cavitation?

[Steve Mahler]

I have a small antique runabout with my trusty late 60s Johnson 20hp mounted, forward controls etc, just had it in the water first time this weekend.

The cavitation plate is dead even with the bottom of the (nearly flat) hull. Yet just as she digs in to go on plane it is like someone steps on the clutch - RPMs shoot up, loose acceleration. I pull throttle back, she bites back in, and will run right up to that same point again.

Can I assume this is cavitation, and the solution is to mount the outboard a bit lower? how much lower? another inch make a difference?

 

[tashasdaddy]

Re: cavitation?

60's motors were longer, 2 inches deeper, than today's motors. there water pump is higher and are not self priming. the older motors, were longer, measure from transom clamp to AV Plate 17 inches for a 15 inch transom, 22 inches for a 20 inch transom.

 

[ondarvr]

Re: cavitation?

What TD said means you may need to lower the motor 2 inches or so, for it to work correctly.

 

[idrownworms]

Re: cavitation?

If the motor isn't trimmed down all the way you might try that first. My boat is the same way as yours and it will do the same as yours if trimmed up to high.

 

[Steve Mahler]

Re: cavitation?

So you think cav plate 2 inches below hull bottom is the safe bet?

 

[swordfish25]

Re: cavitation?

I had the same problem with my e rude but come to find out I spun the rubber hub in the prop , When it got loaded to pull and run it would slip and cativate. Had the hub replaced for 28 bucks and works fine now.

 

[Chris1956]

Re: cavitation?

Steve and Sword, It is a small point, however, we are discussing ventilation, not cavitation. The plate on the motor is the anti-ventilation plate.