Size vs pitch?

[Caveman Charlie]

How does the relationship of size and pitch work? I mean will a (say for round numbers) 20 inch 15 pitch prop perform the same as a 15 inch 20 pitch prop. Obviously not. But, how does that all work?

 

[Silvertip]

Re: Size vs pitch?

The two examples you gave would be totally different -- not only from a performance standpoint, but from an application standpoint as well.

Pitch refers to the amount of forward movement the prop pushes the boat in one revolution. 15P = 15 inches. 20P = 20 inches. Now realize that those numbers are theoretical in that the prop operates in a liquid, not a solid so there is some slippage. On a given engine, a prop with low pitch provides good hole shot but would have terrible top end because it would allow the engine to over rev -- sort of like having your car in low gear all the time. Props with higher pitch have reduced hole shot but have much better top end. Too much pitch though causes the engine to lug -- like trying to climb a steep hill in high gear. Happiness is a prop that allows the engine to operate at wide open throttle in the rpm band recommended by the engine manufacturer.

When it comes to diameter, your example was extreme in that you could not install a 20-inch diameter prop on an engine that was designed for say a 13- 15 inch prop. Conversely, installing a 12 - 15 inch prop on an engine or out drive that could have a 20 inch prop would be rediculously inefficient. Small outboards are designed to take 8 - 10 inch props, mid range engines take in the range of 10 - 13 and large engines in 14 - 16 range.

 

[Caveman Charlie]

Re: Size vs pitch?

When it comes to diameter, your example was extreme in that you could not install a 20-inch diameter prop on an engine that was designed for say a 13- 15 inch prop. Conversely, installing a 12 - 15 inch prop on an engine or out drive that could have a 20 inch prop would be rediculously inefficient. Small outboards are designed to take 8 - 10 inch props, mid range engines take in the range of 10 - 13 and large engines in 14 - 16 range.

OK, lets say you had two props of identical pitch. One was 14 inches in diameter and the other 16. Both were installed on a identical boat with a identical engine. ( I know this is not possible but, let just go with it ) What would be the difference in top speed, top RPM, and hole shot?

 

[Texasmark]

Re: Size vs pitch?

Large diameter props have a lot of torque as a result of the long (moment of inertia) arm caused by the large diameter. They are usually used on slower turning shafts and usually have shallower pitch. Like on a 100' tugboat I worked on once, the diameter of the prop was something like 6' but the pitch was around 12" and max engine rpm's were 1500 or thereabouts....don't remember if it had a reduction gear or not. It had a V12 at 900 hp to twist it and could move half a dozen 100 ton barges 5+ mph.

When I was a kid we raced small light weight (200#) hydroplanes (8') and the prop was more like 6" in diameter and 30" pitch wired 1-1 with the engine which turned 6000+ rpm. The boats ran 50+ mph kinda things.......once you got them to plane out which took awhile.

Each has it's ability and application. So pleasure boats are somewhere in the middle depending upon the requirements of torque and speed.

HTH,

Mark

 

[Silvertip]

Re: Size vs pitch?

Look at this way -- if pitch remains the same, it takes more power to turn the large diameter prop than it does the smaller one. Therefore the boat with the smaller diameter prop would have better hole shot, but the boat with the larger diameter prop would maybe out run the other at the top end since its getting a bigger bite on the water. On the other hand, the engine may not have enough power to spin that prop configuration so to answer your question with any degree of certainty is not possible. Especially to predict speed and rpm with any accuracy. If you want to drive yourself nuts, google "marine propeller theory". You'll have enough reading to keep you busy for months.

 

[Caveman Charlie]

Re: Size vs pitch?

OK , thanks. Is there anyone out there that ever wrote a book or article for laymen ? This is how I learned about engines/ camshafts and such on my car when I was a boy. I subscribed to all the hot rod , car craft magazines and read the tech articles. Eventually I got a basic working knowledge and it helped me pick out what I wanted for my car.

 

[Silvertip]

Re: Size vs pitch?

Do what I suggested. Do a Google or Yahoo search on "Marine Propeller Basics".

Start here:

http://www.iboats.com/images/propfinder/popups/popup.cgi?finder=InfoPopup&page_id=diameter