owen
Member
can anyone enlighten me on the pitch of a propeller. what exactly doe the number refer to and how does one relate that to the force /thrust it produces. thanks
<hr width=100% size=1>
<hr width=100% size=1>
propellor pitch
- Thread starter owen
- Start date
Evadne
Active member
It's the same as a screw thread. One figure refers to the diametre and the other to the distance covered by one full turn of the thread, in inches usually. I can never remember which way around they go, but the larger one is the diametre. Changing these figures into a precise thrust figure is not a trivial exercise; among other things you'd need to know how fast the propellor is moving through the water in the first place.
There are lots of calculators (approximators?) on the web such as <A target="_blank" HREF=http://www.marinewaypoints.com/javaprop/JavaProp.shtml>Javaprop</A> to help you get the right prop. for your boat. They include a fudge factor to account for the drag of the hull, which you have to know or guess.
<hr width=100% size=1>
There are lots of calculators (approximators?) on the web such as <A target="_blank" HREF=http://www.marinewaypoints.com/javaprop/JavaProp.shtml>Javaprop</A> to help you get the right prop. for your boat. They include a fudge factor to account for the drag of the hull, which you have to know or guess.
<hr width=100% size=1>
oldharry
Well-known member
Pitch is the theoretical distance the propellor moves forward in one complete revolution. So an 8" pitch propellor would move forward by that amount if it was pushing against a solid block rather than water.
Ideally Pitch is around 2/3rds to 3/4 of the diameter in a normal displacement hull situation. Propellors for planing hulls are much coarser pitch, and pitch measurement usually well exceeds the diameter.
Propellors are often 'sized' by adding the pitch and diameter measurements, so that a 12 x 8 would be a size 20. Also within practical limits an 11 x9 or 10 x 10 would work equally well. However as the pitch becomes larger, so the speed of water flow through it needs to rise to allow it to operate efficiently, so that our 10 x 10 prop would be less efficient at 5 knots than the 12 x 8.
Conversely a 16 x 4 prop would only work well up to around 3 knots, as the shaft RPM required to develop higher speeds would begin to introduce other undesirable effects such as cavitation, reducing efficiency.
Propellors for displacement hulls rarely work effciently at higher than around 2200 rpm or so, because of cavitation problems, so the designer is seeking an engine/propellor combination that will push the hull at design speed at between 1500 and 2000 rpm.
Increasing the pitch without reducing diameter - or vice versa - increases the load on the propellor, requiring a higher power input. Decreasing the load by reducing diameter and pitch will allow the engine to run faster, producing more power, but will overspeed the propellor so the power is lost in the props' inefficiency.
Symptoms of a wrongly matched propellor/engine/hull: An oversize propellor will mean the boat travels quite quickly at tickover, but as the throttle is opened does not pick up much more speed, and the engine labours rather than revving up - like a car starting off in too high a gear. Similarly an undersize prop will allow the engine to race, but not produce much speed or power.
The calculation for the correct size of propellor is quite difficult, and it is far easier to go to engine suppliers like Lancing Marine, who have it all worked out in tables. I beleive there are also web sites that will do prop sizing calculations.
<hr width=100% size=1>
Ideally Pitch is around 2/3rds to 3/4 of the diameter in a normal displacement hull situation. Propellors for planing hulls are much coarser pitch, and pitch measurement usually well exceeds the diameter.
Propellors are often 'sized' by adding the pitch and diameter measurements, so that a 12 x 8 would be a size 20. Also within practical limits an 11 x9 or 10 x 10 would work equally well. However as the pitch becomes larger, so the speed of water flow through it needs to rise to allow it to operate efficiently, so that our 10 x 10 prop would be less efficient at 5 knots than the 12 x 8.
Conversely a 16 x 4 prop would only work well up to around 3 knots, as the shaft RPM required to develop higher speeds would begin to introduce other undesirable effects such as cavitation, reducing efficiency.
Propellors for displacement hulls rarely work effciently at higher than around 2200 rpm or so, because of cavitation problems, so the designer is seeking an engine/propellor combination that will push the hull at design speed at between 1500 and 2000 rpm.
Increasing the pitch without reducing diameter - or vice versa - increases the load on the propellor, requiring a higher power input. Decreasing the load by reducing diameter and pitch will allow the engine to run faster, producing more power, but will overspeed the propellor so the power is lost in the props' inefficiency.
Symptoms of a wrongly matched propellor/engine/hull: An oversize propellor will mean the boat travels quite quickly at tickover, but as the throttle is opened does not pick up much more speed, and the engine labours rather than revving up - like a car starting off in too high a gear. Similarly an undersize prop will allow the engine to race, but not produce much speed or power.
The calculation for the correct size of propellor is quite difficult, and it is far easier to go to engine suppliers like Lancing Marine, who have it all worked out in tables. I beleive there are also web sites that will do prop sizing calculations.
<hr width=100% size=1>
TonyBrooks
New member
Very nicely put - not sure about those websites though. The ones I have tried have all told me my data is invalid, but I think they will give a guide as long as you have a propper boat and not a tin slug ;-)
Tony Brooks
<hr width=100% size=1>
Tony Brooks
<hr width=100% size=1>
brianrunyard
New member
The larger one is not necessarily the diamater, my prop is 15 x 13.
It's pitch first, (e.g. 15 x 13 = 15" Pitch x 13" diameter).
<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard
It's pitch first, (e.g. 15 x 13 = 15" Pitch x 13" diameter).
<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard
williamshugh
New member
Think you'll find that diameter is usually the first number i.e diameter x pitch. I've never seen it the other way around.
Hugh
<hr width=100% size=1>
Hugh
<hr width=100% size=1>
brianrunyard
New member
Sorry my mistake
Sorry my mistake, just checked my prop and it's marked 13 x 15, dia x pitch (13" Dia x 15" pitch)
<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard
Sorry my mistake, just checked my prop and it's marked 13 x 15, dia x pitch (13" Dia x 15" pitch)
<hr width=100% size=1>Brian
http://homepage.ntlworld.com/brunyard
halcyon
Well-known member
Just looking at my two blade prop, marked 24 / 12 diameter of blades is 13 ".
Local Volvo do not believe that 24 pitch can be right, any thoughts.
Have possible alternative 18 / 13, ?
Boat is 27 long keel yacht.
Anyone ????
Brian
<hr width=100% size=1>
Local Volvo do not believe that 24 pitch can be right, any thoughts.
Have possible alternative 18 / 13, ?
Boat is 27 long keel yacht.
Anyone ????
Brian
<hr width=100% size=1>
oldharry
Well-known member
Unfortunately there is no standardisation of how and where propellors are marked. Possibly it is a size 24 propellor with a 12 degree taper? If so its a 13 X 11.
<hr width=100% size=1>
<hr width=100% size=1>
