Speed Plateaus at 5200rpm

[jimmbo]

Can you take one with the AV Plate parallel to the bottom of the Hull? The ones you have posted look like the engine is trimmed in fully

 

[jlh3rd]

I'd lower the motor. there's a gap at the top of the transom, so it looks like that might be possible?
If that doesn't solve what looks like a ventilation issue, then I guess look a t the prop again.....
The AV plate isn't parallel, but close enough..but looks like trimming the motor level will still put the AV plate at the correct "Starting " point for initial setup....emphasizing starting point...because that's where initial rigging begins, with the hope it works....
I was surprised to see on my boat how lowering the motor just the 1/1/2-2 inches made a significant difference in my ventilation issue.....

 

[Texasmark]

Here are the requested pics. I’m uploading from my phone so hope they turn out ok. Took one extra photo to show how the plate lines up with the transom pretty spot on.

Lineup looks OK but lowering the engine an inch would help. Just commenting on pictures, not other responses on height. Also, I don't detect any "cupping" in the SS prop.....which are usually cupped.....each prop design determines just how much cupping is right for that design.....lots of cup is not necessarily correct in the prop design......per comments from prop design folks. Cupping helps to prevent blow out and is a must for high mounted engines to maintain prop grip close to the surface.

 

[jakec]

So I actually previously had the Mercury 1150 mounted too low. I had the AV plate about 1.5" below the hull. This was because I just set the outboard on the top of the transom and bolted it down, and didn't think about where the AV plate aligned. But I also had a problem with the lip of the transom being a radius creating spray. So in the end it's hard to tell much from that experience because I don't know if my problems were due more to the outboard mounting or the transom lip.

Thread

Anyways now the transom lip is fixed. And the mounting height is theoretically correct. I don't have any spray. And the hole shot is fantastic. The plate above the AV plate is fully out of the water, deflecting spray downward toward the AV. So the AV is out of the main body of water, but is covered in a shower of water from the plate above. Hope that makes sense.

BTW the prop is not stainless, if that's what you meant.

I'm confused why I would want to lower the outboard, looking at this graphic, it seems counterintuitive to what I want (more speed?)

 

[Stinnett21]

Can't tell from the pics of the prop if it's pressed hub but it is possible his prop is slipping above 5200?

 

[Stinnett21]

So I actually previously had the Mercury 1150 mounted too low. I had the AV plate about 1.5" below the hull. This was because I just set the outboard on the top of the transom and bolted it down, and didn't think about where the AV plate aligned. But I also had a problem with the lip of the transom being a radius creating spray. So in the end it's hard to tell much from that experience because I don't know if my problems were due more to the outboard mounting or the transom lip.

Thread

Anyways now the transom lip is fixed. And the mounting height is theoretically correct. I don't have any spray. And the hole shot is fantastic. The plate above the AV plate is fully out of the water, deflecting spray downward toward the AV. So the AV is out of the main body of water, but is covered in a shower of water from the plate above. Hope that makes sense.

BTW the prop is not stainless, if that's what you meant.

I'm confused why I would want to lower the outboard, looking at this graphic, it seems counterintuitive to what I want (more speed?)

Raising the motor can give more speed as long as it's not raised too far. Others are wondering if yours is raised too far and loosing bite as it nears the top end.

 

[jakec]

I believe if you zoom in on the dead on prop pic, you can see the hub, it is a light grey color, and it doesn't appear to have ever slipped. The edges are nice and sharp.

 

[jlh3rd]

" top/comfortable speed increased to 34mph. The speed plateaus at about 5200rpm (max 6200). Since the engine is under 10 hours I've only had the rpm up to about 5800rpm."

isn't this, as you stated, the original problem?....your speed stays at 34 mph starting at 5200 rpm up to 5800 rpm....and probably at 6200 rpm?...so something is "slipping"...
didn't you say it's not the prop hub?.....
which means the prop is "slipping "....
so you had it lower.....and no good....so

maybe you DO need to try another prop.......

 

[Texasmark]

So I actually previously had the Mercury 1150 mounted too low. I had the AV plate about 1.5" below the hull. This was because I just set the outboard on the top of the transom and bolted it down, and didn't think about where the AV plate aligned. But I also had a problem with the lip of the transom being a radius creating spray. So in the end it's hard to tell much from that experience because I don't know if my problems were due more to the outboard mounting or the transom lip.

Thread

Anyways now the transom lip is fixed. And the mounting height is theoretically correct. I don't have any spray. And the hole shot is fantastic. The plate above the AV plate is fully out of the water, deflecting spray downward toward the AV. So the AV is out of the main body of water, but is covered in a shower of water from the plate above. Hope that makes sense.

