Want to save some serious gas money. Diesel too.-gctid387296

[dmcb]

Order a boating magazine that tests boats such as Boating. Study the fuel usage at different speeds. You will see a pattern that is true of almost any boat.

Those who slow down to save fuel many times kid themselves and actually burn more.

A couple of examples.

A 23' Sea Ray with a 5.0 Merc.

At 1000 rpms speed is 5.6 mph economy is 3.7 mpg.

At 1500 rpms speed is 7.4 mpg economy is 2.8 mpg.

at 2000 rpms speed is 8.1 mpg economy is 1.8 mpg.

Look what .7 additional speed did to the economy.

And at 3000 mph the boat got 3.7 mpg.

Now lets look at a bigger boat.

A 40' Cabo with twin 600 hp 8.3 Cummins.

900 rpms, 7.94 mph, 2.65 mpg.

1200 mpms 9.4 mph, .94 mpg. Just 1.1 mph more but look at what it did to the mileage.

at 2700 rpms 36.6 mph .71 mpg.

If you want to slow down to save fuel you have to really slow down. Diesel will show the biggest difference.

I see many boats slowing down, maybe 10 mph, plowing water, and burning more fuel than if it was on plane.

Doug

[Guest]

Showing my ignorance here...

but I always thought the best MPG was the slowest you could go while still maintaining plane...???

[Guest]

3055 wrote:
Showing my ignorance here...

but I always thought the best MPG was the slowest you could go while still maintaining plane...???

Doug's data above holds true for every cruiser I've owned. If we don't have a timetable to keep, we go slow. Less than 7mph slow!

[Guest]

Doug

Very true! However, with a diesel you need to make sure you are at operating temperature, for our CAT is about 180 degrees. I need to run at least 1200 to see that. The engine manual warns against prolonged idling or low speed running.

[Eyeman]

dmcb wrote:
Order a boating magazine that tests boats such as Boating. Study the fuel usage at different speeds. You will see a pattern that is true of almost any boat.

Those who slow down to save fuel many times kid themselves and actually burn more.

A couple of examples.

A 23' Sea Ray with a 5.0 Merc.

At 1000 rpms speed is 5.6 mph economy is 3.7 mpg.

At 1500 rpms speed is 7.4 mp[COLOR]"#FF0000" wrote:
h[/COLOR] economy is 2.8 mpg.

at 2000 rpms speed is 8.1 mp[COLOR]"#FF0000" wrote:
h[/COLOR] economy is 1.8 mpg.

Look what .7 additional speed did to the economy.

And at 3000 [COLOR]"#FF0000" wrote:
r[/COLOR]p[COLOR]"#FF0000" wrote:
m[/COLOR] the boat got 3.7 mpg.

Now lets look at a bigger boat.

A 40' Cabo with twin 600 hp 8.3 Cummins.

900 rpms, 7.94 mph, 2.65 mpg.

1200 [COLOR]"#FF0000" wrote:
r[/COLOR]pms 9.4 mph, .94 mpg. Just 1.1 mph more but look at what it did to the mileage.

at 2700 rpms 36.6 mph .71 mpg.

If you want to slow down to save fuel you have to really slow down. Diesel will show the biggest difference.

I see many boats slowing down, maybe 10 mph, plowing water, and burning more fuel than if it was on plane.

Doug

[COLOR]"#FF0000" wrote:
Just a couple of typo corrections[/COLOR], not to pick, but this is great information. It appears planing speed of 3000 rpm gets the same mileage as just over idle. Interesting, with my gas 5.7L I can only get 2.5 mpg at planing speed of 3150 rpm. Thanks.

[MidnightSun]

Two words, FUEL COMPUTER will give you the best economy for every condition. Heading into strong winds and current or tide can translate things into really bad numbers at very slow speed. Heck in my flying days I actually flew a Cessna backwards relative to the ground because the winds were so strong. Same thing happens with current/tides on a boat.

Single engine operation at slow speed on Midnight Sun gets me over 5mpg.

[Guest]

MidnightSun wrote:
Two words, FUEL COMPUTER will give you the best economy for every condition. Heading into strong winds and current or tide can translate things into really bad numbers at very slow speed. Heck in my flying days I actually flew a Cessna backwards relative to the ground because the winds were so strong. Same thing happens with current/tides on a boat.

Single engine operation at slow speed on Midnight Sun gets me over 5mpg.

You get 5+ MPG on a boat that size?

AWESOME..........

[SomeSailor]

Trawler speeds. I push along 8 knots and burn 3.5 nMPG at 1675 RPM. That's worked out to be just under 3GPH so far (2.857 to be exact).

The money savings is crazy. The fuel economy is about a factor of 2 or so when you do it on a per hour basis. I can operate this boat (42' trawler) at best cruise at about $13/hour. My last boat (2859) would be $67/hour. BUT... 3.5 times faster at cruise (29-30 knots).

