noimnotonfacebook Posted October 5, 2011 Report Share Posted October 5, 2011 Hi all Is there a calculation i can use to help me decide what prop would be best for my boat i know the engine torque gear reduction and also calculated torque at the prop and i know aprox boat dimentions eg draft weight ect can anyone help me? many thanks Link to comment Share on other sites More sharing options...
Proper Job Posted October 5, 2011 Report Share Posted October 5, 2011 You could try: link Link to comment Share on other sites More sharing options...
KenK Posted October 5, 2011 Report Share Posted October 5, 2011 Crowthers Marine 0161 652 4234 Ken Link to comment Share on other sites More sharing options...
Speedwheel Posted October 5, 2011 Report Share Posted October 5, 2011 Let me know the following and I will work out an approximate for you: Length Width Weight (approx) Engine hp Engine max rpm Gearbox reduction Maximum size prop you can swing (if known) Link to comment Share on other sites More sharing options...
noimnotonfacebook Posted October 5, 2011 Author Report Share Posted October 5, 2011 Hi speed wheel narrowboat will be 60ft x 6.9 (ish) @ waterline draft of 2ft so a caculated weight of 18 ton (aprox) hp = 15 @ 1500 rpm (175.5nm of torque @ flywheel) looking at hydraulic drive with 2:1 reduction and estimate room for about 18" prop (allowing space) any help would be great many thanks Link to comment Share on other sites More sharing options...
Speedwheel Posted October 5, 2011 Report Share Posted October 5, 2011 (edited) Right, well I get 22 x 16 as being the optimum size (at 50% B.A.R) so if 18 inch is the largest you can swing then you are going to need something in the range of 18x18 to 18x20. Almost certainly going to be best speaking to someone like Crowthers and ask them if one of their high efficiency type props would be more suitable. Edited October 5, 2011 by Speedwheel Link to comment Share on other sites More sharing options...
noimnotonfacebook Posted October 5, 2011 Author Report Share Posted October 5, 2011 not wanting to sound stupid but can you explain (50% b.a.r) and also the calculation as others have come up with different results not saying your rong but if i understood it better i could make a mor informed choice thanks again Link to comment Share on other sites More sharing options...
Pie Eater Posted October 5, 2011 Report Share Posted October 5, 2011 Hi all Is there a calculation i can use to help me decide what prop would be best for my boat i know the engine torque gear reduction and also calculated torque at the prop and i know aprox boat dimentions eg draft weight ect can anyone help me? many thanks I sent all the details to Crowthers when I wanted to check if my prop was the correct size. The checked and said it was correct. Steve Link to comment Share on other sites More sharing options...
noimnotonfacebook Posted October 5, 2011 Author Report Share Posted October 5, 2011 blade are ratio not so stupid afret all Link to comment Share on other sites More sharing options...
Chris-B Posted October 5, 2011 Report Share Posted October 5, 2011 18 x 20 on my prop calc Chris B Link to comment Share on other sites More sharing options...
Speedwheel Posted October 5, 2011 Report Share Posted October 5, 2011 18 x 20 on my prop calc Chris B I think we use the same program. (BTW France coming back down weekend after next - bring on next years tug of war!) Link to comment Share on other sites More sharing options...
noimnotonfacebook Posted October 5, 2011 Author Report Share Posted October 5, 2011 hi chris still o u a beer from your previous help about hydralics iv got the ruston 2vth just trying to sort the boat out to go aroud it is this common that diferent people come to such diferent answers, is there not a caculation or fumular that gives one answer every time???? Link to comment Share on other sites More sharing options...
BEngo Posted October 5, 2011 Report Share Posted October 5, 2011 (edited) hi chris still o u a beer from your previous help about hydralics iv got the ruston 2vth just trying to sort the boat out to go aroud it is this common that diferent people come to such diferent answers, is there not a caculation or fumular that gives one answer every time???? If you want to stick hundreds or thousands of horsepower into the open water for a warship, fishing boat, container ship or tanker then there are some cracking programs out there which will give you the 'right' answer, every time (for a given value of 'right', which depends on whether the market wants quietness, speed, fuel economy, pulling power or what). They need an awful lot of information about the ship, its engines and what the owner wants to get out of it. They all need an industrial strength computer too. All this software is supported by lots of research, paid for by ship owners clubs, propellor manufacturers and defence departments because there is a lot of market out there, keen to get an edge in their version of 'right'. By comparison there is next to bugger-all research on small craft propulsion, and even less into canal craft propulsion. This is because, even though there are lots of small craft, most of them are in the leisure sector and the various bits of 'right boat' in that market are not really the same as the bits of 'right ship ' for the big commercial market. Think of what people pay extra to have fake rivets on washer-joshers, or the selling features of Broom and Sunseeker et al. Washers are solely about appearance or if the prototype doesn't go like stink the production version will get a more powerful engine to fix that. The diesel consumption doesn't matter if you can afford to buy one and the buyer will be much more interested in the mirrorred ceilingdeckhead in the master suite (they hope) or the size of the saloon cocktail cabinet and the gin tanks. A small craft prop is also pretty cheap in the scale of things- so selection by substitution, aka, keep trying till you are happy, is affordable. On the cut there is the added problem that the shape of the boat is cr@p (from a water-flow perspective at least) and the interaction between the water, the boat, the propellor and the canal is both very complicated and keeps changing as the channel depth, width and shape vary. There is more than one way to skin that particular cat. A program to sort this all out for every boat would be huge, expensive, slow and require even bigger and more expensive computers than the commercial world has. So things have to be simplified. The result is that there are some pretty simple prop-size calculating programs out there, but because these are based on various assumptions, their recommendations do need to be tempered by a good deal of experience. That's why Crowthers invariably get a recommend as the people to talk to. They have a lot of experience of canal craft, know how to apply it and, to me at least, charge very reasonably for their overall product. If you just take the output from the various web propellor calculators, and the recommendations from others on here then the result will probably be acceptable. It almost certainly won't be perfect (unless you are very lucky), but it will get you about at canal speeds, and a bit faster on the rivers and will not noticeably affect your diesel consumption. N Edited October 5, 2011 by BEngo Link to comment Share on other sites More sharing options...
