[[[Edit:The RPM #s in this post are no good. I found that my tach was way off. Corrected #s in a later post.]]]
I have discussed this issue with a few forum members but now that I have more info and a plan I wanted to post. The basic problem is I never reach hull speed, or even close, under power. I believe my hull speed works out to be 7.6 MPH with a 25 ft LWL. I will use MPH due to traveling on the ICW.
I did some test runs the other day and they are very similar to tests in the past in calm conditions clean prop etc.
1200 RPM 3.1 MPH
1400 " 3.5 "
1600 " 4.0 "
1800 " 4.5 "
2000 " 5.0 "
2200 " 5.5 "
2400 " 5.7 "
2600 " 6.0 "
2800 " 6.2 "
I have the 2 to 1 reduction gear and the engine showed no sign of strain or of topping out.
I have sailed faster than this so I do not think it is a hull speed issue. I just think I am under propped. The prop is 14X?.
From the #s, point of diminishing returns, and the 2000 to 2500 RPM continuous use recommendation for the A-4. I will shoot for 2200 as a cruising RPM with a few hundred more in reserve.
From what I understand you lose 100 RPM per 1" of added pitch. I can not go to a bigger diameter due to the MCs aperture. My plan is to add 2" of pitch to my existing prop no matter what pitch it turns out to be.
1. Does this sound reasonable?
2. What is the max a props pitch can be changed?
3. Extra Credit: What kind of performance improvement can I expect?
I have contacted several prop dealers and they have been helpful but none of the props they suggest are 14" and some are to large for the aperture. The smaller ones might be negatively impacted by the width of the keel just 5 or 6 inches forward of the prop. Plus the MC is a homemade boat so there are no boats to reference her to. I would like to modify and use my existing prop if possible. Thanks. Dan S/V Marian Claire
I have discussed this issue with a few forum members but now that I have more info and a plan I wanted to post. The basic problem is I never reach hull speed, or even close, under power. I believe my hull speed works out to be 7.6 MPH with a 25 ft LWL. I will use MPH due to traveling on the ICW.
I did some test runs the other day and they are very similar to tests in the past in calm conditions clean prop etc.
1200 RPM 3.1 MPH
1400 " 3.5 "
1600 " 4.0 "
1800 " 4.5 "
2000 " 5.0 "
2200 " 5.5 "
2400 " 5.7 "
2600 " 6.0 "
2800 " 6.2 "
I have the 2 to 1 reduction gear and the engine showed no sign of strain or of topping out.
I have sailed faster than this so I do not think it is a hull speed issue. I just think I am under propped. The prop is 14X?.
From the #s, point of diminishing returns, and the 2000 to 2500 RPM continuous use recommendation for the A-4. I will shoot for 2200 as a cruising RPM with a few hundred more in reserve.
From what I understand you lose 100 RPM per 1" of added pitch. I can not go to a bigger diameter due to the MCs aperture. My plan is to add 2" of pitch to my existing prop no matter what pitch it turns out to be.
1. Does this sound reasonable?
2. What is the max a props pitch can be changed?
3. Extra Credit: What kind of performance improvement can I expect?
I have contacted several prop dealers and they have been helpful but none of the props they suggest are 14" and some are to large for the aperture. The smaller ones might be negatively impacted by the width of the keel just 5 or 6 inches forward of the prop. Plus the MC is a homemade boat so there are no boats to reference her to. I would like to modify and use my existing prop if possible. Thanks. Dan S/V Marian Claire
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