NEWPORT 17 with 75-HP: Are Low RPM A Tachometer Error

  ContinuousWave
  Whaler
  Moderated Discussion Areas
  ContinuousWave: Whaler Performance
  NEWPORT 17 with 75-HP: Are Low RPM A Tachometer Error

search | FAQ | profile | register | author help

Author Topic:   NEWPORT 17 with 75-HP: Are Low RPM A Tachometer Error

Newtauk1 posted 06-18-2010 03:14 PM ET (US)
[This article has been extensively edited and re-written--jimh.]
Right now a 1988 Newport 17 with a 2001 Mercury 75-HP two-cycle motor mounted in the top holes--the motor transom bracket is at its lowest point resting on the transom and the motor could be raised--has a maximum speed of 35.5-MPH wiith an aluminum 12-3/4 x 21 propeller. The operator weighs 250-lbs and there are 28-gals of fuel on the boat. [The boat goes] 30-MPH at 3,100-RPM and 35.5-MPH at 4,000-RPM. The Mercury 75-HP two-cycle should run up to 5,250-RPM.
I have replaced the voltage regulator, rectifier, and tachometer (which went when the voltage regulator blew). To be clear the tachometer is not moving above 4,000-RPM and is on the correct setting, "6". Should I assume the tachometer is reading wrong? Or, are there other things to look at? I used to hit 36-MPH and 4,900-RPM with this same set up. In the past I have used a stainless propeller at 4,950-RPM and 40-MPH. I prefer to use the aluminum as I fish tight to the rocks.
Sal DiMercurio posted 06-18-2010 05:41 PM ET (US)
You have the wrong size prop on that engine, no matter if it's aluminum, or s/s.
And your engine is mounted way to low.
Are you also running the boat with the engine tucked [ trimmed ] all the way in, or trimmed out.
I would think that set up would need a 17p s/s, & your engine raised up to the 3rd set of holes.
If you keep running that engine on that 21p prop, your going to be shoping for a new one very very soon.
There's no way that engine can twist a 21p prop to 5500 rpms.
Off the top of my head, I would say the factory recomended maximum rpm is 5500 for that engine.
You don't have a tach problem, you have a prop, trim, & engine height problem.
You say you have run aluminum before, but what size was it, certainly not a 21p.
Sal
Newtauk1 posted 06-18-2010 09:32 PM ET (US)
This is the same prop that got me 4900rpm and 36mph for the last 6 years. I am getting 35.5mph now but the tach is at 4000rpm ather than 4750 rpm. Last season the wiring harness came undone under the cowling and fried the voltage regulator and rectifier. I am on my second voltage regulator and rectifier in two seasons. After replacig the voltage regulator and rectifier last season my tachometer was still all over the place. I replaced with a new one this spring and it began acting up. I put on the second voltage regulator and rectifier, and the tachometer works fine, except now it won't pass 4,000-RPM. I am not sure what the problem is , but I am suspecting a bad ground some where.
The top range for the motor is 4750- to 5,250-RPM, so I was hitting the upper range. The stainless is a Rapture 13.25 x 19 which gave me about 5050 to 5100rpm and 40mph. I agree the motor should be raised.
I have been testing with a multimeter but my electrical savy is poor. Battery test 12.53 with motor off. At 1500rpm in advanced neutral it reads 15. Busbar tests at 12.53 as well. and 12.53 at the tach.
Newtauk1 posted 06-18-2010 09:34 PM ET (US)
I think I will put the stainless back on and see what the readings are.
jimh posted 06-18-2010 10:17 PM ET (US)
There is no simple way for you to assess the accuracy of the tachometer by using a voltmeter as a diagnostic test instrument. The only straightforward way to test the accuracy of your tachometer is to compare its reading to the reading you get on a known-good and known-accurate tachometer. Perhaps you can borrow a second tachometer and substitute it for your tachometer as a test.
Newtauk1 posted 06-18-2010 10:26 PM ET (US)
I am checking the plugs in morning. I realised that the plugs in the motor now I use when winterizing with heavy fogging. I forgot to replace them with the other plugs that I use under running conditions. Perhaps one plug is fouled. A compression test may be in order as well.
