1989 Johnson VRO... maybe prop question!

[Cobia177]

So, I'm new to the forums. I purchased a 1986 Cobia Sunskiff 177 that has been partially redone, but I cannot identify the motor and have no idea how to prop it because the current aluminum prop is pretty beat up. He said the motor was a 1989 Johnson VRO, but it looks like a 1986 (the serial numbers are rubbed off all I can provide is the aluminum stamp number). The prop has absolutely no markings on it because it is quiet dinged up and worn. I'm just curious if anybody has any idea what size prop to place onto this motor and boat package (pitch and diameter). If there is 2 people or less on the boat it gets onto plane easily and does about 30mph WOT and trimmed. With 4 adult I have to send someone to the front to get it to plane quickly and does 28mph WOT and trimmer. The tach doesnt work on the boat so I am afraid to just throw a new prop on and see how it's running. Any advice would be appreciated!

 

[Cobia177]

Boat and Johnson 70 OB picture

 

[WesNewell]

According to info I found, the boat weighs 700# dry and the engine has a 2.42 GR and rpm range of 4500-5500. If that's correct I'd start with a 19P prop for 2 people, and maybe 17P for 4 people and gear. Engine mounting height is also critical for max performance and AV plate should not be below hull bottom when trimmed parallel to hull bottom.
Related info:
hhttp://www.kirschbaum.at/ki_prop/pdf/2015/RPM%20Chart.pdfttp://www.kirschbaum.at/ki_prop/pdf/2015/RPM%20Chart.pdf
http://boatspecs.iboats.com/Cobia_Boats__SS_177__1986/bp/66b21444

 

[Cobia177]

Thank you very much for your quick response and the info! This helps alot! I'm finally getting this boat back to it old glory.

 

[Cobia177]

So, I just found the size of the current prop it is 13 3/4 × 15p should I increase it to a 17p or 19p?

 

[Cobia177]

Condition of the current prop

 

[WesNewell]

Something is not right. a 15P prop should shoot that boat out of the hole even with 4 people in it. May be time for an engine tune up or replacement. Check engine. And verify GR. Easy to do. A 2.42 gr should take 2.42 turns of the flywheel to turn the prop one rev.

 

[Cobia177]

I mean it does get up and go, but idk I just expected it to be quicker to plane. Maybe it's because this is my first skiff and I've only had fish and ski's in the past with bigger outboards. The compression was checked and was good, but I cannot remember the numbers. They said everything works well, except I have a leak in my tilt and trim which I knew. I have the rebuild seals arriving monday. They were curious why the oil wasn't hooked up and I explained I opted to just mix the oil in the gas tank because I read a lot of horror stories about the VRO failing. It does run decent I am just wondering if replacing the prop would make a difference because it is beat up. Now that I've seen it is a 15p I am a little worried about the rpms.

 

[ahicks]

Another thought may be to have the boat weighed to see if it's the same or near the advertised weight. If you're lugging around a few hundred pounds of water logged foam, you'll notice that for sure!

Too, 4 guys in a boat that size will affect performance as well. Weight distribution important.

 

[WesNewell]

You really need a working tach and trim to know where you are prop wise. With everything in decent condition you should be able to get close to 40 mph at wot with just you in the boat and a 19p prop. May theo with a 15p is 32 mph @5500 rpm.

 

[Cobia177]

The trim works, it just has a slow leak so I have to top off trim fluid. I ordered the wrong spanner wrench and wasn't able to replace the seals so that will have to wait until I get the right one. I think my next must do will be getting the tach working. I've never had a boat I couldn't monitor where I'm at and it makes me nervous.

 

[Texasmark]

With a 2.42 gear ratio (where ever that as a number came from) I assume we are talking about 70 hp.....either you didn't tell us the HP or I missed it.

70 hp would be 3 cylinder and above that OMC went to V4 and 85 hp (Johnson Stinger excepted...at 75 hp triple), and a 2:1 gear ratio was popular. If in fact you are running a triple stay with what you have. 15 is what I was going to recommend before you posted it was 15. The prop that came with the boat was on there for a reason.....i'd bet.

I SERIOUSLY doubt that a 70 would: "a 15P prop should shoot that boat out of the hole even with 4 people in it."

 

[flyingscott]

How about we start with some correct information for the OP. Your motor is a 56 cid block so your RPM range is 5000-6000 rpm. What are your RPMs at WOT?

 

[Cobia177]

Texasmark, yes it is a 70hp I'm sorry I thought I posted that. It's in the picture but may be hard to see. Flyingscott that is one of my main issues. The tach doesn't work along with the speedometer so I am not currently sure what rpms at wot are. Hence why I have never run at WOT for more than 30 seconds at a time until I get it fixed. I usually back off from WOT after it planes quiet a bit.