Hello everyone ! I own a Hunter 34, 1983. I want to change the 2 blades propeller for a 3 blades. The actual size of my prop is 15 x 12 RH. The engine on my boat is a Yanmar 3gmf ( 22 hp) . Someone suggested that I install a 3 blade propeller the sama size and pitch ( 15 x 12). Someone else told me that the right size was 15 x 15 ....Could somebody help me in choosing the right size ? Denis.
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Do a web search
There are supposed to be some programs to help you select the correct size based on displacement, horsepower, etc.
There are supposed to be some programs to help you select the correct size based on displacement, horsepower, etc.
call a prop shop
You need a prop shop to do a calc for ya. It will save you a lot of grief and expense. You will need to know the gear ratio of the trans. and the max rpm your boat will do with the current prop, the size of the current prop and the shaft size. with this info it should be easy for any reputable prop shop to do the calc for ya. General propeller in bradenton florida is a good one. or Admirial propellor in Tampa fl. they will give you the number you need and then you can compare that to what others use.
You need a prop shop to do a calc for ya. It will save you a lot of grief and expense. You will need to know the gear ratio of the trans. and the max rpm your boat will do with the current prop, the size of the current prop and the shaft size. with this info it should be easy for any reputable prop shop to do the calc for ya. General propeller in bradenton florida is a good one. or Admirial propellor in Tampa fl. they will give you the number you need and then you can compare that to what others use.
My 84 came with a 15 X 15 RH 2-blades
Same engine except the tag on it says 20 hp. Tranny is standard KM3-A and shaft is 1". I get 6.5 knots at 2800 RPM when in calm weather, but it does drop to 5.5 when running into heavy winds/waves. Stopping quickly and backing is not a problem as I can generally use the tendency to walk to port as a help in maneuvering . Another 7 or 8 weeks and we're in the water ! Good luck and have a good season.
Same engine except the tag on it says 20 hp. Tranny is standard KM3-A and shaft is 1". I get 6.5 knots at 2800 RPM when in calm weather, but it does drop to 5.5 when running into heavy winds/waves. Stopping quickly and backing is not a problem as I can generally use the tendency to walk to port as a help in maneuvering . Another 7 or 8 weeks and we're in the water ! Good luck and have a good season.
Sav e your money
So you want to spend money to make your boat sail slower and get no motoring improvement? What's up with that? A properly pitched two blade prop will push the boat hull speed with a lot less drag (and you'll have money 4 beer)
So you want to spend money to make your boat sail slower and get no motoring improvement? What's up with that? A properly pitched two blade prop will push the boat hull speed with a lot less drag (and you'll have money 4 beer)
propeller change
This is exactly what someone suggested : 15 x 12 rh 3 blades prop.... Hope it's the right size....Do you see any improvements facing heavy winds and strong wave ?
This is exactly what someone suggested : 15 x 12 rh 3 blades prop.... Hope it's the right size....Do you see any improvements facing heavy winds and strong wave ?
3-bladed prop
Denis, You should be very sure about the prop's specs, as its load must match your engine/gearbox rpm range in order to neither be ineffective in improving your boat's acceleration nor overload the engine. If it's well-matched, you obviously won't get any higher speed, as that's limited by the DWL of your hull. You will, however, find that the higher thrust will get you there faster - and that acceleration can be a big improvement in close quarters maneuvering in currents. Since effective reverse thrust is much less than forward, the improvement in backing against currents will be quite noticable. I've found that the additional thrust also seems to better hold my speed against the deceleration effects of slicing into bow-on waves in heavy chop over a breaking bar. Since I have a lot more sail area than the standard (let alone furilng mainsail) versions of my boat, I don't find the increase in drag of the 3-blade noticable, even in light air. However, if the prop can thrust harder, it has to drag more (unless it folds or feathers), and racers often feel the trade-off isn't worth it. For me it most certainly is in crossing the Merrimac entrance bar and in docking in a tight slip across heavy currents. However, while I'm no expert on the subject, I believe you may be heading for trouble by keeping both blade length and blade pitch the same while adding 50% more of such blades to push. You may find that you can't reach previous rpm limits and that your engine turns into a vibrating smoke-generator when you try. Accepting this effective "rpm-governor" effect would mean that you couldn't run your engine at 75-to-80% of max rpm, where it's happiest and has the longest life. If, on the other hand, you go with blades that are too small and/or pitched at too slight an angle, you'll get an easily high-revving engine but no performance. I don't know if Hunter ever offered a 3-blade as an option with your boat, but that's the first place I'd check for the proper specs. Ed is right. Consult a prop specialist - don't go by what some random guy (like me) says is "the right size" for your boat's engine and gearbox combo.
Denis, You should be very sure about the prop's specs, as its load must match your engine/gearbox rpm range in order to neither be ineffective in improving your boat's acceleration nor overload the engine. If it's well-matched, you obviously won't get any higher speed, as that's limited by the DWL of your hull. You will, however, find that the higher thrust will get you there faster - and that acceleration can be a big improvement in close quarters maneuvering in currents. Since effective reverse thrust is much less than forward, the improvement in backing against currents will be quite noticable. I've found that the additional thrust also seems to better hold my speed against the deceleration effects of slicing into bow-on waves in heavy chop over a breaking bar. Since I have a lot more sail area than the standard (let alone furilng mainsail) versions of my boat, I don't find the increase in drag of the 3-blade noticable, even in light air. However, if the prop can thrust harder, it has to drag more (unless it folds or feathers), and racers often feel the trade-off isn't worth it. For me it most certainly is in crossing the Merrimac entrance bar and in docking in a tight slip across heavy currents. However, while I'm no expert on the subject, I believe you may be heading for trouble by keeping both blade length and blade pitch the same while adding 50% more of such blades to push. You may find that you can't reach previous rpm limits and that your engine turns into a vibrating smoke-generator when you try. Accepting this effective "rpm-governor" effect would mean that you couldn't run your engine at 75-to-80% of max rpm, where it's happiest and has the longest life. If, on the other hand, you go with blades that are too small and/or pitched at too slight an angle, you'll get an easily high-revving engine but no performance. I don't know if Hunter ever offered a 3-blade as an option with your boat, but that's the first place I'd check for the proper specs. Ed is right. Consult a prop specialist - don't go by what some random guy (like me) says is "the right size" for your boat's engine and gearbox combo.
Fold it.
"The increased handling is well worth any loss under sail." Li'l Sea, what's up with that statement? That not only sounds wrong, that IS wrong. I have a two-blade Flex-O-Fold on my H37C. I dock bow-in in a river where the current fights the windage. I back out and never touch a piling. I motor in wind and waves at hull speed. I sail up to a knot faster compared to dragging a big 3-blade. Why would I want a fixed 3-blade?
"The increased handling is well worth any loss under sail." Li'l Sea, what's up with that statement? That not only sounds wrong, that IS wrong. I have a two-blade Flex-O-Fold on my H37C. I dock bow-in in a river where the current fights the windage. I back out and never touch a piling. I motor in wind and waves at hull speed. I sail up to a knot faster compared to dragging a big 3-blade. Why would I want a fixed 3-blade?
I went thru this so if you want the right pitch
for your Hunter 34 it's 11" ... 12" will only get your engine to 2900 rpms at WOT.. 11" gets the engine to 3400 or so at WOT.. So it's 15" * 11" right hand turn. Made things much smoother in my boat.
for your Hunter 34 it's 11" ... 12" will only get your engine to 2900 rpms at WOT.. 11" gets the engine to 3400 or so at WOT.. So it's 15" * 11" right hand turn. Made things much smoother in my boat.
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