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Old 04-19-2016, 09:50 AM   #1
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Perkins Engines --- 6-354 vs 4-236

This question requires a lot more engine knowledge than I have.

Given that both engines are in " near same age and physical condition "
Both engines married to the same velvet drive gears,
Both engines turning same size shaft - one & one half inch,
Both engines tuning the same size wheel - 24" x 21",
Both engines - dry stack exhaust and keel cooled,
no accessories on either engine to rob horsepower.

Both engines turning at the same RPM's eg. 1200, 1400,

The BIG Question???

Could one expect the same performance from both engines
" in terms of torque and speed over the water " ???.

Your 2 cents, Please
Capt Nemo
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Old 04-19-2016, 10:03 AM   #2
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Quote:
Originally Posted by Capt Nemo View Post
This question requires a lot more engine knowledge than I have.

Given that both engines are in " near same age and physical condition "
Both engines married to the same velvet drive gears,
Both engines turning same size shaft - one & one half inch,
Both engines tuning the same size wheel - 24" x 21",
Both engines - dry stack exhaust and keel cooled,
no accessories on either engine to rob horsepower.

Both engines turning at the same RPM's eg. 1200, 1400,

The BIG Question???

Could one expect the same performance from both engines
" in terms of torque and speed over the water " ???.

Your 2 cents, Please
Capt Nemo
The 6 will have greater torque and run slower at the same speed with the appropriate bigger propeller.

There is no substitute for cubic inches
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Old 04-19-2016, 11:23 AM   #3
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Given the same boat, propeller, & gear ratio; any two engines turning the same RPM will give the same speed. In the case of 2 diesels, the smaller, lower internal friction, more heavly loaded engine will give better fuel efficiency (provided it is not overloaded.)
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Old 04-19-2016, 12:09 PM   #4
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To expand on Brooksie's correct answer and to paraphrase Tony Athens (and no doubt lots of others), "props move boats and engines move props". In other words it doesn't matter what engine is driving the prop (assuming it has enough power to do so), a 24x21" prop absorbs so much power at a given rpm. And that prop will push the boat to the same speed no matter which engine is driving it.


But maybe your question is which engine is better. It all depends on how fast you want to cruise and what your maximum speed requirements are.


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Old 04-19-2016, 02:39 PM   #5
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Assuming non-turbo in a boat that would fit either. For me it would come down to boat size then down to ER size. Something less than 34 ft could get away with the 4 cyl. Above that go with the 6 because sometimes you will need the extra ponies to make a bridge or lock opening. Neither engine is going to push you up on efficient plane unless small and light.
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Old 04-19-2016, 03:33 PM   #6
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It's a matter of engine size. Not really even that .. just power. So it's a matter of how much power is desired for the boat. And of course that leads to how much speed is needed for the boat. And that leads to what speed was the boat designed for? And then how much reserve power is needed? And since these are pleasure boats there's no needs .. just desires.
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Old 04-19-2016, 04:16 PM   #7
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Four cylinders tend to be a good bit more "shaky" at idle, and that can be annoying. With good mounts the issue can go away.

Otherwise, the same engine just one is a six and one a four. Both good engines.
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Old 04-19-2016, 06:28 PM   #8
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Ski wrote ... "Fours shake at idle"

Yup and the bigger they are the more they shake. A higher idle speed (mine is 1000rpm warm) and not over propping both help quite a bit. My Mitsu 4-107 (small) a tad under propped w Poly Flex mounts is almost smooth at idle ... and dosn't go too fast around the marina.
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Old 04-20-2016, 12:09 AM   #9
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Quote:
Originally Posted by Capt Nemo View Post
This question requires a lot more engine knowledge than I have.

Both engines turning at the same RPM's eg. 1200, 1400,

The BIG Question???

Could one expect the same performance from both engines
" in terms of torque and speed over the water " ???.

Capt Nemo
Is this just a theoretical question? A "what if" In which case your question was answered below.

Cuz 1200 - 1400 rpm's is not much more than a fast idle in a 6-354. Correctly propped they top out at 2800 BTTW and normal cruise is 2200 - 2400. The engine and VD's probably wouldn't even come up to operating temperature at that speed. I can't imagine it would be particularly wise to perpetually run one that slow as a routine practice.
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Old 04-20-2016, 12:35 AM   #10
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I am pretty sure this was just a theoretical question....not "which one is better". As long as all things are equal(prop and gear), the same RPM from ANY engine will drive the same boat at the same speed....in theory.
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Old 04-25-2016, 11:41 AM   #11
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Perkins 6-354 vs 4-236

Thank you to all who replied. "Question Answered"

And Yes,the questions were hypothetical.

Hypothetical boat 44' x 14' steel, approximately 27 ton.

But, just say for instance, that for various reasons, among which one being economics, we were to plant both of these engines in the same boat,
and both engines turning the same size wheel.

Other than looking kind of Odd, would there be some advantages
(ie: fuel economy for starters) to this scenario?

There are numerous opinions about idle-ing velvet drive gears while under-way. Not sure yet which is the correct answer.
Just a little food for thought for all you engineers.

Thanks again
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