Prop is either venting or spinning. (over revving)

[mcattac]

We just bought a 1979 Mirrocraft 16' aluminum boat.
It has a 79 Merc 20hp 2-stroke on the rear.
The motor is a 20" long shaft.
The boat is basically flat at the stearn.
The prop was spinning as we would (almost) get up on plane.
We marked the prop to see if the rubber hub was spinning and it was.
So we bought a new prop.
Same thing is happening.
It's over revving!

The anti-cavitation (anti-ventilation) plate is about 2" below the hull.

(Please no lectures on the difference beteen cavitation and ventilation - you know what I am refering to)


Could it be too low in the water?
I wouldn't think this would ever cause the prop to over rev.

It has a new prop. Is it possible the new prop is also spinning on the rubber hub? I guess it is possible, but not probable.

The anti-cavitation plate has 6 small 3/8" diameter holes in it.
A previous owner must have had a trolling plate or hydrofoil on it at one time.

Is it possible that the water is sucking air through those holes like a venturi causing the prop to vent?

Went trying to get up on plane the anti-cavitation plate is right at the water's surface. This gets me to thinking I could still lower it more to get the prop into cleaner water. But like I said, it is already about 2" below the hull.

We will put another mark on the new prop and see if it is spinning on the hub, but I seriously doubt a brand new prop could be spinning.

Any ideas?

Mac

 

[5150abf]

Re: Prop is either venting or spinning. (over revving)

Maybe put some duck tape over the holes just to see if that is the problem, I could see if sucking enough air to cause it to cavitate.

Sound like the engine is low enough and I really doubt a new prop is spun too.

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

The boat is actually my son's boat.
It's his first.
I told him to do the duct tape idea also.
Also told him to mark the new prop and hub with a small dot of white paint using a Q-tip. Do NOT score it like we did to the old prop. (In case it needs to be returned)

Then take it back out to see if the duct tape worked, and if not, if the hub on the new prop spun.

Pouring rain today so I doubt he will go out.
But this will give the paint time to dry.

He's now talking about a permanent jack plate to mount the motor further rearward and lower. I keep telling him this isn't needed.

All he wants to do is spend money.

I'll keep you informed as to what we find.

Thanks.
Mac

 

[ddrieck]

Re: Prop is either venting or spinning. (over revving)

He's now talking about a permanent jack plate to mount the motor further rearward and lower. I keep telling him this isn't needed.

All he wants to do is spend money.

When he's ready to spend his money on a Jack Plate send him my way so I can tell him he needs to go up....not down.

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Up?

To get rid of the over revving?
Or simply to increase speed?

Please explain.

Mac

 

[Nightfisher-]

Re: Prop is either venting or spinning. (over revving)

I am by no stretch of the imagination an expert on marine mechanics, but it might be the prop itself. My 115 Johnson will cavitate at higher rpms with one of the two props I have, but not the other. Can't remember the pitches of the 2 props right now. Is the new prop you have exactly the same as the other? If it is, try another with a different pitch and see if it solves things. Of course changing the pitch will also effect performance too, which could be either good or bad.

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

According to the place we bought the prop, there are only 2 props available for the motor.
A 9" and a 13" pitch.
The 9" is the most common and is what we have.
The 13" is a special order.

Mac

 

[foodfisher]

Re: Prop is either venting or spinning. (over revving)

Cavitation means its getting air where water is supposed to be. At the depth you have the motor at, the only way air can get to the prop is ths shape of the hull. Is there a step allowing turbulence to reach the prop? Otherwise overreving must be a spun prop or wrong pitch. Maybe you need the 13p

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Cavitation means its getting air where water is supposed to be. At the depth you have the motor at, the only way air can get to the prop is ths shape of the hull. Is there a step allowing turbulence to reach the prop? Otherwise overreving must be a spun prop or wrong pitch. Maybe you need the 13p

Actually, ventilation is getting air where the water is supposed to be. But I asked not to open this can of worms, so I should stop.

The hull is flat back there. No step. Just a simple and standard aluminum Mirocraft 16' hull. Nothing crazy.

As for the pitch, it isn't that. I know what that acts like. When your pitch is incorrect, the motor will over rev, but at a smooth rate. It will simply rev too high.

