Test results from prototypes

Page 2 of 2 Previous  1, 2

View previous topic View next topic Go down

Re: Test results from prototypes

Post by topherdawson on Fri Feb 03, 2017 7:24 pm

Today we went rowing in a southerly chop maybe 0.7 to 1m high, and a headwind of 17 to 25mph. We had the solid cedar outboard (Design 2) oar at bow. It was pretty hard rowing with our 60+ mixed crew but we got about a mile to windward before turning for home. The existing long narrow sea blades did catch more than once but the narrow Macon ply blade did not, and my wife reported that it was light and easy to row with. She says that four of these oars in the hands of a strong crew in good conditions would fairly make the boat fly.

Also the ball ended pin and keeper worked well with no tendency to jump off the pin, even though it has no lanyard over the top of the oar to keep it down, as we have for the other oars.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Mon Feb 06, 2017 5:49 pm

Just a quick note to say that the UHF for the hollow outboard cedar/ply 4.3m oar Design 2 hot out of the mould is in the 1.5kg region. The deflection is about 100mm which is softer than target, probably because the flanges were a little undersize due to trying to get two strips out of the 19mm thickness.

Anyway it's the lightest oar yet!

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Thu Feb 09, 2017 5:26 pm

Further info as I progress: The balance point of the oar is close to the pin positions (265mm from the middle position) so the UHF varies quite a lot with gearing. I cut out a bit of the outboard to fit the blade which made the UHF lighter, so now without varnish it's 1.59kg at 3.0, 1.28kg at 2.8 and 0.99kg at 2.6.
Total weight is 4.89kg.
This is the pin area, the arrow at the right is where the balance point is.


This is the pin slot and dimple plate.
" />
This is the blade attached.


topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by Don Currie on Thu Feb 09, 2017 11:21 pm

Did some rough numbers for the hollow loom NZ prototype oar. Using current exchange rates 150x50 yellow cedar is about 17.32 pounds sterling per metre. 4mm Gaboon ply is about 40.86 pounds sterling per sheet. Total cost for timber and ply works out to 148.23 pounds sterling for a set of 4, so about 37 pounds sterling per oar, plus cost of glue and varnish of course. So in accordance with my general approach to life, it might not be particularly good, but by crikey it's cheap!!

That's a scary looking oar Topher - will be interesting to see whether it's strong enough with that great big slot cut for the pin. Could be a bit vulnerable in the intermediate gear with a strong rower. Maybe a tapered doubler, about 600 long, either side of the slot would be good insurance. Won't affect UHF at mid gearing at all, and at max gearing, only slightly.

Don Currie

Posts : 39
Join date : 2016-12-03

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Thu Feb 09, 2017 11:47 pm

Hi Don,

Yes the hollow oar is cheap in timber but expensive in skilled time, a trade-off we will shortly need to decide on. Finlay is making a solid all-Douglas prototype.

The slot is a bit scary but not far from it, two cedar flanges 12mm thick are taking all the stress. I will be putting two big screws through the slot to reinforce the barriers between the gears, and there is an 18mm plate glued over the top of the slot, bridging the gap.

Meanwhile a UHF of 1.28kg in middle gear is not to be sniffed at.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Sat Feb 11, 2017 10:23 am

After coating with epoxy resin, just a priming coat, the deflection is down to 83mm from 98mm. Epoxy is pretty stiff and it has been applied to the outer fibres of the wood so it's in the most effective place. Since it also protects the wood a bit from denting I'd say it's worth doing.

I don't know what the extra weight of the epoxy and subsequent varnish is going to be, but it obviously will increase UHF. It's sticky at the moment but I will measure UHF when it dries.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Sun Feb 12, 2017 7:40 pm

OK varnish and resin dry now, which seems to have made the oar a lot heavier (5.1kg but I don't completely trust my scales and have ordered another) and the UHF's are:
1.871kg@gear 3.0 true
1.5 kg@gear 2.8 true
1.21kg@gear 2.6 true.
Tomorrow the wind is looking pretty strong but if we go rowing I will post.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Mon Feb 13, 2017 10:54 pm

Went rowing today and took some video with a camera mounted on an "oar selfie stick" fastened to the oar pointing at the blade. This is the YouTube link:

https://youtu.be/dNa8ZwgdXnI

The oar is a bit too soft for my taste but that is because I made the flanges too thin by trying to economise on wood. It did not break although I tried. I caught a couple of crabs because the other three oars in the boat are set to 4 degrees of pitch and this one is zero pitch, so the boat was lurching about with the upthrust from the other oars. It was pretty nice to row with once I got used to the 1.87kg UHF. The low Polar Moment of Inertia was nice and it felt as though faster strokes would be easy to do. The slot/ball ended pin worked well and the spoon blade grabs the water well.
It did seem to kick up more water than the other blades, but this may be me not being used to it.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Thu Feb 16, 2017 8:48 pm

What have we learned so far?

Solid designs are coming out at about 2.5kg UHF at middle gear, hollow ones about 1.5kg.

Making the inboard of dense timber is making the oars as heavy as existing ones (5-6kg) but crucially they are better balanced.

