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Old 20-12-2013, 07:29   #31
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Re: Suggestions for things to Test with a Load Cell?

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No, only hand. Dozens of repeats. When the tension comes on there is enough give in the lin to equilize; different than fabric sewing.

Do you know . . .would that also be true in High modulus line (dyneema), or only in nylon/dacron? Is the "give in the line" construction/braid give on in the fiber elasticity? I generally am pretty happy with knots in Dacron/nylon line (when I can't splice) but they are really bad in high modulus line (I have had several explode).

Thrust data point: Yamaha 9.9 high thrust gives 250# forward and 180# reverse (several owner data points, all close).

Terrific. Thanks. Do you know . . . is it geared different in forward vs reverse or the same gearing and the difference is all in the different prop profile forward vs reverse?

That data is consistent with our experience that we can use a dinghy/outboard as a 'bow thruster'/ 'pusher boat' up to about 20kts.

I will be curious to measure Hawk forward vs reverse, as we have an autoprop (where the blades spin around 180 degrees, so it should see the same prop profile and get the same gearing, so in theory should be the same thrust. But I don't think it is, has always feeled more powerful in forward.

......
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Old 20-12-2013, 08:08   #32
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Re: Suggestions for things to Test with a Load Cell?

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......
I don't think the equalixation is fiber stretch. Visually (watching closely as they load up) I think the mechanism is a combination of the rope compressing and the stitches slipping (like a lashing). Since I am certain not all of the stitches are uniformly tensioned (first and last will always be different, and some samples had only 4 stitches), only this sort of mechanism explains the extreme repeatability. I tested with Stayset and Warpspeed.

Gearing is the same forward and reverse (I've had it apart). Just a difference in prop profile and exhaust. Outboards exhaust through the prop, but the Yamaha High Thrust has a deflector that gets most of it out of the suction area in reverse. It is also geared lower and has a larger prop than other 9.9 outboards. At over 50% throtle in reverse the vibration is considerable, particularly when stationary. I need to see if it is vortexing (prop not under the hull).

I feel the difference when backing into the slip; because I have twin screws it's easy to feel that forward is more than reverse.

Of course, boats are not symmetrical. Backing is different.

-----

I think it is an axiom for most sailboats, debunking the wives tale, that there is never enough power to truly set the anchor for a storm, only enough to give it a good start. I've always accepted that. Testing it under power is mostly for the nerves; if you do not know the nature of the bottom and whether it can bury, you have an unknown quantity that no amount of tugging will clarify what is really going on. You may simply have balanced the tip on a rock. Who can know?
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Old 20-12-2013, 08:25   #33
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Re: Suggestions for things to Test with a Load Cell?

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just picked up a 10 ton puller ram (already had the hydraulic pump).

Just need to make up a steel I beam base, with the ram at one end and the load cell at the other and I will be ready start some serious breaking.
Evans,

Probably goes without saying, but put yourself a metal screen cage around
your test frame... You're getting into the pretty big bang range of energy when stuff parts.... Good insurance should a fixture decide to give first...

I'm in the testing business, and have seen lots of eye opening surprises...

Got to see a 1 million lb tensile pull at a neighbor lab on occasion.... biggest boom I've ever heard/felt ...
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Old 20-12-2013, 08:29   #34
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Re: Suggestions for things to Test with a Load Cell?

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I think it is an axiom for most sailboats, debunking the wives tale, that there is never enough power to truly set the anchor for a storm, only enough to give it a good start. I've always accepted that. Testing it under power is mostly for the nerves; if you do not know the nature of the bottom and whether it can bury, you have an unknown quantity that no amount of tugging will clarify what is really going on. You may simply have balanced the tip on a rock. Who can know?
well said. completely agree. . . . Except . . . reverse testing will tell you if it is NOT set at all. So, it is perhaps best defined as a "necessary but not sufficient" condition.

