Assignment America . . . Single vs. Twin Engine

[PATRIOT]

Assignment America . . . Compare the overall performance and theoretical top speed of two identical boats, one powered by a single 40HP motor and the other powered by twin 20HP motors, using these parameters, assuming the combined installed engine weight is identical :

40HP EFI 4-stroke
3-cylinder 747cc
Prop: 13.375"dia X 13"pitch
GearRatio: 2:1
RPM: 6000

20HP EFI 4-stroke
2-cylinder 327cc
Prop: 9.25"dia X 10"pitch
GearRatio: 2.08:1
RPM: 6000

 

[earl60446]

I am not going to try and get scientific about this but I believe the single 40 would outdo the twin 20's in almost every way. I doubt combined engine weight could be the same, btw.
variable = winner
top speed = 40
time on plane = 40
reliability = 20's (2 against 1)
maneuverability = 20's slightly better
fuel consumption = 40
installed cost = 40
trolling = 20's
operating cost = 40
maintenance cost = 40
maintenance issues = 40

Tim

 

[Lowe 2070 CC]

You neglected to address the fact that two lower units and two props in the water equals more drag to overcome.

There are legitimate reasons to run dual outboards in larger hp situations but 2 20's will not outperform a single 40.

 

[Pappy]

The answers are spot on...........the 40 will win every time

 

[BigGill]

You guys are right on with this one ! The 40 all the way.The 20's will not come close.

 

[skipper123]

Man that sucks, I was thinking of running a pair of fours. All jokes aside I plan to run a 4 mud motor beside a 4 Tohatsu just to get back in the weed beds cracker fishing. Dad did run a pair of sixes and when you fired up the second one she jumped on plane and took off like a sling shot. His main reason was fishing back waters that was a long way from home.

 

[NewportNewsMike]

skipper123 » 09 Oct 2013, 14:01[/url]"]Man that sucks, I was thinking of running a pair of fours. .......

I would still consider running your pair of fours. The comparison between the 20's and a 40 is somewhat relevant to your situation, but probably not so relevant that I would not try your 2 X 4 setup.

Keep in mind you have a valid reason for the 2 X 4 setup. You are not really looking to get the best of all worlds, you are trying to solve a specific problem.

If it was me, I would give your 2 X4 setup a try.

 

[Pappy]

Kinda' two different scenarios going here plus small motors are way different than larger ones, performance wise.
If you want pulling power then go for two. If you want top speed stay with one larger one.
Rule of thumb is that if you want to pull a load then find a way to move a large column of water at a slow speed.
If you want to go fast move a small column of water at a high rate of speed.
With the twin 4s you will be able to pull more load and you just may get a hard to plane boat on plane.....who knows.
Will give you an example.
Was running a little Johnson 3hp I had just gone through. Had it on a 14' aluminum boat. If memory serves me correct it went something like 6.9 on the GPS (think I made a post of this somewhere) I put a second one on the boat and it climbed to just over 7mph. Not done yet, I put a third one on! Top speed was something like 7.4 or something. These little engines were limited (on this hull) by the prop and gear ratio. Only way they would have gone faster would have been on a narrow non-planing or displacement hull. Fun but totally meaningless.