I'm trying to determine the best propeller size for my '79 Hunter 30. The boat was repowered at some point with a Yanmar 3HMF (30hp) - I'm not sure what the stock engine was. It currently has a 2 blade 13x10 prop, and motors very slowly even with a freshly cleaned bottom. It seems to me that I should be able to motor close to hull speed (6.8 knots) with a 30hp motor, but the best I can get at the moment is about 5 knots (which is barely adequate for getting in and out of the Intracoastal inlets down here in South Florida). Will changing the prop make any difference, and if so, what size should I be looking at. What else could be causing my performance problems? The engine does not sound like it's straining, but I'm no expert on marine diesels ;-). Any help would be most appreciated! Regards, David.
Propeller size for 1979 Hunter 30
- Thread starter David Hoy
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I used to get at least 6 knots on a 1980 H30 with the 15 hp Yanmar. Are you sure that your engine is performing OK? You should contact at least two different propeller manufacturers. They will need the boat size and displacement, the engine size and gear reduction for the tranny. They can plug all of this in and give you recommendations for the correct diameter and pitch for a prop on your particular boat.
I used to get at least 6 knots on a 1980 H30 with the 15 hp Yanmar. Are you sure that your engine is performing OK? You should contact at least two different propeller manufacturers. They will need the boat size and displacement, the engine size and gear reduction for the tranny. They can plug all of this in and give you recommendations for the correct diameter and pitch for a prop on your particular boat.
RPM's
Have you looked to see what RPM's you are turning? If you are less than 3000 at wide open throttle the prop is too large. If you are more than 3000 at wide open throttle your prop is too small. The diameter of the prop will need to stay close to the same and the pitch is what you need to work with assuming the diameter was correct with the repower. Each change up or down in pitch will usually change your RPM's by approximately 400RPM. At wide open throttle you should be turning around 3000RPM so you can cruise at a little over 2000RPM for efficiency. Hope this helps.
Have you looked to see what RPM's you are turning? If you are less than 3000 at wide open throttle the prop is too large. If you are more than 3000 at wide open throttle your prop is too small. The diameter of the prop will need to stay close to the same and the pitch is what you need to work with assuming the diameter was correct with the repower. Each change up or down in pitch will usually change your RPM's by approximately 400RPM. At wide open throttle you should be turning around 3000RPM so you can cruise at a little over 2000RPM for efficiency. Hope this helps.
No tach, small prop, big(ish) engine...
Doing a little more research, it seems that the stock 13x10 prop (such as what is currently installed on the boat) would have been right for the stock engine, a Yanmar YSM12 12hp motor. When I got the boat a few months ago it already had been repowered with the 3HMF 30hp motor, but they still used the stock prop size. This seems decidedly silly to me... Unfortunately my boat does not have a tachometer, so I cannot tell what the engine RPM is. This seems like a rather strange ommision - how can I add a tach to my motor? The Clymer service manual I have has very little to say about the tach sender unit, and whether the 3HMF even has one. Without actually getting underneath the boat I do not know how much clearance there might be for a larger prop. I guess I need to figure out whether a larger diameter prop is feasible, or whether I can only play with the pitch. I'm beginning to think that a call to my local Yanmar dealer may be in order. They should be able to help me out with the tach, engine information, prop selection, etc. I was hoping to avoid the expense, but it is probably going to be my best option... Regards, David.
Doing a little more research, it seems that the stock 13x10 prop (such as what is currently installed on the boat) would have been right for the stock engine, a Yanmar YSM12 12hp motor. When I got the boat a few months ago it already had been repowered with the 3HMF 30hp motor, but they still used the stock prop size. This seems decidedly silly to me... Unfortunately my boat does not have a tachometer, so I cannot tell what the engine RPM is. This seems like a rather strange ommision - how can I add a tach to my motor? The Clymer service manual I have has very little to say about the tach sender unit, and whether the 3HMF even has one. Without actually getting underneath the boat I do not know how much clearance there might be for a larger prop. I guess I need to figure out whether a larger diameter prop is feasible, or whether I can only play with the pitch. I'm beginning to think that a call to my local Yanmar dealer may be in order. They should be able to help me out with the tach, engine information, prop selection, etc. I was hoping to avoid the expense, but it is probably going to be my best option... Regards, David.
Try the prop shop.
Im sure you already know this, but try Frank & Jimmys Prop shop here in Ft.Lauderdale for the prop size.Also try Complete Yacht Service, also in FLL for tack info, he is the local Yan. dealer, ask to speak to Mark.
Im sure you already know this, but try Frank & Jimmys Prop shop here in Ft.Lauderdale for the prop size.Also try Complete Yacht Service, also in FLL for tack info, he is the local Yan. dealer, ask to speak to Mark.
Tach
You can ask the folks at Yanmar about adding a tachometer. If your engine does not have a Tach sending unit, you can use one of the tachometers that are driven by the alternator (see West Marine or others). You can have any alternator shop add the wire to drive the tach.
You can ask the folks at Yanmar about adding a tachometer. If your engine does not have a Tach sending unit, you can use one of the tachometers that are driven by the alternator (see West Marine or others). You can have any alternator shop add the wire to drive the tach.
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