Well I think we may have a slight case of not considering all the factors. In the past day or so I ran across a sight (somewhere) that said you use short oars for a slow boat and long oars for a fast boat. Force on the water is important, but speed of the oar tip relative to the boat is also important. Work is force times distance, and power is force times distance per unit time; i.e., force times velocity. Moving the pivot point out so that you put more force on the water means you have to move the handle further and faster for a given boat speed, so there is a tradeoff. I find that my oars have the pivot point about 23 inches from the handle on an oar that is about 6.5 feet long for a 10-foot rowboat. So you don't go 4.5 times further with the same effort. There ain't no free lunch. --Gary On Dec 3, 2007, at 5:34 PM, Steve R. wrote:
Howard, Gary, and others:
Let me throw some numbers at you for your comment.
First, assume there is no friction or other losses. :{)
Assume the blade of that 15 ft. oar is 3 ft long. The distance from the handle end to the center of the blade is then 13 and 1/2 ft.
Assume the distance from the handle to the oarlock is 3 ft.
The distance from the oarlock to the center of the blade is then 10 and 1/2 ft.
If you apply 10 pounds of pressure to the oar handle, you only transfer 2.9 pounds of force to the water. That is, 10 pounds multiplied by the ratio of 3 over 10 and 1/2.
If you could locate the oarlock at the midpoint of the 13 and ½ ft. distance, then 10 pounds of force would be transferred to the water for every 10 pounds you pull. So an arm would have to extend roughly 4 ft. from each side to support the oarlock. By doing this, you have multiplied your rowing effort by a factor of 3.5. If you could row for 1 mile before, now you can row for 3.5 miles with the same effort.
There is an additional benefit as well by reducing the arc. By my quick calculations, if you swing an oar through a 90 degre arc, only 63% of your effort goes into propelling the boat forward. The reminder goes to pushing water away from the boat at the beginning of the stroke, and pushing water toward the boat at the end of the stroke. If you could reduce the arc to 60 degrees, your propulsion fraction increases to 70%. For 40 degrees of arc it is 80%. I think with oarlocks on the coaming you would have roughly a 90 degree arc if you do “full extension” rowing. By moving the oarlock to the mid point it is closer to 30 degrees.
By combining both factors, you would go from rowing one mile to about 4.5 miles with the same effort.
I have run these numbers by non-sailors and they agree with this analysis. What do you guys think?
steve
Steve R. M-15 #119 Lexington, KY
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I thought the way Bob put those oarlocks on was pretty neat. Have wondered how well it worked to row the boat. I've also heard of guys fitting oar locks into the top of sheet winches, which are about the right location, but that leaves the oar height somewhat high.
On the Japanese sculling oar, I've built one of those too. First attempt was a crude one to see how well it worked. Initial attempt on the Spindrift was disappointing as it didn't want to rotate in the oarlock, so I went back and modified it some. Didn't have a chance to try it again and it's gotten pretty cold now for boating. I think it might work, but so would the other one.
http://i112.photobucket.com/albums/n165/haudsley/Picture260.jpg
http://i112.photobucket.com/albums/n165/haudsley/Picture262.jpg
http://i112.photobucket.com/albums/n165/haudsley/Picture263.jpg
The Spindrift gets rowed or sailed. The oars on it do pretty well and 1.5 to 2 knots is pretty easy to get. You can cover a lot of ground doing 2 knots. This was just playing around on it. I view the rowing option on the Montgomery as a third, emergency form of propulsion. Something I know will work when there is no wind and the motor won't start.
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Rowing the M17: Check out the Strawanza photos at the following link. Bob Eeg mounted oarlocks on the toe rail track and used 12-foot break- down oars. http://www.MSOGPhotoSite.com/mpage.htm
I'm playing around with the idea of a Japanese sculling oar design, called a "Ro", ironically. It places the blade vertically at rest rather than horizontally and may be more efficient. The following web site discusses that design with video. http://www.amateurboatbuilding.com/articles/howto/sculling_oar/
--Gary Hyde 2005 M17 sailboat #637 'Hydeaway 2' We can't change the wind, but we can trim our sails.
On Dec 2, 2007, at 8:41 AM, Steve R. wrote:
Hi Harry,
Welcome to the group.
You raise some interesting points for an off season discussion.
I have been pondering a post I read here (I think) recently about the difficulty of using a single oar as an Oriental yuloh. I believe the geometry is wrong for efficient use of oars with our small boats. Our boats have a narrow beam, short cockpit and high freeboard. If you place an oarlock on the top of the coaming or transom, you have poor leverage for seated rowing or use of a yuloh (yulohing?). Some people stand up, which increases the angle of the blade in the water, and improves the leverage situation by allowing use of a shorter oar. I would be comfortable standing in calm water, but not in extreme conditions.
So…if I HAD to use an oar I would think about installing oar ports. The ports would be located about waist high along the sides if I wanted to row or in the transom for a yuloh. Bolger has used oar ports as did the Dovekie. In extreme weather the ports could also function as cockpit scuppers or be closed off if necessary. Ports would also have the advantage of allowing the use of shorter oars.
Or..I would consider using some sort of an arm that would locate the oar lock several feet outboard. The leverage situation would be improved, although longer oars would be needed. Modern racing shells often have oar locks mounted outboard.
And..I would give a lot of thought to a sliding seat with a pivoting brace for the feet.
steve
Steve R. M-15 #119 Lexington, KY
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Greetings from N. shore of L. Superior: -15F,snow& wind last couple of nights but at least my M15 is safely tucked away in its dome shelter! Bob Eeg's tale of the specially equipped M17 that he constructed for the Austrian man was fascinating and raised a couple of questions about the M15: (1) Has anyone tried rowing one, and if so how was it? and (2) What were the worst conditions a M15 has survived intact (wind, waves) with all sails down and only motor power? Pardon me if these questions have been asked before- I joined just last year. If they have please point me in the right direction and I'll check it out. Thanks and safe sailing to the lucky souls in the south. Harry Elmslie
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