Prop ventilation?

[jrmarsh]

1978 Stuery 17’ 7ft beam tribulation Johnson 115 2 stroke.
Boat does fine working it’s way up to speed, when it hits around 4,000 rpm and getting up on plane the motor starts racing up to 8k rpm and the propulsion decreases significantly. Is this prop ventilation? If so, what can be done to fix it. There are two caveats at play here, one is that there is a kicker motor that was all the way down at the time. Could it be causing ventilation or cavitation?
The other is the middle of the prop appears to be only 6in below the center bottom of the boat when out of water.

 

[racerone]

What is the height of the transom.-----It should be around 20", and if it is 22 or 23 " the motor is mounted too high.

 

[Faztbullet]

Take that old 2 blade Merc prop off and install a 3 blade and raise the kicker to decrease drag on take off

 

[racerone]

OMC offered 2 blade bronze props as well.------Early 1970's I think.-------Have a few myself , 23" pitch.

 

[stormyj]

What is the height of the transom.-----It should be around 20", and if it is 22 or 23 " the motor is mounted too high.

I think this guy has the right idea. when you get on plane, the top of the prop is out of the water and causes cavitation, introducing air into prop water and allows the prop to over-speed and lose efficiency.

 

[jrmarsh]

I talked to a prop shop today because I’d read that getting a four blade prop as opposed to the three blade I have now could help with the prop ventilation. He told me to try bringing the trim up as I get up to speed (which feels counter intuitive) because it will make the prop dig in as low as it can be as the bow raises. Makes sense but of course an idiot like myself didn’t try that.

 

[Scott06]

I talked to a prop shop today because I’d read that getting a four blade prop as opposed to the three blade I have now could help with the prop ventilation. He told me to try bringing the trim up as I get up to speed (which feels counter intuitive) because it will make the prop dig in as low as it can be as the bow raises. Makes sense but of course an idiot like myself didn’t try that.

Only other thing you may want to rule out is the prop hub slipping. I assume you have an older pressed in rubber hub, these eventually fail and need to be rehubbed.

 

[Texasmark]

Besides the other good ideas, if OMC is like Mercury in that HP range the props suited for it are between 13 and 14" diameter. Divide either by 2 and you wind up with a number larger than 6...your reported depth of your prop shaft......but if your measurement was at the centerline of the boat, once off center the hull rises making the situation worse at the blade tip area.

At slower speeds the prop is fully in the water because the bow is high but as speed increases the bot drops and the water exiting from under the transom increases the angle with respect to the transom....meaning that your 6" puts the tip of your prop blades out of the water and ventilation is getting you on a basic (old) aluminum 2 blade prop.......but if you had a SS prop (3 blade as Faztbullet mentioned) with the attributes they normally possess and you were up to high enough speed, you would get a performance boost in MPH without the prop loosing its bite. BTDT

Also agree on his mentioning that your kicker should only be down when you are actually trolling.

 

[racerone]

Post the actual height of your transom here !!---Start with simple facts.

 

[Texasmark]

Post the actual height of your transom here !!---Start with simple facts.

Measured from where? CL or area that lets the most water across the blades which looks to be at least an inch different. Pros and cons to either dimension.

 

[Chris1956]

Measure the center of the transom down to the center of the hull. Most motors have multiple mounting holes. Advise us on which ones you are using.

 

[racerone]

With actual dimensions of the boat a decision can be made.-----About 20 years ago I had an enquiry about a motor jumping out of gear .----Asked party to measure the transom.----He said that it " can not be the problem " as it worked well before.-----I inspected gears and clutch dog .-----Found to be OK.-----I measured transom.----Asked party to cut the transom to 20" and no more issues after that.