Oar blade shenanigans (here be dragons)

Discussion in 'Hydrodynamics and Aerodynamics' started by NoEyeDeer, Dec 1, 2013.

  1. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, so what is actually known about the hydrodynamics of oar blades? Not "what is sometimes theorised by people who feel like theorising without examining their own logic too closely for fear of finding something inconsistent?", but what is actually known?

    Reason I ask is because I've been looking into this lately, and the top players (Dreissgacker, Dreher, Croker, Bracca) all seem to have different ideas. Not only do they have different ideas to each other, but they keep coming up with different ideas to their own existing products. :p

    So, what makes an oar blade efficient or inefficient?

    Another question that is worth considering is "what makes an oar blade easier to handle without reducing efficiency?", because if the thing is easier to handle that in itself is likely to make you go faster.

    ETA: Oh and another thing to consider is how the difference between fixed seat boats and sliding seat boats will affect the oar blades (if it does). All the top stuff is made for sculls, which are faster than recreational fixed seat boats, and use longer oars and a lower stroke rate.
     
  2. DCockey
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    DCockey Senior Member

    Considerable discussion of varying quality at http://www.boatdesign.net/forums/hy...cs/what-propulsive-efficiency-oars-47468.html Leo's post #47 in that thread has several references, and the following posts discussed oar motion relative to the water. It's very important to do a "vector analysis" of the blade motion relative to the water, not just look at the component of the blade motion parallel to the direction the boat is traveling.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    I've looked at most of the Atkinsopht site already, and it is interesting. However, I'm not so much interested in "What is the propulsive efficiency of oars?" as such since it'll be what it is, if you see what I mean.

    The really interesting thing from my point of view is how to shape the silly things to get maximum whoosh per grunt. Specifically, for a fixed seat boat that will be running at around 6 knots or so, which is rather different to Olympic sculls.

    I'm assuming the difference in oar length and boat speed is going to have some ramifications for blades, but it's hard to tell what those will be.

    There may also have be ramifications for things like catch angle, since the benefit of very narrow catch angles for sculls is tied to their speed. As an obvious example, sculls use a much shorter catch when starting a race, so perhaps a slower boat should also use a slightly shorter catch.
     
  4. michael pierzga
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    michael pierzga Senior Member

    Easy to handle...balance, grip dimensions and power. Think of a transmission.

    Some oars are good for low gear conditions, some are good for hiGh gear.

    Row in permanent high gear and you will not have fun.

    As a general oar I like one with a bit of flex and a bit of slip with good balance..not so tip heavy..and a little short in length.


    If you are making up oars..build your first pair to be low gear stubbies.
    Leave plenty of extra wood stock at the grip so that you may remove and fine tune
    The second pair can be sporty and long
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    I'm a bit past that Oars 101, mate. :D

    I'm specifically thinking of flat water racing in good conditions.

    "Sporty and long" doesn't make sense, because as you increase the boat's speed the resistance increases markedly (around V2.5 in the speed range of interest) so you actually want a lower gearing (shorter outboard) at higher speeds.

    When just poddling around at 2 or 3 knots you can easily pull a really long oar with high gearing because the resistance is so low anyway, but if you try that at high speeds it's like trying to haul a Kenworth up a cliff.

    And yes, I do know this from experience. It's not "just theory". ;)

    Anyway, good balance and grips sized for the rower's hands (I prefer the Culler-style conical grips) are already assumed.

    Slight flex is also assumed (all cantilevered beams will flex to some extent under load) but should not be excessive (IMO Culler's oars are too soft for what I want).

    All blades will slip to some degree as a consequence of basic fluid dynamics, but you don't want too much slip as it wastes power.

    And no, this wont be my first pair of oars. My first pair was made decades ago, had Macon blades, and each oar was laminated up from 9 pieces of wood of 4 different species. They were pretty good. I want these ones better.
     
  6. DCockey
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    DCockey Senior Member

    The implication of the diagrams and discussion in the previous thread I referenced above is that the first step in understanding how oars actually work is to stop thinking in terms of "slip". A well functioning oar blade is not simply a drag producing device. During part of the stroke the blade can be moving "forward" relative to the water and still be producing thrust.
     
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    Question: would it or would it not be logical to assume that someone who is aware of how the tight catch angles work in sculls is aware of how the oar blade is working at such angles? :p

    If you really want to get into that, one should not necessarily stop thinking in terms of slip since it is relevant during some parts of the stroke.

    I really am past Oars 101, mate. Some people may not be. I am. Just sayin'. I'd like to go a bit deeper than that.
     
  8. michael pierzga
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    michael pierzga Senior Member

    ..i speak from the perspective of rowing nice boats . ocean shells...5 miles per day, year round for about 6 years. That is one hell of a lot of miles Hands like leather, callouses on my as s, permanent stiff neck.