What is happening here is it grabs, then over revs, then grabs, then over revs, etc. It wants to push water, then it slips.

But because it catches here and there and almost surges inclines me to believe it is vetilating, not the hub spinning. Typically, once a hubs breaks loose it doesn't grab unless you lessen the RPM.

I'm pretty much convinced that it is air.

He installed some bolts in the holes.
(Dad don't know ship, so he spent cash on ss bolts, nuts, and washers instead of using my duct tape idea)

There is also a small hole up top on the rear of the tail shaft above the prop. The hole is an access hole to allow you to get at the allen bolt that secures the sacrificial anode above the prop. This hole is supposed to have a rubber plug in it but it is missing. And it vents water and exhaust through this hole while running. I had the motor in a trash can and when I put my finger over the hole while it was running I could feel water coming out. So I told him to go spend money (hehehehe) on a small rubber plug to cap that hole. I doubt that is where it is getting the air from. But who knows?

 

[ondarvr]

Re: Prop is either venting or spinning. (over revving)

Post a pic of the rear of the boat so we can see what's being discussed.

 

[Andy in NY]

Re: Prop is either venting or spinning. (over revving)

the hole in the rear of the lower unit should be spitting out water. im not a "mechanic" by any stretch... but i did stay at a holiday inn express once. seriously though, every motor ive had has that hole that spits out water.


9p prop seems awefully small for a 16' boat... even with a 20hp motor.


i doubt its a spun hub because it would slip all the time, not slip-grab-slip-grab like you describe.




at the risk of sounding dumb, is it possible you are poirpising? if the stern is getting pushed up too far due to the motor being too low, then back down, then up again, etc...

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Might be porposing, but not a lot - I go 280+.



Here's a few pics.

BTW, the hole I am speaking about is just above the casting over the prop.
It's not the water pee hole that indicates cooling water flow.

 

[foodfisher]

Re: Prop is either venting or spinning. (over revving)

Where does the exhaust exit that motor? Could the exhaust be causing the cav er problem?

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Where does the exhaust exit that motor? Could the exhaust be causing the cav er problem?

Thru the prop

 

[bonz_d]

Re: Prop is either venting or spinning. (over revving)

Silly question but is this prop one that uses a hub kit? Like the Michigan Wheel Vortex. If so was the hub replaced also or just the prop?

What kind (make) of prop is this and what is the prop size? Diameter and pitch.

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Nope, not like a Michigan.

Prop and hub can't be taken apart.
It can be by a pro, but not like a Michigan unit.

It is a standard Mercury prop.
Unsure of the diameter, but pitch is 9.
9" is standard prop for this motor.
13" is also available, but special order.

The diameter was what the shop manual calls out for, but the diameter escapes me. I'm not at home. I'll check later and post.

 

[ondarvr]

Re: Prop is either venting or spinning. (over revving)

Most of the time using a motor with too long of a shaft will create some issues (poor handling, excessive water spray, less top speed, etc), but still works.

Having it ventilate would be odd, but that's what it sounds like its doing from you're description.

You can raise the motor until the clamps are right at the top of the transom, see if doing this changes anything.

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Most of the time using a motor with too long of a shaft will create some issues (poor handling, excessive water spray, less top speed, etc), but still works.

Having it ventilate would be odd, but that's what it sounds like its doing from you're description.

You can raise the motor until the clamps are right at the top of the transom, see if doing this changes anything.

The clamps ARE at te top of the transom.

 

[Banditz]

Re: Prop is either venting or spinning. (over revving)

Dude, your hunch is correct...just duct tape that hole up...its not a water outlet at all, but s just acting like one cause the friggin plug is out. If that anode was installed you wouldnt even have an issue at all. Your venting at higher speeds cause as the exhaust builds up it pushes out that hole instead of the prop...So tell your son to go spend money on the anode not a plug...but duct tape it first and you will see a marked improvement, I promise...


Oh and tell your son....Sometimes it pasys to be older and wiser, not younger and richer!

 

[mcattac]

Re: Prop is either venting or spinning. (over revving)

Really - thanks but the anode is still there.

Mac