Best Prop Size for twin Mercruiser 165's

[catmandu55]

This is my first post ever so please bear with me.

First some background info. I have a 26 foot 1976 Slickcraft Sport Offshore 255 Cabin Cruiser. I'm not completely sure about the weight but the previous owner told me it weighs somewhere around 10,000 lbs. It has twin Mercruiser inline 6 cylinder 165's in it. The WOT according to the manual is between 3900 and 4300 RPM. It currently has aluminum 3 blade propellers with a 22" pitch and about a 15" diameter.

Now for the problem. I've experienced some strange performance. The port side engine tops out around 3000 RPM. The starboard engine will top out around 5000 RPM if I let it. I did briefly see the port side engine bump up to about 4000 RPM one day so I'm not sure that the prop is completely responsible for the low output. Keep in mind that this boat sat in storage for 10 years before I bought it so it could be something else I haven't discovered yet.

I want to replace both props with new ones since they are pretty old and brittle. I've read that changing pitch can alter revolutions inversely about 200 RPM per inch. To fix my issue, however, would require running props with 2 different pitches... which I'm sure is not practical.

Has anyone seen anything like this before? Does anyone have a similar setup to mine that could recommend a good prop size? Any recommendations/insights would be greatly appreciated.

 

[Scott Danforth]

welcome aboard

are these inboards or I/O's

if the boat has been sitting for 10 years, I assume you did all the maintenance

verify that both props are the same. something is seriously wrong with your tach if you see 5000 RPM on an inline 6 as the motor design alone limits the air flow into the motor to about 4300 RPM max. most likely your tach is not reading correctly.

 

[catmandu55]

Thanks for the response, Scott.

They are I/O sterndrives.

Yes, I've put lots of hours and $$$ into it since last July to get it into working condition. I most recently replaced all of the bellows and shift/throttle cables. I actually started to wonder myself if they may be different pitches. Or maybe they are so old/used that they have deformed. I live about an hour and a half away from it though so I'll have to wait until the weekend to verify.

The tachs seem to work fine. I've got both engines idling at around 600 and at 750 while in gear, per spec. They seem to be very in sync with each other at low RPM based on sound and tach reading. Does it seem likely/possible that the reading would be off only at higher RPM?

 

[Scott Danforth]

use a shop tack to verify. since the tach on the dash can easily be switched from 4-6-8 cylinders, etc, it could be set wrong

take both props to a prop shop and have them repaired

 

[ahicks]

It's possible one of your props, the one turning 5K on occasion, is spinning on the hub. The rubber hub delaminated. You can confirm by marking the prop shaft and hub, then test drive and see if your marks still line up.

You'll want to keep the props the same size. 22" sounds like a lot of pitch to me, but not much experience with twins or boats this size. Point being, your port engine may be the good one!

Sounds like a nice rig. Best of luck with it.

 

[catmandu55]

Well I hit a log last week and took a decent chunk out of one of the starboard blades so that's why I'm looking into getting new props. But the behavior was the same even before that happened.

Yeah I guess it could be spinning. I'm thinking about just getting a couple 19" or 20" pitch props with the exchangeable hubs and going from there. At least then I'll know how each engine performs with identical props.