prop pitch

[wsmcycle]

What is the common method of measuring the pitch of a prop? I am confounded by the continuously varying angle from the hub to the blade tip.

[artemis]

What is the common method of measuring the pitch of a prop? I am confounded by the continuously varying angle from the hub to the blade tip.

There is an article in Steamboats and Modern Steam Launches (I assume that since you're in this hobby, you have a copy of this book - and since it is still published I can't reproduce the article here) by Weston Farmer in the September-October 1962, ppgs 18 thru 22 entitled: Building Your Own Propeller. He explains how to determine when you have achieved a determined pitch. A little thinking will easily show how to determine an existing pitch.

[wsmcycle]

analyzing Farmer's method, the prop pitch is the circumference of the hub concentric circle where the prop angle is 45 degrees. I thought the pitch would be the distance forward the prop moved in one revolution. Like thread pitch on a bolt. I see that it cannot be so simple because the liquid is slipping along the surface of the prop. By Farmers equation, a point on the prop where the prop is 45 degrees will progress forward 1:1 with progression of the circumferential arc length. I am sure this equation was distilled over the years to a very good approximation to the true pitch.

Thanks for directing me

[fredrosse]

Neglecting slip for the moment, a screw propeller works similar to ordinary screw threads.. Assuming a position on a propeller at 6 inch radius, or a 12 inch diameter (1 foot in diameter), the this point will travel 1 x Pi or 3.14 feet in one revolution.

If the blades are tilted 45 degrees, then they will advance forward the same amount as it moves about its axis, or 3.14 feet. For the screw to advance 1 foot in one revolution, the blade angle would be 17.6 degrees, not 45 degrees.

So if you construct a wedge with a 17.6 degree angle, and see where it matches the propeller blade angle, the circle corresponding to the 17.6 degree angle is the theoretical pitch of the propeller.

Of course, in water the propeller does not actually advance as much as theoretical pitch. The term for this is “slip", which is the percentage loss of forward advance of the propeller. Typical values for slip are in the vicinity of 20-25%.

In this example, the propeller with 1 foot pitch (12 inches pitch) would actually advance thru the water is about 9 to 9-1/2 inches for each revolution.

[wsmcycle]

Quoting myself, "a point on the prop where the prop is 45 degrees will progress forward 1:1 with progression of the circumferential arc length." I did not say the forward movement would be 1 foot per 1 revolution. I did say the ratio of forward movement is equal (1:1) to the "circumferential arc length" traveled. the circumferential arc length for 1 revolution would be pi(D). Your term "blade angle" is not something which can be deduced from Farmer's "pitch measure method". I cannot see that a prop could be properly described by "blade angle". Every point on a prop moves forward the same amount as the points on the"45 circle". For a prop of pitch 1 foot, farmer's equation tells us the diameter of the prop at the 45 degree point is 1/pi=.318 feet or 3.819 inches. Therefore the distance form the hub center to the 45 degree point is 3.819/2 >>1.909 inches. A very small prop.
Farmer's method averages many prop features such as initial blade angle, final blade angle and blade curvature. The slowly rotating prop of a steam boat (at speed) simply screws and slips through the water unlike a power boat prop which creates thrust. Any rpm higher than the (hull speed/prop pitch), simply beats the water. Which means there is a simple optimum rpm.
I have an 18 foot boat with a 7 foot beam. It was powered by a Semple 34dw single. The max boat speed was 7.5 mph. I built a Stuart 6 and swapped it out thinking I would have ample HP and the Stuart would waste less steam (power). What I found was that the Stuart 6 would only move the boat at 5mph. This is why I sought a method of prop measurement. I know now that my current prop has a 16 inch pitch. I could change it to a smaller pitch and let the Stuart 6 run at a higher rpm. However, this may result in using the same amount of steam as the Semple engine did. The Semple was certainly less finnicky.

[fredrosse]

Sorry I mis-read your statement, which is correct. Many props (generally higher speed props with low pitch) have a hub diameter large enough that you cannot find the diameter corresponding to the 45 degree blade angle. If that is the case you can determine the pitch based on the information provided in my last post here.

[wsmcycle]

This inquiry has been very thought provoking. I had and have read most of the steam boating book i was referred to but I had not realized it contained the answer i sought. Someone said "we more often need reminding than instructed".
Thanks for your help