Ford Lehman tachometer vs prop speed

[TJM]

Finally had the time to perform a comparison of my dashboard tachometer gages vs the actual shaft RPM to the props.
My engines were idling at 500 RPM's by the tach's but my shafts were spinning at 250 when measured with my handheld meter with a reflective pick up on the shaft.

Is this a function of the cam shaft vs the crank shaft or maybe a 2:1 transmission reduction ? I do not know what my tranny's are rated at.

 

[mike66]

Transmission. You can measure engine off the crankshaft pully. Should be 500.

 

[SoWhat]

Measure your engine rpm at the bottom pulley in the front of the engine.
Transmission ratio of 2:1 would be common.

 

[High Wire]

Is this a function of the cam shaft vs the crank shaft or maybe a 2:1 transmission reduction ? I do not know what my tranny's are rated at.

The trans ratio is normally stamped onto a tag on the tranny. If not the Model number may tell the story.
Post a photo of the ID tag.

 

[catalinajack]

Check the manual. My Lehman120 manual suggests an idle speed of up to 700 RPM.

Transmission. You can measure engine off the crankshaft pully. Should be 500.

 

[rgano]

That's too low for a Lehman 120 to idle. 600 would be better.

 

[mike66]

Sorry, wasn't offering a specific idle rpm, just showing an example of gear reduction with the transmission. OP should definitely set the engine rpm measured at the crank pully to proper spec, then adjust the tachometer to match. Shaft speed will about half that.

 

[koliver]

BW Velvet Drive transmissions have many different reductions.
Mine are 1:1.9 and 1: 2.0
I had a "synchroniser" that shot a magnet clamped to the shaft on each, so I could get so-called "Sychronised" when the shaft speeds were the same, but off by the diff between 1.9 and 2.0 at the engines.
Using the harmonics that I can feel seems like a better way to synchronise, so I no longer use the shaft mounted one.
I also checked my tachs against a digital tach shooting the damper on the front of the crank on each engine. Big difference, so I tweaked the tachs. Over the years since I did this, the diff betwee the wheelhouse tachs and the Fly bridge tachs has increased, from zero, when first calibrated, to about 20%. Aren't they reading the same electrical pulses? Seems tachs are at best, just a guess.

 

[catalinajack]

Yes, many tachs, especially the old Motorola and Stewart's Warner's that are adjusted with the internal potentiometer are notorious for being inaccurate or becoming inaccurate. However, modern programmable tachs driven by a signal generator or magnetic pickup can be adjusted to near precision. I have even had success using the alternator feed to adjust my programmable tachs to near precision but, over time, as belts stretch accuracy lessens but probably not significantly. I have since changed to using a signal generator and do not have to adjust at all. The Aetna Engineering digital readout tachs require no programming are are quite accurate but are also not inexpensive.

BW Velvet Drive transmissions have many different reductions.
Mine are 1:1.9 and 1: 2.0
I had a "synchroniser" that shot a magnet clamped to the shaft on each, so I could get so-called "Sychronised" when the shaft speeds were the same, but off by the diff between 1.9 and 2.0 at the engines.
Using the harmonics that I can feel seems like a better way to synchronise, so I no longer use the shaft mounted one.
I also checked my tachs against a digital tach shooting the damper on the front of the crank on each engine. Big difference, so I tweaked the tachs. Over the years since I did this, the diff betwee the wheelhouse tachs and the Fly bridge tachs has increased, from zero, when first calibrated, to about 20%. Aren't they reading the same electrical pulses? Seems tachs are at best, just a guess.

 

[TJM]

Here is an update with a very happy ending.
Based on my new knowledge from TF. I placed a new hand tach sensor on the crank pulley up front of both engines. Short version is my engine synchronizer is way off by 700 RPM's between engines.
Underway this weekend, I disengaged my engine sync and manually adjusted the throttles to matched RPM's by the hand tach. Hand Tach was 1800 RPM's, shafts were both 900 RPMs and speed jumped up to 10 kts ! I then recalibrated my dashboard gages to match the 1800 RPM's. This week I will be adjusting the engine synchronizer.
Thanks for all of the input.

 

[SteveK]

BW Velvet Drive transmissions have many different reductions.
Mine are 1:1.9 and 1: 2.0
I had a "synchroniser" that shot a magnet clamped to the shaft on each, so I could get so-called "Sychronised" when the shaft speeds were the same, but off by the diff between 1.9 and 2.0 at the engines.
Using the harmonics that I can feel seems like a better way to synchronise, so I no longer use the shaft mounted one.
I also checked my tachs against a digital tach shooting the damper on the front of the crank on each engine. Big difference, so I tweaked the tachs. Over the years since I did this, the diff betwee the wheelhouse tachs and the Fly bridge tachs has increased, from zero, when first calibrated, to about 20%. Aren't they reading the same electrical pulses? Seems tachs are at best, just a guess.

I have wondered about how an engine synchroniser would compensate if both engines are at the same RPM but the tranny output is a different ratio. It seems that you would want the props in sync. OR are props sized different to compensate?

 

[mike66]

Congrats! Come back soon with your next problem!

 

[rgano]

We KNOW you are holding out a juicy problem on us. We are bored.

 

[FF]

"It seems that you would want the props in sync."

The engines are the major source of vibration and noise , so the engines are synced.

For folks that like to play a simple ROT is at 1000 shaft RPM an inch of pitch in the prop is one Knot of boat speed , with NO prop slip.

So if you measure the speed with the GPS ,and the tach is accurate, you can figure the prop slip., at any speed.

 

[koliver]

"It seems that you would want the props in sync."

The engines are the major source of vibration and noise , so the engines are synced.

For folks that like to play a simple ROT is at 1000 shaft RPM an inch of pitch in the prop is one Knot of boat speed , with NO prop slip.

So if you measure the speed with the GPS ,and the tach is accurate, you can figure the prop slip., at any speed.

My numbers in that ROT:

shaft speed: 1000 rpm
engine speed (BW Velvet Drive 1:2.0): 2000 rpm
prop pitch: 17"
theoretical speed: 17 knots
actual speed by GPS: 8 knots.
prop slip: 8/17 = 47%

If that is true and I have 47% slippage, what would I do about it? Why would I want to know this?
I don't find this ROT at all useful.

 

[FF]

"I don't find this ROT at all useful. "

If you don't believe Knowledge is Power,,then forgetabout it.

Someone changing props , or just cleaning the prop or bottom or interested in max range might have a different opinion.


Folks that want to travel at "hull speed" can watch the load and slip climb as they push the throttle.