1977 Johnson 70 vs. 1999 Mercury 50

[johnjsa]

I currently have a 1977 Johnson 70 hp on my 15 1/2 Bomber Bassmaster boat. I have a CMC PT130 on it since the engine does not have power tilt & trim. I have the opportunity to buy a 1999 Mercury 50 HP with power tilt & trim at a good price. Would there be a great difference in power/speed with the 50 considering that the CMC adds about 45 pounds in weight. I am not interested in a lot of speed just reliability. I had trouble last year with the 70 not being able to run wot.

 

[Silvertip]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

Considering you are dropping nearly 30% in HP the logical conclusion would be yes -- you will notice a difference in performance. Very likely a big difference. If you can't run WOT, fix what's wrong. No need to buy a new motor!

 

[jtexas]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

3-cylinder 50hp merc only weighs 30 lbs less than the 3-cyl 70hp johnson. add the CMC, and the weight difference is about as much as my dog, and my speed & hole shot are unaffected by Dusty (15' kingfisher/70 rude). Same boat with 50hp would be excruciatingly slow.

With a minimal amount of care, your johnson is one of the most reliable motors ever made. Easy to work on and parts are readily available.

 

[johnjsa]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

With the Merc 50, it comes with power tilt & trim, so I would take the CMC off my boat, so it would be a total of 75 pounds less weight.

 

[Chinewalker]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

Also, keep in mind that the 1999 is rated at the prop, while the 1977 is rated at the powerhead. Horsepower difference is probably closer to 10, rather than 20...
- Scott

 

[Gary H NC]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

Plus the fact that the wiring harness and controls would all have to be swapped out...It would be better to get the original motor running good.

 

[kynolan2183]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

I would think that you would be really affected at hole shot. A buddy of mine has a Javelin 310a with a 50 hp rude and the hole shot has been a problem, ( the right prop has helped alot though from a 15 ptich to a 14). I have an Ozark SV-17 with a 70hp rude and my hole shot is about twice as fast. On top speed he runs 30 and I run 35. I think that is a displacement issue.
It takes torque to get the boat out of the hole.

Also always follow the 66% rule.
Never power a boat with less than 66% of its rated horse power.

Bubby's boat 70 hp rating with a 50 thus he needs 46.2 hp min and he is on the edge. It works but more ponies would be nice.


Mine boat 90 hp rating with a 70 thus I need to have 59.4 hp and I am over that by quite a bit more.

Kyle

 

[johnjsa]

Re: 1977 Johnson 70 vs. 1999 Mercury 50

Also I found a 2006 Mercury 60 elptefi which trim & tilt, which might get close to the 70 in performance.