Determining Proper Propeller Without Tachometer

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Author Topic:   Determining Proper Propeller Without Tachometer

Rodeath posted 08-13-2008 06:15 PM ET (US)
I just put a [1996 Mercury] 40-HP on my [1965] Boston Whaler [13-foot boat]. The mechanic suggested that I may have the wrong prop on the motor. The only way to check this, he claims is to buy a tachometer (which he will sell me for over $200), take it out and report the [engine speed] at full throttle. Is [measurement of the engine speed at wide open throttle] the only way to find the proper propeller? Or is there a standard size that is appropriate for this motor-boat combination?.
Thanks, Dave.
jimh posted 08-13-2008 08:55 PM ET (US)
Measurement of engine speed at wide open throttle is considered the best indicator of proper propeller fitting.
towboater posted 08-14-2008 01:42 AM ET (US)
One way [to determine if the propeller is properly fitted to the boat and motor] without a tachometer is to listen for engine [speed] to drop substantially when you put the engine in forward, idling. If the engine loses quite a bit of [engine speed] and lugs down, you may have too much wheel. If there is very little change in [engine speed], you might not have enough [propeller].
Dia = power
Pitch = speed
Another way [to determine if the propeller is properly fitted to the boat and motor] without tachometer is to seek advice or ballpark the above formula and try another wheel to compare. Good luck.
mk
Fishmore posted 08-14-2008 03:37 AM ET (US)
Best way [to determine if the propeller is properly fitted to the boat and motor] is with a tachometer, but I just bought a new-in-the-box Mercury tachometer for $70 from my dealer. It is part #79-895283A05. It works with all Mercury engines. [To pay] $200 is way too much.
Another thing you can do is search on this website to see if others have your same setup and ask them what prop they are using.
jimh posted 08-14-2008 09:39 AM ET (US)
I have never heard of the method proposed by towboater for determining the proper fitting of a propeller by audible judging of the change in engine speed then the transmission is moved from neutral to forward at idle speed. I cannot imagine that this method would have any accuracy at predicting the resultant wide open throttle engine speed with any sort of resolution. Could we learn more about how to make this aural measurement. For example, how much engine speed change at wide open throttle will be in predicted by an engine speed change at idle? And how does one learn to make these aural judgments? I suppose that if the propeller where grossly too large that at idle a significant engine speed change might occur, but I can't see any finer resolution would be possible.
Also, sonic memory is very short, perhaps no more than a few seconds. Testing has shown that most people can not make any accurate comparisons of sounds unless they are able to compare them within a few seconds of hearing both. The least possible time that a propeller could be changed would be several minutes. I cannot imagine that anyone, even a person who had trained their ear to be acute for speed change, could remember the previous pitch change after several minutes and compare it to the new pitch change from a different propeller.
Tohsgib posted 08-14-2008 11:51 AM ET (US)
Towboater has merit in what he says but it is not accurate. For instance my 13 w/40 Evinrude drops about 400rpm when dropped into gear because I am swinging a 17" SS prop. Yes it is a big wheel but with the jackplate I can still get 100 over redline. My 15 w/70 would idle at 1200 and then drop down to 750+ in gear when swinging a 20" SS prop but I was just at 6k redline.
towboater posted 08-14-2008 12:00 PM ET (US)
Hello Jim, how's your summer goin?
"listen for engine [speed] to drop substantially"
note...SUBSTANTIAL.
This info will be more relevant to a prop expert or pro than one who isnt. I am neither. Yet, I have been asked this question by pro's on several occasions regarding several different engines even tho they had tachs.
gtg, rare time off this weekend, fish are waiting.
mk
Tom W Clark posted 08-14-2008 12:15 PM ET (US)
Just speculation, but propping a tug boat and propping an outboard powered Boston Whaler may best be done using very different techniques.
towboater posted 08-14-2008 12:30 PM ET (US)
yeah, you are both probably right.
I just refuse to accept the fact you cant "prop" any engine/boat WITHOUT a tach. I am not debating using a tach is not cost effective.
Tom, salmon are still laying outside the mouth of the Columbia. The ocean quota is nearly filled and the weather forcasts and tides are great for this weekend.
MKJ Marine is listed in the Kelso yellow pages.
Moorage N-19, Ilwaco.
I'll be down there til Tues.
mk
jimh posted 08-14-2008 01:00 PM ET (US)
Nick--no fair giving us tachometer readings when you shift to FWD from NEUTRAL. You have to make the judgment by ear, not by tachometer.
