I have a 1981 Hunter 30. I switched from the factory 2 blade to a 3 blade to help me fight currents and tides. I dropped an inch off of diameter and pitch (I believe it was 13X13 and now it is 12X12). The boat does have more thrust, but there is still a decent amout of vibration involved. Also, the boat never really got beyond 5.5 knots under power with either prop, but with sails I can hold 6+ knots on the right point and wind. I achieve 5.x knots under power at about 2500 RPM. The motor is a Yanmar 2QM-15. If I push the throttle beyond 2500RPM, then the stern starts to sink down, and by 2800RPM there is noticable black smoke being produced. Do I have the wrong prop?
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Jim Wolfgram
Sailing vs Motoring performance.
Most boats will actually make better speed under sail than power. As the boat heels, hull shape typically adds a couple feet of waterline. Waterline is the governing factor for speed on displacement craft. Check your waterline spec Square root of waterline * 1.34 equals hull speed, if that equals the 5.5 your doing, then your cool on prop. Hope this helps.
Most boats will actually make better speed under sail than power. As the boat heels, hull shape typically adds a couple feet of waterline. Waterline is the governing factor for speed on displacement craft. Check your waterline spec Square root of waterline * 1.34 equals hull speed, if that equals the 5.5 your doing, then your cool on prop. Hope this helps.
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Jim
Right Prop
I don't know all the answers but here are a couple of resources for figuring out the right prop. There is a link from Yanmar to Gori for what is supposed to be a propeller calculator. To me it looks more like a propeller quoter. But here it is anyway. http://www.gori-propeller.dk/Index.asp?IDS=1195 Here is an article on Choosing the Right Prop on a different forum. http://www.sailnet.com/collections/articles/index.cfm?articleid=suelar164#sidebar We have a Jeanneau SO 37 and figured out that the factory prop was incorrect from a similar set of symptoms (black smoke at high RPM). Picked up a knot with the new prop.
I don't know all the answers but here are a couple of resources for figuring out the right prop. There is a link from Yanmar to Gori for what is supposed to be a propeller calculator. To me it looks more like a propeller quoter. But here it is anyway. http://www.gori-propeller.dk/Index.asp?IDS=1195 Here is an article on Choosing the Right Prop on a different forum. http://www.sailnet.com/collections/articles/index.cfm?articleid=suelar164#sidebar We have a Jeanneau SO 37 and figured out that the factory prop was incorrect from a similar set of symptoms (black smoke at high RPM). Picked up a knot with the new prop.
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RonD
Auxiliary Engines
From all I've read on the subject, most sailboats have auxiliary engines with sufficient HP to approach hull speed only under benign wind/sea/current conditions & a clean hull. Generally, with any sort of choppy waters &/or headwinds your motoring speed will fall off. A hull bottom with lots of sea growth affixed will also drop that significantly. On my C320 (3-blade prop), at 2800 RPM on a dead calm day the engine will move me at over 6.0 knots. Heading into 20 knots of wind with 3 ft seas drops that down to about 3.4 knots or less. (Just an example -- 2800 RPM on my Yanmar is loafing along) Let's face it, most of us (purists aside) that do any cruising will at times need to motor some distances to make a schedule, beat the weather, whatever. Having a calibration of speed vs RPM (and fuel consumption vs RPM) under various headwind conditions is important if you are cruising any distance under power, as it drastically affects your range between refuelings. You will rarely ever get beyond the displacement hull speed under sail, power, or combination unless other factors are working for you. (BTW: that 1.34xSQRT(LWL) is an approximation, at best.) And, the extra foot or so of waterline length (LWL) you might gain from heeling is almost imperceptible in terms of added max hull speed. You probably can't do much about re-sizing the engine. The best you can do is to ensure that your engine & drive chain (transmission, shaft, prop, bearings, etc.) are all working efficiently. A prop's diameter, pitch, and blade configuration must be matched to the engine's efficient RPM range & the transmission ratio. You should consult your engine manufacturer and/or the boat manufacturer to get the recommended configuration. --RonD
From all I've read on the subject, most sailboats have auxiliary engines with sufficient HP to approach hull speed only under benign wind/sea/current conditions & a clean hull. Generally, with any sort of choppy waters &/or headwinds your motoring speed will fall off. A hull bottom with lots of sea growth affixed will also drop that significantly. On my C320 (3-blade prop), at 2800 RPM on a dead calm day the engine will move me at over 6.0 knots. Heading into 20 knots of wind with 3 ft seas drops that down to about 3.4 knots or less. (Just an example -- 2800 RPM on my Yanmar is loafing along) Let's face it, most of us (purists aside) that do any cruising will at times need to motor some distances to make a schedule, beat the weather, whatever. Having a calibration of speed vs RPM (and fuel consumption vs RPM) under various headwind conditions is important if you are cruising any distance under power, as it drastically affects your range between refuelings. You will rarely ever get beyond the displacement hull speed under sail, power, or combination unless other factors are working for you. (BTW: that 1.34xSQRT(LWL) is an approximation, at best.) And, the extra foot or so of waterline length (LWL) you might gain from heeling is almost imperceptible in terms of added max hull speed. You probably can't do much about re-sizing the engine. The best you can do is to ensure that your engine & drive chain (transmission, shaft, prop, bearings, etc.) are all working efficiently. A prop's diameter, pitch, and blade configuration must be matched to the engine's efficient RPM range & the transmission ratio. You should consult your engine manufacturer and/or the boat manufacturer to get the recommended configuration. --RonD
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Pete
check the archives
Fred if you did not have any engine smoke before the prop change you may be over proped! Check the archive under hunter 30 and they are talking about 13x9 and 13x11 3 blade fixed props. Get a good prop shop to run the spec and see what they recomend.If the engine was ok before it sounds like you are overproped.the vibraton can be caused by several thing including the prop,the prop shaft,cutlass bearing and or engine aligment.Was the vibration there before the prop change?
