Johnson 3-cyl vs. Mercury inline-6?

[Rotti]

Hello folks,<br /><br />I know this topic may sound strange but I have a question on this to all the experienced boaters in here:<br />I recently got me a 20" fishing boat with a moderate V-shaped hull design. The boat now has a 1983 Mercury 115hp inline-6 at its stern and the previous owner said its running well. Since Im not interested in top speed or holeshot whatsoever Im thinking about switching this huge motor to another model! I have a 79 model Johnson 3-cyl 70hp still in my shop, itll start but needs a major tune-up job- now my questions:<br />- which motor is more reliable?<br />- will the 70 give me enough power to get my boat into full planing? I only want cruising speeds of about 18 to 20 knots with a little bit of extra power left, thats it!<br />- is it possible to fit the Johnson 70hp with powertrim afterwards and if so, which units from what models and years would fit?<br /><br />Would be great if I could get some answers, thank you already for your interest!<br />Greetings from germany,<br />Rotti

 

[JasonJ]

Re: Johnson 3-cyl vs. Mercury inline-6?

As far as reliability, you are probably about even, depending on the true condition of each of your motors. That being said, the 83 Merc I6 is a very good motor, and is 4 years newer than the johnson. True usefullness, on the other hand, is a different story. On a 20 foot boat, a 115 horse motor is about the minimum power you can get away with. A 70 would be miserable. The key to what you want to accomplish is for the boat to be on plane at a cruising speed that does not have the engine maxed out. You are interested in low speed cruising, so once on plane, that 115 should not be maxed out. It will use less fuel, and will not be worked as hard. Mercury inline 6 outboards are very durable, and can run wide open all day, but you don't have to run them that way. The Johnson 70 you have will be running almost wide open to accomplish what you want, which will use more fuel than the Merc at part throttle. Also, if you have more people in the boat, the 70 may not get it on plane. Once the boat falls out of plane, you are overworking the motor, and wasting fuel. My opinion, stay with the Merc, you will be happier. My other opinion is that 150-175 would be a more appropriate horsepower for that size boat. It is always better to have more power and not use it than it is to not have enough power and need it. Good luck....

 

[DJ]

Re: Johnson 3-cyl vs. Mercury inline-6?

Rotti,<br /><br />The 70 is a reliable engine, for sure. But, remember, it is over 20 years old.<br /><br />In my opinion, a 70 would be grossly overworked on a 20' boat.

 

[JB]

Re: Johnson 3-cyl vs. Mercury inline-6?

Ditto DJohns.<br /><br />I think that 70hp is not enough for your boat.

 

[Rotti]

Re: Johnson 3-cyl vs. Mercury inline-6?

Huuuu,<br /><br />these are clear answer if not advices!! Thanks-a-lot folks! I knew that probably all of you would recommend even more power on my boat, but remember this: Over here a gallon of fuel is about 4.65$ converted to US-Dollars!!! Id go out rather with 175-200hp on my boat but nobody over here can afford this anymore- the ones who could, have already a 35" footer which they also cant afford any longer.... what fuelconsumption could I expect when running the Merc on 65% throttle? Thats what the previous owner did for cruising....<br /><br />greetings,<br />Rotti

 

[Rotti]

Re: Johnson 3-cyl vs. Mercury inline-6?

Oh, just ran into something: Ive made a mistake while converting our metric measurements to your imperial system: so my boat is a 16" footer and this might make a big difference?<br />Sorry for that!<br /><br />Rotti

 

[bossee]

Re: Johnson 3-cyl vs. Mercury inline-6?

