OMC Cobra Prop Selection Help

[drexton311]

So, I broke my prop this past weekend. Good timing as my motor is only running 3800 RPM at WOT and it was suggested on here to change the prop pitch. I currently have a 15 x 17 3 blade on my 1988 OMC Chevy 5.0 I/O. I have confirmed that the motor is running properly (is tuned, no issues) with the exception of low cylinder compression (all cylinders are in the 110 PSI range (cold)…which would explain the inability to hit the 4200-4600 suggested WOT). I have two other props, both are 15 x 17, but I wanted to see if I could get closer to the max WOT by changing to a 15 or 13 pitch. Problem is from what I can calculate I need a 13 pitch prop but this would increase my prop diameter from 15 to about 15-1/2. Could I use a smaller diameter prop to get what I am after? Also, I would be using this boat to pull tubes, say 4 adults, 4 kids, and I want highest top speed and most hole-shot. With the current set up I am hitting about 38 MPH @ 3800 RPM when not pulling a tuber. 3 or 4 blade pros cons?

Additional Info:
18.5' boat
Fiberglass Deep V Hull
No Trim Tabs or Other "lifting devices"
Boat Planes fine with current set up

Any configuration suggestions from the more experienced boaters on here?

Thanks!
Josh

 

[bruceb58]

15 1/2" diameter is fine. That is what I had for my high altitude 15x15 prop with your exact motor on a 20.5' boat.

I used to run a 15x17 on my boat when at sea level. The fact that you can not meany you have an engine issue. It's probably your compression which is very low.

You should also weigh your boat and make sure you aren't carrying around a lot of water slogged foam flotation.

 

[steelespike]

Try both of the props you have in hand.The diameter is usually the result of the pitch.
No way to know if anyone messed with them.Have you verified the throttle opens all the way?
Timing set right and advancing as it should?Bottom clean and smooth?
If you must change props I would stick with the 15" pitch.Perhaps have a prop shop change the pitch.

 

[drexton311]

(Per the OMC manual) I have confirmed timing, new plugs, wires, distributor cap, points, drained fuel tank and replaced fuel, new carb and throttle opens all the way. I did a cold compression test and it read consistently at 105-110 PSI on all cylinders. The engine has not been rebuilt, so I am pretty sure its just worn rings and the little mouse runs great, just a bit under powered. Hull is clean and boat does not appear to be water logged as it has no issue planning out, does not feel heavy when pulling, but I have yet to weigh. I know what the 17 pitch feels like since that is what was on it before, was just worried about going to a 13 pitch from 17 and how much top end I can expect to lose.

 

[drexton311]

Oh and I'm in Oklahoma, so no high altitude here. However, the boat is originally a Wisconsin boat, so not sure if that helps the prop question or not.

 

[drexton311]

Just for additional, info: My tach is functioning correctly, I checked it with my digital timing light and it appears to be pretty close, the speedo has not been checked yet. So knowing that my motor is a little tired, any type of calculation I can use to pick a proper sized prop in order to get the RPMS up?

 

[drexton311]

I talked to Turning Point and they recommended a prop to me, its a 15 pitch prop, I ordered it. Since I still have a brand new OMC 17 pitch prop, I will compare the two when I can and let you guys know the outcome. Thanks for the help!

 

[drexton311]

I tried the 15 pitch and it definitely got my RPMs up, however I think the 17 pitch I had on it before performed a little better. I am going to give it another chance as I had some engine trouble today, which I am going to start a new thread about. Thanks for the advice guys!

 

[steelespike]

You need your speed by gps at wot.Your early numbers calculated at 0 slip ;12% slip is more typical.
Are you trimming up for best speed?

 

[drexton311]

SS, yes that is the next thing I plan to do, however the motor keeps cutting out, until I figure out what going on there I'll have to wait. I want to use the GPS because I am not convinced the speedo is right. Yes I am trimming, however, the 15 pitch did not like ANY trim, it would bobble the boat up and down in any position except full down...the 17 pitch I had on it before liked about 1/4 up trim.

 

[drexton311]

Sorry, I completely forgot to update you guys... I have been running the 15 pitch all summer and the boat runs great! As a matter of fact, I am no longer convinced I have low cylinder pressure as I easily hit the 4600 RPM and I still have throttle left. I verified the Speedo and the Tach are accurate (with GPS and Tach light). however, early in the season, I did have to spend some time getting the mouse motor running well, with several maintenance and tuning issues.

I am going to put the OMC 17 pitch back on this weekend and see if there is a significant drop in RPM. I'd like to hit a faster speed than the 35-38MPH I get with the 15 pitch.

 

[drexton311]

Cruised at 40mph at 4100-4200 RPM, no issues. I think there may be a little more there but happy to hit the lower part of my power band with the 17 pitch. Think I am going to order a turning point 17 pitch to add to my 15 pitch and just switch them out depending on what activities I will do. I really like the TP 15" and I hope the 17" will perform as well as its little brother.