Estimating the pitch of an unmarked propeller.

[alan_fincher]

If a narrow boat propeller proves to have no markings on it that show its pitch, does anybody have suggested methods for making any at least rough estimate of what the pitch is?

 

If so, please explain.

 

How accurate is any such estimate likely to be?

 

(Note this is a speculative question at the moment, as I have not yet been told for certain that no markings can be found anywhere on the prop).

 

This is a largish prop - I believe I was told 25" diameter, but to be confirmed.

[Alan de Enfield]

Make a bowl of jelly (larger than the prop)

 

Put prop on top of the jelly

 

"Screw" prop into the jelly by one complete turn.

 

The depth that the prop 'digs in' is the pitch of the prop.

[BilgePump]

Was looking online the other day as have a spare unmarked outboard prop. Came across this page http://www.propellerpages.com/?c=articles&f=2006-03-27_manual_pitch_measurementbut haven't tried to take the measurements yet.

[Tacet]

Interesting and hopefully someone will know before long.

 

The jelly/butter idea is a good concept but with a twisted blade one part of the bat must grip whilst others strip the thread.

 

Perhaps a slightly more practical approach would be to measure the angle of attach of a bat and do some sums. Again, you would need to choose where to measure - and the widest part might be a start. But how this converts into recognised measurement is another matter; might there be a mystery factor X? Blade size and even number of blades must make a difference to performance but are do they play any part in nominal sizing?

[MtB]

I asked the same question on here three or four years ago and no simple and practical method emerged, IIRC.

[Dalslandia]

Pitch is measured at the 70% radii, along that radi the hight from the table it rest on can be measured at the LE and TE.

the chord length along the radii, not a straight line along the the measuring points is the chord.

the Tan is the height difference / length of the chord.

example, the diff in height is 1" the segment/circle blade chord is 5"

that is 1/5=0.2 (Tan 0.2) that is 11.31 degree

if the diameter is 20" the 70% radii is 7" or circum. is 14"x phi= 43,98"

43,98x0.2 is 8,79"

[Alan de Enfield]

Pitch is measured at the 70% radii, along that radi the hight from the table it rest on can be measured at the LE and TE.

the chord length along the radii, not a straight line along the the measuring points is the chord.

the Tan is the height difference / length of the chord.

example, the diff in height is 1" the segment/circle blade chord is 5"

that is 1/5=0.2 (Tan 0.2) that is 11.31 degree

if the diameter is 20" the 70% radii is 7" or circum. is 14"x phi= 43,98"

43,98x0.2 is 8,79"

 

Or - use a bowl of jelly.

[jake_crew]

I didn't know about the 70% radius rule, but as it happens I guessimated by ocular survey that that that sort of position would give an "average" pitch.

 

I meaured the relative height along that radius on one fluke over an angle of 45deg and multplied by 8.

 

Perhaps this picture helps ?

 

Edited November 1, 2016 by jake_crew

[Tacet]

Pitch is measured at the 70% radii, along that radi the hight from the table it rest on can be measured at the LE and TE.

the chord length along the radii, not a straight line along the the measuring points is the chord.

the Tan is the height difference / length of the chord.

example, the diff in height is 1" the segment/circle blade chord is 5"

that is 1/5=0.2 (Tan 0.2) that is 11.31 degree

if the diameter is 20" the 70% radii is 7" or circum. is 14"x phi= 43,98"

43,98x0.2 is 8,79"

 

 

I think I understand. Is it the same as saying:

 

• All measurements are taken at 70% of radius (presumably measured from the centre)
• Measure how far advanced the leading edge is over the trailing edge (1" in your example)
• Measure the span of the blade - but not across its face (i.e. not as an hypotenuse) but between two notional washers slipped on ahead and behind the prop (5” in your example)
• Divide to get a gradient (0.2 in your example)
• Multiply gradient by the (70%) circumference

Edited November 1, 2016 by Tacet

[Texasboater]

 

 

Try this/

[Dalslandia]

 

Or - use a bowl of jelly.

Dalslandias prop is 32" in diameter.

 

All boat propellers is measured at the 70% radii, (or r/R .70) would be 30% from the tip.

most smaller propps have a constant pitch along the blade, but not all. so 70% is giving the nominal pitch that is stamped on the hub between the blades. or sometimes at the end of the hub.

one way might be to use the smartphone, measure the angle over those two points, (close enough?)

from that angle find the Tan

Tan x circumference at the 70% radii is the pitch

Edited November 1, 2016 by Dalslandia

[MtB]

Use a big bowl then...