I remember a situation two summers ago, when I "set" an anchor (the "R" anchor) with decent rpm, we had a three day blow, it held fine for 2 days, and then let go and we went back for a boat length (quite quickly) before it grabbed again. I asked about the bottom ashore (Nova Scotia, too cold with too bad visibility for a casual dive) and they told me there were 'narrow mud ridges over smooth rock' and that probably I had slowly pulled it right thru one ridge and come out the other side.
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Old 20-12-2013, 16:46   #35
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Re: Suggestions for things to Test with a Load Cell?

Comments on a few fibers in this thread:

My 7.5 hp Johnson outboard develops a measured 125 lb static thrust in forward and 60 lb in reverse at wide open throttle. I think the gear ratio is the same in forward as in reverse. At least in my memory the gears in the lower end look the same. I have three props for the engine (all different), and I can not remember which was in use when I did the measuring this summer.

My Yanmar 3HM35F has a 2.14 ratio in forward and 2.50 in reverse. The 14% lower prop speed for the same engine speed in reverse should reduce the speed of the water exiting the propeller by 14% and also reduce the weight/time of the water exiting the propeller by 14%. It is sort of a double whammy on thrust. In addition the blades of the prop seem to have a bit of an airfoil shape that might not like reverse. I have not measured the static thrust in forward on the boat.

On page 222 in Earl Hinz, "The Complete Book of Anchoring and Mooring" 2nd rev ed, static thrust ranges from 20 lb/hp for displacement hulls, to 15 lb/hp for semi-displacement hulls, and to 10 lb/hp for planing hulls. (1) The book never says forward or reverse. (2) I'm guessing that the differences in the table are due to the use of props really designed for use at cruising speed being used at zero speed. (3) On the next page after talking about powerboats with twin 200 hp engines, he says, "Sailboats faced with storm conditions at anchor may want to seek the services of a powerboat to help them powerset their anchors before the winds arrive."

Digging through a coffee can full of high shank feet years ago looking for a 1/4" cording foot to make the piping on my boat cushions, I found a foot with a zig zag hole and two side-by-side tunnels under the foot. The owner of the shop said the foot had been used to sew the ropes together in thousands of the loops at the end of the guy lines of army shelter-half tents. So, at least the US army machine sews eyes in small diameter lines.

I got a great eBay deal on my 0-2000 lb Dillon dynamometer by mispelling dynamometer "dynomometer". (I can't spell.) A seller also could not spell, and I was the only bidder. I bought it for a song to watch the loads on my anchor line in a breeze with and without an anchor riding sail.

In the Bahamas with clear warm water, I many times find my 44 lb Bruce with only one ear buried by my reverse. Three times after a front, it has completely dug in with only the shank partly showing. I am guessing that the wind pulls harder than my engine. (I am a 34' Pacific Seacraft.)

It might be fun to walk a marina dock with a dynamometer in hand asking the people in the cockpit with a drink in their hand to try reverse at full speed with the tension gauge tied between their bow cleat and a dock cleat. You could get a bunch of reverse/thrust data and maybe some dock cleat strength data. You might do the same thing with anchored boats, but it might not be as much fun.

Bill Murdoch
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Old 21-12-2013, 12:15   #36
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Re: suggestions for things to test with a load cell?

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Just an FYI, I think it was Practical Boat Owner, the UK mag, that did breaking strength tests on a number of common marine knots this year. I probably should have bought that issue...
It was the last issue, I believe. Good article.
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Old 07-01-2014, 16:26   #37
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Re: Suggestions for things to Test with a Load Cell?

Frozen dock lines?
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Old 19-02-2014, 10:52   #38
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Re: Suggestions for things to Test with a Load Cell?

Ground tackle connections, various combinations:

Shackles
Rope to Chain Splice (short)
Rope to Chain Splice (Elongated)

Would be interesting to test the connection eye on various anchors...I suspect HT chain exceeds eye strength of some anchors.



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Old 17-06-2014, 06:47   #39
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Re: Suggestions for things to Test with a Load Cell?

Elsewhere in a discussion about jacklines, the issue of what you make them fast to came up. This has prompted me to wonder if there could be some value in examining various methods of putting a loop on cleat or padeye, and or test the cleat or padeye fastening? Just a thought.
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