Nomad Willy
Guru
I think that propeller speed v/s boat speed affects the idle boat speed. Lets say you have a direct drive gear and another identical boat has 10-1 reduction. I think the 10-1 reduction boat's idle speed will be considerably slower than the direct drive. This is admittedly a very extreme example but sometimes extremes will make it obvious what small differences fail to show.
Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 when warm but I could back out of my slip and possibly into another boat if I set it at 700 because the engine may quit while shifting gears.
koliver wrote;
"
Eric:
I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing.
I have always related idle speed to full speed. For Example, if I have a boat with a full speed of 9 knots at WOT of 3600 rpm, and you have a boat with a full speed of 25 knots at WOT of 3600 rpm, I will expect your idle speed at 750rpm to exceed my idle speed at 750 rpm by the same ratio, 25:9. So if my idle speed is 3.5 knots, I will expect yours to be 9.7 knots.
This explains the guys in the fast sport fishermen who have to go in and out of gear to get below the no wake speed when coming through the marina.
FF wrote;
"The thrust required to move a boat at a set speed does not change because of prop diameter.
Producing the required thrust at slow (under 10K) is always more efficient with the largest diameter that will fit under the boat that it can be geared for..
This assumes the number of blades and blade width is also optimized.
As an example a ride on a tug (HUGE PROP and frequently 6-1 gearing) will show the engine is slowed very little to cut speed while free running .
Going from 10K to 5K does not require half RPM, just far less thrust.
Run a bunch of props thry Skenes to see how Diameter is king of thrust efficiency."
As I think I've said before I'm quite certain that FAPP the bigger prop/lower gear combination will always produce greater thrust at slow speeds and full load. The higher speed smaller prop will always have less thrust ..... at WOT at rated rpm. But I think that at some point of reduced rpm they will become equal and w a further reduction in speed (prop speed) the smaller faster prop will have more thrust and present more load to the engine. The right propeller thrust curves would reject or embrace this theory.
Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 when warm but I could back out of my slip and possibly into another boat if I set it at 700 because the engine may quit while shifting gears.
koliver wrote;
"
Eric:
I don't get what you are driving at. I think boat speed at idle has nothing whatsoever to do with gearing.
I have always related idle speed to full speed. For Example, if I have a boat with a full speed of 9 knots at WOT of 3600 rpm, and you have a boat with a full speed of 25 knots at WOT of 3600 rpm, I will expect your idle speed at 750rpm to exceed my idle speed at 750 rpm by the same ratio, 25:9. So if my idle speed is 3.5 knots, I will expect yours to be 9.7 knots.
This explains the guys in the fast sport fishermen who have to go in and out of gear to get below the no wake speed when coming through the marina.
FF wrote;
"The thrust required to move a boat at a set speed does not change because of prop diameter.
Producing the required thrust at slow (under 10K) is always more efficient with the largest diameter that will fit under the boat that it can be geared for..
This assumes the number of blades and blade width is also optimized.
As an example a ride on a tug (HUGE PROP and frequently 6-1 gearing) will show the engine is slowed very little to cut speed while free running .
Going from 10K to 5K does not require half RPM, just far less thrust.
Run a bunch of props thry Skenes to see how Diameter is king of thrust efficiency."
As I think I've said before I'm quite certain that FAPP the bigger prop/lower gear combination will always produce greater thrust at slow speeds and full load. The higher speed smaller prop will always have less thrust ..... at WOT at rated rpm. But I think that at some point of reduced rpm they will become equal and w a further reduction in speed (prop speed) the smaller faster prop will have more thrust and present more load to the engine. The right propeller thrust curves would reject or embrace this theory.
Last edited: