Sunday evening physics question - prop thrust

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scruff

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Just pondering a question over a glass of wine and thought it apt for on here.

When anchoring, I give the 30hp engine ~1800rpm or half-ish throttle in reverse to check anchor set. Prop is 2 blade fixed. This will put Xkg of strain on anchor chain.

Question being;
What sort of wind speed is that strain equivalent to for a 34ft AWB. Extra points for showing equation/ working. For calculation let's assume no tidal flow or wave induced snatch loads.

Thanks
 
If you motor into a headwind at 1800rpm what wind speed would stop you making any progress? At that wind speed prop thrust equals wind drag. Pretty strong wind I imagine.
 
DJE said:
If you motor into a headwind at 1800rpm what wind speed would stop you making any progress? At that wind speed prop thrust equals wind drag. Pretty strong wind I imagine.
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Interesting take on it. I found this season that 2400revs made 4.5kts progress into 42kt apparent wind so ~38kts true. Your method assumes the same efficiency in reverse as forwards.
 
scruff said:
Interesting take on it. I found this season that 2400revs made 4.5kts progress into 42kt apparent wind so ~38kts true. Your method assumes the same efficiency in reverse as forwards.
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You might be getting even more efficiency in astern gear if the prop was working in less disturbed water.
 
scruff said:
Interesting take on it. I found this season that 2400revs made 4.5kts progress into 42kt apparent wind so ~38kts true. Your method assumes the same efficiency in reverse as forwards.
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Yes but I don't suppose there's much experience of motoring backwards against the wind. :) It's a fixed prop so that error is probably no worse than several of the other uncertainties in this question.
 
scruff said:
Just pondering a question over a glass of wine and thought it apt for on here.

When anchoring, I give the 30hp engine ~1800rpm or half-ish throttle in reverse to check anchor set. Prop is 2 blade fixed. This will put Xkg of strain on anchor chain.

Question being;
What sort of wind speed is that strain equivalent to for a 34ft AWB. Extra points for showing equation/ working. For calculation let's assume no tidal flow or wave induced snatch loads.

Thanks
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If you're assuming no wave induced loads, you're wrong.

You could estimate the thrust at 1800rpm, and you could estimate the windspeed to priduce that force from the cross section of your superstructure etc, but even very small waves deserve respect.
 
The correct answer is that it is a trick question without enough detail. Most likely it is between 30 and 40 knots, as others have said, but the devil is in the details.

If you want an accurate answer, get a load cell and measure your rode tension in a breeze (tension ~ V^2) and then measure reverse thrust at the dock.
 
An answer is in PBO, somewhere.

There was an article in PBO of prop efficiency in which they tested various props on a yacht (I think a yacht) and I think in reverse (but it might have been in forward gear.)

I did not pay much attention to the article as I have props, 3 bladed folding Volvos - and having made the investment I was not going to jettison them for some minor improvement (or in fact for a major improvement!).

They secured the vessel and simply ran the engine measuring the tension developed.

If anyone has a copy of the article - AN answer, maybe not THE answer, is in there.

A common rule of thumb was 100kg of tension for every 10hp - but this averages prop efficiency.

That defines the tension - but now you need to know the windage of your yacht - as it is the windage that allows the yacht to develop the tension and a 40' ketch with a deck saloon, dinghy on the foredeck etc is obviously going to develop more windage than some sleek ULDB. However the ULDB might be quite flighty nd yaw all over the anchorge

The tension is in itself a bit meaningless as you really need to look at the extremes - what is the tension developed if the yacht yaws and hobby horses in a bit of chop (unless your wind, unusually, does not develop waves and you can guarantee swell never enters your anchorages) - the average is thus a bit of a waste of time.

I have done as Thinwater suggests and at 35 knots had snatch loads of 650kg on our cat (same windage as a 45' AWB) - which were frightening - and I terminated the tests. the 650kg was the extreme - averages were much lower. This was at short scope - and the anchor did not move (I measured its location as well). Arguably you are not going to anchor at short scope (3:1) - but you may want to retire from an anchorage and as you reduce the rode length as you retrieve the anchor - you will be anchored in 3:1 scope - and then 2:1.

I also, using the same load cell, measured the tension developed when running our 2 x 20hp engines in reverse at 3,000 rev, 2 x 3 bladed folding props, when attached to our mooring and achieved a maximum tension of around 400kg.

It is actually difficult to define the actual kgs - the tension is masked by the momentum of the yacht and the catenary of the chain - you run the engines flat chat and the yacht bounds (backwards) but the energy developed is quite high. Some of that energy lifts the chain and some moves the yacht aft. if you slowly reverse up and increase revs the yacht develops less momentum, its moving slowly - so the 2 tensions developed are different.

Of course once the momentum is lost (and some was lifting the chain and the chain now has potential energy stored in the catenary) - the yacht moves forward.

So the maximum tension varies with the speed of the yacht, whether it is the weight, catenary, of a mooring or your chain.

So.....you can get any answer you want - but if you define the parameters you can achieve the answer you need.

But when someone queries the size and design of our aluminium alloy anchor 8kg and use of 6mm chain - I know that I have tested it (without snubbers), not to its limits (or it would have dragged) but know it is all pretty robust and efficient - and that the anchor can stand the tension - but the crew will withdraw long before.

Jonathan
 
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LONG_KEELER said:
... John Goode sailing/berthing guru used to say that one knot of tide equals a F4 wind.
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There is only one way that makes sense (since 1 current knot on the nose is nothing), and that is if the tide gets you sideways. Which it will.

Wind against tide is a really tough one to calculate. It has a lot to do with underwater shape and balance.
 
all i remember from the PBO article is that when testing anchor is firmly in ground, simply going astern did not generate enough load to test it, but that you have to go forwards, then astern to get enough. hope that makes sense?
 
bikedaft said:
all i remember from the PBO article is that when testing anchor is firmly in ground, simply going astern did not generate enough load to test it, but that you have to go forwards, then astern to get enough. hope that makes sense?
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You can use the momentum of the boat to generate storm-worthy forces, but ONLY if you have a lot of rode out. You need some shock absorption. Otherwise you may only yank the anchor out. Effectiveness also depends on the bottom material. Additionally, the speed must be rather low. Like many aspects of anchoring, the math is too involved for a forum post. It's a chapter.

But in most cases this jerking around will only loosen the anchor. I think most feel that letting the storm do the work is more effective. The bump-setting method is really only for boats that have little reverse thrust, such as outboards. They may only have enough thrust to test to 20 knots.
 
LONG_KEELER said:
John Goode sailing/berthing guru used to say that one knot of tide equals a F4 wind.
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thinwater said:
There is only one way that makes sense (since 1 current knot on the nose is nothing), ...
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Seems quite reasonable to me. Sea water is about 850 times the density of air. F4 is about 13 knots so if the area x drag coefficient above waterline is five times that below waterline the two forces will be about the same. (Five times thirteen squared is 845.)
 
Personally I take the view that applying a bit of astern just checks the anchor is basically working. I.e., it's in the seabed and not wrapped around with its own chain, in the middle of a huge loose ball of weed, skating across smooth rock etc etc. It doesn't test it like a storm would. A gale of wind will plough the anchor in deeper, but a squirt of astern shows that it's starting to plough itself in, that's all. But it's still a useful test, because I have had an anchor fail to grip.
The force from backing the mainsail is much less than from the engine, but still does the job.
 
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