Is it possible to get 35mph with a bad cylinder? This is a three cylinder motor.
Newtauk1 posted 06-20-2010 06:28 AM ET (US)
New plugs and still not exceeding 4000rpm. Speed continues to be 35.3mph. I did notice a weak tell tale stream.
I plan to have water pump replacd this week.I will see if the mechanic can also raise the motor by one or two holes. It should be an easy job.
I did push some weed whacker sting down the tell tale hole as well as remove, inspect and clean up the thremostat and poppet valve. The telltale is strong again. I also put the motor on muffs at the slip and ran fresh water through. I have not had the channce to re-test the motor at higher speeds after repairing the weak tell tale.
Would a poor performing themostat restrict rpm and top speed?
newt posted 06-21-2010 09:02 AM ET (US)
It seems extremely unlikely to me that you are achieving the same top speed as you always have with the same prop, yet the rpms are down 750. Does your tach behave properly at all other times?
Sal DiMercurio posted 06-21-2010 10:03 AM ET (US)
Now that we know your useing the same props as you used before, I tend to lean towards Newts thinking.
Your getting the same speed as you were when the engine was turning 4950 rpms, but your tach is 1950 rpms off.
Did you put a Mercury tach in, or an off brand such as Teleflex ?
I don't know about Mercurys, but when useing a Teleflex on an OMC type engine, I've had nothing but problems getting them to read right.
In one of your posts, you mention your engine is rated between 4750 & 5250 rpms, yet your twisting only 4950 rpms & your saying your at the upper end of the rpm range.
4950 rpms is not the upper range of that engine, [ 300 rpms below it ] & the way the engine is set up, the only direction the rpms can go, is straight down with any added weight.
I would check that tach, raise the engine, & trim it out, & see where your rpms go.
If they don't push 5250 rpms real hard, I'd than drop down 2" of pitch in your prop size.
Sal
davej14 posted 06-21-2010 10:35 AM ET (US)
Wouldn't hurt to do a compression test either.
Tom W Clark posted 06-21-2010 10:45 AM ET (US)
Low compression will in no way affect the tachometer reading. While it wouldn't hurt to change your underwear, that won't affect the tachometer either.
If the boat travels at the same top speed with the same propeller, there is no way the RPM has changed.
Newtauk1 posted 06-21-2010 10:37 PM ET (US)
Update: Sea trialed boat with mechanic today. Hit 36mph and the same 4000rpm. Mechanic belives faulty tach and I agree, but I am concerned that there is an electrical gremlin. I still would like to know why Voltage regulator failed for the second time.The first time was due to wiring harness detaching. The new tach is the exact make and model that was originally installed, Mercury/Quicksilver.
I asked him to raise motor up one hole and replace steering cable. He called me at the end of the day advising not to raise the motor as it is even with bottom of the hull. I told him that Whaler suggests up one hole and he basically brushed it off. I guees he does not want my money. He wants to sea trial my old stainless prop which I ran for a year. I explained to him that I ran the SS prop for a year , but went back to aluminum as I was now lobstering in shallow , rocky waters and would rather ding up a aluminum prop then smash a ss prop and damage my lower unit.
We also debated the the upper rpm rage for the motor. I assuurred him it was 4750-5250 despite him insisting it was closer to 5800rpm.
I am begging this guy to take my money. I am starting to believe that these Northeast mechanics are not current on all engines due to the fact that they are not working on motors all year. I think the seasonality creates a lack of engagement by these NE mechanics.
Bottom line:
*Tach is off due to pulse being sent.
*I will raise the motor on my own and trial the existing aluminum prop.
*Mecanic to replace water pump
I feel like I am in the twilight zone on this. I belive there is something electrical happening to create the tach reading false. I guess raisiing the motor and installing a new steering cable is not profitable work for my local mechanics. I don't want to push this mechsnic to raise the motor as he clearly does not want to or I guess does not belive it should be.
Fishmore posted 06-22-2010 12:04 AM ET (US)
I have a 73 16' Montauk with the same engine (but not the same year).
You are correct on the RPM range 4750 to 5250.
I am 300 lbs 6'4" and all speeds from GPS, tested with a full tank (20 gallons minus a gallon or so to get to clean water). Also have a 7.5 hp kicker on the back with two group 24 batteries in the console.
