Maximum efficiency of a 3.0 litre Mercruiser

[Per]

I presently have a 4.3LX V6 Mercruiser with an Alpha drive in my 21 foot
boat, and cruise at

25 mph. However, fuel costs are going through the roof, so to make the boat
more affordable

when I am retired in the near future, I want to replace the engine with a
3.0L Mercruiser, which

I will buy new, bobtail, and hereafter go slow. (I love the 3 litre because
I had it in a previous

boat.) From tests I have done, I get 3.7 mpg (imperial gallons) on flat
water with the V6 and

a speed of 5.5 mph at 1400 rpm.

What speed and fuel efficiency can I expect from the 4-cylinder under the
same conditions?

By rough estimates I figure it to be about 6 mpg and a speed of 6 mph with
the same prop

and gears. This is the minimum I hope to get. My other question: if it
should't be that good, is

there anything I can do to the engine or the boat to improve things? The
engine of course is

never going to plane the boat, but run all day, for days at 1400 - 1500 rpm,
and be relatively

lightly loaded, putting out something like 10 or 12hp. I don't mean to do
anything too drastic;

just tweaking things for max efficiency at low rpm.

I need a lot of reserve power in heavy seas and strong currents.

[Woodchuck]

Going to a smaller engine may not net more MPG because the smaller will have
to turn more RPM to maintain the same MPH.

"Per" <phno...@canada.com> wrote in message
news:vt3Kc.49823$Rf.12299@edtnps84...

[Tony Thomas]

What your talking about is turning this boat into a displacement hull slow
mover. I would get a small diesel engine. The fuel will be less costly and
it will get a lot more fuel economy.

Other option would be to get a very large pitch prop that will push you a
lot faster at idle which will increase your mpg.
Try something like a 30 pitch prop. You will have to increase your idle
speed to make up for the pitch but should really increase the speed to make
up for it.
Of coures you probably won't ever be able to plane with this prop.

Assuming you have a 15 pitch prop now at 1400 rpms w/ the 2:1 gears that
gives you a speed of 10 theory. Efficiency of the prop stinks at this speed
so you are only getting 5.5 on gps I assume.

Now w/ the 30 pitch ast 1400 rpms you would get 20 mph theory. Again
efficiency will stink but you will probably get closer to 10 mph which will
come close to doubling your mpg without an engine swap. However, you will
be lugging the engine.

--
Tony
my boats at http://t.thomas.home.mchsi.com

"Per" <phno...@canada.com> wrote in message
news:vt3Kc.49823$Rf.12299@edtnps84...

[Per]

The prop I use now s a 19" Mirage plus. Gear ratio is 1.84:1. The boat is a
planing hull 21 foot so hull speed is only about 6.2mph. I should be able to
push it to 7mph with a 3.0L engine without sacrificing mileage too much. You
are right aboput getting more speed for the same rpm and what I can do there
is changing the gear ratio to 1.65:1 or even 1.47:1 using the same prop, but
I have to be able to maneuver around the marina also. Putting in a small
diesel is an option but with a lot of complications and at a much higher
cost. And how do you do things like make it cut out when shifting out of
gear? I like the 4-cylinder GM engine because it's easy to work on in the
boat.
The boat will have an oversized engine for the power required. That should
work for good durability, and it will have a lot of reserve power.
So what can be done to a basic carburated gas engine to get more efficiency
at low power and low rpm, without impairing things like starting, idle and
durability?
Ignition timing? Valve timing? Valve springs? Carburator settings? Different
spark plugs? Operating temperature? It will have closed cooling.
It seems there is more discussion about about power and speed than about
fuel efficiency in these newsgroups.

[Jim Kelly]

You could put up a mast and sail and make it into motor sailer.

[Steve Lawson]

Having had a 3.0L in an 18' boat and barely getting 42 or 43 MPH out of it,
and now having a 5L in a 21' boat and just getting 50 MPH out of it.....i
don't see any way that a 3L can push a boat that large. Standard motor for
my boat is a 4.3L 190HP.....that is nearly 50% more HP than the 3L can put
out.

Steve L.

[Matt Lang]

If you want to go very slow for longer periods of time then maybe a
high thrust 4 stroke kicker (8-25 HP) would do the trick. As long as
you dont want to go faster than hullspeed it will use less gas than
your main engine and you can still keep a larger main engine.

Matt

"Per" <phno...@canada.com> wrote in message news:<vt3Kc.49823$Rf.12299@edtnps84>...

[JamesgangNC]

If you want to run at displacement speeds you really should get a
displacement hull and a nice diesel engine. What you're trying to do is
akin to trying to make a formula one racer out of a volkswagen beetle.

You're not going to double the mileage by switching from a gas 6 cylinder to
a gas 4 cylinder engine. What you're going to do is make a boat that no one
will want.

"Matt Lang" <web...@shaw.ca> wrote in message
news:a0e12e54.04072...@posting.google.com...