 

 

Mind you, I think a big bowl of jelly is a daft medium to use as it's bendy and elastic. A better medium would be a block of something soft, but rigid. A one tonne block of butter for example. Or perhaps cheese.

 

 

Edit an incorrect wrod.

Edited November 1, 2016 by Mike the Boilerman

[Arthur Brown]

Which gives us, http://www.propellerhub.com/

[FadeToScarlet]

I tried doing it when the boat was on dock last.

 

After lots of faffing with tape measures, notebooks and calculators, I came to the answer.

 

"sufficient".

[Dalslandia]

I tried doing it when the boat was on dock last.

 

After lots of faffing with tape measures, notebooks and calculators, I came to the answer.

 

"sufficient".

Lagom is a good Swedish word = Just right, not to much or little, don't have to be perfect

Edited November 1, 2016 by Dalslandia

[alan_fincher]

Thanks All,

 

I have now heard that the current prop is 25 1/4" diameter, but seems to have very little pitch. Currently the yard have estimated 16.75", based on a calculation they have done.

 

The general feeling both of the yard, and based on a prop size calculator I have used, is that for this diameter, the pitch should be around 21", so if that is right, and the 16.75" estimate anywhere near correct, then it is severely underpitched for this application, (and probably explains a very poor stopping performance!).

 

Here are some pictures I have been sent - certainly looks very "flat" to my eye.

 

 

Edited November 1, 2016 by alan_fincher

[Dalslandia]

looks like it is very flat.

25x21 would be some 21 degree at 70% radii

Edited November 1, 2016 by Dalslandia

[MtB]

 

Here are some pictures I have been sent - certainly looks very "flat" to my eye.

 

 

 

 

Quite probably just the right pitch for a 1:1 transmission.

 

Changing the final drive ratio *might* turn out to be easier/cheaper than finding another blade as big as that.

[alan_fincher]

 

Quite probably just the right pitch for a 1:1 transmission.

 

Changing the final drive ratio *might* turn out to be easier/cheaper than finding another blade as big as that.

 

Nope - it would be too big even on a 2:1, I think.

 

Current box is 3:1, and only other option on a Blackstone box would, I think, be 2:1, but they are apparently very hard to acquire.

 

Re-pitching existing prop seems current favourite, (assuming it is OK to re-pitch) unless someone has a second hand blade that suits.

[MtB]

 

Nope - it would be too big even on a 2:1, I think.

 

 

Form my limited experience of prop-sizing for 2:1 and 3:1 ratios on the same boat, I'm inclined to think that highly unlikely.

 

I suggest you run some figures through a prop calc and compare the results with all parameters the same except final drive ratio. Not that it will help much if 2:1 reductions are in the hens' teeth category.

Are you putting the HA2 back in or do you now have something else?

[alan_fincher]

Form my limited experience of prop-sizing for 2:1 and 3:1 ratios on the same boat, I'm inclined to think that highly unlikely.

 

I suggest you run some figures through a prop calc and compare the results with all parameters the same except final drive ratio. Not that it will help much if 2:1 reductions are in the hens' teeth category.

 

The Vic Prop calculator someone suggested the other day as not giving silly results suggests;

 

3:1 25.1" x 20.8"

2:1 19.7" x 13.9"

 

I have 25.25" x 16.75" (estimated) - so I would say too "small" for 3:1, but far too big for 2:1.

And yes, HA2, either the current one or a "new" one.

Edited November 1, 2016 by alan_fincher

[monkeyhanger]

 

Nope - it would be too big even on a 2:1, I think.

 

Current box is 3:1, and only other option on a Blackstone box would, I think, be 2:1, but they are apparently very hard to acquire.

 

Re-pitching existing prop seems current favourite, (assuming it is OK to re-pitch) unless someone has a second hand blade that suits.

I had my prop repitched by Norris's of Isleworth. I was very pleased with the job they did.

[alan_fincher]

I had my prop repitched by Norris's of Isleworth. I was very pleased with the job they did.

 

Thanks,

 

Funnily enough, that is exactly where this (and the prop) may be heading.

 

Muggins may be doing a lot of driving tomorrow!

[blackrose]

Would a helix calculator help?

 

http://www.oocities.org/web_sketches/calculators/HELIX.html

 

That prop looks as flat as a pancake. When I had my prop repitched Norris' told me not to reduce the pitch too much as a flat prop would lack "bite". It would be interesting to know how that flat prop would perform.