No tach ...yet, is this safe?

[ErikF]

Hey Everyone,

Apologies in advance for posting a question in the prop forum without all the necessary information.

I have an 18' Blue Fin Sea King, it's an aluminum boat very similar in design to a Starcraft Holiday, it has a maximum horsepower rating of 120. I recently picked up a 1973 115HP Evinrude to replace the '72 65HP Evinrude that blew a lower unit.

The 115 currently has a 14"x19 pitch prop that's in good condition. Running on the lake I don't think the engine is even hitting 4k RPM at WOT going by ear. I was thinking of swapping the prop off of my blown 65HP and going back on the water to see if the top end changes significantly and verify that I do have a prop problem (over-propped).

Do I run a real risk of over revving the engine by doing this or does the outboard have a rev limiter?

I will have a good tachometer soon so real numbers will be forthcoming, I'd just like to do this experiment now if it's safe as today is looking like a nice day and the forecast is for rain the next five days.

Advice?

 

[Bondo]

Re: No tach ...yet, is this safe?

Hey Everyone,

Apologies in advance for posting a question in the prop forum without all the necessary information.

I have an 18' Blue Fin Sea King, it's an aluminum boat very similar in design to a Starcraft Holiday, it has a maximum horsepower rating of 120. I recently picked up a 1973 115HP Evinrude to replace the '72 65HP Evinrude that blew a lower unit.

The 115 currently has a 14"x19 pitch prop that's in good condition. Running on the lake I don't think the engine is even hitting 4k RPM at WOT going by ear. I was thinking of swapping the prop off of my blown 65HP and going back on the water to see if the top end changes significantly and verify that I do have a prop problem (over-propped).

Do I run a real risk of over revving the engine by doing this or does the outboard have a rev limiter?

I will have a good tachometer soon so real numbers will be forthcoming, I'd just like to do this experiment now if it's safe as today is looking like a nice day and the forecast is for rain the next five days.

Advice?

Ayuh,... Does the 65hp prop even fit the 115hp motor,..??

If so, what pitch is it,..??

If so, try it...
Just be wary, 'n don't over-rev it too badly if the pitch is that much lower....

Generally speakin', the Prop is the rev-limiter on old outboards...

 

[Philster]

Re: No tach ...yet, is this safe?

The prop is the rev limiter IF it's not too small. If it's too small, it's not the rev limiter.

Short answer: You need a tach to play with props.

 

[steelespike]

It does appear they are the same prop. Just be aware that a 1" piutch change can add or subtract up to 200 rpm.
17" and 19" are typical sizes for the 115.
Won't hurt to try the prop just be aware of the rpm.
Won't hurt to be sure the 115 is in tip top tune. Timing set rightThrottle opening all the way.Don't assume that because its a new install everything is right.Also check the height
The antivent plate (just above the prop) should be even with the bottom or slightly above.
Should be set to a point just before venting becomes a problem.

 

[Texasmark]

Re: No tach ...yet, is this safe?

Back in the early 70's 60 to 75 omc loopers were common and 15 and 17P were the props of choice depending upon whether you wanted to water ski or not. So he is probably looking at a 2-4" reduction.

Mark