prop upgrade and engine performance

[rzkas]

hi and how are you? i have 1995 120 force. currently it gives me 33mph max speed. i like to go faster than that. will changing my prop increase my speed and accelaration?. if yes, what type or kind of prop should i put in? two blade, three blade, four blade etc, what pitch, tainless steel or aluminum? i want to put in a 26 pitch 13.5" in diameter with three blades.

At 10mph my bow rise up to the air to the point that i can't see where am going. i will have to stand to see where am going but as speed increases it comes down. is this normal or something is going on with my engine.

my boat doesn't have a tacometer, so how do i check my boat RPM at full throttle?

is it normal for me to have to crank my engine several time like two to four times or sometime even more time for the egine to fire up/ start up. but whenever i turn the key it turns over, the flywheel spins and the engine cranks. The battery is fully charge. sometimes one turn starts the engine.

thanks

 

[roscoe]

Re: prop upgrade and engine performance

Wow, you are covering a lot of ground here.

hp, weight load, prop, all play a role in getting on plane (holeshot) , and top speed at WOT (wide open throttle).

So does having a properly running engine.

Assuming the engine is running properly, and that the motor is sized appropriately for the boat, and that the hull is not waterlogged.....


Too much prop pitch will bog down the engine and kill your holeshot, and cause too much stress/load on the engine.

You really need a tach to get this right.

Even a TINYTACH, or other inductive type tach will work.

The prop should be pitched so that at WOT, you are near the top end of the recommended rpm range for your motor.
I think that is approx 5000-5400 rpm.

Best holeshot will be achieved with a prop that will spin 5400 rpm or more. You should get fully on plane in 2-7 seconds.
If you have time to stand up, and look to see where you are going, the holeshot is unacceptable and the prop already has too much pitch.

Going to a 26p prop could very well leave you in the bow up condition forever.


Cannot recomment a prop without knowing the current prop size and the WOT rpm.



Starting procedure for your motor:


Open fuel tank vent

Check the kill switch, place in "run" position

Squeeze fuel primer bulb till firm.

Advance the throttle 3/4 way, without putting it in gear. -- This is done by either pushing a button in the center of the controls, or pulling the shift handle toward the driver, or raising the fast idle lever, all depending on what control unit you have.

Activate the choke (fuel enrichment valve) by pushing in on the key. The valve is only open while the key is being pushed in.

Turn the key to start while continuing to hold the key in.

Release the key and choke when it starts.

3-5 seconds of "choke", is usually enough.

Be ready to pull the throttle back toward the idle/neutral position, when the engine starts and the revs increase.

Continue to pull the throttle back as the engine warms up.

You should not have to use the choke much, if at all, once the engine has warmed up.

Engine should start within 10 seconds of turning the key.

 

[rzkas]

Re: prop upgrade and engine performance

thanks for the info. i will look online for a tach. where or which wire should i connect the tach signal wires to get RPM RESULTS.

 

[rzkas]

Re: prop upgrade and engine performance

more input and explanation pls

 

[fucawi]

Re: prop upgrade and engine performance

The reason your boat points at the sky is because you do not have the engine trimmed under ( down) enough...it should be right down when you open the trottle ..check the pin in the mount to see its in the bottom hole ...if with it tucked under fully it still is not right put some fins on the ob.. you never travel at 10 kts ....its not fast enough to plane thats 15 kts and too fast for displacement ....you go from 5 to 15 kts as fast as you can ......

regarding prop ..get a rev counter ..run at WOT and check rpm ..if at top end of engines range prop is ok ..you will not improve it ....if over reving or under then for every 2 inches of pitch you change the rpm will change 400 rpm

17 or 19 pitch would be normal maybe 20 if its a light boat

33 mph is 36.5 kts thats about it for a 120 hp