When an engine runs at 1,200-RPM, that is a pitch of 1200/60 or 20-Hz. When it slows to 800-RPM, the pitch drops to 13.33-Hz. These extreme low frequencies are in the range normally noted as sub-audible, that is, below the range of human hearing perception.
I just don't see a lot of application for this method in fitting propellers to Boston Whaler boats. I can't hear the frequencies involved.
Tohsgib posted 08-14-2008 01:02 PM ET (US)
Actually years ago not many boats had tachs because the dealers knew how to set them up. If you bought a montauk from an experienced dealer with either a 70 or a 90, they KNEW what prop it took and what was optimal. The problem is we now have these boats 30 years later and are dropping on modern engines...how can one tell without a prop or info from an owner with similar setup?
To answer your question Dave...what prop do you have now and why does your mech think it is wrong?
Tohsgib posted 08-14-2008 01:06 PM ET (US)
Jim I know but when I put this 70 on my 15 I told my mech that it was idling to high. He said what is it in gear and I said perfect. He then said if I had a 17 alum prop it would idle where it should but since I am slinging a large SS it has to have enough idle to move the boat when engaged into gear or it will lug or stall. Ever notice when a gofast is near and comes idling in, then clicks into neutral and it sounds like it is screaming? It is when you have a 27" prop on there, probably like 1500+ in nuetral. Has to reak havoc on the clutch dogs.
cooper1958nc posted 08-14-2008 01:51 PM ET (US)
?When an engine runs at 1,200-RPM, that is a pitch of 1200/60 or 20-Hz. When it slows to 800-RPM, the pitch drops to 13.33-Hz. These extreme low frequencies are in the range normally noted as sub-audible, that is, below the range of human hearing perception."
Well not exactly.
Sound is created by moving pieces and by exhaust pulses. Exhaust pulses for a 4 cyl 2 stroke at 1200 RPM are at a primary frequency of 80 Hz, well within hearing range. Moving pieces create a host of harmonic frequencies that I could not begin to estimate. Exhaust pulses also appear to have multiple harmonics embedded, probably because of reflections and wave superpositions within the exhaust system.
fishgutz posted 08-14-2008 02:53 PM ET (US)
Tohsgib,
You are sooo right. A good dealer/mechanic should pretty much know which prop to use within and inch or two, especially if they sell and set up lots of small boats.
My dealer up north could tell me which prop to use for the kind of performance I wanted. In fact they were right on the money.
jimh posted 08-14-2008 05:13 PM ET (US)
Coop'--no doubt there are overtones from the engine, and variations due to the number of cylinders and the combustion cycle. I will leave it for you to perfect the acoustic method of propeller selection. For me, I'll go with good advice based on experience or a tachometer.
jimh posted 08-14-2008 07:42 PM ET (US)
By the way, with gasoline at $5/gallon, it won't take long to burn up $70 worth of gasoline with a badly fitted propeller. It would be better to invest the $70 in a tachometer. Besides helping to save gasoline, a tachometer is really an essential instrument for running an outboard motor. I cannot imagine not having one on a motor--anything bigger than a dingy ought to have a tachometer.
cooper1958nc posted 08-14-2008 10:31 PM ET (US)
O cummon.
A 40 hp outboard needs a tach?

Run at 25 mph. There won't be a dimes worth of difference in fuel consumption no matter what prop you use.
I'll go into the details later.
seahorse posted 08-15-2008 08:08 AM ET (US)
Ask your mechanic how much to rent a shop tach for an evening.
If he is not agreeable to that, ask how much to rent him and his tach for a boat ride?
jimh posted 08-15-2008 08:30 AM ET (US)
With this new audible method, can boat speed be determined by ear? It would save the fuss and bother of getting a GPS receiver.
sloppyrudder posted 08-15-2008 08:42 AM ET (US)
Have you been here yet? http://sites.mercurymarine.com/portal/page?_pageid=126,53050,126_53052&_dad=portal&_schema=PORTAL
Tohsgib posted 08-15-2008 11:47 AM ET (US)
Yes Coop a 40hp needs a tach, why should'nt it? I have one on my 40hp and even have a tach for my 20 but I doubt I will hook it up since it is a pull start and not sure how or if possible. Tachs would not be that necessary if engine manufacturers would use a rev limiter that is at REDLINE, not 200-400 above it. This way you could run WFO until it hit the limiter then go up in pitch until it did'nt.
