't quite wide enough but you don't want to pay for the folding metal outriggers, consider making some bent ply outriggers. These are used a lot out here in the SF Bay Area by TSCA rowers, and can be easily removed and turned inwards for transport.



    Last edited by Thorne; 01-22-2019 at 12:53 PM.
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  6. #6
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    Default Re: Oar gearing thread

    If using that formula how will your hands be aligned?

    ( I've gotten in the habit of rowing slightly cross-handed (hands pretty much above one another) in order to use some longer oars on a narrower boat and stay geared down a bit. I'm not really sure if it's a good thing or if it's just what I've gotten used to, but the boats I've used with hands spaced further apart feel awkward.)

  7. #7
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    Default Re: Oar gearing thread

    I shift hand positioning depending on how rough it is. Normal (high gear) is hands near each other with the handle ends maybe an inch apart. Sometimes if that's work I'll go to hands one above the other which shifts the pivot point of each oar 2" to 3" down. And if I really need bollard pull power, like the time I towed a fifty something foot motor boat with a failed engine the last half mile to a marina, I take the oars to opposite hands - starboard oar to right hand - for maximum leverage.

    The heavier the load, the more (at least to get going) you want to move the pivot down the loom and the faster and shorter the stroke. Working oars are used in varous ways.

  8. #8
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    Default Re: Oar gearing thread

    Around here the traditional formula is:
    Oar lenght is a little over one and a half times the width of the boat measured outside the gunwales at the oarlocks.

    This makes sence with our traditional boats which have a moderate freeboard and fixed seats. Traditionally our oars have a rather narrow blades. In my oppinion that makes them easier to use in choppy weather. They are also counterweighted with a very thick inboard part.

    Previously I had a pair of early factory made oars which still retained some of the traditional shape. Last autumn they wore out so I had to buy a pair of modern Lahnakoski oars as that was what I could get. I have already felled the trees for a new pair of traditional oars. The Lahnakoski ones are awful.
    Amateur living on the western coast of Finland

  9. #9
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    Default Re: Oar gearing thread

    I tried using 9'6" oars on my row/ sail boat, RanTan, whose spread was the same as a racing boat, 5' and change. Not good. What the formulae tend not to include is the height off the water. I had to angle the oars down so much that I was pulling up around my chest somewhere. I grabbed the 10'2" oars from my åfjords faering and they were far better. I got some 10 footers out of Nova Scotia ( nice spruce) but they were also hard. The blades were too big, bigger than the area on the 9'6" sculling oars and bigger than the faering oars. So I laid the faering oars on the Nova Scotia ones and whacked off about a 2" chunk slice on the shoulders of the blade. Much better. The åfjords faering oars also have distinct ridges running from the kniep down the loom continued as a ridge on the blade making the loom shaped almost like two cathedral ceilings base to base. An oval with ridges. All of this to lighten the outboard weight. It works.
    Ben Fuller
    Ran Tan, Liten Kuhling, Tipsy, Tippy, Josef W., Merry Mouth, Imp, Macavity, Look Far, Flash and a quiver of other 'yaks.
    "Bound fast is boatless man."

  10. #10
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    Default Re: Oar gearing thread

    Thanks for your information on the doryman's stroke rigger design.


    As I am not limited by beam of boat, I guess I will have to so some experimenting with a range of different oars.

  11. #11
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    Default Re: Oar gearing thread

    Quote Originally Posted by heimlaga View Post
    Around here the traditional formula is:
    Oar lenght is a little over one and a half times the width of the boat measured outside the gunwales at the oarlocks.

    This makes sence with our traditional boats which have a moderate freeboard and fixed seats. Traditionally our oars have a rather narrow blades. In my oppinion that makes them easier to use in choppy weather. They are also counterweighted with a very thick inboard part.

    .
    We use double the width at the 'oarlock' + 12 cm for the grip. Balance point is 2/3 of the distance from blade tip to where the grip begins.
    On my 596 cm (19'6") long færing that gave 324 cm (10'6") long oars for the longest pair. (156 cm beam at the rowing station). 110 cm long blade, 4 cm wide.
    Last edited by lagspiller; 01-28-2019 at 11:26 AM.

  12. #12
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    Default Re: Oar gearing thread

    The oars should be matched to the boat and usage. The formulas mostly work for heavier, slower boats. Fast narrow boats like wherries, guideboats, and Rangelys tend to benefit from longer oars and shorter blades. If you sail more than row, stowing the oars out of the way is more important so you wind up with shorter oars.

    Oar blade tips slip backwards through the water as you pull, at some point on the blade they pivot, and farther from the tip a long oar blade can move forward through the water. The oars slip more on a heavier, slower boat so the whole blade is pushing the boat. The oars slip a lot less on a fast narrow boat, so long blades on long oars can have half the blade pushing the wrong way. If the blade is too long and wide on a fast boat it can be like driving with the emergency brakes on. Larger blades are great downwind, but a smaller blade is less tiring upwind. The only person I knew who matched oar blades to conditions was Andy Steever. Andy went into great detail in Oars for Pleasure Rowing. (I don't like the temporary flea bay link, but Abe Books has it for twice the price and Amazon 3x).
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  13. #13
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    Default Re: Oar gearing thread



    The twin holes in Vivier oars offer two different gears. An example of workboat refinement - faster easier stroke into the wind I believe.

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