Hi Rotti,<br />Your boat:<br />20 feet, moderate V shape.<br />How much is moderate V? 10, 15 degrees?<br />More V shape and You usually need more power to get up on plane. More V shape usually handles waves better.<br /><br />I have myself a 19 feet bowrider with 21 degrees V shape and a 115 hp 4-stroke.<br />I would never dream to have a 70 hp on my boat since it would not even plane then.<br />But maybe your boat will if it is not so much V shape and You are 1-2 person onboard. If You are say 3-4 or more persons onboard I doubt it will plane.<br /><br />There are some arguments to use the 115 hp on your boat:<br />1) easy to plane even with many persons onboard.<br />2) you can lower the RPM after you get on plane and get a comfortable cruise speed at much lower RPM and probably lower sound level than You could have with a 70 hp (if 70 hp is capable of get your boat to plane). Fuel consumption is probably equal or lower at cruise with the 115 hp compared to the 70 hp (at much higher RPM to achive and maintain plane speed).<br />3) 6 cyl on the 115 hp will probably mean less vibrations than the 3 cyl 70 hp.<br /><br />I do not know what a 115 hp Mercury -83 weight but can not be that much diffrence compared to a 70 hp Johnson? (50 kg more?)<br /><br />Any negative thing to use the Mercury 115 hp outboard?<br />Hard to find any negative arguments... Little more wight back there. Service cost maybe some more on the Mercury 115 hp? Maybe a thief will find a 115 hp Mercury 1983 more tempting to steal than an older Johson 70 hp.<br /><br />To fit a electric Power Trim and Tilt afterwards on the Johnson 70 hp is probably both costly and will require some work I imagine. I would not do it, but that is me... Hope I'm wrong so it is not so expensive and not to complicated to modify. Electrical Trim and Tilt is very usefull, I had it on my older Suzuki DT65 (1991) 2-stroke. I think I could not use a boat without it... to use the Trim to get on plane easy and Trim to get a nice cruise and top speed is so much easier.<br /><br />Good luck and happy fishing!<br />/Bo<br /><br />PS.<br />I see You write that Your boat is only 16 feet now. A 70 hp should probably get it on plane easy then. But the advantages of using the 115 hp is still valid above, I think.

 

[roscoe]

Re: Johnson 3-cyl vs. Mercury inline-6?

5 meter, 16'<br /><br />6.3 meter, 20'<br /><br />The 70 would probably work, but the cost of adding tilt and trim would be great, unless you find used parts.<br /><br />At full throttle, the 115 may use 40L of fuel per hour.<br />But at 20 knots, it would be running below half throttle, so perhaps 15-20L per hour.<br /><br />Although the 70 would use less at full throttle, it would be very comparable to the 115 at 20 knots.<br /><br />If you do extensive trolling, I would get a 15hp kicker motor, and save gas that way.

 

[rickdb1boat]

Re: Johnson 3-cyl vs. Mercury inline-6?

16'er with a 70 johnson, perfect!

 

[JasonJ]

Re: Johnson 3-cyl vs. Mercury inline-6?

Yeah, the 70 would be adequate on the 16 footer. I still believe the fuel consumption at part throttle on the merc will be reasonable. I run a 125 horse on my 16.5 footer, and if I just cruise at around 3200 rpms, I get decent economy. I say try both motors and compare. Its a pain, but if you do the work yourself, you will know for sure which motor works better.

 

[mellowyellow]

Re: Johnson 3-cyl vs. Mercury inline-6?

it really depends on the hull. what is the max.<br />HP rating? my 16' is rated for 135hp and 70 would<br />be way underpowered. my cousin has v.similar 16'<br />that is max. rated for 85 and performs very well<br />with a 70hp.

 

[Rotti]

Re: Johnson 3-cyl vs. Mercury inline-6?

Hello guys,<br /><br />thanks-a-lot for all your qualified answers again- now I get the idea! I was just asking becuase the big Mercs dont have the best reputation over here but maybe thats just because we dont know how to operate them correctly? The boat I own is a rather heavy 16-17 footer with a 20° V, so I guess itll need some horses to run..... The problem is that Ive owned only inflatable boats (RIBs) so far so I have no idea how certain horsepower work on a rigid boat! I had the idea with a good kicker as well, just for strollin around and I have a nice Honda BF100 4-stroke 10hp which should work quite well.... Ill leave the Merc on the boat now, might feel better to have some extra hp every now and then and if you guys think that the Merc can stand cruising at half open throttle the motor is fine for me! Over here they say that Mercs are only good for WOT and would not perform well at lower rpm for a longer time? I havent had a Mercury outboard yet so I can only rely on what I get from here- but so far Ive got a lot!!!<br />Thanks again,<br />Rotti