# Prop Cavitation/Ventilation (pic added)



## gator1gear (Jun 7, 2013)

I have a question for you guys. I have a short shaft motor on a 20" transom and buying a long shaft motor is not an option right now. I took the motor out for the first time and at about half throttle, the rpms came up, but the speed didn't. I added a homebuilt "compression plate" and it helped out a TON! That being said, something still didn't feel quite right in the upper rpms. I added a few pics to show the position of the cavitation plate. My next step is to lower the outboard with a fixed setback plate or mini jack plate. I am definitely encouraged by the fact that it ran better after adding the plate to keep water around the prop. At this point, I can't change the boat or the motor, so I'm looking for a compromise that will "work" with the options I have. Thanks guys and Gals! May not look like it in the pics, but the cavitation plate is really close to even with the bottom of the boat.


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## Johny25 (Jun 7, 2013)

It is really hard to tell in the pics where the cav plate is in relation to the bottom of the boat but I will say this...... A short shaft will not work properly on a 20" transom. It can go the other way.....long on a short transom but not vice versa. The funny feel you are getting is prop cavitation likely. This is not good for 2 reasons, 1- your motor can and will over rev and 2-if your water pickup screen is not full submerged you will over heat and fry the impeller and motor.


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## lckstckn2smknbrls (Jun 7, 2013)

If you can post a picture with a straight edge coming off the bottom of the hull so we can see how close the cav/vent plate lines up. 
If it's as close as it seems you might only need the appropriate amount of set-back.


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## gator1gear (Jun 7, 2013)

Thanks so much for the replies! It's dark here now, but I will get a pic tomorrow that shows the position of the cav plate and the bottom of the boat!


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## gator1gear (Jun 8, 2013)

Here's a pic with the level


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## lckstckn2smknbrls (Jun 8, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=317925#p317925 said:


> gator1gear » Sat Jun 08, 2013 9:05 am[/url]"]Here's a pic with the level


That looks just fine.


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## Johny25 (Jun 8, 2013)

Is the level your using running on the same plane as the keel of the boat? Or do you just have it touching the transom and then again at the cavitation plate? It looks like if you pin the motor down one more slot it may be better.

Oh and I am about 100% sure your motor will cavitate on tight turns where it is at right now


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## gator1gear (Jun 8, 2013)

That's where I've been running it and it wants to cavitate/ventilate in the upper rpms, even in a straight line. I've checked the prop and it's ok. I made a plate that bolts to the cavitation plate and it helped a LOT, but it still wants to rev up without increasing speed at a certain point. I was told to use a jackplate to set the motor back away from the boat, but wanted some input from you guys first.


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## tnriverluver (Jun 8, 2013)

Sounds to me like your props rubber hub is slipping and worn out, not a cavitation problem. The hubs will start to slip when you start applying max throttle. You can check for slippage by putting a mark on the prop and one one the shaft and take it for a spin. If those marks aren't lined up when you return you need a new prop or have the old prop re-hubbed. The cavitation plate can be up to an inch above the bottom of the transom and still work just fine.


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## Charger25 (Jun 8, 2013)

[url=https://www.tinboats.net/forum/viewtopic.php?p=317935#p317935 said:


> Johny25 » Today, 11:13[/url]"]Is the level your using running on the same plane as the keel of the boat? Or do you just have it touching the transom and then again at the cavitation plate? It looks like if you pin the motor down one more slot it may be better.
> 
> Oh and I am about 100% sure your motor will cavitate on tight turns where it is at right now





I agree, looks to me like thats tilted out wayyyyyy too far. If it were me ,I'd move it in a slot at a time till it didn't cavitate . After ya get a feel for it you'll know where to set it with different loads in your boat. For example, on my 16 w/the 25, if wife goes with me I can set the motor out two slots, by myself its got to go all the way in.


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## Charger25 (Jun 8, 2013)

If I run it alone the motor is set all the way in.




If someone goes with me depending on the weight carried I can move it out and still run the same speed. Last weekend I kept up with a bud of mine , he has a new 14' mod V w/a new 25 hp yammy . mine ,16 ft semi V w/a 92 25 Rude


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## Johny25 (Jun 8, 2013)

Hey I like that cord/string you put on your trim pin charger  I need to do that with mine so I don't lose it in 200' of water. Good idea you got there =D>


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## cgaengineer (Jun 8, 2013)

Johny25 said:


> Hey I like that cord/string you put on your trim pin charger  I need to do that with mine so I don't lose it in 200' of water. Good idea you got there =D>



No doubt, everything else has methods to prevent falling in the water except that pin!


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## Charger25 (Jun 9, 2013)

thanks, replacement for that pin is about 40 bucks, ask me how I know #-o


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