I bought a 9.8 Tohatsu for my Hunter 26, the motor came with a mid range prop, I notice that at full throttle I get a lot of cavitation bubbles swirling in the prop wake, also the boat is only making about 5.8 kt, and the motor should push it along at over 6 ...?? should I go to the full displacement prop, or something in between..I figure some of you must have pondered this issue and found the best pitch, hope to plumb your knowledge..if it helps..the boat weighs in around 3 tons loaded and crewed Thanks for the help
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Similar question - for reverse
I have a similar question posted but I'll add my request here as well - my issue is a prop that cavitates thus killing thrust - especially in reverse. I've got the 9,8 Evinrude.
I have a similar question posted but I'll add my request here as well - my issue is a prop that cavitates thus killing thrust - especially in reverse. I've got the 9,8 Evinrude.
robin, you *might* be a bit over-propped but
your theoretical max speed is ~6.4 knots. What are you using to measure speed with? If a paddle wheel, you may be .5 knots off anyway and your motor/prop is getting you to hull speed at less than full throttle. At any rate, maybe you could borrow a prop 1" pitch less or buy it with the understanding you could return it if unhappy. Same for BillP except if you throw that puppy into R and rev sharply, you'll probably cavitate regardles of pitch.
your theoretical max speed is ~6.4 knots. What are you using to measure speed with? If a paddle wheel, you may be .5 knots off anyway and your motor/prop is getting you to hull speed at less than full throttle. At any rate, maybe you could borrow a prop 1" pitch less or buy it with the understanding you could return it if unhappy. Same for BillP except if you throw that puppy into R and rev sharply, you'll probably cavitate regardles of pitch.
I also have a new 4 cycle 9.8
Tohatsu on my 260. When I ordered the O/B it came with a mid-range prop. I ordered the prop for the heavy displacement and keep the orginial one as a spare. The combiation seems to work well, I just purchased a Tach and will see if the max RPM is reached at full throttle. Just for information sake I bought a 25 inch extra long shaft length. From what I know if the outboard reaches Max RPM at full throttle than you have the right pitch. If the max RPM is not reached that it is overloaded, if your RPM is higher than max RPM than you are over reveing the engine (also not good). For our engine 9.8 Tohatsu I remember only three props available. You pick the closes one to match max RPM at full throttle John USNret
Tohatsu on my 260. When I ordered the O/B it came with a mid-range prop. I ordered the prop for the heavy displacement and keep the orginial one as a spare. The combiation seems to work well, I just purchased a Tach and will see if the max RPM is reached at full throttle. Just for information sake I bought a 25 inch extra long shaft length. From what I know if the outboard reaches Max RPM at full throttle than you have the right pitch. If the max RPM is not reached that it is overloaded, if your RPM is higher than max RPM than you are over reveing the engine (also not good). For our engine 9.8 Tohatsu I remember only three props available. You pick the closes one to match max RPM at full throttle John USNret
I've got a Yamaha 9.9 w/high-thrust prop
also with a long shaft lengh like John. At full throttle my speed indicator and both GPS units all register about 6.5 - 7 knots. My engine doesn't cavitate, and I think that might because it has the long shaft. If you think it would help, I'd suggest you consider the "powerthruster" available in the chandlery here. If it works you could then consider trying other, higher-thrust props.
also with a long shaft lengh like John. At full throttle my speed indicator and both GPS units all register about 6.5 - 7 knots. My engine doesn't cavitate, and I think that might because it has the long shaft. If you think it would help, I'd suggest you consider the "powerthruster" available in the chandlery here. If it works you could then consider trying other, higher-thrust props.
clarification
My motor is (from advice here) the ultra long shaft (25") so I think the cavitation is from wrong prop selection, there are 2 more props available on the displacement end of the range, I did some more research and think it best to get the prop with the least pitch (6.5") unless I hear otherwise
My motor is (from advice here) the ultra long shaft (25") so I think the cavitation is from wrong prop selection, there are 2 more props available on the displacement end of the range, I did some more research and think it best to get the prop with the least pitch (6.5") unless I hear otherwise
Reverse cavitation
is usually caused by the prop sucking in exhaust, which is exiting through the middle of the prop.
is usually caused by the prop sucking in exhaust, which is exiting through the middle of the prop.
Prop Shop
As Mike H I also have the Yamaha High Thrust. It has a large propeller that is geared to turn slower at a higher RPM. I have not had cavitation problems either. I would recommend that you look for a local propeller shop. Most marine areas have one. These guys really understand props and could best recommend a solution for you. Good luck.
As Mike H I also have the Yamaha High Thrust. It has a large propeller that is geared to turn slower at a higher RPM. I have not had cavitation problems either. I would recommend that you look for a local propeller shop. Most marine areas have one. These guys really understand props and could best recommend a solution for you. Good luck.
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