Propellor prices

[casper ghost]

My Dad used to say that pitch was the distance the prop would screw in, after one revolution, if you could somehow turn it in clay..

Gary

[Timleech]

From the same link I posted earlier:

 

"Pitch

Pitch is defined as the theoretical forward movement of a propeller during one revolution — assuming there is no "slippage" between the propeller blade and the water.

Pitch is the second number listed in the propeller description."

 

 

 

 

 

 

Just means the pitch is the same as the diameter. A longer pitch than diameter is 'oversquare' and generally regarded as a Bad Thing. I dunno why. I expect some will expand on this soon!

 

 

Mike

 

Big pitch is not a Good Thing for heavy, displacement craft. It belongs in the World of Phylis

 

Tim

[rallyfan]

If you end up at Crowthers, don't let them talk you into a "Kelvin Pattern" prop. I did, to start with, and it was a disaster with my K2. I kept glazing the bores until I changed it for a 24x24 "proper" (pardon the pun!)shaped one.

[BEngo]

I suspect slip increases as prop speed decreases. I think it is also inversely proportional to the DAR (developed area ratio) of the prop. The DAR seems imortnat to me but rarely gets mentioned in prop size discussions. We have a very low DAR 'Kelvin' pattern prop 21" x 16" on ours now and the boat hardly moves in the water at tickover (K1). I'm planning a prop change too for similar reasons to Graham but need to avoid over-propping the engine as it doesn't have the grunt of a K3 and already struggles to spin the 21 x 16 up beyond about 650rpm in forward gear.

 

Mike

 

 

This is where it gets complicated. A propellor with no slip would be perfectly efficient, but produce no thrust because there would no change in the velocity of the water passing through the prop disk. So the prop will slip enough so that the thrust produced by increasing the momentum of the water balances the overall drag of the boat. At slow speeds other things, which can be ignored at high speed because they have much less effect on the overall picture, come into play- things like the loss of energy and thrust caused by the water going round with the prop, stalling of parts of the propellor blades, interference from the swim, skeg and counter etc. If the prop is not 'seeing' the water easily then it is hard to accelerate it to get thrust, so the slip rises. All canal boats have this problem to some extent because water normally likes to flow under rather than round a hull- hence the double curves on working boat swims and more speed for the same power on deeper water.

 

The other problem is that as far as I know there is next to no research on propellor design for hulls with a high block coefficient and b@gger all power requirement in restricted channels. (If you want to put 80MW into a carefully designed and hydrodynamically efficient container ship, sleek grey war canoe or a floating block of flats then the situation is different.) I am sure that somewhere like Southampton or Loughborough university might be interested in filling the gap- provided you have a lot of money. Until then it's down to experience from people like Crowthers, trial and error for anyone who doesn't have a back catalogue of knowledge to draw on.

 

N

[Phoenix_V]

Its a minefield out there, very hard to know if you are comparing apples with apples

 

e.g. take a look at the range of prices for the same size propeller from Lancing marine

 

lancing pricebook

 

FIXED 2 BLADE 3 BLADE 3 BLADE 3 BLADE 3 BLADE 4 BLADE

Blade area ratio 33% 43% 50% 52% 65% 67% MAX SHAFT

Blade shape Turbine Radice Turbine Radice Equipoise Hyperform 73 SIZE

Diameter, ins Mag. Bronze Mag. Bronze Mag. Bronze Mag. Bronze Mag. Bronze N.A.B. ins/mm

12 £121 £200 £161 £215 £215 – 11⁄8/28

13 £135 £230 £180 £245 £240 – 11⁄8/28

14 £137 £240 £183 £255 £245 – 11⁄4/31

15 £148 £250 £198 £260 £264 – 11⁄2/40

16 £161 £275 £215 £300 £287 – 11⁄2/40

17 £176 £295 £234 £320 £313 – 11⁄2/40

18 £207 £325 £276 £345 £369 £501 11⁄2/40

19 £253 – £337 £395 £451 £531 13⁄4/45

20 £284 – £378 £420 £505 £624 13⁄4/45

21 £310 – £413 £525 £552 £729 13⁄4/45

22 £389 – £452 £645 £603 £790 13⁄4/45

23 £406 – £541 – £724 £845 2/50

24 £466 – £613 – £820 £891 2/50

25 – – £704 – £940 £982 2/50

26 – – £757 – £1011 £1098 21⁄4/60

27 – – £859 – £1147 £1237 21⁄4/60

28 – – £912 – £1219 £1367 21⁄2/65

29 – – £963 – £1287 £1544 21⁄2/65

30 – – £1020 – £1363 £1698 21⁄2/65

31 – – £1130 – – £1852 23⁄4/70

32 – – £1279 – – £2004 23⁄4/70

33 – – £1321 – – £2158 23⁄4/70

34 – – £1500 – – £2319 23⁄4/70