If I can remember all my figures correctly:
Running a 13.25 x 19 pitch aluminum at 5200 RPM I get 38 mph. Running a 13.25 x 17 pitch aluminum at 5400 rpm I get 35-36 mph.
Running a 13.25 x 17 SS prop that was reworked to a 13.25 x 19 pitch at 5200 RPM I get 39 mph.
Motor is mounted up and the plate is about 1 inch above the bottom of the hull.
It might pay for you to order the Service Manual for the engine.
I had a similar performance problem when one spark plug was damaged internally. Try changing the plugs. Could also be a low output on one of your coils. The service manual has the specs for testing all of your electrical components.
Tom W Clark posted 06-22-2010 01:42 AM ET (US)
John - I think it is fairly obvious by now that you need to find a new outboard mechanic. The one you have now doesn't know what he is doing.
Chase down your electrical/tachometer problem before making any other changes. That way you will know what you are getting when you do raise the motor and buy a better prop.
Newtauk1 posted 06-22-2010 07:42 AM ET (US)
Thanks guys. I do have the sevice manual and it is well worn. I don't think I would be hitting 36mph with two adults on board if he engine had a performance problem. I do think the tach is getting a bad signal. When I ran the ss i hit 41.3mpg with the motor mounted down as it has been always. I will raise the motor this week and run with the 21' alum first then perhaps buy another alum dropping down the pitch.
jimh posted 06-22-2010 09:48 AM ET (US)
The tachometer signal on an older two-cycle motor is usually developed from the rectifier. The tachometer signal is a pulsating DC signal which is proportional to engine speed. The tachometer acts like a frequency counter. It measures the frequency of the tachometer signal and converts it into a reading that corresponds to engine speed.
The tachometer signal frequency is proportional to the engine speed and the number of poles in the battery charging winding of the stator of the outboard motor permanent magnet alternator. The number of poles varies among different engines, and this is why the tachometer typically has a calibration control. You match the tachometer calibration to the number of poles in the alternator on the motor.
The usual arrangement in the alternator is to have a full-wave rectifier. The tachometer signal can be derived from a half-wave tap in the rectifier, creating the pulsating DC signal needed.
If your motor has a rectifier and regulator that are combined into one assembly, there may be some other circuitry in the assembly that is used to condition the tachometer signal.
If the rectifier fails, the tachometer signal is also affected, and this malfunction is usually seen as a loss of tachometer readings or as erratic tachometer readings.
A very simple problem with error in tachometer reading can be due to the calibration control setting. It is common that the connections in the moving part of the calibration control become a bit oxidized or intermittent. I suggest you carefully rock the calibration control on the tachometer through its range many times, and this will help to clean the contacts. Then restore the setting to its proper calibration. Usually the calibration control has a detent for each setting.
As I mentioned before, there is no simple procedure to check the tachometer accuracy using just a voltmeter. The best way to check a tachometer is by comparison with another tachometer that is known to be good.
Determining if the problem is in the tachometer or in the electrical system of the motor can also be done by substitution. We proceed as follows:
Assume Boat A has the problem. Boat B does not have the problem.
Install the tachometer from Boat A on Boat B. Check for the problem. If the problem follows to Boat B, the tachometer from Boat A is bad. If Boat A's tachometer works fine on Boat B, the problem was likely in Boat A electrical system.
Install the tachometer from Boat B onto Boat A. Check for the problem. If the problem remains on Boat A, the tachometer was probably not the source of the problem. If Boat B's tachometer works fine on Boat A, the problem is likely the tachometer from Boat A.
Tom W Clark posted 06-22-2010 10:17 AM ET (US)
And for those of you who would like to see the Video of Jim giving that explanation, you can view it here:
http://www.youtube.com/watch?v=oIS5n9Oyzsc
Newtauk1 posted 06-22-2010 10:18 AM ET (US)
Thanks Jim. I have read several of your posts on this topic and it has helped narrowing down the problem. The boat will have to wait another day.
I am off to Portland Maine today and will work on the boat Weds.
A Portland trip for me always includes Hamilton Marine and J's Oyster.
www.jsoyster.com
www.hamiltonmarine.com
Sal DiMercurio posted 06-22-2010 08:25 PM ET (US)
Good one Tom.
