Propellers compared

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Propellers compared

Postby electrosys » Wed Sep 14, 2011 2:52 pm

At last I've fulfilled one of my ambitions - to own a 'Barge-Pusher' - and a few days ago I took delivery of a 1958-9 102 with its 4:1 reduction box and mighty 13" 'cloverleaf' propeller.

The prop on this engine seems so much 'bigger' than the Hydrofan prop on my S/C/Plus's (which have the same gearbox ratio and engine cubic capacity), so I made a stab at comparing them this morning.

By tracing each blade outline onto graph paper, the Hydrofan prop has a measured total blade area of some 247.5 sq.cm., which equates to 38 sq.in. or 0.26 sq. ft. This compares with the 102 'cloverleaf' prop of 496 sq.cm., that's 77 sq.in. or 0.53 sq.ft.

So the 'Barge-Pusher' has twice as much grip on the water as the Silver Century Plus. Curious.

I then checked the 'swept area' (the area of water cut by the prop - the prop diameter minus hub diameter), and the S/C/P is 95 sq.in., compared with the 102's cloverleaf at 120 sq.in.

I don't have the necessary tools to measure prop pitch, but they look pretty similar - maybe the cloverleaf has a slightly coarser rake angle, but there's not much in it.

From all I've read, the S/C/P is supposed to be a direct equivalent of the 102 barge-pusher, but right now I'm finding this hard to accept. Looking forward to testing it on the water, alongside the others.
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Re: Propellers compared

Postby Rex NZ » Wed Sep 14, 2011 7:57 pm

Welcome Electrosys

Finding the pitch of your prop is straight forward

The prop is a just helix & we want to establish the theoretical forward climb per revolution

If you 'unroll' the helix onto a flat plane (ie take a developmental view), then you will find you have a right angle triangle

Our right angle triangle has these values;
* opposite length ~ this is the Pitch
* adjacent length ~ this is the working circumference of the prop. Say 80% of the tip diameter (inches) x 3.142
* angle in degrees ~ sit the prop flat on the bench & measure the 'working blade angle' (at 80% of the diameter) from the bench

Calculator method for lazy people like me;
Pitch = Tangent of angle (degrees) x working circumference (inches)

Alternative practical method if you have no calculator or protractor;
Sit the prop on the bench. Project a straight-edge from the bench-top across the working face of a blade. Mark out the working circumference of the prop as a straight line on the bench from where your straight-edge touches the bench. Measure the perpendicular height of the straight-edge at the working circumference. No trigonometry required. Just a tape measure & chalk.

Either way, After measuring your prop you will discover the seagull prop is poorly shaped being a grossly imperfect helix. Having said that, they perform surprisingly well.

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Re: Propellers compared

Postby Rex NZ » Wed Sep 14, 2011 8:28 pm

Electrosys

As for the thrust of a 102 barge vs Silver Century Plus

I've wondered about this too. They look to be fairly similar.

It was proposed to have a tug-o-war between the two to prove the theory. However, this is yet to happen

Did do a tug-o-war between a Silver Century Plus & a regular silver century. The difference in thrust was quite significant.

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Re: Propellers compared

Postby Daryl » Wed Sep 14, 2011 11:26 pm

Is this the next in upcoming competitions Seagull Tug of wars, could be interesting.
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Re: Propellers compared

Postby Collector Inspector » Thu Sep 15, 2011 12:46 pm

I think that "The Bigger Box" may win a tug?

Maybe Mark can organize a competition heat during the OZ cruise Aye?

Should be a draw card to any local meet.

B

PS: Nothing has been mentioned from the UK about this type of interest? Bollard Pull etc................why?
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Re: Propellers compared

Postby electrosys » Thu Sep 15, 2011 1:53 pm

Collector Inspector wrote:Nothing has been mentioned from the UK about this type of interest? Bollard Pull etc................why?

Well - I'm the new boy here, so can't comment on that ... but whatever the outcome (btw, my money's on the 102 cloverleaf) ... it will raise some interesting questions:

If the cloverleaf wins, then what was British Seagull's thinking re. the hydrofan ? As this result would suggest that the hydrofan is undersized.

However, if the hydrofan is equal to, or more powerful than the cloverleaf - why hasn't the commercial sector gone over to using square blades instead of the cloverleaf-type which appear to be de rigueur - with the notable exception of ducted propellors which tend to use square blades for obvious reasons ?


So - just being curious - will you guys have a tug-of-war 'just for fun', or will you adopt a scientific approach by swapping the props over on the two boats (which would suggest using 2 x 102's - one fitted with a S/C/P prop) between runs ?
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Re: Propellers compared

Postby charlesp » Thu Sep 15, 2011 4:33 pm

Collector Inspector, if you type 'bollard' into the search box at the top right of the SOS forum screen you'll see it's been discussed at length.
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Re: Propellers compared

Postby charlesp » Thu Sep 15, 2011 7:30 pm

When the Hydrofan was introduced British Seagull sold it as an upgrade; in fact if memory serves me right they quoted a 10% increase in power. Mind you, this was well before the Trading Standards Act. They has also been advertising the Villiers magneto as being fitted with 'Instarter Magnition' which was supposed to make the engine 'simply refuse to not start'. Excruciating, eh!

It's been suggested that it looked a little (if you have a fevered imagination) like the turbo props on the airliners of the day, very modern, very hi-tech.

Me I just don't know which is more powerful. I did have the opportunity a couple or so years ago when I lent a Kingfisher to a chum so he could compete in the Lerryn racing. We sorted out a few props for him to try but never got round to it.

