Measuring the power output

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charlesp
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Measuring the power output

Post by charlesp » Mon Oct 13, 2008 10:40 pm

The other Charles and myself have been mulling over the measurement of power output using the most rudimentary methods.

I would like to compare bollard thrust, from a fixed point, with a spring balance or similar.

The other Charles points out that this is an inaccurate method.

So I'll throw it open to the forum - any ideas?

We would like a method that anyone can employ using the cheapest and most easily available equipment that will enable us to compare motors.

What do you think?

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timberman2004
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Post by timberman2004 » Mon Oct 13, 2008 11:19 pm

m'dear chaps this is SO easy ...

a std test craft ....( I happen to have one )

engine mounted, and in full singing fettle .....

a spring balance metred out in KPMG's (how topical ) and off you go

thrust to the max and record the results ...

these figures returned, matched to the Heisenburg Constant will return an accurate HP ......in old money (=divide by todays exchange rate to the Bolivian peso )

a test result of 1:1.56 off the oscilloscope on the given criteria, on full amplitude, merits a gold star

to the less technical, ...yes ,they can pull the skin off porridge...if in the right conditions,.....just

Do not confuse this with the completely different custard-skin test

regrettably seagulls so tested, will remain oily. smokey, noisy, and rather endearing, .....despite all the previous

which is probably why we delight in mucking about with them
Neal...errrr... an ON, OP, 2xSD, F, 3xSJP, LLS, 2xFV, FVP, FPC, CPC WPCL, WSC, and a few eggs hatching, hopefully

rosbullterier
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Post by rosbullterier » Mon Oct 13, 2008 11:48 pm

Bollard pulling?! Visit deep darkest Cornwall on Dec 20th (10am) for the Lerryn Seagull Race day.

- I understand this year (21st anniversary race) there will be a seagull tug 'o war . . .

I didn't quite understand the technical details explained in the Ship Inn the other day - clock into www.Lerryn.net after November 5th and the latest details will be revealed with poster to download.

Pints of testosterone are available all day at the hostelry.

My latest weapon, a short white catamaran, will introduce me, and the new blonde bullterrier (English). I would be amazed and delighted to meet and greet you sad buggers from the the south east, preferably as entrants . . . ( and Austrafalia and the Nederlands)

Andy

pistnbroke
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Post by pistnbroke » Tue Oct 14, 2008 9:10 pm

I dont see why bollard pull would be a problem ..the boat does not matter as its stationary ..you just going to get a reading say 90 lbf.....BUT how you convert that to HP is not possible as you are including propeller efficiency gearbox losses etc .. And because the propeller is not moving through the water its not operating efficiently ...

Conventionally you would get the engine to drive a generator ..not via the gearbox ,,the output shaft ...and then load the generator to hold the revs down ..

So say at 3500 rpm the generator makes 750 watts ...then IF generator is 50% efficient you have a 1500watt output which is just over 2 hp ( 746watts equals 1 hp you could use a car alternator but you need to get it up to 6-8000 rpm not very paractical for a seagull ...

A traditional engine dynamometer is basically paddles in water and you measure the input temp output temp and volume of water heated to see how much power was put in.....

the only other way is to try a bollard pull with say a modern 10HP and then compare with seagulls ....??

phil
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power output

Post by phil » Wed Oct 15, 2008 1:17 pm

It seems to me the important thing is how it moves the boat. The horsepower measure is of work done over a period of time. This suggests to me at least, a sea trial situation, eliminating as many variables as possible the most influential first. In practice, running a course, flat water, no current, no wind, same temp. same barometer etc.etc., same boat with same loading, same lower end on a variety of power heads. The displacement length of the boat should allow should support of a higher hull speed than any of the motors are likely to produce.
After using various Seagulls on my 21 ft, 2150 lb. sailboat this summer, it appears there was very little actual difference in performance between a 1971 WSPCL rated at 5.5 hp 4:1 gearbox large 5 blade prop., and a WSL rated at 4.5 hp 3.5:1 gearbox 9.5" 5 blade prop., and a 102 cylindered WSPC power head on a 2.5:1 gearbox, short shaft and 2 blade bow-tie style prop. As age creeps up, the real question becomes which one do I want to have to haul up the vertical wharf ladder at low tide. Its definitely not the 1971 monster motor.

rosbullterier
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Post by rosbullterier » Fri Oct 17, 2008 1:28 am

As you say, the 102cc engines (with the same carb) all ought to put out the same power with the different propellers performing their tasks with appropriate gearing to suit different hulls.