Sal

Hop to:

Contact Us | RETURN to ContinuousWave Top Page

Powered by: Ultimate Bulletin Board, Freeware Version 2000
Purchase our Licensed Version- which adds many more features!
© Infopop Corporation (formerly Madrona Park, Inc.), 1998 - 2000.

Author	Topic: NEWPORT 17 with 75-HP: Are Low RPM A Tachometer Error
Newtauk1	posted 06-18-2010 03:14 PM ET (US) [This article has been extensively edited and re-written--jimh.] Right now a 1988 Newport 17 with a 2001 Mercury 75-HP two-cycle motor mounted in the top holes--the motor transom bracket is at its lowest point resting on the transom and the motor could be raised--has a maximum speed of 35.5-MPH wiith an aluminum 12-3/4 x 21 propeller. The operator weighs 250-lbs and there are 28-gals of fuel on the boat. [The boat goes] 30-MPH at 3,100-RPM and 35.5-MPH at 4,000-RPM. The Mercury 75-HP two-cycle should run up to 5,250-RPM. I have replaced the voltage regulator, rectifier, and tachometer (which went when the voltage regulator blew). To be clear the tachometer is not moving above 4,000-RPM and is on the correct setting, "6". Should I assume the tachometer is reading wrong? Or, are there other things to look at? I used to hit 36-MPH and 4,900-RPM with this same set up. In the past I have used a stainless propeller at 4,950-RPM and 40-MPH. I prefer to use the aluminum as I fish tight to the rocks.
Sal DiMercurio	posted 06-18-2010 05:41 PM ET (US) You have the wrong size prop on that engine, no matter if it's aluminum, or s/s. And your engine is mounted way to low. Are you also running the boat with the engine tucked [ trimmed ] all the way in, or trimmed out. I would think that set up would need a 17p s/s, & your engine raised up to the 3rd set of holes. If you keep running that engine on that 21p prop, your going to be shoping for a new one very very soon. There's no way that engine can twist a 21p prop to 5500 rpms. Off the top of my head, I would say the factory recomended maximum rpm is 5500 for that engine. You don't have a tach problem, you have a prop, trim, & engine height problem. You say you have run aluminum before, but what size was it, certainly not a 21p. Sal
Newtauk1	posted 06-18-2010 09:32 PM ET (US) This is the same prop that got me 4900rpm and 36mph for the last 6 years. I am getting 35.5mph now but the tach is at 4000rpm ather than 4750 rpm. Last season the wiring harness came undone under the cowling and fried the voltage regulator and rectifier. I am on my second voltage regulator and rectifier in two seasons. After replacig the voltage regulator and rectifier last season my tachometer was still all over the place. I replaced with a new one this spring and it began acting up. I put on the second voltage regulator and rectifier, and the tachometer works fine, except now it won't pass 4,000-RPM. I am not sure what the problem is , but I am suspecting a bad ground some where. The top range for the motor is 4750- to 5,250-RPM, so I was hitting the upper range. The stainless is a Rapture 13.25 x 19 which gave me about 5050 to 5100rpm and 40mph. I agree the motor should be raised. I have been testing with a multimeter but my electrical savy is poor. Battery test 12.53 with motor off. At 1500rpm in advanced neutral it reads 15. Busbar tests at 12.53 as well. and 12.53 at the tach.
Newtauk1	posted 06-18-2010 09:34 PM ET (US) I think I will put the stainless back on and see what the readings are.
jimh	posted 06-18-2010 10:17 PM ET (US) There is no simple way for you to assess the accuracy of the tachometer by using a voltmeter as a diagnostic test instrument. The only straightforward way to test the accuracy of your tachometer is to compare its reading to the reading you get on a known-good and known-accurate tachometer. Perhaps you can borrow a second tachometer and substitute it for your tachometer as a test.
Newtauk1	posted 06-18-2010 10:26 PM ET (US) I am checking the plugs in morning. I realised that the plugs in the motor now I use when winterizing with heavy fogging. I forgot to replace them with the other plugs that I use under running conditions. Perhaps one plug is fouled. A compression test may be in order as well. Is it possible to get 35mph with a bad cylinder? This is a three cylinder motor.