BTW the prop is not stainless, if that's what you meant.

I'm confused why I would want to lower the outboard, looking at this graphic, it seems counterintuitive to what I want (more speed?)

No doubt, the higher you can get the LU out of the water the lower the drag and higher the speed and RPMs. Just how high depends on a lot of factors and mainly selecting a prop that runs shallow and doesn't blow out. Without that you are relieving yourself into the wind......ugh....grin.

I had an original Mercury Laser SS on my Ranger bass boat and I had it raised off the transom, forget how much and that hull was a pad hull with offset transom (6") behind the end of the pad. That prop was comfortable with the blade tips at or near the surface. That I6 115 ran that boat about 50 and at 6k RPMs. Ran it like that for 7 years and sold it to my BIL who ran it for who knows how many more years. One of my interesting aspects of that boat and that top end is that I could hear the powerhead of the engine singing while the tips of the prop were apparently airborne and you could hear a second sound....prop humming as the blades beat the water.

 

[jlh3rd]

maybe he's one of the few who could actually benefit from a cavitation plate device.....

 

[Texasmark]

maybe he's one of the few who could actually benefit from a cavitation plate device.....

Er ah Antiventilation Plate. Cavitation is most prominent on propeller tips and the more dings the worse it is. Antiventilation is the purpose of the plate above the prop.....reduces the propeller's ability to suck surface air into the slipstream and cause ventilation....air where water should be.

I am guilty of the misnomer. All my life it was the AC plate. Join this forum and get SMACKED.........I am now a convert.

 

[jlh3rd]

Er ah Antiventilation Plate. Cavitation is most prominent on propeller tips and the more dings the worse it is. Antiventilation is the purpose of the plate above the prop.....reduces the propeller's ability to suck surface air into the slipstream and cause ventilation....air where water should be.

I am guilty of the misnomer. All my life it was the AC plate. Join this forum and get SMACKED.........I am now a convert.

lol..
you're right....and I know the difference....

but, eh, I'm not alone...including the navy (see last one)

 

[Texasmark]

lol..
you're right....and I know the difference....

but, eh, I'm not alone...including the navy (see last one)

Well, obviously I picked it up in my limited boating circles when a teen. I see the misnomer is widespread. Course, people who write articles for the news media don't necessarily know what they are talking about either. Case in point the word "re-iterate". it has to be said and repeated (iterated) to qualify for being re-iterated. Course to use the wrong word sounds like you know what you are talking about.

 

[jakec]

Given the difficulty of moving the engine on the transom, I'm not going to experiment there right now. I would be fine with my current speed, rather than go to that length.

I do have an AV plate whale tale that came on my previous outboard. I could put it on.. but not sure if I want to go drilling holes in the new outboard. I would be more likely to get a new prop first I think. Maybe I need to call the local prop shop and see what they say, and if they will loan out some props so i don't have to buy them just to experiment. Seems like after all this discussion we are still pretty much stumped on what the source of the problem is.

 

[Willyclay]

Seems like after all this discussion we are still pretty much stumped on what the source of the problem is.

After so many years of that beauty sitting on a trailer, could your hull possibly have some deformation, hook or rocker? Good luck!

 

[jakec]

I don't think so. It was on a nice 4 bunk trailer when I got it, and I swapped it to another 4 bunk trailer. Anything Is possible I suppose but there's nothing I can see with the naked eye.

 

[jlh3rd]

yeah...frustrating as heck....nice looking boat, ready to enjoy except for this...
I wouldn't want holes in a new av plate....
owning a pontoon boat for a while coupled with some trial and error learning curves and reading about pontoons, the general consensus concerning 4 blade props is that they help with ventilation issues.....that's what "they" say.... I have no practical experience.....
They do make "whale tails" that clamp on....no drilling....I couldn't recommend any...but...If I was in your shoes and frustrated with a problem like this.....trial and error might be a last resort.....

 

[jakec]

I was thinking maybe I can take the boat to the prop shop, that way they can look at the transom, hull shape, outboard mounting, current prop etc, take all that into consideration, and then make a suggestion.

 

[Texasmark]

I believe if you zoom in on the dead on prop pic, you can see the hub, it is a light grey color, and it doesn't appear to have ever slipped. The edges are nice and sharp.

You need to remove the prop to see whether or not the hub has slipped, monitoring YOUR scribe line connecting the splined hub to the outer barrel.

Glad you made it back on here.....the saga continues. Good luck.

 

[jakec]

We didn't get to take the boat out for a couple months and just had it out this weekend. I was messing around in the upper range again and found this.

I pushed past 5,200rpm and maintained for a little longer. The speed peaked at 35mph at 5,800rpm. If I pushed throttle forward further, rpms seem to stop climbing.