My 1997, Hunter 310 sailboat came with a 15 x 12, 2 bladed prop. The engine turns up to 3900 RPM before it starts to blow black smoke. The speed at 3600 is 6.5 knots. Normally, I don't cruise at more than 2600 to 2800 RPM. I checked the reduction on the transmission and it reads 2.62. Hunter suggested a 15 x 11, three bladed prop. I barrowed a 15 x 12, three bladed to try. The engine topped out at 3100 RPM with much black smoke and only 5.6 knots at 2800 RPM. What prop size should I use to get the same performance as the two bladed.
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This does not sound right but....
Ray: This does not sound right but I would have both props check to see if they are the size and pitch that you think they are. The general rule of thumb is that 1 in. of diameter or 1 degreee of pitch will usually give you about 200-300 rpms. It sounds like the 2 bladed prop was perfect for your engine. Make sure that you know for sure what the pitch of each prop is before you do anything. They need to be taken to a prop shop and be measured. You may need a 15 x 10 to get you back in the ball park. PS: Why do you think you want to change props anyway.
Ray: This does not sound right but I would have both props check to see if they are the size and pitch that you think they are. The general rule of thumb is that 1 in. of diameter or 1 degreee of pitch will usually give you about 200-300 rpms. It sounds like the 2 bladed prop was perfect for your engine. Make sure that you know for sure what the pitch of each prop is before you do anything. They need to be taken to a prop shop and be measured. You may need a 15 x 10 to get you back in the ball park. PS: Why do you think you want to change props anyway.
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Ray Chapman
Steve, The three bladed 15 x 12 came from a prop shop so should be OK. The two bladed is from Hunter. I will be taking in the two bladed to get it balanced there was considerable vibration over 2200 RPM. As I under stand props the pitch determines speed and the diameter power. Is this correct? If so, should I be looking for a 14 x 12 in a three bladed prop? The reason for the three blades is I feel that they have less vibration problems. Ray
Yes.
Ray: That sounds right, 14 x 12. But these are just numbers to work with as rules of thumb. There are other considerations with tip clearance too. I think that there is a measurement of 15% of the diameter should be the minimum tip clearance. So the 14" prop would also offer additional clearance. I personally would not want the extra drag of the 3 blade prop. It is very seldom that "I" need the extra push of the three blade. According to the owners that have changed to three bladed props, they have mentioned the same thing with the vibration. As far as your speed calculations how are you measuring them (knot meter or GPS). The way I figure speed calc with the knot meter, it's just a relative number for your vessel. They are not necessarily your true speed, unless you have recalibrated the unit.
Ray: That sounds right, 14 x 12. But these are just numbers to work with as rules of thumb. There are other considerations with tip clearance too. I think that there is a measurement of 15% of the diameter should be the minimum tip clearance. So the 14" prop would also offer additional clearance. I personally would not want the extra drag of the 3 blade prop. It is very seldom that "I" need the extra push of the three blade. According to the owners that have changed to three bladed props, they have mentioned the same thing with the vibration. As far as your speed calculations how are you measuring them (knot meter or GPS). The way I figure speed calc with the knot meter, it's just a relative number for your vessel. They are not necessarily your true speed, unless you have recalibrated the unit.
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Ray Chapman
Used Both GPS and Knot meter
Steve, I used both GPS and Knot meter. My knot meter is a little off depending on the hull speed. I have calibrated it so the maximum error based on GPS is about .3 knots. It used to be over .5 knots. I was at my prop shop and he doesn't have a used 14 X 12 to try, so I guess I will just get the two bladed balanced and use it. Then when I get to Florida I will look around. Thanks for the responce. Ray
Steve, I used both GPS and Knot meter. My knot meter is a little off depending on the hull speed. I have calibrated it so the maximum error based on GPS is about .3 knots. It used to be over .5 knots. I was at my prop shop and he doesn't have a used 14 X 12 to try, so I guess I will just get the two bladed balanced and use it. Then when I get to Florida I will look around. Thanks for the responce. Ray
Ray:
I would like to know if the 3 bladed prop took care of the basic problem that you had or were you just looking for some more thrust in a seaway? Anyway, if you have the time I would like you to call Kilian Prop in San Leandro, CA. Tell them what your problem is and see what they can do for you. They have some very high tech stuff that they can do to a prop. I have talked to people that understand props and say the stuff that they do won't work, but the people that have had the work done swear by them. Their phone number is 510/614-0596 and their website is http://www.kilianprop.com/
I would like to know if the 3 bladed prop took care of the basic problem that you had or were you just looking for some more thrust in a seaway? Anyway, if you have the time I would like you to call Kilian Prop in San Leandro, CA. Tell them what your problem is and see what they can do for you. They have some very high tech stuff that they can do to a prop. I have talked to people that understand props and say the stuff that they do won't work, but the people that have had the work done swear by them. Their phone number is 510/614-0596 and their website is http://www.kilianprop.com/
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Gordon Torresen
Prop sizing
It might be a good idea to check your tach. The governor should be set for a max of 3600 RPM. If you can turn 3900, in gear, something doesn't ring true. Blade area is the main factor in getting the most out of a prop. Unfortunately, adding area increases drag while sailing. Short of a folding or feathering prop, this is a fact of life. Some rules of thumb regarding props: Add a blade, decrease diameter or pitch one inch; add an inch of diameter, subtract an inch of pitch - visa versa works too.
It might be a good idea to check your tach. The governor should be set for a max of 3600 RPM. If you can turn 3900, in gear, something doesn't ring true. Blade area is the main factor in getting the most out of a prop. Unfortunately, adding area increases drag while sailing. Short of a folding or feathering prop, this is a fact of life. Some rules of thumb regarding props: Add a blade, decrease diameter or pitch one inch; add an inch of diameter, subtract an inch of pitch - visa versa works too.
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