Fred if you did not have any engine smoke before the prop change you may be over proped! Check the archive under hunter 30 and they are talking about 13x9 and 13x11 3 blade fixed props. Get a good prop shop to run the spec and see what they recomend.If the engine was ok before it sounds like you are overproped.the vibraton can be caused by several thing including the prop,the prop shaft,cutlass bearing and or engine aligment.Was the vibration there before the prop change?
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Kevin
Prop loads
Hello Fred, I would say yes you have the wrong propeller. The load the propeller produces as it is being driven by the motor will act as a governor limiting the motors ability of obtaining Rpm’s. The greater the pitch the fewer the Rpm’s, likewise the smaller the pitch the greater the Rpm’s. In your case where the Yanmar is rated to 3000 under full load you will want to adjust the pitch in order to allow the 3000 to be obtained. I myself would reduce the pitch by 1” or down to 11. This should increase the motors full load Rpm’s by apx. 7%. I would first check to see if your tack is reading accurate before the prop change. It is not uncommon to see a tack off. The black smoke you are describing could be caused by the overloading of the current prop. Even though maximum Rpm’s are not obtained the injectors will still open to 100% when asked to do so by the throttle. If the Rpm’s are not obtainable you are forcing more fuel into the cylinder then it is capable of using, thus some fuel will exit the exhaust as black carbon/unused fuel. Your boat speed sounds actually good to me. If the vessel’s transom appears to be squatting when underway you are approaching the point that any faster and the vessel will try to plain. The reason you see greater sailing speeds is that the calculation you use to determine sailing hull speed, square root WL x 1.35, takes into consideration and assumes the boat is at heel, which extends the WL. Very few boats ever motor as fast as they sail. The hull speed calculation is also very generic. The formula you use for your H30 is the same formula used on a very traditional Block Island 40, so you will see differences for sure. In my opinion if the propeller diameter is in line with hull length and motor is within range of needed HP the speed you achieve at a particular Rpm is what you will see, as long as the motor is capable to obtaining its rated maximum. Good luck Kevin
Hello Fred, I would say yes you have the wrong propeller. The load the propeller produces as it is being driven by the motor will act as a governor limiting the motors ability of obtaining Rpm’s. The greater the pitch the fewer the Rpm’s, likewise the smaller the pitch the greater the Rpm’s. In your case where the Yanmar is rated to 3000 under full load you will want to adjust the pitch in order to allow the 3000 to be obtained. I myself would reduce the pitch by 1” or down to 11. This should increase the motors full load Rpm’s by apx. 7%. I would first check to see if your tack is reading accurate before the prop change. It is not uncommon to see a tack off. The black smoke you are describing could be caused by the overloading of the current prop. Even though maximum Rpm’s are not obtained the injectors will still open to 100% when asked to do so by the throttle. If the Rpm’s are not obtainable you are forcing more fuel into the cylinder then it is capable of using, thus some fuel will exit the exhaust as black carbon/unused fuel. Your boat speed sounds actually good to me. If the vessel’s transom appears to be squatting when underway you are approaching the point that any faster and the vessel will try to plain. The reason you see greater sailing speeds is that the calculation you use to determine sailing hull speed, square root WL x 1.35, takes into consideration and assumes the boat is at heel, which extends the WL. Very few boats ever motor as fast as they sail. The hull speed calculation is also very generic. The formula you use for your H30 is the same formula used on a very traditional Block Island 40, so you will see differences for sure. In my opinion if the propeller diameter is in line with hull length and motor is within range of needed HP the speed you achieve at a particular Rpm is what you will see, as long as the motor is capable to obtaining its rated maximum. Good luck Kevin
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Ed Allen
lots of issues at hand.
the correct prop for your boat will allow the engine to turn its reccomended max. rpm, no more no less. to use the caluclators you need to know the gear ratio of your transmission. they are not all the same. look on the top of the gear to findout what ratio you have. run the engine to max atainable rpm and make note of the engine speed and boat speed. call a good prop shop with the numbers that will get you to the right prop quickest.
the correct prop for your boat will allow the engine to turn its reccomended max. rpm, no more no less. to use the caluclators you need to know the gear ratio of your transmission. they are not all the same. look on the top of the gear to findout what ratio you have. run the engine to max atainable rpm and make note of the engine speed and boat speed. call a good prop shop with the numbers that will get you to the right prop quickest.
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J. Tesoriero
Same boat, same problem
I have the same boat and motor and also had to change my prop. I went from the original two-blade 13X13 to a three blade Campbell Sailor at 13X9. The original prop did not give me good speed and I had an exhaust stained transom. Just going to a three blade should have reduced the pitch to 11 to keep everything the same. I reduced the pitch an extra inch on the new prop (from 11 to 9) and got at least an extra knot at 28,000 RPM's and the engine does not smoke. If you contact any of the prop manufacturers, they can calculate the optimum prop for your boat.
I have the same boat and motor and also had to change my prop. I went from the original two-blade 13X13 to a three blade Campbell Sailor at 13X9. The original prop did not give me good speed and I had an exhaust stained transom. Just going to a three blade should have reduced the pitch to 11 to keep everything the same. I reduced the pitch an extra inch on the new prop (from 11 to 9) and got at least an extra knot at 28,000 RPM's and the engine does not smoke. If you contact any of the prop manufacturers, they can calculate the optimum prop for your boat.
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