will smaller prop help?

[rebars1]

1990 high Johnson 60hp, 1976 18' boston whaler type boat. The engine runs fine, new water pump, flushed out water passages with a hose on the water tube, good pee flow, 5000 rpm's at WOT.

However, the engine overheats at high rpms. Some say it is because the engine is too small for the boat, or the boat may be heavy from water in the foam.

I was wondering if switching to a smaller pitched prop would help the engine, even though boat speed would be reduced?

 

[Joe Reeves]

Re: will smaller prop help?

I don't have the full throttle rpm specifications for that engine BUT I would think that it is rated somewhat higher than the 5000 rpm that you are experiencing.

Check with your dealership as to what the top rated rpm of that engine should be.

If it is something like 6000 rpm, then your top rpm should be just slightly less than that, 5800 rpm, something in that area.

If your engine is in top operating condition and the top rated rpm is not being met, then yes, dropping to a lower pitch propeller would help with that problem, BUT if not in top condition, there may be an underlaying problem pertaining to overheating which remains to be seen.

 

[wavrider]

Re: will smaller prop help?

Over heating is not a prop problem, it is a cooling system problem, inspect your impeller, thermostats and flush your system completely.


For the RPM's you are looking for there are alot of variables to consider.
Set up, height of motor, etc, etc. Post questions in the prop section, alot of experts there can help to get you dialed in to the correct RPM for your set up.

 

[tashasdaddy]

Re: will smaller prop help?

just because you have a good tale tell flow, does not mean the thermostats are opening and cooling the motor. i just means the impeller is pumping water. if this is a salt water boat i would definitely look at the tstat.

 

[rebars1]

Re: will smaller prop help?

I double checked the rpm's. We were getting 5500 rpm, about 26 knots at WOT. Manual lists 5000-6000 rpm at WOT.

I replaced the thermostat and water pump when I first got the boat a few months ago. I'm pretty sure I installed them correctly, but maybe the tstat is not working right. How would I know if it is opening correctly on the engine? Is there a way to install a water temperature guage and sensor?

 

[freddyray21]

Re: will smaller prop help?

did you replace the pump housing or just the impeller? Overheat at high rpms is indicative of a pump sucking air. If you did not replace the housing I would do so.

 

[rebars1]

Re: will smaller prop help?

I replaced the complete assembly. I noticed that the new base plate was slightly different from the plate I removed in that the arcing slot was a little smaller from the original.

Also, the hose for the taletell starts at the lower side of the head, but does not loop over the top of the engine. Would that make a difference. I have read some threads recommending the hose be re-routed over the top, or reconnected to the top.

 

[ezeke]

Re: will smaller prop help?

Your motor has one of a series of three cylinder engines that had overheating problems and consequent damage related to the location of outlet nipple and hose for the tell tale. The recommended work is outlined in OMC Service Bulletin 2276 Rev. 1. Click the thumbnails below, please.

Additionally, because of damage to wiring and ignition components caused by the overheating, CDI Electronics published additional information re: SB 2276 in their Troubleshooting Guide. See Page 77: http://cdielectronics.com/portals/0/downloads/Troubleshooting Guide 2006.pdf

 

[rebars1]

Re: will smaller prop help?

Thank You. Ezeke, I could not open the last PDF link regarding electronics. Is there an alternate to "OMC Gel-Seal II" such as Perfect Seal?

 

[Joe Reeves]

Re: will smaller prop help?

I couldn't open that PDF file either Zeke. If mermory serves me correctly Rebars, I believe that LocTite 518 is the same as GelSeal. If wrong, I'm sure I'll be corrected shortly.

 

[ezeke]

Re: will smaller prop help?

The pdf file is very large as you might expect, which is why I copied page 77 and posted it as a thumbnail. Even on a large drive, with good memory and using cable, it still takes a good bit of time to open it.

Saving the Troubleshooting Guide to your drive is worth the space; it is the basis for almost all of the guides on other sites.

If you want to download a large pdf document, right mouse click the document link and select "save target as".

Permatex 80063 # 98 High Tack Gasket Sealant will work. It just needs to stand the temperature and not be affected by gas and oil.

 

[rebars1]

Re: will smaller prop help?

Thanks. I got the PDF.

I'll let you know how the reroute operation goes.

Could I use the old nipple point to install a temperature sensor for a guage?

 

[rebars1]

Re: will smaller prop help?

Finally got the time to relocate the tell-tale nipple connection to the top of the engine per the Service Bulletin and the info in the "Add a Tell-Tale" thread in FAQ. It was a lot easier to drill and thread the hole than I thought it would be. The grease worked wonders to keep the shavings out.

Now comes the weird part. With the new installation complete I hooked up the hose and started the engine to test it out. The engine started, but no water peed out. I thought maybe it just takes time for the water to fill up the engine and pump out. I walked to the back and noticed there was not even water coming out prop. Then, the SHBT occurred to me...I had forgotten to turn on the hose! Yikes! I quickly shut off the engine, turned on the hose, restarted the engine and the water flowed perfectly. Unfortunately, now I will need to check the impellar for damage......