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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

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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 by Speedwheel
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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

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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 by BEngo
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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)

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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

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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.

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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 by Phoenix_V
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  • 2 years later...

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