90hp e-tec OB 20' pontoon prop question & cavitation issue

[jeffkrol]

Well we bought a new 90 hp 2012 e-tec to put on a 20' Lowe Trinidad pontoon and we were set up w/ a 14-11 3 blade aluminum prop.
Problem is 1)we cavitiate occasionally and the motor is set all the way down.
2)Full throttle RPM's (per gauge) is about 5200. Evinrude recommends 5000.

now the question is we need probably a slightly bigger pitch and was wondering if a slightly different diameter would also be recommended. Would that help the cavitation issue? Usually occurs at mid-high throttle range and is somewhat tilt dependent. I don't expect trim/planing miracles but it does seem when I get the nose up a bit too high w/ the 2 of us in the back it occurs more frequently..seem odd to me.

Next thing is I do have a prop (4 blade Vortex 14-21) that I meant to try for fun and to see "what it does"..
Problem is I need a hub (interchangeable) since my current insert is for a Merc. Alpha drive. Mich. Wheel chart is confusing and I'm not sure which one (any advice on both the hub and feelings on what would happen w/ that prop?) I'm also looking at getting a 3 blade 13 3/4?-14 vortex Aluminum (SS just too costly for an ol pontoon in my mind) so the insert will kill 2 birds w/ one stone...
bottom line I will probably keep the 14-21 (got an old Chris-Craft deckboat w/ the Merc 470 that I may refurbish.. yea right ) and the new replacement for the 14-11 which seems only a bit off (If I solve the cavitation)

 

[jeffkrol]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

nothing huh............ oh well.

 

[steelespike]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

nothing huh............ oh well.

I think if you stay with the same size and go to a 4 blade it will help your rpm and ventilation problem.
If there is a choice of diameter go with the larger.

 

[jeffkrol]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

I think if you stay with the same size and go to a 4 blade it will help your rpm and ventilation problem.
If there is a choice of diameter go with the larger.

Thanks,
14 is about the largest size that will fit and an 11 is harder to find in a 4 blade.
I do need (5000 is the reported sweet spot) to knock the RPM's (roughly 5300 @ WOT) down a bit and can go as low as 4500 per manuf spec's......

I ordered a 4 blade 13 pitch 13 1/4 prop. When I get it on the boat I will report back.. It's only money....
Worse comes to worse it's a good spare. The hub insert is compatible w/ my 14-21 4 blade that I had on the Merc (Mich. Wheel Vortex) that never hit the water.
W/ luck the RPM hit will be about 200 or so.. by rough calculation (I believe). We shall see.
Actually it is: lose 400 RPM by pitch, gain 600 rpm by smaller prop size (200 per 1/4"), lose 400-600 by 4 blade.. Hope the 3 vs 4 is accurate enough. Going up would be the worst case
http://www.rundquist.com/how_change_pitch.htm
Will let you know when it's tested.. probably about a week or so..........

 

[jeffkrol]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

AS promised, a report..............
RPM (per boat gauge) tops out at 47-4800.. Not as high as I'd prefer and but not too bad.. (4500-5500 is the recommended range for this motor)
Seems to me the best w/ this 4 would be a 12 pitch.......

Didn't seem to lose much off the top speed BUT I need to verify w/ a GPS. forgot it........ sad.

Using the above formula I posted the "calculated" rpm range would have been 4900-5100 roughly.

Couple of comments wife thought it was "smoother".. I was wondering about a bit of added vibration for some odd reason....I could feel it on the deck a bit.. any comments on this??
Prop is a Solus Rubex...........

It did vent once at the beginning for some odd reason ( though it may have been trimmed a bit high at that time.. seems our trim gauge decided to stick) but another hour of test driving and it didn't reoccur.......
no regrets on going 4 blade

 

[jeffkrol]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

Just an update...
Took the 14-11 prop in to our local shop (North American Propeller Inc 5704 US Highway 51 S Schofield, WI 54476-1332) who had suggested I just get that one cupped a bit more..

Have to report it worked like a charm... No venting, 23mph(per GPS)/@4900rpm(per dash tach) ... I was literally amazed at the difference. STILL want to re-pitch the 4 blade though. It is a bit "smoother" and seems a bit more energy efficient at mid range rpms..

I was thinking of un-cupping it (the 3 blade) a bit to hit the 5000 spot but I'm not sure 1)if the tach is accurate) and 2)not really worth it except for pride since it does already vary by load. I think I could hit 5000 w/ a REAL LIGHT LOAD but 100 RPM'S is really no big deal.

So mostly just FYI's for others...........

 

[sublauxation]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

I have an '06 90 Etec and they told me to shoot for about 52-5400 RPMs. You can have a shop run the diagnostics and I'm pretty sure they can tell you the max RPM's the motor was run at for comparison vs your dash tach.

 

[steelespike]

Re: 90hp e-tec OB 20' pontoon prop question & cavitation issue

Check out the etec test reports they routinely run the 90 between 5000 and 5500
and specs say 5000 to 5500.
The 4 blade 21 is way too high. Even if you repitch your only going to gain 2 ".
And at that extreme it may compromise perfornmance.
Problem with your present setup is in the best case your below the minimum rating.
When you start loading up you drop even farther out of the range.Marine engines work hard
like going uphill all the time. For their health they need to be able to rev easily.
When you force a motor to run below its rpm range its kind of like hooking the boat and trailer to your car and run it up a never ending hill in overdrive.
While a high pitch might sometimes produce slighly higher speed at reduced rpm
very likely throttle setting will be higher and over time lugging the motor will create premature repair problems.