Engine won't turn bigger prop

[TD_Maker]

Mr. Moderator. I did post this same question in the "Props" section of this forum Please don't be too angry with me.

I have a 2003 Johnson 90 (Bombardier) V-4 mounted to a 17 ft McKee Craft CC Boat. 20 Gallon gas tank mounted under console. Motor runs great, idles great....no issues at all.

However,

To get the motor to run in the 5500-5800 RPM range, I can only turn a 14 x 13 pitch prop. Everyone says I should be turning a 14 x 17 minimum, so there must be something wrong with my motor. I did a compression test and it is very close to 125lbs in every cylinder. Is there something wrong?

Oh, I forgot to mention. Motor is mounted in the third hole up, and the cavitation plate is about 1/2" above the bottom of the hull.

 

[Faztbullet]

Re: Engine won't turn bigger prop

What brand props?

 

[TD_Maker]

Re: Engine won't turn bigger prop

I have a couple of props I am playing with (Trying to determine what SS Prop to purchase) Any way, I have an original OMC (with vent holes) 17 Pitch Aluminum cupped prop which was only turning 4700 at WOT I also have a Michigan Prop (Aluminum) 14 x 13 which seems to be exactly what I need. What do you think? Am I OK? I was thinking maybe the lower unit was geared a little different.

 

[kjdunne]

Re: Engine won't turn bigger prop

I think 5500 should be your target WOT, lightly loaded. I'm running a 15 pitch 4 blade alum. on a 90 pushing a 2000 lb. Manatee, gets me 5000 WOT w/ only me aboard on flat water. What does your boat weigh? I understand the McKees are built heavy compared to a similar size Whaler for instance. A 13 pitch SS prop will probably drop your WOT 100-200 RPM compared to alum., a 15 pitch SS will probably put you in the 5300 range. The 13 will give you a quicker hole shot, the 15 a higher top speed by a few MPH. See if you can borrow a 15 to try out (I know it's not easy to get a loaner prop), and see what happens. Also, how do you usually use the boat, lightly loaded or more fully loaded? Do you need lots of pulling power for skiers or boarders? Ignore the people telling you you should have a "17 minimum", final prop selection depends on to many variables, including how you use your boat.

 

[phillnjack2]

Re: Engine won't turn bigger prop

Whats with the 14 inch diameter ?
your all going to have problems running these props on heavy set ups, too much diameter for anything less than 150hp.

you should be on a maximum of 13.5 diameter, and yes it does make a huge difference.

with the original posters engine that will easy push a 13x 19 pitch stainless prop and give a better hole shot and more top speed and more revs.

the diameter is the problem, its just too much diameter for anything less than a 150hp engine.

ive got a 12 3/4 x 23 that goes on those 90hp engines and a 13 3/4 x 23 that will fit anything from a 3 cylinder 60 to a 140hp v4.

14 diameter is for a cruiser at low speeds or for very lightweight boats.

try a 13 x 17, that's what they came standard with and will rev to 5500 rpm.
Just by putting on an extra 1 inch of diameter is adding a lot more metal than you might think.
the outer edge of the blade has to travel so much more distance its just not going to happen with a 90hp on a 17ft boat.

there is plenty of props around to try out, if it was me I would be looking at around 12 3/4 by 19 or 21 to get it moving for top end
and a 13x19 for all round prop and a 13x15 to blat away on the hole shot for ski-ing.

14x13 needs to be put in a bucket as a spare prop in emergency only, its no good for anything else with that engine.

can always vent a 19 pitch and get good hole shot as well as top end.

is this a 13 spline prop shaft ?
if it is then just go borrow a prop from anyone with a 60/65/70 rude to see how it performs.


phill

 

[kjdunne]

Re: Engine won't turn bigger prop

Good point on the diameter, mine's a 13".

 

[boobie]

Re: Engine won't turn bigger prop

Is your tach accurate ??

 

[TD_Maker]

Re: Engine won't turn bigger prop

Thanks for the help guys:

Boobie: Pretty sure my tach is correct. It is a systems check tach.

I went back out and checked the actual size of the OMC prop. It is actually a 13 1/4 x 17 (lightly cupped with two vent holes) and it turns at 4800 RPMs WOT with full tanks and one person. Prop # 391199

The Michigan Prop is a true 14 x 13 but allows the motor to rev at the 5800 range.

Oh, I do not pull skiers or tubers. I am generally alone, or may have one other person in the boat with me. My little boat is a skinny water fishing boat and nothing fancy. This old workhorse is my old project baby, and I love her. I am just having issues getting her propped correctly.

 

[boobie]

Re: Engine won't turn bigger prop

Do you have the correct pole setting on it ??

 

[TD_Maker]

Re: Engine won't turn bigger prop

Hello Boobie. Yes, I pulled the tach out today and it is set for the 6 Pulse 12 pole setting.