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Author Topic:   Long shaft kicker cavitating

minnow posted 08-05-2005 09:15 PM ET (US)
I took my 2000 Montauk out to the bay on Wednesday to test out the used Honda 8hp long shaft kicker I just bought. I was suprised to find the motor states to cavitate at half throttle (the boat was barely moving). I clamped the kicker on the port side as discribed by some on this web site. The kicker was trimmed all the way down. I doudle ckecked the lenght to make sure it's 20". Can someone tell me why?
Plotman posted 08-06-2005 10:50 AM ET (US)
The prop apllicable to a kicker will be very different than what the same motor will use pusing a small, light boat at speed.
A kicker prop will generally have 3 large, almost mouse-ear blades, and have a very low pitch.
My guess is that you have a regular prop on your kicker, and need to change it.
minnow posted 08-06-2005 10:59 PM ET (US)
The prop has three blades but I don't know what's the pitch on it. Why would that makes any different?
bsmotril posted 08-08-2005 12:31 PM ET (US)
The max speed of a boat at displacement speed is limited by the length to beam ration of the wetted waterline length of the hull. On a boat like the Montauk, that will be around 5 mph. Unless you have an abundance of horsepower to push the hull above displacement speed and up onto plane, you're not going much faster than 5 mph. The prop that is likley on that motor is designed to run around 25 mph. When you reach half throttle, the motor is turning the prop the rpms that equal approximately 5mph. Any move throttle, and the RPMs of the prop execeed the displacment speed limit of the hull, there is not enough HP to push the boat faster, the prop looses traction, and slips or cavitates. You need a prop with much less pitch, probably 4 - 6 inches less pitch to that the 5mph displacement speed of your hull matches the prop and motor rpms nearer the full throttle range of the motor. Running a prop with too much pitch like you have now is not good for the motor and puts an excessive load on it. It's like trying to climb a hill in your car, towing a trailer, and leaving it in 5th gear vers downshifting to a lower ratio. The lower pitch prop gives you the downshift to move a heavy load at slow speed at the motors most efficient operating range. BillS
minnow posted 08-09-2005 12:01 PM ET (US)
Thanks BillS for the explanation. How do I know what’s the pitch on my prop now? I found some numbers on the prop, 9 ½ & 8 5/8, 240 & 220. What kind of numbers should I look for to be used as a kicker.
bsmotril posted 08-09-2005 02:02 PM ET (US)
Usually you see prop pitch represented as a number like 13.5 X 17. That would be a prop 13 1/2 inches in diameter, and 17 inch pitch. Measure the diameter of the circle formed by the tip of your blades. It will likley be either 9 1/2, or 8 5/8. The prop pitch is then most likley to be the other of those two numbers which is NOT the diameter measurement. BillS
minnow posted 08-09-2005 02:31 PM ET (US)
I think the 9 1/2 & 8 5/8 are in inches and the 240 and 220 are in mm(metric).If that's the case, does that mean I have a 9 1/2in prop and 8 5/8in pitch and 4-6in less pitches will end up with 3in. Is this correct BillS?
bsmotril posted 08-09-2005 10:08 PM ET (US)
That would be correct. But, 3 inches of pitch is almost flat, and I doubt they make them that low. Smaller outboards often only have 2 or 3 prop choices that fit, one for a light load or light boat, and another lower pitch for a "Load Prop". Get whichever one you can find that has the lowest pitch number, even if it is only 2-3 inches less than the one you have now, it will still make your motor live longer. Also, with the new prop, don't open the throttle any more once you reach your max hull speed. You can tell when you are there by watching the speed on a GPS. At a certain point, the speed won't go up anymore, other than small tenth of mph increases. BillS
Teak Oil posted 08-10-2005 11:03 AM ET (US)
Just contact your Yamaha dealer and tell him you need a high thrust kicker prop for your little 8hp. There is probably only one high thrust prop made for that model so you dont have to worry about screwing it up.
newt posted 08-10-2005 02:15 PM ET (US)
This discussion has me wondering about something. Before I ask the question, I will point out that I have almost zero experience with small outboard engines used as a kicker.
