I have a 376 which appears to have the incorrect pitch prop. She revs high with very little speed. Can anyone tell me what pitch and diameter I should have to achieve best performance?
Prop pitch on 376?
- Thread starter husky
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1- A call to Hunter should help tell you the spec'd prop they suggest, provided you have the stock engine. You can then compare this to what you actually have and this will be a good start.
2- The prop that allows your engine to turn max rated RPM at wide open throttle with a clean bottom and prop is the correct pitch. If you can not attain the max rated RPM at WOT, with clean bottom and prop in smooth no current conditions, the prop is over sized or over pitched. If you can easily hit max rated and go beyond it the prop is undersized and under pitched.
RPM should be determined with a digital photo tachometer and the tachometer on the gauge panel should then be calibrated if you have discovered that it is out. The on-board tach may be spot on but often times, they are not. This is why they should not be used to determine something as important as prop sizing & pitch. More info on how you determined it was wrong would really help, RPM, speed through wate, conditions etc..
MS: I would have to disagree with you on contacting Hunter about props. I think that there has been untold accounts of Hunters (and other boats) with the wrong prop.
My advise is to get this information from a prop shop. They have computer programs to properly size/pitch a prop for each vessel. I believe a lot of the information that Hunter has comes from Yanmar and they do not seem to have the magic formula either.
Most of the time when you have a prop that reaches max rpm, but does not get to the max. theoritical speed, it is limited by the govenor.
My advise is to get this information from a prop shop. They have computer programs to properly size/pitch a prop for each vessel. I believe a lot of the information that Hunter has comes from Yanmar and they do not seem to have the magic formula either.
Most of the time when you have a prop that reaches max rpm, but does not get to the max. theoritical speed, it is limited by the govenor.
My settings
I have experimented with pitch adjustment on my feathering prop on my H376 and find the value given by "Propcalc" - downloadable from Castle Marine.co.uk in Wales (http://www.castlemarine.co.uk/pitch.htm) is about right.
This assumes LWL=32', Displ=16000lb, Engine BHP=37 and gearbox Ratio=2.83:1 which is the standard Yanmar 3JH2E fitted to the H376.
The result is 17½ x 14 three blades.
The engine can just reach 3600RPM -(beware Yanmar tacho reads 6% low).
Programme indicates the Theoretical Hull speed of 7.7 knots and I get pretty near to this at WOT.
Give us your speed through the water in calm conditions and how you measured it. Also give the engine RPM and we may be able to comment on your boats' performance.
Regards.
I have experimented with pitch adjustment on my feathering prop on my H376 and find the value given by "Propcalc" - downloadable from Castle Marine.co.uk in Wales (http://www.castlemarine.co.uk/pitch.htm) is about right.
This assumes LWL=32', Displ=16000lb, Engine BHP=37 and gearbox Ratio=2.83:1 which is the standard Yanmar 3JH2E fitted to the H376.
The result is 17½ x 14 three blades.
The engine can just reach 3600RPM -(beware Yanmar tacho reads 6% low).
Programme indicates the Theoretical Hull speed of 7.7 knots and I get pretty near to this at WOT.
Give us your speed through the water in calm conditions and how you measured it. Also give the engine RPM and we may be able to comment on your boats' performance.
Regards.
Perhaps Hunter is worse at prop sizing than Catalina and others? That is too bad.. I know Catalina does actual in-water testing of their suggested stock props and they have always been nearly spot on when recommending a prop, when checked against my digital tach. They suggested the three blade MP prop for my C-36 and it was within -50 RPM of max rated.
As for prop shops and sizing...well? They to often miss the mark though most boaters never know. The Michigan Wheel calculator misses quite often especially with sailboats. Sailboats are not as easy to size props for as some think. A friend of mine, who is now retired, was a marine surveyor. I used to help him on occasion. He was one of the few surveyors who checked max rated RPM against a photo tach on sea trials. In his surveys he found that nearly 50% of the boats out there are either over or under pitched and a large number of boats have tachs off 100-300 RPM or more. Most owners don't understand what a properly sized prop should do so even folks who got the wrong prop, from a prop shop, think they got the right one.
Hitting max RPM should not happen to fast it will take a little time at WOT to attain max speed and RPM. If you hot it quickly and easily, and are not at hull speed as you mentioned, it is undersized/pitched. Many engines will run slightly over max rated RPM when limited by the governor, though some actually stop before max rated so this needs to be confirmed before you sea trial. If you can get the engine to 3100 in neutral and 3000 is max rated you should be able to hit 3000 at WOT, or darn close, with the right prop. If you can only hit 2900 in neutral then your governor needs to be addressed.
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