Mercury I6 115 '83 -/+?

100333624

Petty Officer 1st Class
Joined
Apr 24, 2012
Messages
205
Hi all, I've decided to dump my motor ('78 mercury 800, 4cyl) and go with an inline 6 mercury tower of power 115... I just know there are some differences depending on which one I go for, and there's quite a few available in my area from 1966 - 1988! Lots!

So the first question is, I've heard that up to 1983 the mercs were 115hp at the crank/flywheel, correct?? This would mean roughly 95hp at the prop? And 1983 and after they were 115 at the prop, which would mean roughly 140hp at the crank, correct? Please correct me if I'm wrong...!

Also, what were the weight differences between those 2 models, let's say a 1978 and a 1983? I'm trying to find the best power to weight ratio motor to put on my 16' but I'm thinking 145hp at crank would be way too overpowered? I currently have an 80hp mercury 800 (65hp at prop?) and she pushes me at 40mph. I wanna get into the mid 50s, do I go with a 115 pre 1983 or a 115 after 1983? I don't wanna go with the 90hp tower because it's roughly the same weight as the 115 and so why not have 25hp more out of the same motor right? The way I see it, 115 pre 1983 is probably my better choice as it'll be roughly 90 or so hp at the prop, thats almost 30 more hp ontop of what I now have, and considering the weight differences and a high pitch stainless prop, this should get me into the mid 50s? Also, the 115 after 1983 would mean I'd have almost 50hp MORE at the prop, if I'm already doing 40mph just imagine how fast this would be, too fast don't you think? I don't wanna fly I just wanna be a bit faster then where I am and fuel efficient as well, but I guess the fuel efficiency went out the window with wanting a 115 tower of power hahah :p

My boat is fairly light for a 16' fiberglass, it's probably a bit over 200 lbs dry. I'm 200 lbs, my buddy is 150 lbs, plus a 15 gallon tank full and 2 small bluetop batteries and some light gear.. Probably around 1000lbs including the weight of the motor! Average load is usually with a buddy, so let's say 1100 lbs wet! What is my best choice for getting into the mid 50s? Give me some insight on this you merc gods!
 

mr 88

Commander
Joined
Nov 3, 2010
Messages
2,262
Re: Mercury I6 115 '83 -/+?

All the inline 6's weigh about the same.The newer the 115 the better,differences with distributor-,exhaust,carbs,pistons-hi dome vs lo dome reed set up and fuel flow all come into play. Your boat probably weighs closer to 500 than 200. My 15' power cat which has no subfloor or windshield etc comes in around 400 and that is light. Looking for some merc speed gurus? go to facebook and search- inline 6 mercury outboard motor fans- there are some old time speed guys on there. There was and it may still be listed in the Buffalo Craigs a 1972 twister with a SSM lower and bp1250 mid,asking 2 grand,that would move you into the 60's and higher if setup correctly. The 115 will put you into the high 40's,maybe and I mean maybe 50 depending on your hull design and how you setup your motor on the transom.My PC which is the original tunnel boat with a rebuilt shorty 1500,new hi dome pistons,crank balanced,rods and pistons weighed out and motor jacked about 2" hits mid 50's gps with one person. Can I make it faster,yep by jacking the motor some more,getting a low water pickup and spending some money on a cleaver,happy with mid 50's and riding a few inches off the water right now.
 

100333624

Petty Officer 1st Class
Joined
Apr 24, 2012
Messages
205
Re: Mercury I6 115 '83 -/+?

All the inline 6's weigh about the same.The newer the 115 the better,differences with distributor-,exhaust,carbs,pistons-hi dome vs lo dome reed set up and fuel flow all come into play. Your boat probably weighs closer to 500 than 200. My 15' power cat which has no subfloor or windshield etc comes in around 400 and that is light. Looking for some merc speed gurus? go to facebook and search- inline 6 mercury outboard motor fans- there are some old time speed guys on there. There was and it may still be listed in the Buffalo Craigs a 1972 twister with a SSM lower and bp1250 mid,asking 2 grand,that would move you into the 60's and higher if setup correctly. The 115 will put you into the high 40's,maybe and I mean maybe 50 depending on your hull design and how you setup your motor on the transom.My PC which is the original tunnel boat with a rebuilt shorty 1500,new hi dome pistons,crank balanced,rods and pistons weighed out and motor jacked about 2" hits mid 50's gps with one person. Can I make it faster,yep by jacking the motor some more,getting a low water pickup and spending some money on a cleaver,happy with mid 50's and riding a few inches off the water right now.

Well I can tell you my boat isn't 500 lbs, that's for sure! It's 16' long and 60" wide so it's a narrow boat.. Without the motor I can lift it and move it on the trailer with little effort... Compared to my brother in laws 15' Sunray that can't even budge with 4 ppl moving it... My boats design doesn't have a "cap"... Sounds funny but it's a light boat and she flies with the 80hp I have on it now... It planes so fast and gets top speed very very fast... It looks almost like a Boston whaler and cuts the water nicely... And I'm pretty sure the 115 will take it into 50 easily... Mind you, the 80 I have on now runs good, and is using the original prop so I havent even messed with props yet, and thats on my transom, no jackplate yet... I am confident the 115 might even hit like 50+ don't kid yourself it does 40mph right now, and it hits it so easily, and that was on gps, with me and a buddy, and 100lbs worth of water weight! I have already removed all the water weight...

I wanted to know the differences between the years of the 115 motors, that's all! I don't even know what an SSM lower is or the bp1250 mid you mentioned...? Could you clarify those please? Thanks!!
 

100333624

Petty Officer 1st Class
Joined
Apr 24, 2012
Messages
205
Re: Mercury I6 115 '83 -/+?

Ok so I did a little homework and checked out the Twister.. Man are they nice motors! Where did you see this you said?? Can't find that ad...
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,554
Re: Mercury I6 115 '83 -/+?

With that 115HP motor, your top speed will likely be influenced by the design of your hull. I will guess that the 115HP will allow speeds that the hull was not designed for. The question will be: "How does the hull handle that increased speed". If the hull is flat bottomed, look for sliding when turning. If she is a vee hull, anything is possible. She could pitch-pole, porpoise, list, plow water, dive the bow, etc.

You will want PTT on that motor. Good luck, take it slowly, especially at first, and let us know how she handles.
 
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