100 gallons of fuel will now take me about 350 miles and gives me a total range of around 2000 miles, so I buy all my fuel at home where its cheaper.

[iceclimber]

You think that's cool, I burned an estimated 75 gallons in under two hours. Then again, it was old gas and I wanted to burn it. One more tank to go.

[MidnightSun]

LazyCrusr wrote:
You get 5+ MPG on a boat that size?

AWESOME..........

Yep however that is only running a single engine, if I fire up both and run the same slow speed It drops to around 3mpg. On cruise at 30+ mph I get 1.25 mpg.

[dmcb]

Eyeman wrote:
[COLOR]"#FF0000" wrote:
Just a couple of typo corrections[/COLOR], not to pick, but this is great information. It appears planing speed of 3000 rpm gets the same mileage as just over idle. Interesting, with my gas 5.7L I can only get 2.5 mpg at planing speed of 3150 rpm. Thanks.

It was not a typo. It was a direct quote from the mag.

All boats, hulls, length, props and so on are different so results may not be the same.

However as Whiskywizard says, the basic idea is the same for all boats.

The mag was one I just happened to have at my fingertips. June 2012 issue of Boating. Read it yourself.

Ron, about temps. You are correct. On my Hino's the operating temp is always there, idle, fast idle or heavy throttle.

I usually run 1000 to 1100 rpms but idle products 170 degrees which is the temp at any throttle position.

Doug

[Guest]

but I always thought the best MPG was the slowest you could go while still maintaining plane...???

Most people assume that, and it's absolutely incorrect. Slowest planing speed has never been optimum on any boat I know of. Most all small (16-34 foot) recreational fiberglass planing hull boats with gas engines will be doing their best MPG around 3600-3800 RPM. This will be well past the initial planing point, which is very inefficient as the hull is still not free from the water. Also if the engine/hull and prop are well matched, the engine will be turning below its most efficient RPM (which for all the normal boat motors in this size is in the 3500-4000 range).As far as your optimum hull speed, that's a mathematical function of the wetted waterline length. Normally you can be efficient to just below the maximum hull speed.

http://baylinerownersclub.org/media/....png[/img]where:"LWL" is the length of the waterline in feet, and"vhull" is the hull speed of the vessel in knots

[dmcb]

What Carlos says is very true. You can see that on the test data also. You can see and feel it yourself.

If another 1000 rpms gets you 2 or so more miles per hour, you are most likely running more efficient.

You want the boat out of the water.

I didn't print all the data because I was talking about best economy.

And things kinda reverse. At below hull speeds a little more rpms will quickly reduce mpg.

When on plane this reverses and your economy again starts to improve the faster you go. But not wot.

Like Hans said earlier, flow data will show you how YOUR boat reacts.

My focus was to get people off the 10 mph speeds which is usually about the worst for mileage.

Doug

[Guest]

SomeSailor wrote:
Trawler speeds. I push along 8 knots and burn 3.5 nMPG at 1675 RPM. That's worked out to be just under 3GPH so far (2.857 to be exact).

The money savings is crazy. The fuel economy is about a factor of 2 or so when you do it on a per hour basis. I can operate this boat (42' trawler) at best cruise at about $13/hour. My last boat (2859) would be $67/hour. BUT... 3.5 times faster at cruise (29-30 knots).

100 gallons of fuel will now take me about 350 miles and gives me a total range of around 2000 miles, so I buy all my fuel at home where its cheaper.

Have you gotten used to the slower speeds yet?

[dmcb]

Wayne, I am sure SS will answer but I would like to give my feelings on this.

From my view of someone who has some pretty fast boats (and still have) over the years and enjoy speed as well as the next person, I have found cruising at hull speed to be one of the best experiences I have had in boating.

My 38xx will never be a fast boat so rule out the thrill of speed. If I want that I tow a 16.5 Sea Nymph with a 90 hp Evinrude or I will use my airboat with a 502 hp engine.

So my choice is 16 mph or about 6 mph.

At 6 mph, it is quiet. It is much easier to scan ahead for problems. The auto pilot removes the tiring work of keeping a course and does it better than I can anyway.

I have found I do not even want to make a lot of noise by speeding up.

I am seeing things in an area I have boated almost 50 years that I have not seen before. I have time to really see instead of having to keep my eyes glued straight ahead.

In today's world of $5 and more fuel, the savings are not something to disregard either. At 6 mph I am getting over 4 mpg. At 16 maybe 1 mpg.

And finally its a boat ride. I could get to the marina 2 1/2 times faster and sit at a dock or enjoy a boat ride at about 1/4 or less of the cost for the same miles.

We'll see what SS says.

Doug