On a planing hull, propeller pitch should be determined based on the maximum RPM range for the engine, which is fairly easy to figure out - wind the engine up to full throttle and see what the RPMs are. Once a boat is up on plane, the correlation between pitch, rpms, and speed is fairly predictable.
When using a small kicker engine, boat speed is limited to the displacement speed of the hull. So, at some point in the RPM range, boat speed will become a constant and any increase in RPMs means an increase in slippage or cavitation or whatever. So how does one determine the propeller pitch for a kicker?
bsmotril posted 08-10-2005 02:47 PM ET (US)
Use either your GPS, or the hull speed formula to calculate your max displacement hull speed. Then plug those numbers into the prop calculator on this site to solve for pitch. Or, plug in the number for the flattest pitch prop available, and solve for RPM that equals your max hull speed. That would be the do not exceed rpm for the motor.
since the formula depends on length measured at the wetted waterline, which is difficult, the GPS would be preferred. Here's a link with a good discussion of the math involved in the hull speed formula:
http://potter-yachters.org/manyways/hullspeed/
BillS
newt posted 08-10-2005 03:31 PM ET (US)
Bill, I understand what you are saying, but something still doesn't click for me.
For the purpose of my argument, lets pretend that there is no such thing as hull speed. Let's say we put an 8 hp kicker on a Montauk and we were able to get 10 mph speed and the engine was running in the proper RPM range at WOT. Then, we run this boat around most of the time at 4 mph and we all agree that the kicker is propped correctly. So even though we never run at WOT, we still know that we are propped right and the engine is not lugging.
Now apply real life physics to this same boat and we find that we can't get over 5-6 mph hull speed, but the engine is still propped right and not lugging.
By your method above, we could take a 15 foot hull, calculate hull speed, and work backwards to find the right prop. If we use that same engine on a 17 foot boat, then hull speed is a little faster, so working backwards gives us a taller prop. So, for the same engine, we would use a taller prop with a larger, heavier boat, which seams backwards to me.
bsmotril posted 08-10-2005 04:33 PM ET (US)
It is not illogical at all when you consider real life physics, and my suggestions were to get you in the ballpark. In reallity, there is likely only a single kicker prop available ina single pitch. At those speeds, pushing a heavy boat at displacement speed, the prop slip factor becomes a fairly large number. As a result, a single prop pitch number will accomodate a large range of hull sizes, maybe 14 - 25 feet. Regardless of the weight of those boats, the kicker will still only take them to hull speed. That size range of boats, assuming equal beam, yields a pretty narrow range of hull speeds, and the prop slip is the cushion that lets the motor work for all of them with a single prop. There's a good discussion of this at:
http://www.geocities.com/Yosemite/Forest/2727/flicka_formula.html
BillS
Joe Kriz posted 08-10-2005 05:06 PM ET (US)
I have a 8.5 x 9 pitch on my 8hp Evinrude...
I have never experienced any problems using the motor on 3 different Whalers.
1. 1978 Montauk
2. 1997 Outrage 17
3. 1985 Outrage 18
I know Yamaha sells a High Thrust T-8 hp model. I am not sure what prop they use.
By the way, the preferred side to mount the kicker is on the Starboard side.
We can't always do this but it is preferred...
Teak Oil posted 08-10-2005 10:52 PM ET (US)
Plugging numbers into formulas for kicker motors is mostly a waste of time since most only have one or two different props for any given model. Usually there is a high pitch prop for running fast aluminum boats and dinghies, and there are low pitch props for use as kickers. Thats it. You have a very small selection
bsmotril posted 08-11-2005 01:13 PM ET (US)
Agreed. But newt was asking a hypothetical question to solve and that is where the prop calculator came into play.
BillS
newt posted 08-11-2005 01:30 PM ET (US)
Guys, thanks for the education.

