Cavitation and Props

[Joer2]

All,<br /><br />I recently repowered an 18CC Angler with a 115 Suzuki 2 Stroke outboard. Engine runs fine, but I think I'm having a problem with prop. The first prop I tried was a 3 x 13.25 x 18 stainless. It seemed fine, but could not get to the recommended RPM range, which is 4800-5500. Second prop was a 3 x 13.25 x 17 aluminum which got me to 5200 rpm, but cavitated when I tried to trim up to the correct bow angle at max RPM. <br /><br />One thing I noticed was that the cavitation plate on the boat is not at the keel level – it’s about 4” off the bottom. The folks who did the repower tell me that is how it should be. My question is: when at Max RPM, is the maximum trim angle less than it would be when I am running below max RPM? The boat seems to plow when I am at 5200 with the 17” pitched prop. Related question: Is there a prop that will get me to max RPM AND higher bow trim angle without cavitating? <br /><br />Sorry if this sounds dumb; I’m not that experienced with Prop problems, and trying to figure it all out. <br /><br />Joe

 

[Dhadley]

Re: Cavitation and Props

Are you saying the AV plate is 4" aboove the botttom when parallel? Or below the bottom?

 

[Joer2]

Re: Cavitation and Props

it's 4 - 5" above bottom when parallel

 

[Dhadley]

Re: Cavitation and Props

Well, that's gonna be tough to overcome. We'll take a shot but let's go slow.<br /><br />The motor is mounted to the transom, right? No bracket or jackplate, right?

 

[Joer2]

Re: Cavitation and Props

Right

 

[LubeDude]

Re: Cavitation and Props

Sounds like this isnt a performance hull, so its probably not a stepped transom. Sounds like they sold you a short shaft engine and should have sold you a long shaft.

 

[Joer2]

Re: Cavitation and Props

LubeDude. <br /><br />I measured the transom, just to make sure; it's a cutaway, 20". The engine has a mounting bracket with holes to adjust height, and it looks like it would be easy to lower it a notch or two. Boatyard guys tell me that it is prop issue - not sure I agree.

 

[Dhadley]

Re: Cavitation and Props

I dont think there were any short 115 Suzys, just long and Xlong (maybe). Anyway it looks like a 17 or 18 pitch may be where we want to end up. We just gotta find one that works that high.<br /><br />So far all you've tried are aluminum props, right?

 

[Joer2]

Re: Cavitation and Props

Dhadley,<br /><br />The engine came stock with a 3 x 13.25 x 18 stainless. That one gave me no cavitation, but didn't go to max RPM. I bought an aluminum 3 x 13.25 x 17 and that one got me to the right RPM range, but cavitated at higher RMP when I went to adjust trim. I think I should ask them to drop the engine a notch before trying a new prop. The engine is a 20" shaft.

 

[Dhadley]

Re: Cavitation and Props

So the motor is mounted on the transom but not all the way down?

 

[Joer2]

Re: Cavitation and Props

right, there is plenty of room to drop her, but the mechanic tells me that the right way to mount it is to have the anti cav plate a few inches higher than the keel (when parallel). Not sure I agree.

 

[Dhadley]

Re: Cavitation and Props

No, thats right but it depends on the prop youre going to use. Or I should say the style of prop. <br /><br />Is the AV plate a true, even 4 inches above the bottom?

 

[Joer2]

Re: Cavitation and Props

the engine is trimmed up just a bit for winter storage (wrapped in plastic all the way down to the skeg), so It's an estimate, but it looks to be just about 4" above keel

 

[Dhadley]

Re: Cavitation and Props

OK, the ss was fine but not enough rpm. The aluminum probably will vent that high. We just need to find a good ss prop with less pitch. What rpm is the ss 18?

 

[Joer2]

Re: Cavitation and Props

The SS 18 got me to around 4800. The aluminum 17 got me 5200. One other thing; I'm confused about RPM range. In the official Suzuki service manual, in the section labeled “Lower Unit,” the recommended speed range is listed as 4800-5500 rpm, but in the specs chart, it lists "recommended operating range" as 5500-6000 rpm. I don't quite get it - but I think the prop should allow me to hit 5500 RPM. <br /><br />Thanks for your patience - I really appreciate it. <br /><br />Joe

 

[Dhadley]

Re: Cavitation and Props

On todays fuel I would look for 5800 with the average load. <br /><br />What brand prop was the 18? I have a 18 Turbo I was thinking about trying but that's not gonna get more rpms.

 

[Joer2]

Re: Cavitation and Props

both props were from suzuki