2021 Mercury 115 Pro XS cavitating

[jerseydevil]

Got the break in period in. Around 3000 rpm or so the motor feels like it's cavitating. Trimming the motor up seems to help a bit & steadies out around 4500 rpm. Called the dealer & they suggested moving the motor down one mounting hole. It only has one hole left to move the bolts to. Plus I would think if that's the case, it would shutter at the lower rpm's as well. Anyone have similar experiences or suggestions?

 

[jimmbo]

Cavitating or Ventilating? Lowering the motor will reduce Ventilation, but not Cavitation. Normally trimming the motor in, not out, reduces Ventilation, as it buries the Prop deeper in the water. Can you post a few Well Taken Pics of your setup. By 'well taken', I mean shot parallel to the ground, at Boat Bottom-Anti-ventilation Plate height, with AV Plate parallel to the Boat Bottom

 

[jerseydevil]

Cavitating or Ventilating? Lowering the motor will reduce Ventilation, but not Cavitation. Normally trimming the motor in, not out, reduces Ventilation, as it buries the Prop deeper in the water. Can you post a few Well Taken Pics of your setup. By 'well taken', I mean shot parallel to the ground, at Boat Bottom-Anti-ventilation Plate height, with AV Plate parallel to the Boat Bottom

Roger that. Will post soon as i can get some pics after work. Thanks

 

[QBhoy]

Hi. I think I might know what this is. Although I thought they had sorted the issue by the time your motor was made. But certainly on earlier models like mine 2016 and just after…this was the known issue with them.
So behind the prop is a white plastic exhaust bore seal ring. Terrible design and can come loose a little. It then allows exhaust bubbles to flow over the blades of the prop at certain rpms…and cause a venting effect. Can also sound like the engine is miss firing too, at lower rpm. Certain props are more affected by it.

 

[QBhoy]

Just in addition. This can also happen if the prop isn’t far enough home and able to seal next to this sealing ring. What prop is on it now ? But see if you can see where the back of the prop is in relation to this white seal ring. definitely take the prop off and have a look of there is any doubt.
The same affect can also happen if you have a pvs vented prop and have the black plugs holed or missing altogether perhaps. The exhaust bubbles will flow over the prop and feel like a venting (almost clutch slipping effect) until way up the rpms. I tried running more pitch on the prop on my 115 not long ago, meaning I had to use holed pvs plugs for better planing. They didn’t seal and stop the venting affect until nearly 4000 rpm. Horrible feeling I thought. Just went back to using the 19” and 20” with full plugs and no venting.
To solve the exhaust bore seal. I stuck it in place

 

[jerseydevil]

Hi. I think I might know what this is. Although I thought they had sorted the issue by the time your motor was made. But certainly on earlier models like mine 2016 and just after…this was the known issue with them.
So behind the prop is a white plastic exhaust bore seal ring. Terrible design and can come loose a little. It then allows exhaust bubbles to flow over the blades of the prop at certain rpms…and cause a venting effect. Can also sound like the engine is miss firing too, at lower rpm. Certain props are more affected by it.

Great info. I'll definately check the position of the prop for sure. Thanks.

 

[jerseydevil]

Just in addition. This can also happen if the prop isn’t far enough home and able to seal next to this sealing ring. What prop is on it now ? But see if you can see where the back of the prop is in relation to this white seal ring. definitely take the prop off and have a look of there is any doubt.
The same affect can also happen if you have a pvs vented prop and have the black plugs holed or missing altogether perhaps. The exhaust bubbles will flow over the prop and feel like a venting (almost clutch slipping effect) until way up the rpms. I tried running more pitch on the prop on my 115 not long ago, meaning I had to use holed pvs plugs for better planing. They didn’t seal and stop the venting affect until nearly 4000 rpm. Horrible feeling I thought. Just went back to using the 19” and 20” with full plugs and no venting.
To solve the exhaust bore seal. I stuck it in place

Not sure what prop is on it now. I don't believe it's vented. I'm posting some pics for jimmbo shortly.

 

[jerseydevil]

Cavitating or Ventilating? Lowering the motor will reduce Ventilation, but not Cavitation. Normally trimming the motor in, not out, reduces Ventilation, as it buries the Prop deeper in the water. Can you post a few Well Taken Pics of your setup. By 'well taken', I mean shot parallel to the ground, at Boat Bottom-Anti-ventilation Plate height, with AV Plate parallel to the Boat Bottom

jimmbo here's some pics that hopefully capture what you wanted to look at.

 

[jimmbo]

A pic at that height from behind would be handy, as it looks like your hull has a Setback transom. Height should be easy to adjust as you have a Jackplate

 

[jerseydevil]

A pic at that height from behind would be handy, as it looks like your hull has a Setback transom. Height should be easy to adjust as you have a Jackplate

Will do.

 

[merc850]

It looks to me that the motor is just too high, the anti-cav plate should ride slightly below the surface of the water when running. Have someone drive it when you're running it on the water and you look at its height.

 

[jimmbo]

On my last Outboard, the Anti-Ventilation plate was a good 2 inches above the Bottom of the hull, and the prop had a good bite on the water

 

[jerseydevil]

On my last Outboard, the Anti-Ventilation plate was a good 2 inches above the Bottom of the hull, and the prop had a good bite on the waterView attachment 360890

Jimmbo
Here's the view from the stern like you wanted. I have no set back & the prop looks like it's vented with a 23 pitch?

 

[jimmbo]

I can tell that's a Mercury Prop. What Model is it, Laser, Mirage, Tempest, or other?(I'm feeling lazy and don't want to look it up)

It looks like you boat places the Transom a few inches back from where the Bottom ends ahead of the engine(setback), that allows for a slightly higher engine height. Also you have the engine mounted on what looks like a Jack Plate, which provides a bit more Setback. With the correct Prop, the Engine will be fine at that height, and could be even a bit higher.

 

[QBhoy]

Jimmbo
Here's the view from the stern like you wanted. I have no set back & the prop looks like it's vented with a 23 pitch?

Looks like you’ve got the prop vented at the pvs plugs. That’s what’s happening.

 

[QBhoy]

Jimmbo
Here's the view from the stern like you wanted. I have no set back & the prop looks like it's vented with a 23 pitch?

Hi...Just looked on the laptop there, at the pics...you have no vent plugs in that prop at all...That is 100% your issue. The 3 holes you have there (one between each blade at the forward end of prop) should have some form of PVS black circular plugs in there. from memory, there are 3 options. Some have a hole about half diameter, some a tiny hole and others are full closed plugs for no slip at all. The way yours are are full bore and must feel terrible until really high up the rpms. It will be useless for anything less than fast planing speeds and feel horrible at it too. Look up "mercury propeller pvs plugs" I can bet my life that this is your issue..no doubt about it.

 

[QBhoy]

Jimmbo
Here's the view from the stern like you wanted. I have no set back & the prop looks like it's vented with a 23 pitch?

In fact...looking again...it actually looks like (on picture 3) you have one hole (top of prop in picture) with a plug in it, an one hole without one at all (bottom of prop in picture) defo the issue.

 

[jimmbo]

When I looked at the 2 pics, I could see Plugs in the Holes.

 

[QBhoy]

When I looked at the 2 pics, I could see Plugs in the Holes.

Look at that first picture again. The lower one has no plug in it. You can see the lip it should sit against when it’s in place. Totally missing, looks like.
Edit***lower one looks like it’s there but is vented. Other one is full bore closed. Weird

 

[jimmbo]

All the Plugs have curvature, even the ones with vents. The one at the top is rotated enough to make a determination difficult, and is only about 69 pixels across in that image