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Marlin275
07-30-2005, 05:43 PM
My marina was closed for gas and my backup marina had a fuel spill
and closed down while I was waiting.
Question, when you are running out of gas, is it better to go 1100 rpm slow or 2800 rpm on plane ?
How do you go further, fast or slow? I thought keep the carb on two barrels not four.
I ran out about a mile away, at 2800 and now I wonder?
http://www.donzi.net/forums/attachment.php?attachmentid=13571

ChromeGorilla
07-30-2005, 07:20 PM
You should run at what ever RPM will give you the best MPG. Usually lower rpm's. Yes it will take longer cuz your slower, but getting more miles per gallon. If you have 2 gallons left and WOT you get 1 MPG, in theory you got 2 miles left if running. If at idle you get 2.5 miles per gallon, in theory you can idle for 5 miles. It's all about covering the most ground with the least fuel.

fasttrucker
07-30-2005, 07:49 PM
I think that I would go for the min. :confused: speed to keep it on plane.So I would have done the same thing as you.

cobra
07-31-2005, 01:32 AM
Generally, you'll get the best MPG at minimum planing speed. Idle or anything under planing uses more fuel. Your 2800 RPM ,if on plane at that speed, was the best choice, but few boats plane at that low RPM. If you have a hot motor (big, long duration cam) you may be beter off above min. plane speed if the RPM is below the efficient speed of the cam.

mattyboy
07-31-2005, 07:25 AM
whatever angle gets the most gas torward the fuel pick up, mine will run at idle for 5 minutes on just what's in the line, and filters,
shut your gas supply off and go slow then do it again and go fast see which way you go further??

Craig
07-31-2005, 08:16 AM
Recently, was running out of gas on my motorcycle on the interstate (no gage- thought I had plenty - but was wrong).

I switched to my reserve and thought I'd be fine for a few exits, but within what seemed like only 5 miles or so, I started to sputter again.

Wondered the same thing; take it easy? or blast it? Figured, like you I should probably just "putt" (of course the motorcylce is "always on plane" ;) ).

However, it seemed to be sputtering at any speed so I cracked the throttle and it took off so I held it there knowing I only had about a mile to a gas exit.

I made it there (amazingly).

Now I fill up my little GS500 about every 125-130 miles. Used to figure about 140.

Regarding boats, I'd pay real close attention to the rule of thirds if I was ever on anything bigger than an inland lake (which I'm usually not). I'm usually within paddling or swimming distance of help!

ChromeGorilla
07-31-2005, 09:40 AM
Ok I am copying this out of a BOATING mag I have sitting here on my desk. These are the #'s they got testing a 22 Classic with 496HO.

RPM_____MPG_____MPH
1000 ----- 2.2 ------- 6.7
1500 ----- 1.6 ------- 8.5
2000 ----- 2.9 ------- 19.9
2500 ----- 3.3 ------- 33.4
3000 ----- 3.1 ------- 41
3500 ----- 2.8 ------- 49.8
4000 ----- 2.2 ------- 57.3
4500 ----- 1.9 ------- 64.8
5000 ----- 1.9 ------- 70.5


Looks like I would be best running at 2500 to make sure I made it to the fuel doc. :yes:

Marlin275
07-31-2005, 10:54 AM
Thanks Chrome for nailing the answer.
Tank is full before I launch today, cruising at 2500 :yes:

Blewbyu
08-01-2005, 02:25 PM
Marlin- Please be aware that your 18 is a whole different ballgame than a 22 w/a 496. If you have access to a flow meter and a GPS, you should get the specs for YOUR boat. My X18 has best fuel efficiency @ 3400. Wetted hull/waterline length, prop, final drive ratio, engine HP/torque,bottom design and strake lengths,bla-bla-bla........all contribute to each boats performance and fuel consumption. Only the identical boat will have the same mpg as yours. If you change your hull form from 24 degree (full length) to a boat that flattens toward the stern, you can slash your fuel bill in half!
Before GPS, this process took "forever" to figure out. Now, it takes less than an hour.
Run a chart in 500 RPM increments from 1000 RPM to WOT. Be sure to let the boat stabilize for a few minutes at each setting. Your final readings will be only for wind/seather/sea condition for the day you are testing.....but a good place to start.
Regards- Jeff
"Blewbyu"

marcdups
08-01-2005, 04:57 PM
Dunno, Ask Forrest :rlol: :rlol: :bonk:

Moody Blu'
08-01-2005, 09:35 PM
well ill add my 2 cents to this and hope no one bashes me :rolleyes:

getting the boat on plane sucks down more gas so its a catch 22, get it on plane and hope it doesnt suck down that vital fuel. or idle in?

