Anyone ever measured a propellor?

[C08]

I just bought a new 4 blade propellor made by Solas, marked on the propellor as 10x7. Came in a Solas 10x7 4 blade box from a very reputable prop supplier but when I measure the prop I can cannot get it to measure more than 93/4".
Am I missing something?

 

[Bobc]

Am I missing something?

about 1/4" by the sound of it

 

[Lodesman77]

Are you allowing for the angle of the blade or measuring at right angles to the shaft?

 

[C08]

Are you allowing for the angle of the blade or measuring at right angles to the shaft?

That is a good point but on a radius can you measure any other way? I did wonder about how the diameter is correctly measured. Even adding 2x the biggest radius measurements that may not lie on a single straight line going through the centre I still come up short. I will ask the supplier about this and feed back the answer.

 

[Aurai]

You might try

Measuring from edge of one blade to centre and X 2 (double it)

 

[scottie]

Nominal size!?

 

[penberth3]

Nominal size!?

Good point. So the difference is 3mm each side, not bad assuming it's hand finished?

 

[Tranona]

Good point. So the difference is 3mm each side, not bad assuming it's hand finished?

They are diecast outboard motor propellers so no hand finishing. The size is nominal and 3mm off the length of each blade will make no difference to performance. The loss of area at the tip is tiny.

 

[C08]

They are diecast outboard motor propellers so no hand finishing. The size is nominal and 3mm off the length of each blade will make no difference to performance. The loss of area at the tip is tiny.

A good point that would be true for a normal propellor but the 4 blader I have bought has an outer where the end of the blade is quite flat and runs around a radius apart from a lead of a few mm, unlike a normal propellor which has a much more pointed tip.

 

[Tranona]

A good point that would be true for a normal propellor but the 4 blader I have bought has an outer where the end of the blade is quite flat and runs around a radius apart from a lead of a few mm, unlike a normal propellor which has a much more pointed tip.

Still won't make much difference. The 4 blade will presumably have a much greater blade area ratio than a 3 blade of same nominal diameter. Think you are reading too much into the quoted size.

 

[C08]

Still won't make much difference. The 4 blade will presumably have a much greater blade area ratio than a 3 blade of same nominal diameter. Think you are reading too much into the quoted size.

Yes bigger total blade area and smaller pitch-is supposed to be better for heavy craft particularly in rough water and amazingly better going astern as the blades are more symetrical on a 4 blade than a 3 blader. The downside is marginally less top speed but I can live with that. I agree I probably am taking the quoted size too literally, just surprised that the nominal size is so different to the actual.. I intend on fitting it this weekend and should know then if the claimed improvements stack up!

 

[lw395]

9 3/4 inches = 247.65mm, maybe that's near enough 250mm, which is ten nominal (sales talk) metric inchy things.

 

[Quandary]

9 3/4 inches = 247.65mm, maybe that's near enough 250mm, which is ten nominal (sales talk) metric inchy things.

I had thought of that too, but Solas are an American company and the only metric measurement they seem to have heard of over there is 9mm.

 

[JumbleDuck]

Didn't this come up some time last year? I'm sure I remember a thread about a smaller-than-expected propeller.

 

[Tranona]

Didn't this come up some time last year? I'm sure I remember a thread about a smaller-than-expected propeller.

That was a custom built 4 blade feathering propeller. This is a mass produced standard size outboard prop.

 

[JumbleDuck]

That was a custom built 4 blade feathering propeller. This is a mass produced standard size outboard prop.

Ah, yes. But perhaps there are industry standards and tolerances for measuring propellers which apply to both?

 

[penberth3]

I had thought of that too, but Solas are an American company and the only metric measurement they seem to have heard of over there is 9mm.

Ah yes, I was a bit slow working that one out - I was thinking screw threads.

 

[Tranona]

Ah, yes. But perhaps there are industry standards and tolerances for measuring propellers which apply to both?

Most unlikely as exact dimensions are not critical. I expect OE outboard manufacturers have their own internal standards and aftermarket suppliers like this one probably ape OE as far as possible.

Remember prop sizing is not an exact science and there is a fair degree of tolerance when fixed pitch props usually go in fixed increments. An outboard manufacturer may well decide on a prop for a particular engine that does not "fit" and has a non standard dimension like 6 7/16" which will be described as 6 1/2". From memory don't think any of our Seagull props had precise to the inch (or even 1/2") diameter or pitch (we worked in angles for pitch and then converted to inches for the description).

 

[JumbleDuck]

Most unlikely as exact dimensions are not critical. I expect OE outboard manufacturers have their own internal standards and aftermarket suppliers like this one probably ape OE as far as possible.

Remember prop sizing is not an exact science and there is a fair degree of tolerance when fixed pitch props usually go in fixed increments. An outboard manufacturer may well decide on a prop for a particular engine that does not "fit" and has a non standard dimension like 6 7/16" which will be described as 6 1/2". From memory don't think any of our Seagull props had precise to the inch (or even 1/2") diameter or pitch (we worked in angles for pitch and then converted to inches for the description).

You've just agreed with me, you know. Thanks.