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Author	Topic: Long shaft kicker cavitating
minnow	posted 08-05-2005 09:15 PM ET (US) I took my 2000 Montauk out to the bay on Wednesday to test out the used Honda 8hp long shaft kicker I just bought. I was suprised to find the motor states to cavitate at half throttle (the boat was barely moving). I clamped the kicker on the port side as discribed by some on this web site. The kicker was trimmed all the way down. I doudle ckecked the lenght to make sure it's 20". Can someone tell me why?
Plotman	posted 08-06-2005 10:50 AM ET (US) The prop apllicable to a kicker will be very different than what the same motor will use pusing a small, light boat at speed. A kicker prop will generally have 3 large, almost mouse-ear blades, and have a very low pitch. My guess is that you have a regular prop on your kicker, and need to change it.
minnow	posted 08-06-2005 10:59 PM ET (US) The prop has three blades but I don't know what's the pitch on it. Why would that makes any different?
bsmotril	posted 08-08-2005 12:31 PM ET (US) The max speed of a boat at displacement speed is limited by the length to beam ration of the wetted waterline length of the hull. On a boat like the Montauk, that will be around 5 mph. Unless you have an abundance of horsepower to push the hull above displacement speed and up onto plane, you're not going much faster than 5 mph. The prop that is likley on that motor is designed to run around 25 mph. When you reach half throttle, the motor is turning the prop the rpms that equal approximately 5mph. Any move throttle, and the RPMs of the prop execeed the displacment speed limit of the hull, there is not enough HP to push the boat faster, the prop looses traction, and slips or cavitates. You need a prop with much less pitch, probably 4 - 6 inches less pitch to that the 5mph displacement speed of your hull matches the prop and motor rpms nearer the full throttle range of the motor. Running a prop with too much pitch like you have now is not good for the motor and puts an excessive load on it. It's like trying to climb a hill in your car, towing a trailer, and leaving it in 5th gear vers downshifting to a lower ratio. The lower pitch prop gives you the downshift to move a heavy load at slow speed at the motors most efficient operating range. BillS
minnow	posted 08-09-2005 12:01 PM ET (US) Thanks BillS for the explanation. How do I know what’s the pitch on my prop now? I found some numbers on the prop, 9 ½ & 8 5/8, 240 & 220. What kind of numbers should I look for to be used as a kicker.
bsmotril	posted 08-09-2005 02:02 PM ET (US) Usually you see prop pitch represented as a number like 13.5 X 17. That would be a prop 13 1/2 inches in diameter, and 17 inch pitch. Measure the diameter of the circle formed by the tip of your blades. It will likley be either 9 1/2, or 8 5/8. The prop pitch is then most likley to be the other of those two numbers which is NOT the diameter measurement. BillS
minnow	posted 08-09-2005 02:31 PM ET (US) I think the 9 1/2 & 8 5/8 are in inches and the 240 and 220 are in mm(metric).If that's the case, does that mean I have a 9 1/2in prop and 8 5/8in pitch and 4-6in less pitches will end up with 3in. Is this correct BillS?
bsmotril	posted 08-09-2005 10:08 PM ET (US) That would be correct. But, 3 inches of pitch is almost flat, and I doubt they make them that low. Smaller outboards often only have 2 or 3 prop choices that fit, one for a light load or light boat, and another lower pitch for a "Load Prop". Get whichever one you can find that has the lowest pitch number, even if it is only 2-3 inches less than the one you have now, it will still make your motor live longer. Also, with the new prop, don't open the throttle any more once you reach your max hull speed. You can tell when you are there by watching the speed on a GPS. At a certain point, the speed won't go up anymore, other than small tenth of mph increases. BillS
Teak Oil	posted 08-10-2005 11:03 AM ET (US) Just contact your Yamaha dealer and tell him you need a high thrust kicker prop for your little 8hp. There is probably only one high thrust prop made for that model so you dont have to worry about screwing it up.