    I still prefer a flexible, stubby, low power oar for general purpose. Whenever i fit out with the latest gran prix power blades, they wore me out and disrupted my natural rhythm. The high power blades were difficult in a seaway and useless when on a tack...with one arm overloaded.

    Even when surfing hard downwind i preferred the low power blade.

    Im sure that every oarsman s style and body shape is different.

    Hence the need for thought when laying out your personal oars.

    And as i mentioned..the grip is critical so that you can easily feather upwind or blade sail downwind.
     
  9. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, that's fair enough, but at the moment I'm not interested in general purpose oars for rough water. That's a situation where ease of handling trumps just about everything else, so I get what you're saying.

    What I'm curious about right now is the optimum blade shape for flat water, fixed seat racing. There's a lot of information out there, but it's all for sculls. There seem to be quite a few people here that are interested in fixed seat racing (I'm one of them) so I figure maybe we should try and nail down how what is known about blades (and rigging and catch angles and all the rest) can be adapted to fixed seat racing boats.
     
  10. michael pierzga
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    michael pierzga Senior Member

    I was able to borrow oars from a rowing club. This might be a good way to approach the problem if you have a club nearby.

    Shafts and handles are pretty standard...its the blade shape and balance

    In the perfect world you could change blade tips to suite the day .

    And i prefer fixed seat , with a good set of foot rests that allow you to extend your power stroke with the front of your foot...kinda like a bicycle, complete with toe straps so that you can pull yourself back into rhythm
     
  11. michael pierzga
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    michael pierzga Senior Member

    I. would embed one of these guzzler style foot pumps into the foot rest to bail the boat as you rowed

    Worthwhile if you think you might be picking up water

    [​IMG]
    subir foto
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Borrowing oars from a club wont be any use, IMO. I have a couple of clubs nearby, but their oars are for sculls, which means they are completely the wrong length for a fixed seat boat and useless for trials. The only way I could use them for testing is if I took a saw to the shafts and shortened them to suit myself, which the club is not going to be happy about at all.

    Bailers aren't exactly relevant to oar shapes either. :D I am thinking of painting the boat a nice shade of green though. I have a personal theory, backed up by impeccably inconsistent logic, which says that painting the boat green will make the oars blades more efficient. I can elaborate on said theory if you would find it interesting. :)
     
  13. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, I thought of a better way of putting this.

    There is already information available for sculls. This includes forces, speeds, stroke rates, catch angles, total stroke arcs, etc, etc.

    Any of the latest blades are probably pretty damned good for sculls. There are minor differences between the manufacturers, but there wont be a lot of difference in performance. All of them seem to win races (sometimes).

    This means I think it's safe to assume that the basic dimensions and curvature of any of the top blades are close to optimum for their designed purpose.

    The problem is translating that to another purpose: namely fixed seat and slower boats. The possible effects that I can see offhand are:

    1/ The lower speed of a fixed seat boat may render the extreme catch angles of sculls ineffective, because the blade wont be moving ahead through the water at the same speed at the catch, and lift drops off as the square of the speed (all else being equal).

    Given that sculls travel around 6 ms-1 and a fixed seat boat will be travelling around 3 ms-1, the same blade at the same angle will only be giving around 1/4 of the lift (assuming flow doesn't break down due to lower Reynolds number or whatever).

    This may mean that it would be more effective to row the fixed seat boat more like a scull that is accelerating from a standing start: shorter catch/less extreme angle. One of Valery Kleshnev's articles about starting in sculls indicates a catch angle around 80% of maximum at around half maximum boat speed.

    If that extrapolation works (ha) that'd mean optimum catch angles around 55 degrees for the fixed seat boat, instead of the 70 of a scull.

    To know if that will work you'd have to have an idea of what the blade can handle in terms of angle of attack/lift/etc before things break down.

    2/ Another other possible effect is on blade curvature. Given the shorter oars will be moving the blade through a smaller radius, it may be that a more highly curved blade would be better. Again, would need to have some idea of what the flow will do, at least in qualitative terms.

    There are bound to be other things to consider, but those are the two that stand out at the moment, AFAICT. :)

    ETA: Just remembered another one.

    3/ Given that the fixed seat boat isn't relying on the legs to thump power into the catch, designing blades and rigging for the catch may not be as effective as it is in sculls.
     
  14. Baltic Bandit

    Baltic Bandit Previous Member

    So you are anti-Orkz? http://en.wikipedia.org/wiki/Orkz
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    No, of course not. I'm pro-Orkz. Orkz are green and "live for war and constantly fight anything in sight, including each other". The latter is a perfect representation of attempting discussion on internet forums.
     
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