I too have a soft spot for the 'Barge Pusher, it's so delightfully big and clunky. It looks great photographed against a Little Model Forty. I haven't tried one on my boat, but this correspondence has had me thinking of getting mine running and giving it a try-out. Those propeller blades look a little like someone has glued table tennis bats to an old prop boss.
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Re: Propellers compared

Postby electrosys » Thu Sep 15, 2011 8:38 pm

charlesp wrote:I too have a soft spot for the 'Barge Pusher, it's so delightfully big and clunky. It looks great photographed against a Little Model Forty.

I recently did exactly that: http://www.ybw.com/forums/showthread.php?t=286793

I'm glad to hear someone else has a soft spot for the BP - in fact every time I look at mine, it puts a smile on my face. No other engine has ever done that. My guess is that psychologists would say it's a similar response one has to the huge paws of lion cubs, or the oversized eyes of kittens ... 'the cuteness factor'. Or maybe it's something to do with Thomas the Tank Engine: the unassuming but very capable machine often subject to ridicule ... but when the going gets tough ...

Jeez - I should have been in advertising.
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Re: Propellers compared

Postby Charles uk » Thu Sep 15, 2011 9:11 pm

I have tried both props, hydrofan & the late model weed free on a espcl that I was using in a race in NZ the hydrofan was 1 mph faster.

Guess which one I used for the race?
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Re: Propellers compared

Postby Rex NZ » Thu Sep 15, 2011 9:24 pm

Seagull Tug-O-War

My money is on the 102 based on the superior thrust face. I think this is the dominating factor as it relates to slippage (ie traction).

A bit like crawler tracks vs wheels I suppose. Gear ratios & pitches may have little effect.

Maybe somebody can clarify what the silver century plus box ratio is (maybe 5:1)

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Re: Propellers compared

Postby Charles uk » Thu Sep 15, 2011 9:49 pm

Seagull gear ratios are as follows

4 :1 Century +, Barge pusher (102+)
3.5 :1 40+, LLs,
3.1 :1 FNR
2.5 :1 Clutched 40, curlew, model 75, 102 clutch & fixed
2.1 :1 Featherweight.

Blade area required is dependant on how hard it is to push the boat.

Gear ratio's & pitch are inversely proportional to each other & proportional to the power available.
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Re: Propellers compared

Postby electrosys » Fri Sep 16, 2011 7:37 am

Rex NZ wrote:Seagull Tug-O-War

My money is on the 102 based on the superior thrust face. I think this is the dominating factor as it relates to slippage (ie traction).

A bit like crawler tracks vs wheels I suppose. Gear ratios & pitches may have little effect.

Maybe somebody can clarify what the silver century plus box ratio is (maybe 5:1)

Rex

As Charles says, they're the same (at 4:1) - indeed, that's the underlying reason behind my initial post: same cc; same compression ratio (I assume); indeed, same everything right down as far as the horizontal drive shaft ... it's only the prop which is different, and I agree with your bet - as the 102+ has twice the blade area.

I respectfully disagree with Charles that the Static Bollard Pull is an invalid method of either measurement or comparison: a tug-of-war situation mirrors a Static Bollard Pull exactly and, in the case of tugs, Static Bollard Pull (usually expressed as tbp - 'tonnes bollard pull') is still used as a standard method of reference measurement between vessels with various permutations of hull design, propellor configuration, engine horsepower and so on.

Although the figures for Static Bollard Pull are not the same as for Dynamic Pull, any percentage difference between them will be the same for any engine/gearbox/propeller configuration installed on that hull. Dynamic Pull cannot be used as a valid method of reference between various engine configurations on different hulls, precisely because of the number of unknown variables involved in a dynamic situation.
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Re: Propellers compared

Postby Charles uk » Fri Sep 16, 2011 9:11 am

Why don't you come to John's canal cruise & we'll give it a try.

I do have a 100lb spring balance, do you want me to bring it?
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Re: Propellers compared

Postby Buzzook » Fri Sep 16, 2011 1:56 pm

I think, before the 'discussion' slips any further, that perhaps we ought to get people to clarify what is important to them and why?

While a bollard pull may be a factual and accurate method of measuring the available "thrust" from a particularl engine/prop combo, and providing all other factors (same boat, same tide, same weather, same fuel, same air temperature and density...) are identical, then one would have merely established which has the greater or lesser "measured bollard pull".

So if one required a vessel and motor capable of a higher bollard pull - say for towing a barge - then that might indicate which motor/prop combo to go for.

But if, as Charlesuk has pointed out, one is after "forward speed throough the water" then a bollard pull is not a real indicator of that.

It's all about the gearing and the pitch, but the lower gearing provides for less revolutions, ergo the larger prop with lower geared box is going to travel slower going forwards than the hydrofan on a similar cc motor with a higher ratio gearbox. Smaller thrusts, but more of them, more often.

It is for the same reson that trucks have lower diff ratios than sports cars, and why, in my long ago squandered youth we swapped out the 3.37:1 diff in the old Holdens for a 4.11:1, as it lowered the back axle ratio and allowed more power to be put on the ground, making the car accelarate harder and faster, but it was ultimately slower at top speed, revving much harder to attain the 100km/h than before.

Au contraire, if one wanted lower revs at highway speeds, say for improved fuel mileage, one swapped out the standard 3.37:1 for a 3.08:1, as this allowed the back axle to spin faster for the same engine revs.

Outboards operate on the same principle. Energy in = force out. Gearing determines where and at what point in the rev range the force comes out.

Hope that helps! :)
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