AZdave posted 08-15-2008 02:55 PM ET (US)
In terms of cost, Harbor Freight has an in-dash tach for as little as $20. They also have a model that counts reflections from a moving part. That's about $50, and has the advantage of needing no electrical connection to the outboard.
Tohsgib posted 08-15-2008 03:05 PM ET (US)
I have used "Tiny-Tachs" on moorcycles but I think most are for 1 cyl engines and connects to the sparkplug wire. Not sure if it would work on multicyl engines but worth looking into for about $25.
cooper1958nc posted 08-15-2008 11:26 PM ET (US)
Nah.
If you go down in pitch, either because you want to tow skiers or pull houseboats around, the engine will run at light load at high RPM. This sounds terrible and you won't run it there much. If you do, horsepower falls off so fast about about 5500 that most 2 strokes will just stop revving eventually without damage.
If you go up in pitch, because you want quiet, economical midrange cruising, then you won't hit much over 3800 WOT, and who cares? You will cruise fine.
A tach for a 2 stroke 40? Millions of old motors ran for decades with no such thing.
frisco pete posted 08-16-2008 11:06 PM ET (US)
After reading this thread, I`ll go with jimh all the way. The only reliable method is with a tach and a GPS. If your too cheap to buy a tach, your probably too cheap to buy a GPS or even a speedo, so your in the dark all the way. Mind as well run whatever prop is on the outboard, and enjoy yourself. Ignorance is bliss, so they say.
rich
jimh posted 08-17-2008 10:34 PM ET (US)
Rich--Maybe you and I can enroll at training classes in the auditory school of propeller selection, and if we do well in our studies, we can ween ourselves off of this silly reliance on tachometers to tell us engine speed and GPS receivers to tell us boat speed.
kwik_wurk posted 08-18-2008 12:55 PM ET (US)
Or you could use a hand-held tach (laser or strobe) on the flywheel. You can do a couple tests with the cowling off with no wiring and stuff. (You can check the timing at the same time too.)
And on a side note: As cooper1958nc stated, in most cases the noise you hear from the engine is either engine firing rate (which depends on cylinders and cycle), or another harmonic (multiple) of the the shaft speed.

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Author	Topic: Determining Proper Propeller Without Tachometer
Rodeath	posted 08-13-2008 06:15 PM ET (US) I just put a [1996 Mercury] 40-HP on my [1965] Boston Whaler [13-foot boat]. The mechanic suggested that I may have the wrong prop on the motor. The only way to check this, he claims is to buy a tachometer (which he will sell me for over $200), take it out and report the [engine speed] at full throttle. Is [measurement of the engine speed at wide open throttle] the only way to find the proper propeller? Or is there a standard size that is appropriate for this motor-boat combination?. Thanks, Dave.
jimh	posted 08-13-2008 08:55 PM ET (US) Measurement of engine speed at wide open throttle is considered the best indicator of proper propeller fitting.
towboater	posted 08-14-2008 01:42 AM ET (US) One way [to determine if the propeller is properly fitted to the boat and motor] without a tachometer is to listen for engine [speed] to drop substantially when you put the engine in forward, idling. If the engine loses quite a bit of [engine speed] and lugs down, you may have too much wheel. If there is very little change in [engine speed], you might not have enough [propeller]. Dia = power Pitch = speed Another way [to determine if the propeller is properly fitted to the boat and motor] without tachometer is to seek advice or ballpark the above formula and try another wheel to compare. Good luck. mk
Fishmore	posted 08-14-2008 03:37 AM ET (US) Best way [to determine if the propeller is properly fitted to the boat and motor] is with a tachometer, but I just bought a new-in-the-box Mercury tachometer for $70 from my dealer. It is part #79-895283A05. It works with all Mercury engines. [To pay] $200 is way too much. Another thing you can do is search on this website to see if others have your same setup and ask them what prop they are using.
jimh	posted 08-14-2008 09:39 AM ET (US) I have never heard of the method proposed by towboater for determining the proper fitting of a propeller by audible judging of the change in engine speed then the transmission is moved from neutral to forward at idle speed. I cannot imagine that this method would have any accuracy at predicting the resultant wide open throttle engine speed with any sort of resolution. Could we learn more about how to make this aural measurement. For example, how much engine speed change at wide open throttle will be in predicted by an engine speed change at idle? And how does one learn to make these aural judgments? I suppose that if the propeller where grossly too large that at idle a significant engine speed change might occur, but I can't see any finer resolution would be possible. Also, sonic memory is very short, perhaps no more than a few seconds. Testing has shown that most people can not make any accurate comparisons of sounds unless they are able to compare them within a few seconds of hearing both. The least possible time that a propeller could be changed would be several minutes. I cannot imagine that anyone, even a person who had trained their ear to be acute for speed change, could remember the previous pitch change after several minutes and compare it to the new pitch change from a different propeller.
