odd ventilation issue - twin F150s

[911grizzly]

I have a pair of 2006 F150s mounted on a 25' Pursuit. Factory installation, no mods. Boat sees little use, has low hours and is in mint condition, as are the motors. Stored covered on a trailer. I've owned it since new and have never experienced this problem before.

Today on a seatrial the starboard engine was intermittently ventilating (or cavitating) wildly when running at speeds of 3,000 RPM plus. (If the port engine was running 3500 RPM the starboard engine would suddenly accelerate to 4300 +, but with no difference in boat speed). This happened in conditions I would not ordinarily expect this to occur. Straight course, no turns, flat seas. It seemed to be only the starboard engine and it seemed a little more prone to this when trimmed up a bit, but it also did it when not trimmed. Backing off the throttle cured the problem, but then it would happen again. 3 or 4 times in a 25 minute run at moderate planning speeds.

The only unusual factor is that the boat has been laid up in storage for 18 months, but it was properly stored and the engines seem to otherwise run fine.

Any ideas?

[Punch953]

Prop hub slipping?

[ausnoelm]

Maybe looking at the trim gauge rather than where the engine really was?

[TownsendsFJR1300]

Sure sounds the prop hub slipping...

You can confirm this by trimming the other engine all the way up,
trim the "bad" engine down (all the way) and accelerate hard.

With the hub bad, the rev's (especially with a good load on it)
will go up quickly with no /little increase in speed..

There's NOT much else it could be..

[911grizzly]

OK thanks guys. I will pull both props today to find out what is going on. What causes the prop hub to slip (other than improper installation)?

[DennisG01]

Don't worry, it doesn't sound all that uncommon.

Some prop hubs have a pressed in rubber piece to act as a shock absorber between the hub/shaft and the actual propeller. The easiest way to think of it... like a car's manual clutch starting to slip. Works fine with a light load, but as the load increases it slips.

Another easy "try" (if engines aren't counter-rotating) is to swap propellers from side to side. Or simply try a spare prop and see if anything changes. But doing what was mentioned above about "loading it up" is quick and easy.

[TownsendsFJR1300]

It just wears out...

Pulling the prop, you likely WON'T see anything. The rubber innerds
aren't one piece anymore but two and slip NOW with load on it.

If left in gear, you may be able to rotate the prop (a good hub WON'T ROTATE OR spin)

I don't think you could screw up installing it if you tried...

Depending on the style, the prop shop presses out the old hub and presses in a new one....

[cpostis]

Don't worry, it doesn't sound all that uncommon.

Some prop hubs have a pressed in rubber piece to act as a shock absorber between the hub/shaft and the actual propeller. The easiest way to think of it... like a car's manual clutch starting to slip. Works fine with a light load, but as the load increases it slips.

Another easy "try" (if engines aren't counter-rotating) is to swap propellers from side to side. Or simply try a spare prop and see if anything changes. But doing what was mentioned above about "loading it up" is quick and easy.

You might want to try this:

How To Tell If You've Spun Your Propeller Hub | Boating Magazine

[ausnoelm]

It's not that hard, simply swap props with the other motor, and go for a test.

[99yam40]

swapping will not work well if one is counter rotating.

I have read about overheating the hub can cause them to fail

are the hubs the pressed in rubber or the newer plastic ones that can be replaced by anyone?

[pstephens46]

Put it on backwards

[ausnoelm]

Did someone mention he has counter rotation?

[TownsendsFJR1300]

Did someone mention he has counter rotation?

If it's a twin engine vessel, doesn't it have to have a counter rotating prop?

[DennisG01]

Did someone mention he has counter rotation?

The OP didn't, but that's why I mentioned "if" up above

[99yam40]

If it's a twin engine vessel, doesn't it have to have a counter rotating prop?

That is why I brought it up, most twins have one going each direction.

I have heard of them not having them and have heard of a single motor being counter. guess they got a good buy on it
lots of things we do not know about on this thread, like what kind of prop/hub it has