Trying to match the correct Prop to Boat/Motor

[Spartan811]

Hey Guys,

Just want to say, this is a GREAT Forum with lots of info and advise!!
Happy to be apart of it. Thanks to the guys with all the Answers!

Here's my issue,
I own a 21' Parker W/A and I just replaced my motor to a 1998 225HP Yamaha OX66 EFI. This motor did not come with a Prop so, I'm trying to match the correct Diameter and Pitch for this boat.

I asked around and purchased a Michigan 15-1/4 x 15" RH Vortex Aluminum
3 Blade Prop but after test running it, I feel like I'm not getting the full output potential. My RPMS are at 5k and I'm running 20 GPS miles per hour.

Anybody out there with the same boat/Motor combo that can shed some light?
Thank you in advance!

[TownsendsFJR1300]

Here's a link to get you in the ball park;

Performance Bulletins | Yamaha Outboards

It should obviously be turning much closer to 5,500 RPM's. (corrected for a two stroke)



I have a 20'4" CC Angler with an F150 (a bit lighter) and with a SS, 3 blade prop, Skeg guard, at 6k, I'm seeing a max of about 46 MPH-per GPS.. 3500 RPM's-25 MPH, 4,000 RPMs, 30 MPH, etc..

[99yam40]

There is a big difference between 2 and 4 stroke motors WOT RPM.
usually 6K on 4 stroke and 5500 on 2 stroke .

Is the 5K your WOT RPM or are you just running at 5K

[Spartan811]

I'm just running at 5k.
When I hit around 5500 RPM
my Rev Limiter shut off my motor

So I run it around 5K for safety sake.

[99yam40]

rev limiter should not shut off motor, just limit too high of a RPM.

Maybe running out of fuel at WOT, because that is the only thing that I know that will shut off motor

Did you look up a rig like yours on the link he posted to see what prop and performance it got?

[rodbolt17]

my 21 seapro cc with a hard top and second station with an F150 runs about 43 at 5700.
its swinging a 13 1/4x18 ss.
you should be able to swing a 14 1/4 x 19 or 21.
what your shooting for is with a full load at WOT the motor can hit about 5300-5500 RPM.

[Spartan811]

The Final Outcome,

Took it to my local Yamaha Expert,
Performed full Diagnostics and found
one Mechanical Fuel Pump not working.

Replaced all three pumps and replaced
the original Prop with a 14.5 x 17" Aluminum
Yamaha Prop.

Boat runs at 5700 RPM no problem in the 40 GPS MPH.

Thanks to all for your Help!!

[99yam40]

WOT RPM should be 5500 , you need to read your owners manual.

Depends on what kind of load you had on the boat at the time you hit 5700, but you may be able to prop a little higher if it was loaded as you normally would run it and still were able to hit 5700. You might even be able to get some better speed/performance

Did you look up a rig like yours on that web site to see what prop they used?

[Spartan811]

5700 was with 2 people

and, Yes I looked it up and
they give this recommendation:
13 3/4 x 17 S/S

[99yam40]

The aluminum and SS are different , if you look at their WOT RPMs listed with the SS I would think it does not get over 5500.

But with a full tank of gas, more people ,lots of ice and drinks and tackle yours probably will not getting to the 5500 .
If it does look at getting a prop like Rodbolt recommended, or see if you can borrow one to try

[Spartan811]

More Problems, Fuel related

Hey guys, Check this out, the Drama continues!

Took the boat out yesterday, thinking I'm all good to go, only to get stuck.
The motor was running ok in the beginning, but by the end she had no propulsion, If I can get her to idle long enough to put her in gear but once in gear it felt like no power at all.

Today, I checked everything,
Electrical system OK.
Compression 120 PSI across all cylinders

Plugs looked very fouled, maybe from idling to long, will this happen?

Anyway, Replaced them, and Motor started right up.
I started checking Fuel Pressure and that was about
28-30psi, books calls for 35.6psi

Books also says, check EXTERNAL RESISTOR if pressure low
- Ohms test on resister
should be 0.53 - 0.57ohms.

My Resister has no readings 0.00 ohms

I tried looking up to find out what exactly does this resister do
for the fuel system but could not find anything.
So if anybody has any info on what's it's purpose and should I replace it??

and will this make a difference to the overall output Pressure..?
or do I need a new Pump?

Rodbolt17 HELP!
thanks in advance!

[boscoe99]

"I tried looking up to find out what exactly does this resister do
for the fuel system but could not find anything.
So if anybody has any info on what's it's purpose and should I replace it??"

Its purpose is to vary the speed of the fuel pump.

Pump speed is reduced when current flows through the resistor (1500 RPM or less) and the pump runs normal speed when fuel pump current does not run through the resistor (above 1500 RPM).

Less engine speed = less need for fuel. More engine speed = more need for fuel.

[boscoe99]

"Anyway, Replaced them, and Motor started right up.
I started checking Fuel Pressure and that was about
28-30psi, books calls for 35.6psi"

Where in the SM are you seeing a pressure value of 35.6 psi?

I see a value of 3.56 psi but that is for the amount of pressure reduction to be applied to the fuel pressure regulator during the test of same.

The output pressure from the fuel pump varies with intake manifold pressure. With the key on but the motor not started you will see one value. When the motor is started the manifold pressure drops. Fuel pressure should drop by the same amount of psi.

[99yam40]

also, make sure your setting on the meter you are using is set to the lowest setting and take into count the resistance of the meter leads to read lower than one ohm

[Spartan811]

Thanks for the info - Boscoe99,

I rechecked the Resister with the DVOM at
the lowest setting, and now it readings
are 1.2 ohms.

But still not in range @ 0.53 - 0.57ohms

would this be acceptable or replace?

Thanks for your help 99yam40!