My 18'6" Whitehall skiff has a fast hull shape and is pushed along easily at 8 knots by a standard Silver Century, but the 11' catamaran I tested today seemed slower using a 170 and a 12.30 box with large three blade 102 prop. The skiff is heavy at 200kg, the catamaran is easily lifted by two people.

Tomorrow we are testing other props including Mercury using revmeter and GPS. It would appear there is no Seagull factory information or specs on the arcane or elegant shaped propellers they produced.

Was the R&D dept run by artistic designers rather than marine engineers?

pistnbroke
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Post by pistnbroke » Fri Oct 17, 2008 4:18 am

Perhaps the two charles have not made it clear if they want to measure engine power or something else ..if its engine HP then the gearbox and prop have nothing to do with it and the measuring equipment should be connected direct to the engine output shaft. and as you say if they are all 102 cc with the same induction and exhaust timing /compression and piston head shape /exhaust..they should all be the same .

I love the comments on the prop though dont be fooled ....the props made by Brunell ..big flat paddles on the end of arms are as efficient as a modern prop ...

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charlesp
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Post by charlesp » Fri Oct 17, 2008 9:55 am

I'm not looking for a measurement of power output based on nhorsepower, or anything like that.

I'm looking for a rough measure that can be used by anyone. That would mean someone could write in to here and say 'I get about 23 on the SOS scale', and we wopuld know what he meant.

That's why I'm mkeen on the 'bollard pull' type of thing. Yes I know it's meaningless when measuring a static craft. Yes I know the revs will be artificially kept low.

But it's accessible. Everyone involved in life afloat has a length of rope; everyone either has or can borrow a spring balance; everyone would be instantly familiar with the method.

So I coul;d write in here and say that my CPC gives, say, 50 pounds of thrust, and someone would reply and say that theirs gives 75, so I must have a problem.

The conversation started when discussing the different 102 cc power heads and their power outputs. The racing boys prefer some to others as there are earlier blocks that have better porting etc. I t would be fun to try some method of verifying these claims.

pistnbroke
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Post by pistnbroke » Sat Oct 18, 2008 10:20 pm

If thats it charles I dont see a problem ..spring balance ..rope and fire it up ....boat does not matter as its stationary but keeping the prop clear of the transom would be best say on a dinghy....you could just say at full throttle . or buy a tach/hour meter from ebay ..just attaches to plug lead .
comparisons between props could be made ...
converting a pair of bathroom scales comes to mind everyones wife has thrown a set because they read too high ,,,

If you dismantle it may be possible to drill a hole "between the feet" and put a rope through so it would read when the rope is pulled

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Charles uk
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Post by Charles uk » Fri Oct 31, 2008 12:27 am

I spent an hour earlier this evening composing a long post explaining why a static bollard pull was not the best way of doing this & why it often gave a false impression, only to have my wonderful new computer crash as I was posting it.

I then had a brain wave.

How about a century transom bracket with the thrust block replaced with the load cell from a set of electronic bathroom scales then connect the battery & the rest of it's internals to the transom somewhere.

Hey presto! a digital readout of the thrust, in pounds, kilos or stones, take your choice.

It would not give you an accurate figure as it operates as a lever with the fulcrum being the tilt pin & there would be a difference between a long & a short shaft as the lever length changes.
It might also need some kind of shock absorber to stop the reading jumping all over the place.

But you now would have a way of measuring the thrust at any speed & with a rev counter you could ensure you were running in the right area in the rev range.

Perhaps one of our southern hemisphere members could try this out, it's far too cold here for playing, it snowed yesterday. Snow in Berkshire in October first time since 1937, so much for global warming!

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charlesp
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Post by charlesp » Fri Oct 31, 2008 10:18 am

Now that's an interesting thought, Charles. By the way, welcome back to the Interweb!

Similar to the method used at the factory, as you know.

It is certainly too cold here in Poole, but perhaps we have a reader who would like to take the bathroom scales to bits. Tell the current primary woman you think they're over-reading and she'll be fine with it.

But what about the display - would you use the opriginal?

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