How can I be sure that I have the proper prop size for my 35.5 with the Yanmar 3GM30F engine? I keep thinking that I'm not getting top performance when I am motoring. The boat is 13 years old and I have owned it for the past 2. According to my knot meter, I get about 4.5 knots in flat seas. Shouldn't I be able to do better?
- Status
- Not open for further replies.
J
Justin Wolfe
Check the knotmeter
Jim, First off isn't an 86/87 actually a 35? Careful with this as there is a lot of difference between a 35 & 35.5 although hull shape may not be one of them. Certainly deck & interior though. As for prop size. Are you sure your knotmeter is accurate? Have calibrated it according to the manual (this may never have been done since new). Have you checked it against a Loran or GPS (not a good calibration tool, but at least a point of reference)? Is your bottom clean? Is your knotmeter paddle wheel clean? What is your maximum RPM's while motoring? You should be able to get around 3500rpm, but not more. If you prop is indeed undersized you MAX would be higher. 9 times out of 10 (maybe 8 out of 10) the prop is not the culprit. Uncalibrated knotmeters can drive people nuts or make them look like Dennis Connor. Ever heard of a Catalina 28 hitting 12 knots in a Bay? The knotmeter said so! Yeah right! Our 35.5 would motor around 6.7 knots at 2800 rpm with a clean bottom as I recall.
Jim, First off isn't an 86/87 actually a 35? Careful with this as there is a lot of difference between a 35 & 35.5 although hull shape may not be one of them. Certainly deck & interior though. As for prop size. Are you sure your knotmeter is accurate? Have calibrated it according to the manual (this may never have been done since new). Have you checked it against a Loran or GPS (not a good calibration tool, but at least a point of reference)? Is your bottom clean? Is your knotmeter paddle wheel clean? What is your maximum RPM's while motoring? You should be able to get around 3500rpm, but not more. If you prop is indeed undersized you MAX would be higher. 9 times out of 10 (maybe 8 out of 10) the prop is not the culprit. Uncalibrated knotmeters can drive people nuts or make them look like Dennis Connor. Ever heard of a Catalina 28 hitting 12 knots in a Bay? The knotmeter said so! Yeah right! Our 35.5 would motor around 6.7 knots at 2800 rpm with a clean bottom as I recall.
Different but the same
Jim: J.W. is correct there is a difference. The 35 calls for a 17x12 and the 35.5 calls for a 16x13. These are essentially the same prop (BUT) you probably cannot put the 17" on the 35.5 (BUT) you CAN put the 16" on the 35 and would get the same relative performance. These props are simple but all of the things together are very complex. Typically you can remove or add 1" from the diameter OR pitch and this will add or reduce your rpm's by 250-350 rpm's per inch. There are problems with sailboats have oversquare prop 10 x 15 would typically be a nono on sailboat but this "could" work on a power boat..... The real deal here how do you know how fast you are going or not going?
Jim: J.W. is correct there is a difference. The 35 calls for a 17x12 and the 35.5 calls for a 16x13. These are essentially the same prop (BUT) you probably cannot put the 17" on the 35.5 (BUT) you CAN put the 16" on the 35 and would get the same relative performance. These props are simple but all of the things together are very complex. Typically you can remove or add 1" from the diameter OR pitch and this will add or reduce your rpm's by 250-350 rpm's per inch. There are problems with sailboats have oversquare prop 10 x 15 would typically be a nono on sailboat but this "could" work on a power boat..... The real deal here how do you know how fast you are going or not going?
P
Paul Akers
Spedo questionable?
I'd first question the speedo. They can give false readings. What does your LORAN/GPS say for speed? These may be a bit more reliable. Also, check with the Yanmar people on the specs for the engine. You should be able to achieve top RPM at full throttle if the prop is correctly tuned to the engine.
I'd first question the speedo. They can give false readings. What does your LORAN/GPS say for speed? These may be a bit more reliable. Also, check with the Yanmar people on the specs for the engine. You should be able to achieve top RPM at full throttle if the prop is correctly tuned to the engine.
J
Jack G.
log calibration
Jim, As others have already noted you should check to see of your prop allows your engine to reach it's rated RPM. There should be a rating plate on top of the engine. On my 35.5 the 3GM is rated for 3400 RPM continously, and 3600 RPM max. Check your rating plate. You should be able to reach the MAX listed RPM at wide open throttle. If you cannot, the prop is oversize (or overpitched). If you could go to a higher RPM the prop is undersized (or underpitched). If the difference isn't very large, say a few hundred RPM, you can either live with it, or take the prop off the boat and to a prop shop. A good prop shop will be able to use a computer program to help them measure your prop and "tune" it to eliminate imperfections (slightly different pitches from blade to blade, which can make a difference)and adjust the pitch overall to increase or decrease "bite". This will slightly change the RPM - more pitch (bigger bite) less RPM and so forth. If the prop is about right i.e. the engine is close to max rpm at wide open throttle you need to look at the boats condition (i.e. is the bottom clean) and calibrate the log. I wouldn't compare the log to the GPS directly. There are to many other factors which affect SOG shown by the GPS to rely on it to accurately reflect log speed. You don't say what type of log you have or if you have the manuals for it. My boat has the Autohelm ST 50 in it, and it gives proceedure for log calibration. You can use a GPS to help you with this by setting a one mile course. I would suggest you run the course at least 4 times. This will give a fairly close calibration. Good luck
Jim, As others have already noted you should check to see of your prop allows your engine to reach it's rated RPM. There should be a rating plate on top of the engine. On my 35.5 the 3GM is rated for 3400 RPM continously, and 3600 RPM max. Check your rating plate. You should be able to reach the MAX listed RPM at wide open throttle. If you cannot, the prop is oversize (or overpitched). If you could go to a higher RPM the prop is undersized (or underpitched). If the difference isn't very large, say a few hundred RPM, you can either live with it, or take the prop off the boat and to a prop shop. A good prop shop will be able to use a computer program to help them measure your prop and "tune" it to eliminate imperfections (slightly different pitches from blade to blade, which can make a difference)and adjust the pitch overall to increase or decrease "bite". This will slightly change the RPM - more pitch (bigger bite) less RPM and so forth. If the prop is about right i.e. the engine is close to max rpm at wide open throttle you need to look at the boats condition (i.e. is the bottom clean) and calibrate the log. I wouldn't compare the log to the GPS directly. There are to many other factors which affect SOG shown by the GPS to rely on it to accurately reflect log speed. You don't say what type of log you have or if you have the manuals for it. My boat has the Autohelm ST 50 in it, and it gives proceedure for log calibration. You can use a GPS to help you with this by setting a one mile course. I would suggest you run the course at least 4 times. This will give a fairly close calibration. Good luck
- Status
- Not open for further replies.