[William G. Andersen]

If you could really swap engines, it would take a lot of fuel savings to pay
for the cost of the new engine. I think you should forget about changing
engines.
I was concerned that I wouldn't be able to afford using my boat when I
retired. It has turned out to be one of the least expensive things I do,
despite the increas in the cost of fuel.
My boat has the 3 liter MerCruiser (19' bowrider). A day of fishing only
uses a couple of gallons of gas. A day of watersports or a day of patrolling
uses about 16 gallons of gas.

"Matt Lang" <web...@shaw.ca> wrote in message
news:a0e12e54.04072...@posting.google.com...

[alejandro...@gmail.com]

You can review the gas mileage in gallons per hour (GPH) in the following site:

http://www.boat-fuel-economy.com/mercury-mercruiser-3.0-181-fuel-consumption-us-gallons

The table show the consumption at max speed (4800 RPM). With a Mercruiser 3.0 you basically get the following:

RPM GPH

500 0.50
1000 1.00
1500 1.50
2000 2.90
2500 3.80
3000 4.80
3500 6.20
4000 7.40
4500 9.30
4800 10.50

Pretty decent for a boat. I have a Bayliner 185BR with that engine and the number in that site are pretty accurate. Assuming a gas price of $3, you should spent about $ 10 / hour. Assuming you use the boat 50 % of the time when you are out on the water, you should spend $ 40 for an 8 hours boating day.

Of course you can spend much more money on drinks !!! :-)

Be safe!

[Tim]

Thanks for the chart. I was going to say that a smaller engine would be disappointing in that boat. Lower performance and more fuel consumption. If a person was going to upgrade, they'd be better to go with a 305 or 350 GM.

But I'd agree with previous posters that the fuel savings for economy wouldn't justify the upgrade

[Username]

That information can only tell you one engine's performance compared to
another. Boat type, use, prop specs, etc. will play a big part in the
results. For some applications, those results may not hold true.

[Tim]

Exactly right, but does provide a 'rule of thumb'

[Tim]

On Friday, July 16, 2004 at 11:14:51 PM UTC-7, Per wrote:

>
> I need a lot of reserve power in heavy seas and strong currents.

That's exactly why you don't want a 3.0 in a 21 ft. boat.

[whoda...@gmail.com]

This may be unrelated, But how the hell did this thread exist in 2004??? I thought google was nothing more than a search engine back then.

[Tim]

Jul 11whoda...@gmail.com
- show quoted text -

This may be unrelated, But how the hell did this thread exist in 2004??? I thought google was nothing more than a search engine back then.

.....
Google is a search engine but it digs through theUSNET archives back into the mid 80's. It digs up stuff older than itself lol

[Its Me]

Yup. From wiki:

Google Groups became operational in February 2001, following Google's acquisition of Deja's Usenet archive. (Deja News had been operational since 1995.)
<snip>
The second kind are Usenet groups, for which Google Groups acts as gateway and unofficial archive (the Google Groups archive of Usenet newsgroup postings dates back to 1981).

[Tim]

Its Me

- show quoted text -

Yup. From wiki:

Google Groups became operational in February 2001, following Google's acquisition of Deja's Usenet archive. (Deja News had been operational since 1995.)
<snip>
The second kind are Usenet groups, for which Google Groups acts as gateway and unofficial archive (the Google Groups archive of Usenet newsgroup postings dates back to 1981).

....

'81? I was thinking '86. News to me...

[gfre...@aol.com]

On Wed, 12 Jul 2017 06:55:15 -0700 (PDT), Tim <tsch...@gmail.com>
wrote:

The home repair group gets decade old posts all the time from Google
users. Typically someone who sees the date posts something back like
"that bad breaker caused a fire that killed Bob and his whole family
in 2010".
I do find it interesting that Google seems to respect the "no archive"
bit because when I use it, nobody who uses GG can see my posts. I
guess they assume once they see something they will always archive it
for posterity. It makes you wonder how much DASD they have.

[Its Me]

It's hilarious that Don thinks Justan has him "filtered" for that very reason!
:)

[True North]

Still calling your fellow turd a 'liar', eh, JackOff?
You're not much of a friend.
Justine claimed to have me filtered...said it right in here.

[Its Me]

Wow. YOU claimed you couldn't see *his* posts because *he* had *you* filtered.

You really can't be that dense, can you? SMH

[John H]

I'll answer for him:

"YES!"

[True North]

SFB!

A while back a number of other posters were quoting him on a regular basis.
They seemed to have wised up lately...

[True North]

Hey JohnnyMop...did your dog ever come back
Time to buy a banjo and take some lessons, eh

[egraham...@gmail.com]

Get a bow mount trolling electric, fix it app it won't turn, keep the engine running too charge, and use the wheel as an udder, I'm assuming you're fishing

[unreal-...@comcast.net]

max efficiency is measured on plane. Usually 3200 to 3500 rpm why would you want to plow water all day. lol The more boat out of the water the more efficient it is.

[Tim]

Per

——

You’re best to leave things alone. The v6 will supliment your needs far better than the 3.0