Max RPM for 1982 Mercruiser 260???

[ingalp01]

I have a 1982 25ft. Chris Craft (Viking) Cabin Cruiser with a Mercruiser 260 I/O with 370 hours on it.

Runs like a champ, but I am curious as to what the max operating RPM's should be.

Currently in the 5000 to 5200 RPM range at WOT with a 15 inch, 16 pitch prop on it. I thought that was a little high, so I have been throttling back to keep it under 5000.

I also have a planing question, but am not worried about that here. Once on plane, it wants to run in the 5000 to 5200 range at WOT, so is that OK, or high???

Thanks...

 

[Bondo]

Re: Max RPM for 1982 Mercruiser 260???

Ayuh,... You need a Bigger prop,... Wot should be 4800rpms...

 

[ingalp01]

Re: Max RPM for 1982 Mercruiser 260???

Bigger, as in higher pitch, correct??? I have a 15 X 16, 4 blade on her now, and there is no way a larger diameter prop will fit. seems like a 14 1/2 by 18 would do the trick, and should drop WOT RPM's by about 400? I think...

 

[Aloysius]

Re: Max RPM for 1982 Mercruiser 260???

Those stock small block Chevy based engines were not meant to run at sustained full throttle operation. Sustained cruise should be 3500-3800 rpms or so.

It doesn't hurt to be slightly underpropped, as this will compensate for times when your boat is heavily loaded.

 

[ingalp01]

Re: Max RPM for 1982 Mercruiser 260???

OK, 3500 to 3800 would never keep my boat on plane, especially without the trim tabs, which I don't have at the moment. ( I have another post on that Topic. )

I thought the idea was to match the prop to the best maximum WOT operating range. You know, have a prop that will let you peg the throttle and still keep the boat under it's max sustained RPM recommendation...

Or at least where the HP and Torque numbers meet in the middle.

Now I'm really confused.

3500 to 3800???

4800???

5000 to 5200???

 

[Aloysius]

Re: Max RPM for 1982 Mercruiser 260???

horsepower and torque "numbers" ALWAYS cross at 5252 rpms. The peak TORQUE value is usually the desired cruise number. The maximum horsepower point, or slightly above it, is usually the desired maximum rpms you can attain.

If you can't keep on plane at 3800 rpms, you don't have enough power. Frankly, most straight inboard guys, including myself, seldom run much over 3500 rpms sustained, with older, stock engines.

 

[Gary H NC]

Re: Max RPM for 1982 Mercruiser 260???

I think about a 17 to 18 pitch prop would get you close to what you need.

 

[ingalp01]

Re: Max RPM for 1982 Mercruiser 260???

Thanks guys... Come to think of it, when not fully laden, 3800 probably would do the trick, but for the amount of time we spent cruising and tubing over the labor day weekend, I was pretty much maxed out most of the time. I had to throttle back to keep the RPM's under 5000, which I did, but the motor never seemed stressed and we ran around for two days like that.

When I get the tabs, thinking of going with the Smart Tabs II with 80lbs x 2 actuators = 160lbs of lift per side, then I may be able to get the right, say 18 pitch prop on her and have everything where I want it to be...

Thanks again...

 

[Utahboatnut]

Re: Max RPM for 1982 Mercruiser 260???

Well look at the redline in any older truck with a 350 in it at 5k your running in the redline. Yes I know a boat is not a truck but the engines are VERY similar between the two, even a motrohome redlines at 5k and that is basically a marine engine. Yes it will probably take it but for how long? I would stay more in the 4500-4800max range, once the prop is figured out 3800 should be more appropriate for cruising.

 

[6meter]

Re: Max RPM for 1982 Mercruiser 260???

4800 max, 3500-3800 cruising. Have you checked your tach with another known accurate one?

 

[rbh]

Re: Max RPM for 1982 Mercruiser 260???

There should be a stock red mercruiser tag on the rocker cover that
should/could say max 4700-4800 RPM.
Neighbour runs a 15x19 prop on a boat about your size and hit 50+- mph trimmed out at 4700 rpm.

 

[ingalp01]

Re: Max RPM for 1982 Mercruiser 260???

Sweet, I'd say 50+ would be very optimistic on my boat, but I think 32-35mph is attainable with the trim tabs and the right prop. Thanks again for all the help...