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What propeller


Amk1986

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1 minute ago, Amk1986 said:

Thank you. I dont know much about props would the 17 x 12 make much difference from 16 x11?

It might prevent the engine reaching maximum revs but for canal work that pretty academic.

 

Both those are for a 2:1 reduction ration, yours may or may not be that.

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13 hours ago, Pie Eater said:

Contact Crowthers who will give you accurate advice.

 

Crowthermarine@co.uk

Although they would be my go-to supplier for inland props they may decline to tell you the size until you order. They did for me. They seem to have got fed up with offering their experience and than having the customer go elsewhere.

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An inch on dia. and an inch on pitch doesn't sound much but water is 'thick' stuff and assuming the shape of the blades is similar it would add a bit to the load on the engine. You might not get a dramatic difference in speed or stopping distance but in my (often wrong) opinion it feels better for the engine to be doing its job by working it a bit rather than doing its job by rattling away lightly loaded and relying on higher revs to get anywhere. Prop. science has got a lot of equations and big sums in it so sometimes you just have to experiment a bit.

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19 hours ago, Amk1986 said:

Thank you. I dont know much about props would the 17 x 12 make much difference from 16 x11?

The question is why do you think the 16 x 11 is wrong? Do you know that it’s not the size fitted from new? My BMC 1.5 fitted 44ft boat was quite troublesome with a 17 x 12, too fast at idle (which was already set low) and in and out of gear much too often when manoeuvring slowly.

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10 minutes ago, Amk1986 said:

Mainly because it dug its arse in and didn't go quicker when in the higher rev range. Though I'd try and find what is the standard fit for it 

That may well be because you were trying to drive it too fast for the width and depth of the channel. If you eased the throttle it may well have speeded up despite lower revs because its no longer dragging its back through the mud.

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16 hours ago, Amk1986 said:

Mainly because it dug its arse in and didn't go quicker when in the higher rev range. Though I'd try and find what is the standard fit for it 

Changing the prop will make no difference in this situation. As Tony says, the width and depth of the channel determines the water flow to the prop. You may reduce the engine revs by fitting a coarser prop, but will end up sitting in the mud at exactly the same speed as before. The 4mph limit on canals is rarely achievable; even planning a journey at 3mph is a struggle. Slow down, relax and enjoy being overtaken by pedestrians.

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  • 4 weeks later...

There are a number of propeller size calculators on line which can be easily found, but I remember about25 years ago PBO published a chart that worked very well for me. I was skippering a 60-tonne 72' schooner with a 112 bhp Mercedes straight-six engine, which seemed a bit gutless, although it would eventually reach a cruising speed. The PBO chart recommended a larger engine, and I fitted a Vetus-Deutz DT65 straight six of around 180bhp. Super result; using the same prop, the boat accelerated and decelerated promptly, and cruised on a whiff of throttle. The DT65 was turbocharged, but without an intercooler, which would have taken the power up to 231bhp. 

 

So why not contact PBO and ask for the chart?

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My river barge is currently out of the water awaiting the end of lockdown before I take her to France via the Thames. I removed the prop, which was undamaged but grubby, as I proposed to fit the pictured rope cutter. As the prop was off, I chose to have it polished by "AmianJ", (sic) a metal polisher in Fenton, Stoke on Trent, who did a great job fat a very reasonable cost, and it's almost a shame to hide it underwater. Anyway, a clean prop is an efficient prop, and is less likely to gather growths.  (Note both the anti-cavitation plate and the Schilling rudder, which is said to be particularly efficient at low speeds).

Prop aft copy.jpg

rope cutter 2.jpg

20200517_214000.jpg

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On 26/04/2020 at 20:46, Amk1986 said:

Mainly because it dug its arse in and didn't go quicker when in the higher rev range. Though I'd try and find what is the standard fit for it 

 

As Tony said, that could be digging in too shallow water, but I notice you say you are in Stockport - which canal were you on when trying this?  You would get very different results on the Bridgewater to those you would get on the Trent and Mersey!

 

Also, propeller size depends on the shape and size of the swim on your boat, not just your engine and gearbox.  In an ideal world you build a boat to suit your chosen engine and prop, but a secondhand boat can be on it's second or third engine by the time you get hold of it ...

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