How to check my outboard performance?

[Jason F-18]

I am running a Honda BF20 tiller model. It does not have any gauges with it, just and oil temp warning light. I have read in a few places that there are some portable tachs that can be easily installed on the spark plug wire. I would like to do some tests to see what RPM my motor is running at and if I am getting the most performance out of it.

I have it on a G3 1548. When I'm fishing, I would guess that the whole rig weighs approx 900 lbs with me and the dog. I have taken my portable GPS from the car and had it up to 24mph. This may be all that its capable with this motor, but I've wondered if another prop could get more speed.

The current prop is a 9 1/2x 10 three blade. Honda lists a power prop option that is 10x 7 5/8.

Any help would be great!

 

[Bubbakat]

The name of the tachs you are refering to is tiny tachs, They have a web site.

 

[Jason F-18]

Yeah I looked at those. They have a new one that is adjustable for a variety of engines. I would imagine it would be a good investment of $47.

I did find a prop slit calculator http://www.go-fast.com/Prop_Slip_Calculator.htm Put in my gear ratio and prop pitch, and WOT RPM max, it gave me a theoretical speed of 27mph. I guess that means I'm about maxed out.

What is a normal slip %?

 

[wrechin2]

Nominal slip depends on a lot of things. Most boats will range from 6% to 15%. The calculators are pretty good tools to get a GENERAL ideal. They are not exact as they are calculated theoretical speed. Not sure I understand the power prop size you up. Props are measured by diameter and pitch. 9.5 X10 means 9.5" in diameter and 10" pitch. Pitch is how far the prop will move in 1 revolution in theory. Slip is how far this is off. So a 10pitch prop moving 9" through the water is 10% slip. The power prop number doesn't make any sense to me. You will also not most of the time when a prop goes up in pitch, the diameter usually goes down. Just my .02

If you want to simply test your RPM, I have a fluke 88 with a RPM adaptor that will tell you your RPM down to the tenths, in other words, 4867 RPMS. I would be happy to set it up on your boat and let you make a pass next time I go to the ramp in soddy or I have a tiny tac some where you can borrow me being there. Just a thought!

 

[fischnrod]

GOOD SPEED WITH A 20 HP

 

[Jason F-18]

Yeah James, I was wondering the same thing too. I guess it is a prop that gives more pulling power than high end speed. I may give you a shout so to see when you will be around the ramp. I'm not sure how much more I could wring out of it, but am curious about my rpm anyway.

 

[wrechin2]

No problem! I will be heading that way sooner or later as I have 7 engines going on and I lake test each one. What ramp I go to depends on where the guy lives. You are welcome to come over and get the tiny tac and use it as long as you need to and I will pic it up from you. Two strokes kinda confuse it so I don't use it.