Amk1986 Posted April 23, 2020 Report Share Posted April 23, 2020 Hi guys, any ideas what propeller should be on a 44ft colecraft boat 1984. With a bmc 1.5. I've tried to look online and it's a mine field any help would be grateful Link to comment Share on other sites More sharing options...
Tony Brooks Posted April 23, 2020 Report Share Posted April 23, 2020 What is the reduction ratio - probably on the gearbox specs. The there are a number of online prop calculators for you to put your numbers in. Link to comment Share on other sites More sharing options...
Amk1986 Posted April 23, 2020 Author Report Share Posted April 23, 2020 Prm 101 2-1 it has a 16 x 11 on it. I tried a few online ones and had different results Link to comment Share on other sites More sharing options...
Slim Posted April 23, 2020 Report Share Posted April 23, 2020 BMC 1.8 (there ain't a lot of difference between it and a 1.5), PRM 150 2:1 box (I think actually 2.05 0r 2.1) Prop 17 x 12 Link to comment Share on other sites More sharing options...
Amk1986 Posted April 23, 2020 Author Report Share Posted April 23, 2020 Thank you. I dont know much about props would the 17 x 12 make much difference from 16 x11? Link to comment Share on other sites More sharing options...
Tony Brooks Posted April 23, 2020 Report Share Posted April 23, 2020 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. Link to comment Share on other sites More sharing options...
Pie Eater Posted April 23, 2020 Report Share Posted April 23, 2020 (edited) Contact Crowthers who will give you accurate advice. Crowthermarine@co.uk Edited April 23, 2020 by Pie Eater Website details Link to comment Share on other sites More sharing options...
Tony Brooks Posted April 24, 2020 Report Share Posted April 24, 2020 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. Link to comment Share on other sites More sharing options...
Bee Posted April 24, 2020 Report Share Posted April 24, 2020 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. Link to comment Share on other sites More sharing options...
Eeyore Posted April 24, 2020 Report Share Posted April 24, 2020 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. Link to comment Share on other sites More sharing options...
Amk1986 Posted April 26, 2020 Author Report Share Posted April 26, 2020 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 Link to comment Share on other sites More sharing options...
Tony Brooks Posted April 26, 2020 Report Share Posted April 26, 2020 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. 1 Link to comment Share on other sites More sharing options...
Eeyore Posted April 27, 2020 Report Share Posted April 27, 2020 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. 1 Link to comment Share on other sites More sharing options...
Amk1986 Posted April 27, 2020 Author Report Share Posted April 27, 2020 Thanks guys will leave it as is. Cheers guys 1 Link to comment Share on other sites More sharing options...
Trevor Lyons Posted May 26, 2020 Report Share Posted May 26, 2020 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? Link to comment Share on other sites More sharing options...
Trevor Lyons Posted May 26, 2020 Report Share Posted May 26, 2020 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). 1 Link to comment Share on other sites More sharing options...
TheBiscuits Posted May 26, 2020 Report Share Posted May 26, 2020 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 ... Link to comment Share on other sites More sharing options...
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