I was wrong to think weight would be a proxy for Polar Moment of Inertia (PMI). Having the weight close to the pin makes the PMI vastly less. A rower who had just tried Design 2 the solid oar, guessed it had half the weight of our existing ones. In fact it is slightly heavier but the UHF and PMI are so much lower it feels like a much lighter oar.

Having a low PMI wastes less of the rower's effort in accelerating and decelerating the oar. If the crew want to stroke faster a low PMI helps a lot.

We now know that optimising the lowest UHF for a given stiffness target needs the innermost section to be stiff, and the flexible part to be the outer section. This is because allowing the inner part to flex makes a big deflection, but taking wood off the outer section has the best effect on UHF.

Making the lightest possible blade helps a great deal, and curving it in the vertical plane stiffens and strengthens a very thin blade. The shorter it is, the nearer the centre of area is to the end, and so the overall oar can be shorter for the same gearing.

Rower feedback is that the spoon blades grip the water better.



topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Mon Feb 20, 2017 9:47 pm

Today we went out in maybe 20mph of wind from the SW but got a gust of maybe 35mph and had a hard row against it. We had the solid prototype at 1 and the hollow one at 3. We were rowing in middle gear so the UHF of the hollow one would have been about 1.5kg.

The rower complained the UHF was too small and his arms got tired lifting the handle up and pushing the oar against the wind. In other words the UHF was too low for his heavy arms.

It may be that when the UHF gets less than the rower's arm weight the oar is unpleasant to row with in strong wind, and if so we may be better to go for the solid oar, which this rower preferred. Or it may just be what you're used to.

The spoons continue to behave well even in broken water.

Another rower had problems with the oar jumping off the pin as he had not tied it down, but this seems to be an individual problem.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by Robert Graham on Fri Mar 03, 2017 2:40 pm

Have almost finished the balancing of the hollow oar. It started with an overall weight of 7kg and UHF 1.3kg.
Current results are;
Length 4700m
Weight 5.85kg
UHF 2kg
Stiffness 94mm
Giving it a coat of epoxy has thrown the initial predictions out the window.
It is a slotted oar and now in the process of making a rowlock from 16mm solid SS pin mounted in a block and a rocker plate.


Robert Graham

Posts : 15
Join date : 2016-12-06

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Fri Mar 03, 2017 3:58 pm

Hi Robert,

Epoxy and varnish make a surprising difference! Did you measure the stiffness at the standard place, 2.5m from the pivot? 2kg UHF is good and so is 5.85kg in an oar this long. What blade are you going to use? Looking forward to hearing what it's like to row with.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by Robert Graham on Fri Mar 03, 2017 9:33 pm

Yes, the stiffness of 94mm was measured at 2.5m. Disappointed with the result after putting lattice stiffening inside to try and improve the deflection.
The blade is flat and currently oversized at 1000 x 150mm. It will be reduced to 1200cm2 max after trials by making it narrower and/or shorter from one end or the other. Any reduction in blade size will be used to further reduce the overall weight rather than reducing the UHF from the design spec of 2kg.

Robert Graham

Posts : 15
Join date : 2016-12-06

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Fri Mar 03, 2017 10:48 pm

The effect of lattice struts in a space frame is to carry shear stress from the compression side to the tension side, also to brace the compression side from buckling. Since the top and bottom webs of the box section do the same thing, I don't think the lattice work is contributing much. Having said that, the wing bones of birds have lattice work inside the hollow bone. This is a vulture wing bone:



My guess based on my first hollow oar, deflection 100mm, is that this one will feel soft, although strong enough. My second hollow oar shaft which came out of the mould today has a deflection of 70mm which is better.

UHF at 2.8 gear with no epoxy or varnish, but with the blade balanced on the end, is 1.5kg. Weight 5kg. Length 4.3m.

topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by Robert Graham on Thu Mar 16, 2017 11:33 pm

Following trials at the weekend under race conditions and feedback from club members a few modifications have been carried out to improve the prototype oar.

1. To reduce windage effect and vibration the oversize blade has been reduced in width from 150mm to 140mm and area from 1500cm2 to 1120cm2.

2. The weight reduction at the blade has allowed the oar to be rebalanced to a UHF of 2kg while reducing the overall weight to 5.5kg.

3. A rounded end piece has been fitted to the handle end to provide a more comfortable grip increasing the overall oar length to 4760mm.

4.During trials in a rough choppy sea the oar jumped of the rowlock which was put down to either the wide blade or the short rowlock pin. The blade has been narrowed and a restraint will be fitted over the oar to hold it on the rowlock pin. The next option will be to lengthen the pin so that it protrudes above the top of the oar.

Robert Graham

Posts : 15
Join date : 2016-12-06

Back to top Go down

Re: Test results from prototypes

Post by topherdawson on Fri Mar 17, 2017 8:00 am

Hi Robert, We also had the oar jumping off the short pin but having a rope restraint over the top of it as we already have with our swivels, completely cures that.


topherdawson
Admin

Posts : 211
Join date : 2016-11-24

http://sasoo.forumotion.co.uk

Back to top Go down

Re: Test results from prototypes

Post by Sponsored content


Sponsored content


Back to top Go down

Page 2 of 2 Previous  1, 2

View previous topic View next topic Back to top


 
Permissions in this forum:
You cannot reply to topics in this forum