Prop cavitation?

[Johny25]

Ok I don't know if this is the right area to ask or not but seems as good as any. I mounted my 25hp johnson on my boat tonight and I am not sure if the jack plate is to high? See I had a 9.9 mounted on the jackplate and the 25hp transom bracket lined up exactly with the bolt holes for the 9.9hp that I took off so I didn't want to drill more holes in my transom. Anyway here is a pic of the prop and cavitation plate at the bottom of the boat. Looks like the prop will still be completely in the water and the cavitation plate may be out of the water by a 1/4 inch or less. I won't be able to get the boat out for another week or so and want to fix it now if it looks to high. So if anyone can look at it and say "NO" that won't work or "YES" that should be fine that would be great I just don't want to get to the lake and find out the prop cavitates and my day is shot. Any info is welcomed.

 

[tpenfield]

Re: Prop cavitation?

Actually you would be concerned about 'ventilation' as opposed to 'cavitation'. My vote is YES you should be fine. some set-ups have the plate higher than what is shown in your picture.

So, it is well within the ballpark and you are not likely to have any issues. Any adjustments would be more for speed optimization.

 

[etracer68]

Re: Prop cavitation?

Looks, good to go to me.

 

[steelespike]

Re: Prop cavitation?

Looks very good to me though with a jack plate and that really nice prop you could shoot for more height.
The Jack plate may rerquire more height to get the "antivent" plate above the water.
Do a test run get your prop size and max wot rpm and speed to us and check if you can see the plate at speed.

 

[Johny25]

Re: Prop cavitation?

Hey thanks guys, I will run it the way it is and hope that I don't have to move it. I just don't want more holes in the transom. I don't think I want to get anymore speed out of it because I should be hitting 30mph plus with this motor and boat which is plenty fast to me You see thats actually a 30 hp motor (did a mod to it) on a 14ft boat so I will fly already.

 

[Johny25]

Re: Prop cavitation?

Ok I don't like threads that don't have and ending so here is an update. I ran the motor and the motor height was fine as long as I did not trim it in either direction. If I put it down one slot it sprayed water up in the air (rooster tailed) and water was getting in the back of the boat at slow speeds. And If I trimmed it up one slot it caused blow out. So although it was fine in one position and the boat is fast as H E double hockey sticks I am going to drop the motor all the way down on the transom and put new holes in to bolt it I believe. I have more than enough power to plane so height is not as crucial with the bigger motor. I might just drop it one more inch? Cause in big water situations I want to be able to trim motor up to raise my bow so I don't take a wave over the bow.

 

[Sea Rider]

Re: Prop cavitation?

Distribute passengers weight accordingly, trim boat on 2-3 hole out transom so small upper cavitation plate is shown slightly out of water flow and paralell to sea level when at full plane. To achieve this will probably need to play with transom heigh a bit if you want boating perfection and speed optimization. It's a matter to test at sea...

Happy Boating

 

[steelespike]

Re: Prop cavitation?

Don't go crazy lowering the motor. If the water is flowing at the line indicated in the last pic.I think its too high.(Motor too low)
Normally the antivent(cavitation) plate is barely visable at speed.
It looks like there are 2 keels about 14" or 18" apart Is there a 3rd down the middle?
Maybe they are disturbing the water more than some do.Setup for the least motor in the water without venting.
For the most speed. You don't have to run wide open all the time.Usually the faster it will go the more fuel you will save as you throtle back.

 

[Johny25]

Re: Prop cavitation?

Yeah the pic above I think would be to low in the water and I don't think I can even get the motor that low? I have it on a jack plate right now so it will only go down 1.5 inches tops anyway if I remove the jack plate.

And yes steelspike there is one down the middle, in fact there are 3 in the middle, one dead center and 2 just to the side of the center one. To have the vent plate in line with the water coming off the back I actually have to have the motor one (actually in between the pin holes would be perfect) notch higher which I can't do without blowout right now so the motor has to come down on the transom at least an inch. I think I am just going to put it all the way down on the transom and see how it works. Then I will decide if I want to put more holes in the transom. Like I said before, the motor has more than enough power to plane my boat now so getting an extra 2-3mph from motor height is not a concern to me this year. When I had the 15hp on last year I was trying to get every mph I could out of it due to not having enough power. I have doubled my HP this year so no worries there, I just want it to run without rooster tailing and I also want to be able to get to the next notch higher to be able to raise my bow on command when in big waters. I took part of a wave over the bow last year because my 15 couldn't keep my bow high enough due to lack of power. When the 40+ foot boats come out in the summer and the idiot operators run them at half throttle and throw 6ft rollers I want to be able to lift my bow if need be. I usually don't go out on the big lakes here in the height of idiot season but when I am chasing the giant summer pike I can't help myself lol. And I have to cross the main channel where the big boats play (3 miles wide) to get back to the launch.