noimnotonfacebook Posted October 5, 2011 Author Report Share Posted October 5, 2011 so what we can derive from this is there is no sience to this its almost hit and miss with some experiance to back it up. now i know nothing about this issue hence the original question but if it is so "hit n miss" why do people go on about getting there boat "proped" to achive maximum speed, stopping power and minumal diesel consumption. a prop is by no means in the cheaper section of building a boat and the last thing i want after spending my money is to get it in the water to find that its cr@p for the want of a better term, i have been planning this for some time now and just want to get it rite first time as when its finished i wont have a bean to correct duck ups i hv now spoken to crowthers and they were very helpfull but could not give a final answer as they need to check the engines power band i weill update as to the result (if i ever get one) Link to comment Share on other sites More sharing options...
BEngo Posted October 5, 2011 Report Share Posted October 5, 2011 so what we can derive from this is there is no sience to this its almost hit and miss with some experiance to back it up. now i know nothing about this issue hence the original question but if it is so "hit n miss" why do people go on about getting there boat "proped" to achive maximum speed, stopping power and minumal diesel consumption. a prop is by no means in the cheaper section of building a boat and the last thing i want after spending my money is to get it in the water to find that its cr@p for the want of a better term, i have been planning this for some time now and just want to get it rite first time as when its finished i wont have a bean to correct duck ups i hv now spoken to crowthers and they were very helpfull but could not give a final answer as they need to check the engines power band i weill update as to the result (if i ever get one) Well, sort of..... There is a good deal of science to it, but not enough to give an affordable, single definite answer- In your case the science says it should be about 22 inches in diameter (but there is not room for that) and between 22 and 16 inches in pitch, allowing, or not, for the shape of the boat There will some further variation depending on the design of the prop (which the program does not know much about.) 'Knowing' what to choose is where the experience comes in. In the scale of many boat costs a propellor is quite a small expense- about £500, or less than 1/2 a percent of even a £60k new-build. Under a quarter of a percent of a new Hudson or other top-whack boat builder. Peanuts in a £350k+ Broom/Sunseeker. You would need to sell alot of £500 props to pay for ressearch into which size is best- not going to happen when the suppliers can get close enough without spending all their profits. I suppose David Beckham's ability to kick a foot ball on a curving path is similarly explained; given enough engineers and a good big computer you could work out more or less where and how hard to kick it and what direction is should set off. Actually kicking it just right might need some experioence development. DB already has sufficient experience that he can just give it the right amount of Dr Marten/Timpson/Nike/Adidas in exactly the right place. I appreciate that you want to just be able to go boating and I think that if you go to a good and experienced supplier (not just Crowthers, there are others) you need not worry- They will not be a mile out. If by some mischance they were then I would think that it would be put right. Crowther may not tell you exactly what size they would supply- if they do some people go to cheaper suppliers and Crowthers have provided the advice and knowledge the cheap guys don't have, but not had a sale to fund it. That seems fair to me- particularly since I don't think you will be able to find much negative comment on Crowthers service and end results. BTW I don't have a connection with Crowthers, or even have a Crowther prop, though they did re-pitch mine some years ago and will be the supplier of choice when the shopping trolleys finally wear out the one I do have. N Link to comment Share on other sites More sharing options...
Neil2 Posted October 5, 2011 Report Share Posted October 5, 2011 as when its finished i wont have a bean to correct duck ups Buying a boat- any boat- by spending your entire budget and not holding something back is unwise, in my opinion. Something is bound to be/go wrong even if you do get the prop size right. Link to comment Share on other sites More sharing options...
Phoenix_V Posted October 5, 2011 Report Share Posted October 5, 2011 (edited) 20 x 20 optimum or 18 x 25 on mine let us know what you fit and how it works The outboard motor people usually offer a choice of just one or two propeller sizes and most seem to work so maybe it's not so important, then again their fuel consumption is pretty crap quite often Edited October 5, 2011 by Phoenix_V Link to comment Share on other sites More sharing options...
Redeye Posted October 6, 2011 Report Share Posted October 6, 2011 I think we use the same program. (BTW France coming back down weekend after next - bring on next years tug of war!) I'll give you a go on the tug of war if Bison is finished! Cheers Andy Link to comment Share on other sites More sharing options...
Speedwheel Posted October 6, 2011 Report Share Posted October 6, 2011 I'll give you a go on the tug of war if Bison is finished! Cheers Andy Done Link to comment Share on other sites More sharing options...
FMC Ling 317 Posted October 10, 2013 Report Share Posted October 10, 2013 Can I be cheeky and ask if someone will run the numbers for Ling Length 70ft Beam 7ft Weight 14 ton and rising lol depends on load in hold hp 20 @ 800 rpm 1:1 drive Currently has Kestrels 24 x 20 on and is over propped Link to comment Share on other sites More sharing options...
FMC Ling 317 Posted October 11, 2013 Report Share Posted October 11, 2013 been told 22 x 15 Link to comment Share on other sites More sharing options...
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