Newtauk1	posted 06-20-2010 06:28 AM ET (US) New plugs and still not exceeding 4000rpm. Speed continues to be 35.3mph. I did notice a weak tell tale stream. I plan to have water pump replacd this week.I will see if the mechanic can also raise the motor by one or two holes. It should be an easy job. I did push some weed whacker sting down the tell tale hole as well as remove, inspect and clean up the thremostat and poppet valve. The telltale is strong again. I also put the motor on muffs at the slip and ran fresh water through. I have not had the channce to re-test the motor at higher speeds after repairing the weak tell tale. Would a poor performing themostat restrict rpm and top speed?
newt	posted 06-21-2010 09:02 AM ET (US) It seems extremely unlikely to me that you are achieving the same top speed as you always have with the same prop, yet the rpms are down 750. Does your tach behave properly at all other times?
Sal DiMercurio	posted 06-21-2010 10:03 AM ET (US) Now that we know your useing the same props as you used before, I tend to lean towards Newts thinking. Your getting the same speed as you were when the engine was turning 4950 rpms, but your tach is 1950 rpms off. Did you put a Mercury tach in, or an off brand such as Teleflex ? I don't know about Mercurys, but when useing a Teleflex on an OMC type engine, I've had nothing but problems getting them to read right. In one of your posts, you mention your engine is rated between 4750 & 5250 rpms, yet your twisting only 4950 rpms & your saying your at the upper end of the rpm range. 4950 rpms is not the upper range of that engine, [ 300 rpms below it ] & the way the engine is set up, the only direction the rpms can go, is straight down with any added weight. I would check that tach, raise the engine, & trim it out, & see where your rpms go. If they don't push 5250 rpms real hard, I'd than drop down 2" of pitch in your prop size. Sal
davej14	posted 06-21-2010 10:35 AM ET (US) Wouldn't hurt to do a compression test either.
Tom W Clark	posted 06-21-2010 10:45 AM ET (US) Low compression will in no way affect the tachometer reading. While it wouldn't hurt to change your underwear, that won't affect the tachometer either. If the boat travels at the same top speed with the same propeller, there is no way the RPM has changed.
Newtauk1	posted 06-21-2010 10:37 PM ET (US) Update: Sea trialed boat with mechanic today. Hit 36mph and the same 4000rpm. Mechanic belives faulty tach and I agree, but I am concerned that there is an electrical gremlin. I still would like to know why Voltage regulator failed for the second time.The first time was due to wiring harness detaching. The new tach is the exact make and model that was originally installed, Mercury/Quicksilver. I asked him to raise motor up one hole and replace steering cable. He called me at the end of the day advising not to raise the motor as it is even with bottom of the hull. I told him that Whaler suggests up one hole and he basically brushed it off. I guees he does not want my money. He wants to sea trial my old stainless prop which I ran for a year. I explained to him that I ran the SS prop for a year , but went back to aluminum as I was now lobstering in shallow , rocky waters and would rather ding up a aluminum prop then smash a ss prop and damage my lower unit. We also debated the the upper rpm rage for the motor. I assuurred him it was 4750-5250 despite him insisting it was closer to 5800rpm. I am begging this guy to take my money. I am starting to believe that these Northeast mechanics are not current on all engines due to the fact that they are not working on motors all year. I think the seasonality creates a lack of engagement by these NE mechanics. Bottom line: Tach is off due to pulse being sent. I will raise the motor on my own and trial the existing aluminum prop. *Mecanic to replace water pump I feel like I am in the twilight zone on this. I belive there is something electrical happening to create the tach reading false. I guess raisiing the motor and installing a new steering cable is not profitable work for my local mechanics. I don't want to push this mechsnic to raise the motor as he clearly does not want to or I guess does not belive it should be.
