Gary I can not promise you a free lunch. But I believe the principals I touch
on will save you energy thereby reducing both your appetite and grocery bill.
You have raised an excellent point about the relationship between boat speed
and oar speed. Please feel free to review and comment on these numbers.
Let us assume that an oar with a 10 ft. lever arm is used to propel a boat at 1
mile per hour. The boat speed may also be expressed as being roughly 1.5 feet
per second (fps). Assume the rower swings through an arc of 90 degrees and
recovery time equals rowing time.
Consider the rate that the rower strokes.
If the rower makes 30 strokes per minute, the oar blade is traveling at 15 fps,
or 10 times faster than the boat speed.
At 15 strokes per minute, the oar is traveling 7.5 fps, or 5X boat speed.
For 10 strokes per minute, the oar is traveling just under 4 fps, or 2+ times
boat speed.
I must conclude that the rower is wasting a phenomenal amount of kinetic
energy. Since kinetic energy is proportional to the mass of the displaced
water multiplied by velocity squared, the faster you move an oar blade through
the water the more energy it takes. It is better to have a large blade moving
slowly than a small blade moving rapidly.
I whole heartedly agree with your statement regarding short oars for slow
boats. These calculations show that long oars are a waste of energy for slow
boats.
Therefore I repeat what I said in my initial post. I believe the geometry is
wrong for efficient use of oars with our small boats. To improve the status
quo, it would be better to use a short oar through an oar port, or a longer oar
with an oarlock mounted on an outrigger arm. I did not say it would be easy to
do. I said it would be more efficient. I hope that collectively we could come
up with some idea of how to do either or both of these things. There is no
doubt in my mind that some optimum combination of oar length, blade size and
oarlock position will improve rowing our boats.
I did not arrive at my initial post by calculation, but by observing a number
of Oriental craft in magazine photos and travel logs that use a single oar hung
over the transom. The oar tends to be slightly longer than the vessel, and is
usually rowed by someone standing near the bow while stroking at a leisurely
pace. The operator is often a small child. This method of propulsion has been
described by European writers for centuries. It may very well be several
thousand years old. If this principal of leverage works in the Orient it seems
to me that it would work on this side of the globe as well. However, those who
try it on this side of the Pacific stand six feet from the transom and proclaim
that it is difficult to do. If you moved to the bow it seems to me that it
would be easier, but that is basically impossible with the cabin and deck
structure of our boats. Is there some other method that we could try? If you
have a suggestion I am all ears.
steve
Steve R.
M-15 #119
Lexington, KY
--- [EMAIL PROTECTED] wrote:
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
>
>
> *************************************************************
>
> 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.
>
> *****************************************************
>
> 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
>>
>> ************************************************
>>
>> 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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>
>
>
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