bill_burke
Member
"Seems every discussion here e
"Seems every discussion here ends up revolving around RPM's so I'm hoping we might consolidate some good information in one thread.
In another thread I asked:
Is the prop RPM directly related to the engine RPM? In otherwords, if I'm running the engine @3000 RPM, is the prop turning that fast as well? I ask because I was just reading how the pitch affects the RPM. Does this mean the type of prop will dictate what the engine's WOT RPM is?
Answers (followed by a new question so skip down if you want):
(Peter Chamberlain)It depends on the ratio of your outdrive, for example if the ratio is listed as one to one then your prop speed is a match for your engine RPM but if it is any other ratio than that it is not.
The pitch and diameter will always have a bearing on the max RPM of your engine, aside from propelling your boat through the water the prop creates a load on your engine, the greater the load the harder it will be for your engine to reach its rated max RPM.
The trick is to get the balance right for your boat as far as pulling power and max RPM and that can mean a bit of fiddling with the prop as far as diameter and pitch.
It is generally recognised that Stainless props out perform Alloy props, the stainless props do not flex as much so you do not get as much variation in pitch under load. Stainless props can cause a bit quicker deteriation of your anodes but that is a small price to pay.
(Rod Stewart) Your drive ratio should be stamped on your drive leg. My book shows it should be a 1.84 ratio for the 170. That means your engine turns over 1.84 times as fast as the prop. Various engines use different ratio drives, with the larger engines using lower numeric ratios such as 1.50 or 1.32.
This is done to keep the prop pitch in the range of one to two inches of pitch per inch of diam; where they are most efficient. This means for a 14 inch diam prop, you want to be in the range of 14 to 28 inches of pitch.
(Crow) Remember you must have your engine trimmed correctly to get maximum RPMs. (usually about halfway on your up/down indicator) If you are still not turning high enough RPMs then you have too much pitch in your prop. Every degree of pitch ususally relates to 200 RPM @ WOT trimmed. So a boat running 3900 with a 14.5 x 19 may need to go to a 14 x 17 (decrease in diameter also has similar effect as decrease in pitch)
*************
All this leads to my next question:
If I recall correctly (don't have the specs with me), the WOT for the 170 is in the 4200-4700 RPM range. Now, when I go out I get up to about 4500 RPM and still have some throttle left. (I admit I'm kinda nervous about going full WOT). What is the maximum RPM I should go to if it does in fact go past book WOT? And if it does, how do I wrassle the beast back down; some engine\carb adjustment or screw around with the prop?"
"Seems every discussion here ends up revolving around RPM's so I'm hoping we might consolidate some good information in one thread.
In another thread I asked:
Is the prop RPM directly related to the engine RPM? In otherwords, if I'm running the engine @3000 RPM, is the prop turning that fast as well? I ask because I was just reading how the pitch affects the RPM. Does this mean the type of prop will dictate what the engine's WOT RPM is?
Answers (followed by a new question so skip down if you want):
(Peter Chamberlain)It depends on the ratio of your outdrive, for example if the ratio is listed as one to one then your prop speed is a match for your engine RPM but if it is any other ratio than that it is not.
The pitch and diameter will always have a bearing on the max RPM of your engine, aside from propelling your boat through the water the prop creates a load on your engine, the greater the load the harder it will be for your engine to reach its rated max RPM.
The trick is to get the balance right for your boat as far as pulling power and max RPM and that can mean a bit of fiddling with the prop as far as diameter and pitch.
It is generally recognised that Stainless props out perform Alloy props, the stainless props do not flex as much so you do not get as much variation in pitch under load. Stainless props can cause a bit quicker deteriation of your anodes but that is a small price to pay.
(Rod Stewart) Your drive ratio should be stamped on your drive leg. My book shows it should be a 1.84 ratio for the 170. That means your engine turns over 1.84 times as fast as the prop. Various engines use different ratio drives, with the larger engines using lower numeric ratios such as 1.50 or 1.32.
This is done to keep the prop pitch in the range of one to two inches of pitch per inch of diam; where they are most efficient. This means for a 14 inch diam prop, you want to be in the range of 14 to 28 inches of pitch.
(Crow) Remember you must have your engine trimmed correctly to get maximum RPMs. (usually about halfway on your up/down indicator) If you are still not turning high enough RPMs then you have too much pitch in your prop. Every degree of pitch ususally relates to 200 RPM @ WOT trimmed. So a boat running 3900 with a 14.5 x 19 may need to go to a 14 x 17 (decrease in diameter also has similar effect as decrease in pitch)
*************
All this leads to my next question:
If I recall correctly (don't have the specs with me), the WOT for the 170 is in the 4200-4700 RPM range. Now, when I go out I get up to about 4500 RPM and still have some throttle left. (I admit I'm kinda nervous about going full WOT). What is the maximum RPM I should go to if it does in fact go past book WOT? And if it does, how do I wrassle the beast back down; some engine\carb adjustment or screw around with the prop?"