newt	posted 08-10-2005 02:15 PM ET (US) This discussion has me wondering about something. Before I ask the question, I will point out that I have almost zero experience with small outboard engines used as a kicker. On a planing hull, propeller pitch should be determined based on the maximum RPM range for the engine, which is fairly easy to figure out - wind the engine up to full throttle and see what the RPMs are. Once a boat is up on plane, the correlation between pitch, rpms, and speed is fairly predictable. When using a small kicker engine, boat speed is limited to the displacement speed of the hull. So, at some point in the RPM range, boat speed will become a constant and any increase in RPMs means an increase in slippage or cavitation or whatever. So how does one determine the propeller pitch for a kicker?
bsmotril	posted 08-10-2005 02:47 PM ET (US) Use either your GPS, or the hull speed formula to calculate your max displacement hull speed. Then plug those numbers into the prop calculator on this site to solve for pitch. Or, plug in the number for the flattest pitch prop available, and solve for RPM that equals your max hull speed. That would be the do not exceed rpm for the motor. since the formula depends on length measured at the wetted waterline, which is difficult, the GPS would be preferred. Here's a link with a good discussion of the math involved in the hull speed formula: http://potter-yachters.org/manyways/hullspeed/ BillS
newt	posted 08-10-2005 03:31 PM ET (US) Bill, I understand what you are saying, but something still doesn't click for me. For the purpose of my argument, lets pretend that there is no such thing as hull speed. Let's say we put an 8 hp kicker on a Montauk and we were able to get 10 mph speed and the engine was running in the proper RPM range at WOT. Then, we run this boat around most of the time at 4 mph and we all agree that the kicker is propped correctly. So even though we never run at WOT, we still know that we are propped right and the engine is not lugging. Now apply real life physics to this same boat and we find that we can't get over 5-6 mph hull speed, but the engine is still propped right and not lugging. By your method above, we could take a 15 foot hull, calculate hull speed, and work backwards to find the right prop. If we use that same engine on a 17 foot boat, then hull speed is a little faster, so working backwards gives us a taller prop. So, for the same engine, we would use a taller prop with a larger, heavier boat, which seams backwards to me.
bsmotril	posted 08-10-2005 04:33 PM ET (US) It is not illogical at all when you consider real life physics, and my suggestions were to get you in the ballpark. In reallity, there is likely only a single kicker prop available ina single pitch. At those speeds, pushing a heavy boat at displacement speed, the prop slip factor becomes a fairly large number. As a result, a single prop pitch number will accomodate a large range of hull sizes, maybe 14 - 25 feet. Regardless of the weight of those boats, the kicker will still only take them to hull speed. That size range of boats, assuming equal beam, yields a pretty narrow range of hull speeds, and the prop slip is the cushion that lets the motor work for all of them with a single prop. There's a good discussion of this at: http://www.geocities.com/Yosemite/Forest/2727/flicka_formula.html BillS
Joe Kriz	posted 08-10-2005 05:06 PM ET (US) I have a 8.5 x 9 pitch on my 8hp Evinrude... I have never experienced any problems using the motor on 3 different Whalers. 1. 1978 Montauk 2. 1997 Outrage 17 3. 1985 Outrage 18 I know Yamaha sells a High Thrust T-8 hp model. I am not sure what prop they use. By the way, the preferred side to mount the kicker is on the Starboard side. We can't always do this but it is preferred...
Teak Oil	posted 08-10-2005 10:52 PM ET (US) Plugging numbers into formulas for kicker motors is mostly a waste of time since most only have one or two different props for any given model. Usually there is a high pitch prop for running fast aluminum boats and dinghies, and there are low pitch props for use as kickers. Thats it. You have a very small selection
bsmotril	posted 08-11-2005 01:13 PM ET (US) Agreed. But newt was asking a hypothetical question to solve and that is where the prop calculator came into play. BillS
newt	posted 08-11-2005 01:30 PM ET (US) Guys, thanks for the education.