I now have a Hunter 37.5 that I bought last July. I understand that there was some considerable discussion about the prop sizes used on Hunter 37.5s a while back. Over the winter it was necessary to replace the prop shaft, cutlass, and the stuffing box. The shaft had a groove cut in it both at the cutlass and at the stuffing box for unknown reasons. Needless to say, the stuffing box leaked a bunch, even after it was repacked. Engine alignment was checked and was just fine. It did not need readjustment after the installation of the new parts. The boat has a Yanmar 3HM35F engine and a one inch shaft. What is the optimum two and/or three bladed prop for this arrangement that will produce maximum efficiency for motoring and control? I also understand there is a simple test that I can perform underway that will reveal if the present prop is the correct one? Please advise? Thanks, Steve s/v Sun Runner
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Try this.
Steve: Check your engine plate. I think that this engine also is rate at a max RPM of 3600 and 3200 rpm. When you have the throttle wide open your engine should be able to achieve 3600 RPM's. The optimum operating speed for these engines is 2800-3000 rpms. This is based on a sustained operating range of 70-80% of the max. RPM. If you cannot get these RPM's then you are PROBABLY overpropped. If this is the case make sure that your engine will rev to 3600 or higher while the transmission is in neutral. If you cannot get to 3600 in neutral then you should have your engine checked. We had this problem with our 2GM20F. We had to have the governor adjusted.
Steve: Check your engine plate. I think that this engine also is rate at a max RPM of 3600 and 3200 rpm. When you have the throttle wide open your engine should be able to achieve 3600 RPM's. The optimum operating speed for these engines is 2800-3000 rpms. This is based on a sustained operating range of 70-80% of the max. RPM. If you cannot get these RPM's then you are PROBABLY overpropped. If this is the case make sure that your engine will rev to 3600 or higher while the transmission is in neutral. If you cannot get to 3600 in neutral then you should have your engine checked. We had this problem with our 2GM20F. We had to have the governor adjusted.
Steve - Correction
The 3HM35F has a max RPM of 3200. If the prop is matched correctly to the engine, you should achieve 3200rpm at full throttle. My '88 Legend 37 has that same engine and I've had the same question in the past. When I showed the Yanmar manual to a yard mechanic, he agreed on the 3200RPM. Good Luck
The 3HM35F has a max RPM of 3200. If the prop is matched correctly to the engine, you should achieve 3200rpm at full throttle. My '88 Legend 37 has that same engine and I've had the same question in the past. When I showed the Yanmar manual to a yard mechanic, he agreed on the 3200RPM. Good Luck
Legend 37.5 Prop
I Don't know where your info came from. I have a 1992 37.5 with the same engine (rated @34HP) I called Yanmar and they told me that max RPM's should be 3450. My two blade was 17X15 and maxed at 3350 full throttle doing about 7.1 Kts. I put on a 3 blade 17X12 and maxed at 3425 doing 7.5 Kts. Check that out first with a prop shop because I can't find my receipt and it's been awhile. They use a formula to convert from a two blade. Of course, now I have an autoprop and do 7.5 kts. @2500 RPM. Tied down in the slip I can get 3425 RPM's in gear and about 3700 in neutral. I'm still inclined to think my 3 bladed prop was 17X12. ChuckWolfe@Mail.com
I Don't know where your info came from. I have a 1992 37.5 with the same engine (rated @34HP) I called Yanmar and they told me that max RPM's should be 3450. My two blade was 17X15 and maxed at 3350 full throttle doing about 7.1 Kts. I put on a 3 blade 17X12 and maxed at 3425 doing 7.5 Kts. Check that out first with a prop shop because I can't find my receipt and it's been awhile. They use a formula to convert from a two blade. Of course, now I have an autoprop and do 7.5 kts. @2500 RPM. Tied down in the slip I can get 3425 RPM's in gear and about 3700 in neutral. I'm still inclined to think my 3 bladed prop was 17X12. ChuckWolfe@Mail.com
37.5 prop
Dear Steve Hornberger: Thank you for your letter. The grooves, which you witnessed on the shaft is scoring. The scoring can take place when a device like a packing gland or a cutlass bearing rides either too tightly to the shaft or runs without lubrication. When scoring takes place it is found that it can be very difficult to seal the packing gland simply because the packing does not have enough flexibility in constricting around the shaft. The cutlass bearing scoring, usually, is a matter of use and time. You’re lucky in that the system did not need realignment. If the cutlass bearing was worn before and an alignment was done during its worn state, once the items are changed the alignment should have changed. Sounds as if you escaped that one though. The two standard propellers for the 37.5 are: 1ea. 17 X 15 1” shaft two blade 1ea. 17 X 14 1” shaft three blade The simple test to confirm if the propeller is loading the engine up sufficiently is to motor the vessel and apply maximum throttle. If the rpm’s achievable under max throttle meets and does not either exceed or fall short of it’s rated full load amount, you will know that the propeller is match correctly to the engine. Hope this helps. Sincerely, Kevin L. Woody
Dear Steve Hornberger: Thank you for your letter. The grooves, which you witnessed on the shaft is scoring. The scoring can take place when a device like a packing gland or a cutlass bearing rides either too tightly to the shaft or runs without lubrication. When scoring takes place it is found that it can be very difficult to seal the packing gland simply because the packing does not have enough flexibility in constricting around the shaft. The cutlass bearing scoring, usually, is a matter of use and time. You’re lucky in that the system did not need realignment. If the cutlass bearing was worn before and an alignment was done during its worn state, once the items are changed the alignment should have changed. Sounds as if you escaped that one though. The two standard propellers for the 37.5 are: 1ea. 17 X 15 1” shaft two blade 1ea. 17 X 14 1” shaft three blade The simple test to confirm if the propeller is loading the engine up sufficiently is to motor the vessel and apply maximum throttle. If the rpm’s achievable under max throttle meets and does not either exceed or fall short of it’s rated full load amount, you will know that the propeller is match correctly to the engine. Hope this helps. Sincerely, Kevin L. Woody
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