# 60/40 merc 4 srtk rpm's , myths , and questions answered



## rockdamage (Nov 8, 2014)

I see guys asking lots of questions about the 40 jet... speeds to expect , rpm's the engine should turn, size of boat suggested....
I would like to put these all to bed right now. 
first off what speed to expect __ most hulls should run in the upper 20's to low thirties. I have seen several 40 jets run 32 mph> and a few even broke beyond 35MPH. now these results are very dependent on C Of G and weight placement. 
"what rpm should my engine run ?" A new 40jet will turn about 5000rpms.. I have seen a few spin up 5080. So many factors can effect how these engines perform that NO ONE can narrow down an exact #. When its above 90 degrees out, these engines struggle to see 4950. but don't let that worry you because all other engines loss power when it's extremely hot out. Another factor is impeller. A damaged impeller will kill your rpm's and destroy top speed. ALL THESE MOTORS COME WITH A STAINLESS. stainless impellers use both round and square keys so we can squash that myth right now. Also before you go telling everyone that your dealer swapped your stainless for an aluminum, make sure you can tell the difference!!!! ( a stainless WILL develop surface rust and it doesn't come factory polished like a prop)
finally hull size. . . . . I don't suggest installing the 40j on anything smaller than a 17/48. I have seen a few on 16/42's but they draft low in the water. I prefer the 56" bottoms but the most common hull you will see is a 52" with lengths from 17-19
how big of a hull will it push? this is a loaded question..... Ive seen a 16/48 roughneck that would barely plane off! It's all about weight fellas
the engine will easily push an 18/72 hull to speeds approaching 30mph. It's only when you load these boats down that these engines fall short. 

And don't get me started on tunnel hulls. There isn't a man on this board that needs a tunnel hull. If you are hitting the shoe on rocks, getting a tunnel is going to keep you from swimming back to the boat ramp. Every time I've hit my shoe is because I wasn't paying attention to my trim. when you are on plain Shoe should be level with the bottom of your boat. If it isn't then you need to use the trim and get that engine out of the water. tunnels cost more $$$ and they kill your speed and are more difficult to plane off


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## catferret (Nov 10, 2014)

Thank you very much for the information. Cold you expand a bit on why you prefer a 56" bottom over the 52" ? 
Is the performance negligible for the increased space?


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## smackdaddy53 (Nov 11, 2014)

catferret said:


> Thank you very much for the information. Cold you expand a bit on why you prefer a 56" bottom over the 52" ?
> Is the performance negligible for the increased space?


Draft


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## rockdamage (Nov 12, 2014)

Main reason.... there is minimal if any performance loss with the wider bottom. Shallower Draft and more floor room. Also the sides are taller which gives you deeper storage. Boat will plane at much slower speeds also !


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