Tohsgib	posted 08-14-2008 11:51 AM ET (US) Towboater has merit in what he says but it is not accurate. For instance my 13 w/40 Evinrude drops about 400rpm when dropped into gear because I am swinging a 17" SS prop. Yes it is a big wheel but with the jackplate I can still get 100 over redline. My 15 w/70 would idle at 1200 and then drop down to 750+ in gear when swinging a 20" SS prop but I was just at 6k redline.
towboater	posted 08-14-2008 12:00 PM ET (US) Hello Jim, how's your summer goin? "listen for engine [speed] to drop substantially" note...SUBSTANTIAL. This info will be more relevant to a prop expert or pro than one who isnt. I am neither. Yet, I have been asked this question by pro's on several occasions regarding several different engines even tho they had tachs. gtg, rare time off this weekend, fish are waiting. mk
Tom W Clark	posted 08-14-2008 12:15 PM ET (US) Just speculation, but propping a tug boat and propping an outboard powered Boston Whaler may best be done using very different techniques.
towboater	posted 08-14-2008 12:30 PM ET (US) yeah, you are both probably right. I just refuse to accept the fact you cant "prop" any engine/boat WITHOUT a tach. I am not debating using a tach is not cost effective. Tom, salmon are still laying outside the mouth of the Columbia. The ocean quota is nearly filled and the weather forcasts and tides are great for this weekend. MKJ Marine is listed in the Kelso yellow pages. Moorage N-19, Ilwaco. I'll be down there til Tues. mk
jimh	posted 08-14-2008 01:00 PM ET (US) Nick--no fair giving us tachometer readings when you shift to FWD from NEUTRAL. You have to make the judgment by ear, not by tachometer. When an engine runs at 1,200-RPM, that is a pitch of 1200/60 or 20-Hz. When it slows to 800-RPM, the pitch drops to 13.33-Hz. These extreme low frequencies are in the range normally noted as sub-audible, that is, below the range of human hearing perception. I just don't see a lot of application for this method in fitting propellers to Boston Whaler boats. I can't hear the frequencies involved.
Tohsgib	posted 08-14-2008 01:02 PM ET (US) Actually years ago not many boats had tachs because the dealers knew how to set them up. If you bought a montauk from an experienced dealer with either a 70 or a 90, they KNEW what prop it took and what was optimal. The problem is we now have these boats 30 years later and are dropping on modern engines...how can one tell without a prop or info from an owner with similar setup? To answer your question Dave...what prop do you have now and why does your mech think it is wrong?
Tohsgib	posted 08-14-2008 01:06 PM ET (US) Jim I know but when I put this 70 on my 15 I told my mech that it was idling to high. He said what is it in gear and I said perfect. He then said if I had a 17 alum prop it would idle where it should but since I am slinging a large SS it has to have enough idle to move the boat when engaged into gear or it will lug or stall. Ever notice when a gofast is near and comes idling in, then clicks into neutral and it sounds like it is screaming? It is when you have a 27" prop on there, probably like 1500+ in nuetral. Has to reak havoc on the clutch dogs.
cooper1958nc	posted 08-14-2008 01:51 PM ET (US) ?When an engine runs at 1,200-RPM, that is a pitch of 1200/60 or 20-Hz. When it slows to 800-RPM, the pitch drops to 13.33-Hz. These extreme low frequencies are in the range normally noted as sub-audible, that is, below the range of human hearing perception." Well not exactly. Sound is created by moving pieces and by exhaust pulses. Exhaust pulses for a 4 cyl 2 stroke at 1200 RPM are at a primary frequency of 80 Hz, well within hearing range. Moving pieces create a host of harmonic frequencies that I could not begin to estimate. Exhaust pulses also appear to have multiple harmonics embedded, probably because of reflections and wave superpositions within the exhaust system.