Fishmore	posted 06-22-2010 12:04 AM ET (US) I have a 73 16' Montauk with the same engine (but not the same year). You are correct on the RPM range 4750 to 5250. I am 300 lbs 6'4" and all speeds from GPS, tested with a full tank (20 gallons minus a gallon or so to get to clean water). Also have a 7.5 hp kicker on the back with two group 24 batteries in the console. If I can remember all my figures correctly: Running a 13.25 x 19 pitch aluminum at 5200 RPM I get 38 mph. Running a 13.25 x 17 pitch aluminum at 5400 rpm I get 35-36 mph. Running a 13.25 x 17 SS prop that was reworked to a 13.25 x 19 pitch at 5200 RPM I get 39 mph. Motor is mounted up and the plate is about 1 inch above the bottom of the hull. It might pay for you to order the Service Manual for the engine. I had a similar performance problem when one spark plug was damaged internally. Try changing the plugs. Could also be a low output on one of your coils. The service manual has the specs for testing all of your electrical components.
Tom W Clark	posted 06-22-2010 01:42 AM ET (US) John - I think it is fairly obvious by now that you need to find a new outboard mechanic. The one you have now doesn't know what he is doing. Chase down your electrical/tachometer problem before making any other changes. That way you will know what you are getting when you do raise the motor and buy a better prop.
Newtauk1	posted 06-22-2010 07:42 AM ET (US) Thanks guys. I do have the sevice manual and it is well worn. I don't think I would be hitting 36mph with two adults on board if he engine had a performance problem. I do think the tach is getting a bad signal. When I ran the ss i hit 41.3mpg with the motor mounted down as it has been always. I will raise the motor this week and run with the 21' alum first then perhaps buy another alum dropping down the pitch.
jimh	posted 06-22-2010 09:48 AM ET (US) The tachometer signal on an older two-cycle motor is usually developed from the rectifier. The tachometer signal is a pulsating DC signal which is proportional to engine speed. The tachometer acts like a frequency counter. It measures the frequency of the tachometer signal and converts it into a reading that corresponds to engine speed. The tachometer signal frequency is proportional to the engine speed and the number of poles in the battery charging winding of the stator of the outboard motor permanent magnet alternator. The number of poles varies among different engines, and this is why the tachometer typically has a calibration control. You match the tachometer calibration to the number of poles in the alternator on the motor. The usual arrangement in the alternator is to have a full-wave rectifier. The tachometer signal can be derived from a half-wave tap in the rectifier, creating the pulsating DC signal needed. If your motor has a rectifier and regulator that are combined into one assembly, there may be some other circuitry in the assembly that is used to condition the tachometer signal. If the rectifier fails, the tachometer signal is also affected, and this malfunction is usually seen as a loss of tachometer readings or as erratic tachometer readings. A very simple problem with error in tachometer reading can be due to the calibration control setting. It is common that the connections in the moving part of the calibration control become a bit oxidized or intermittent. I suggest you carefully rock the calibration control on the tachometer through its range many times, and this will help to clean the contacts. Then restore the setting to its proper calibration. Usually the calibration control has a detent for each setting. As I mentioned before, there is no simple procedure to check the tachometer accuracy using just a voltmeter. The best way to check a tachometer is by comparison with another tachometer that is known to be good. Determining if the problem is in the tachometer or in the electrical system of the motor can also be done by substitution. We proceed as follows: Assume Boat A has the problem. Boat B does not have the problem. Install the tachometer from Boat A on Boat B. Check for the problem. If the problem follows to Boat B, the tachometer from Boat A is bad. If Boat A's tachometer works fine on Boat B, the problem was likely in Boat A electrical system. Install the tachometer from Boat B onto Boat A. Check for the problem. If the problem remains on Boat A, the tachometer was probably not the source of the problem. If Boat B's tachometer works fine on Boat A, the problem is likely the tachometer from Boat A.
Tom W Clark	posted 06-22-2010 10:17 AM ET (US) And for those of you who would like to see the Video of Jim giving that explanation, you can view it here: http://www.youtube.com/watch?v=oIS5n9Oyzsc
Newtauk1	posted 06-22-2010 10:18 AM ET (US) Thanks Jim. I have read several of your posts on this topic and it has helped narrowing down the problem. The boat will have to wait another day. I am off to Portland Maine today and will work on the boat Weds. A Portland trip for me always includes Hamilton Marine and J's Oyster. www.jsoyster.com www.hamiltonmarine.com
Sal DiMercurio	posted 06-22-2010 08:25 PM ET (US) Good one Tom. Sal