Low RPM but good performance

[Citori421]

My boat is an 18' alum bayrunner with wood decking and center console, 2007 70 HP yamaha 2 stroke main, 8 HP yamaha four stroke kicker. Current prop is OEM aluminum 13.5x15, with a small bend at the end of one of the fins being the only defect. WOT is only 3200 (spec for motor is 5000-6000). Hole shot is great, engine feels and sounds good, and easily hits 30 mph with two people, dog, and a mess of camping/fishing gear, which I'm no expert but seems pretty good. My question is whether it could just be the tach malfunctioning, or if I should drastically reduce the pitch/diameter. Or see about engine issues even though it seems to run smooth. My electrical engineer friend installed the tach last year when I bought the boat. Not his area of expertise per se but better than me installing it! My gut feeling is that a tach either works or doesn't work, not just off a bit. Thanks all!

 

[Scott Danforth]

first, check your tach

all tachs can be set for various configurations and you may be spinning 4500 RPM or 6000 RPM. you must verify your tach to know

then use a GPS to verify speed.

then if you are still low on WOT, look for reasons (look in the stickies)

then fix your prop

 

[Citori421]

first, check your tach

all tachs can be set for various configurations and you may be spinning 4500 RPM or 6000 RPM. you must verify your tach to know

then use a GPS to verify speed.

then if you are still low on WOT, look for reasons (look in the stickies)

then fix your prop

Thanks! I've been doing some digging and may have stumbled on the answer. My tachometer is a yamaha pro series II analog, and the yamaha website states:
"Pro Series analog gauges are offered in an illuminated, black-and-orange faceplate design. They’re engineered for those running F20 to F300 outboards"
Could it be that my two stroke 70TLR is giving an input that a tach designed for a four stroke would interpret as half of what the actual RPM is? It is frustrating how difficult it is to find manuals, instructions, and specs for these gauges.

 

[Scott Danforth]

no, your tach has either a rotary swith with 8 positions or a series of dipswitches with 8 or more positions. it is most likely set wrong for your motor. so spend the $30 and go get a photo tach and actually read the revolutions of the flywheel and compare to what you are seeing on the dash.

EHDIS Digital LCD Photo Tachometer Non-Contact RPM Meter Motor Speed Gauge Gun Style Includes 10 Reflective Tapes: Amazon.com: Industrial & Scientific

EHDIS Digital LCD Photo Tachometer Non-Contact RPM Meter Motor Speed Gauge Gun Style Includes 10 Reflective Tapes: Amazon.com: Industrial & Scientific

www.amazon.com

 

[WesNewell]

Not familiar with yam guages, but I would think there's a setting on the back for different engines. Might should contact Yamaha support with engine model and tach model for proper setting.

 

[Texasmark]

Your tach says you are way high in your prop pitch....your hole shot says you are fine with your prop pitch. 70 HP isn't a lot especially a Yammie (opinion due to cubes) on your rig but 15 isn't a lot of pitch either. I'd say it's doing a fine job.

I'll check your tach for you...guessing your gear ratio at 2.33. If wrong, do the math on the difference and tweak my response by that percentage'

Bam go-fast.com prop slip calculator says: For a WOT speed of 30 and 3200 RPMs getting it with the 15" prop and that gear ratio, your prop slip is Negative 50.....-50. Now a screw turning in a block of wood has a slip factor of zero...0. Fast boats for normal fishing rigs run single digits positive slip, like 4%....10%....positive numbers.

The calculator doesn't lie nor does the gear ratio, the prop pitch, nor your speed if GPS verified.....what lies is your tach!

 

[Citori421]

Your tach says you are way high in your prop pitch....your hole shot says you are fine with your prop pitch. 70 HP isn't a lot especially a Yammie (opinion due to cubes) on your rig but 15 isn't a lot of pitch either. I'd say it's doing a fine job.

I'll check your tach for you...guessing your gear ratio at 2.33. If wrong, do the math on the difference and tweak my response by that percentage'

Bam go-fast.com prop slip calculator says: For a WOT speed of 30 and 3200 RPMs getting it with the 15" prop and that gear ratio, your prop slip is Negative 50.....-50. Now a screw turning in a block of wood has a slip factor of zero...0. Fast boats for normal fishing rigs run single digits positive slip, like 4%....10%....positive numbers.

The calculator doesn't lie nor does the gear ratio, the prop pitch, nor your speed if GPS verified.....what lies is your tach!

Thanks everyone pretty sure I figured it out- it is the tach. I was finally able to pull my tach off the dash and sure enough there is a selector on the back that was set to 12 poles, and my motor is 6 poles. So I'm expecting it to read much closer to expected values next time I take it out. Sure wish they made it easier to find manuals and diagrams on these things, that would've answered this question in 2 seconds.

 

[Texasmark]

The engine service manual gives you the poles. The tach comes with instructions for the number to set for the number of poles. All tachs aren't in English....aka they don't have a 12 position for 12 poles and 6 position for 6 poles....they have a code you have to use ...in the instructions.....Neat huh!!!!!