fishgutz	posted 08-14-2008 02:53 PM ET (US) Tohsgib, You are sooo right. A good dealer/mechanic should pretty much know which prop to use within and inch or two, especially if they sell and set up lots of small boats. My dealer up north could tell me which prop to use for the kind of performance I wanted. In fact they were right on the money.
jimh	posted 08-14-2008 05:13 PM ET (US) Coop'--no doubt there are overtones from the engine, and variations due to the number of cylinders and the combustion cycle. I will leave it for you to perfect the acoustic method of propeller selection. For me, I'll go with good advice based on experience or a tachometer.
jimh	posted 08-14-2008 07:42 PM ET (US) By the way, with gasoline at $5/gallon, it won't take long to burn up $70 worth of gasoline with a badly fitted propeller. It would be better to invest the $70 in a tachometer. Besides helping to save gasoline, a tachometer is really an essential instrument for running an outboard motor. I cannot imagine not having one on a motor--anything bigger than a dingy ought to have a tachometer.
cooper1958nc	posted 08-14-2008 10:31 PM ET (US) O cummon. A 40 hp outboard needs a tach? Run at 25 mph. There won't be a dimes worth of difference in fuel consumption no matter what prop you use. I'll go into the details later.
seahorse	posted 08-15-2008 08:08 AM ET (US) Ask your mechanic how much to rent a shop tach for an evening. If he is not agreeable to that, ask how much to rent him and his tach for a boat ride?
jimh	posted 08-15-2008 08:30 AM ET (US) With this new audible method, can boat speed be determined by ear? It would save the fuss and bother of getting a GPS receiver.
sloppyrudder	posted 08-15-2008 08:42 AM ET (US) Have you been here yet? http://sites.mercurymarine.com/portal/page?_pageid=126,53050,126_53052&_dad=portal&_schema=PORTAL
Tohsgib	posted 08-15-2008 11:47 AM ET (US) Yes Coop a 40hp needs a tach, why should'nt it? I have one on my 40hp and even have a tach for my 20 but I doubt I will hook it up since it is a pull start and not sure how or if possible. Tachs would not be that necessary if engine manufacturers would use a rev limiter that is at REDLINE, not 200-400 above it. This way you could run WFO until it hit the limiter then go up in pitch until it did'nt.
AZdave	posted 08-15-2008 02:55 PM ET (US) In terms of cost, Harbor Freight has an in-dash tach for as little as $20. They also have a model that counts reflections from a moving part. That's about $50, and has the advantage of needing no electrical connection to the outboard.
Tohsgib	posted 08-15-2008 03:05 PM ET (US) I have used "Tiny-Tachs" on moorcycles but I think most are for 1 cyl engines and connects to the sparkplug wire. Not sure if it would work on multicyl engines but worth looking into for about $25.
cooper1958nc	posted 08-15-2008 11:26 PM ET (US) Nah. If you go down in pitch, either because you want to tow skiers or pull houseboats around, the engine will run at light load at high RPM. This sounds terrible and you won't run it there much. If you do, horsepower falls off so fast about about 5500 that most 2 strokes will just stop revving eventually without damage. If you go up in pitch, because you want quiet, economical midrange cruising, then you won't hit much over 3800 WOT, and who cares? You will cruise fine. A tach for a 2 stroke 40? Millions of old motors ran for decades with no such thing.
frisco pete	posted 08-16-2008 11:06 PM ET (US) After reading this thread, I`ll go with jimh all the way. The only reliable method is with a tach and a GPS. If your too cheap to buy a tach, your probably too cheap to buy a GPS or even a speedo, so your in the dark all the way. Mind as well run whatever prop is on the outboard, and enjoy yourself. Ignorance is bliss, so they say. rich
jimh	posted 08-17-2008 10:34 PM ET (US) Rich--Maybe you and I can enroll at training classes in the auditory school of propeller selection, and if we do well in our studies, we can ween ourselves off of this silly reliance on tachometers to tell us engine speed and GPS receivers to tell us boat speed.
kwik_wurk	posted 08-18-2008 12:55 PM ET (US) Or you could use a hand-held tach (laser or strobe) on the flywheel. You can do a couple tests with the cowling off with no wiring and stuff. (You can check the timing at the same time too.) And on a side note: As cooper1958nc stated, in most cases the noise you hear from the engine is either engine firing rate (which depends on cylinders and cycle), or another harmonic (multiple) of the the shaft speed.