Diminishing returns on motor size

It stands to reason that a bigger motor will make a boat faster, but I would only expect for this to be true up to a point. Then I would expect for there to be diminishing returns for larger and larger motor sizes. If my boat is a 1652 what would be the ideal motor size to get the biggest bang for my buck before hitting that point of diminishing returns?

I would like to hear how big some of your boats are along with motor sizes and speed.

Ummm 1652 what? Your transom should have a tag that tells how much HP it can safely run. Because your statement is untrue about diminishing returns for a larger motor (that is assuming you were talking about speed). Throw a 90hp motor on that baby and the only thing that will diminish is passengers that are willing to get on your boat :lol: ........... 25-40hp is pretty typical on 16' tins though

I don't think that a 50 hp motor will push a boat twice as fast as a 25 hp motor, even if the boat is rated for a 50 hp motor. It would push it faster for sure, but not twice as fast.

It all depends on the boat, it most certainly could. If the 25 doesn't get the boat on plane and you are running at 12 mph. The 50 could push it at 30 mph on plane. There are way to many variables for such a general queation. So no. it will probably not push it twice as fast, but it could in the right situation.

You're on the right track in that you normally have to quadruple hp in order to double speed due the the added weight and increased friction/wind resistance. I don't know what your boat is rated for, but a 16' with console steering and 40 is pretty common and will run @32 on GPS with a light load and the right prop. A 60 is even better but will only get you up to about 38.

This has been my experience, there are lots of folks on here with 16 footers so opinions may vary.

I found this hull speed calculator:

https://www.psychosnail.com/boatspeedcalculator.aspx

A Jon boat is a planing hull. If you select the button for planing hull and hit next, it takes you to a screen to enter the total displacement. The discussion says that with a planing hull, the only thing that matters is total displacement. But, if you hit next again it takes you to a screen to enter the length waterline. This seems to be in contradiction to the discussion about displacement being the only thing that matters for a planing hull. It is also confusing to me because when the boat is on plane the waterline is very small. As far as the formula is concerned, the longer the waterline the better, but we know that with a planing hull, the less boat you have in the water the better.

I have tried to find something like this that would be more along the lines of a Jon boat, but haven't found anything.

But, actual experience is better than theory anyway. That is why I wondered what boat/motor/speed combinations you guys have experienced. I would like to know the various combinations you guys have used and what the results were.

Lowe 2070 CC said:

You're on the right track in that you normally have to quadruple hp in order to double speed due the the added weight and increased friction/wind resistance. I don't know what your boat is rated for, but a 16' with console steering and 40 is pretty common and will run @32 on GPS with a light load and the right prop. A 60 is even better but will only get you up to about 38.

This has been my experience, there are lots of folks on here with 16 footers so opinions may vary.

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X-2 Your numbers or pretty darn good in my exp...cva34

Jeffrey said:

I don't think that a 50 hp motor will push a boat twice as fast as a 25 hp motor, even if the boat is rated for a 50 hp motor. It would push it faster for sure, but not twice as fast.

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Guess I didn't understand that "diminished returns" meant not being able to double your speed with double the HP #-o. Well in this case you should start with the smallest motor that will get your boat on plane..... likely a 20-25hp, because after that you will start seeing this "diminished return" thing happen :wink:

Johny25 said:

Jeffrey said:

I don't think that a 50 hp motor will push a boat twice as fast as a 25 hp motor, even if the boat is rated for a 50 hp motor. It would push it faster for sure, but not twice as fast.

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Guess I didn't understand that "diminished returns" meant not being able to double your speed with double the HP #-o. Well in this case you should start with the smallest motor that will get your boat on plane..... likely a 20-25hp, because after that you will start seeing this "diminished return" thing happen :wink:

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Can you loan me a few motors with different horsepower ratings so I can find the right motor size through a process of trial and error?

Rule of thumb I like to stick by...... find the transom tag that tells the max HP for your boat and put that size motor on. Anything less always seems to be not enough. And a lot of us go over that recommendation by 5-10hp :mrgreen:

muskiemike12 said:

There are way to many variables for such a general question.

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I think that says it all in a nutshell. You want information that is too specific for parameters that are too general. A 1652 could be a semi v, mod v, or just a flat bottom jon, and all 3 would have different results with the same motor. You'd also have the potential of different outcomes based on your test environment. One might do better on rough water and into the wind while on a different day the other boats do better. Then you have the total weight of boat/motor/gear/etc to keep in mind. All 1652's don't weight the same. And the same type hull from different mfg's could have different speeds because of subtle changes like lifting chines. And I think you'll exceed the boats hp rating before you start to see a real "diminishing returns" effect.

Heres my 2Cs weight and displacement in the water. Go to the lake and observe boaters and evaluate what the load is they are carrying. Then decicde what boat type you have and get the matching motor. Trim and a jack plate will give you more control than trying to buy straight horepower!!!!!! If you load a 16 ft. you could use a 70hp at max for fishing on the jon boat side of the equation. Example a 1648 alumacraft decked out at home with 3 fat fishermen a 40-70 could be the ticket. I have a 15 alweld decked out with a 40 with trim. Toooooooooooooooooo much motor for the boat it will not safely carrry enough weight water line theory. tiller go 40 and below and decked alweld go 40-70 . You can thank me later that information usually cost 3 or more boat purchased. 51 years of outboard experience and a tin freak to say the least!!!!!!!!!!!!!!!! HOOK

I have a 1448MV. 1st motor was a 72 25hp tiller johnny, no fun. Then jumped up to a 76 40hp tiller johnny, started to get a smile. Added a cmc pt135 TnT with set back, then prop limitations.

I did like the balance of the boat with the big twin, so I obtained a 81 50hp console steer johnny, better. Obtained a 81 60hp johnny. After mods and having the propped stretched with lots of cup, that yielded the sheet eating grin. But once again I am prop bound. Very tough to find a 23 plus pitch SS prop for the small gear case. Light load runs mid to high 40's. Once again prop bound, the motor is pulling to many rpm's

I did have a notion to put a triple on it, but did not want the extra weight.

A you tube video on the river https://www.youtube.com/watch?v=TD8YkECUPGM

I own a 1652,has a 6 degree deadrise with a jet tunnel.
First motor was a 40hp Suzuki 2 stroke--top speed 33 mph.
Current motor an old 30hp Mariner---squeeze 28mph out of her.(by myself, fishing gear and the is under 400lbs)
The tunnel hull hurts my top speed, you might do abit better.
Both motors are tiller steer, and my boats rated 35hp tiller 60 console.

You should consider the rated hp form your boat(there not all the same) and
how much of load/weight,do you normally travel with.

Are lakes are small here so speed isn't a real big issue.I,95% of the time fish by myself so as long as I plane off and get above 25mph,it's good for me.

Zum said:

I own a 1652,has a 6 degree deadrise with a jet tunnel.
First motor was a 40hp Suzuki 2 stroke--top speed 33 mph.
Current motor an old 30hp Mariner---squeeze 28mph out of her.(by myself, fishing gear and the is under 400lbs)
The tunnel hull hurts my top speed, you might do abit better.
Both motors are tiller steer, and my boats rated 35hp tiller 60 console.

You should consider the rated hp form your boat(there not all the same) and
how much of load/weight,do you normally travel with.

Are lakes are small here so speed isn't a real big issue.I,95% of the time fish by myself so as long as I plane off and get above 25mph,it's good for me.

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Thanks Zum. That is some useful real world data right there.