Messing with Props

[biglurr54]

1) Performance issue you are trying to correct.
Unload the old engine to allow for better towing and easier life on the motor.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Manufacturer unknown. Aluminum 3 blade
3) Current prop diameter and pitch (required).
I have a 19P and a 17P
4) Wide open throttle RPM and speed with an average load (very helpful)
Take with a grain of salt ... Taken from 1989 instruments. 19P 38MPH at 4100 RPM 17P 42MPH at 4900RPM Trimmed.
5) Engine/drive make, model, year, and HP
1989 Evinrude 120 with extremely low hours.
6) Boat make model, year, length and weight
1989 Crestliner Phantom 17.5 with 26 gallons of fuel.

Should I make the jump to 15P. I use the boat for putting around our small lake and towing tubes and wake boards occasionally. Never go full throttle unless testing to see WOT RPM. I want to unload the engine so it can have a happier, longer life.

 

[biglurr54]

Double post. Can a Mod delete one?

 

[SkiDad]

if you can verify with GPS that would help on the speeds - the numbers don't make too much sense. I would think that a 17p would be fine but if the RPM is correct then yes, definitely 15p - Toss the 19p on craigslist and keep the 17p for a spare. just my .02

 

[Maclin]

Seems to me 4900rpm with the 17p is the right prop, if maybe even slightly underpropped. Going to a 15p is in the wrong direction in my opinion.

 

[Maclin]

Oops, saw 120hp and thought I/O. You need to get the rpm's up, so 15p is the next in line to try.

 

[biglurr54]

Can the stock 1989 tach be trusted. I dont trust the speed gauge and i know they are not reliable. The Tach seems to read the same every time. I might try to find a 15p at the local marines prop tub on my way home today.

 

[steelespike]

I don't think your tach is accurate You can't reach those speeds at those rpm.
​ The 17 and 19 props are pretty typical.For the 120 and I believe the ratio is 2.00.
The 120 is rated at 5500 rpm.
With an accurate tach I think you will find the motor close to the rpm range.
5500-6000

 

[biglurr54]

how can one find an accurate Tach to borrow or pay very little for.

 

[jestor68]

The projected performance of that 1500 lb(dry weight) boat is:

2.1 gear ratio turning a 19 pitch prop at 5300 rpm shows a theoretical 48 mph, corrected for 10-13% slip is 42-43 mph.

That shows how far off your readings are. A boat's speedo is a joke(unless it's a GPS unit). Your tach readings are likely too low.

The 19 pitch should be the correct all around prop, if the motor is healthy.

A 17 pitch would most likely take it to 6000 rpm.

 

[biglurr54]

Well the boat has some added weight. All the fuel, the floor is replaced with 3/4 unsealed ply that has soaked up plenty of water, a 10 hp kicker, my fat butt at 260. Mos tof the water logged foam is out but there is still some in there. will any tach for a 4 cylinder motor show accurate readings.

 

[steelespike]

An induction tach is reasonable, easy to install and will work on almost any motor.
The wire needs to be installed so it won't pickup stray pulses
There may be a selector switch on there back of your tach note its position and operate it a few times to
clean the contacts.It's al;so possible it is set wrong.