BaldEagle
08-01-2005, 09:36 PM
First of all I finally got my 06 18C. Never had a Donzi but I'm lovin this.
Its fun being scared.
It would be easy to plot fuel consumption against Rpm if you were running a Floscan Totilizer but most, like me, don't have them in an 18-22 fter. With motorcycles/cars the air resistance is easy to calculate ( the drag co-efficient a bit more difficult). But the bottom line is, whether air or water, resistance or drag goes up exponentially after a mathimatical constant is exceeded. Because planning hulls are dynamic in regards to the wetted surface area vs the air or wing area, it becomes more sophisticated.
EX: in your motorcycle or car.
The formula for converting air velocity to resistance in inches of water column is: ((feet/minute)/4005)2. That means that after you exceed 4005 feet/minute the drag or resistance goes up by a squared relationship and typically the brake horsepower goes up by a cubed relationship. Ever wonder why it's hard to get the last few MPH top end when though you've added HP. When you are running low on fuel in a car stay below 45 mph.
This applies to water too. The kicker is the drag co-efficient which is a percentage of the drag, and thus goes up by the same relationship.
Maybe the low rpm plane idea makes sense and when you hear the sputter change the hull pitch to empty the tk.
Sorry for all the BS - but its true.
(I'd at a pic of my new 18C except I don't see the att. icon).
j

MOP
08-01-2005, 10:38 PM
Had to jump in being as I also was on the wrong end of a tow rope about a week ago, if you are low and trying to stretch what little you have the posts above are best! But once the engine coughs and you know it is going to die the best thing is to lower the rpm and hold it in a bow high attitude,the reasoning is the tank pickup is aft so make sure every drop is as far back as possible. This was told to me by the guy from the marina that towed me in, it does make sense wish I had thought of it. That would have gotten me well past where I had to go for fuel, I still had four gallons in the tank when I filled it.

Phil

Moody Blu'
08-01-2005, 11:26 PM
Had to jump in being as I also was on the wrong end of a tow rope about a week ago, if you are low and trying to stretch what little you have the posts above are best! But once the engine coughs and you know it is going to die the best thing is to lower the rpm and hold it in a bow high attitude,the reasoning is the tank pickup is aft so make sure every drop is as far back as possible. This was told to me by the guy from the marina that towed me in, it does make sense wish I had thought of it. That would have gotten me well past where I had to go for fuel, I still had four gallons in the tank when I filled it.

Phil

that reminds me, another thing is to change the fuel pick up. I changed mine and had about a inch and a half more to go till it hit the bottom. I lengthened it a inch, that one inch is probably at least 4-5 gallons.

mattyboy
08-02-2005, 08:39 AM
Phil,
thought that's what I said :rolleyes: :p ;)
You have to take into account ,wind, current and which way the gas dock is too I would think the longer pick up makes sense but I wouldn't want it too close to the bottom might pick up sludge and stuff as time passes

MOP
08-02-2005, 10:20 AM
I had the wind and the river on my side, I ran out just past Singer Castle heading toward Blind Bay. It was so nice out I did not mind the wait except for Bonnie bellowing I Told You So and other choice words! :bonk:

CHACHI
08-02-2005, 11:53 AM
I had the wind and the river on my side, I ran out just past Singer Castle heading toward Blind Bay. It was so nice out I did not mind the wait except for Bonnie bellowing I Told You So and other choice words! :bonk:"YES DEAR" Phil, I feel your pain.

Tim Morris
08-02-2005, 12:12 PM
"YES DEAR" Phil, I feel your pain.

Hasn't happened to me yet, but I'll 'hear the pain' long before
I feel it.

MOP
08-02-2005, 02:20 PM
We all row the same boat!

ChromeGorilla
08-02-2005, 03:14 PM
Bald Eagle...email me some pics of the new boat. I'll post them for you.

varsanyi@tds.net

Formula Jr
08-02-2005, 07:27 PM
Its almost always, displacement speed. Even with a planning hull. Jeff, I would be amazed if your boat got better MPG at 3,400. Did you do low to high number idle speeds also in you tests? The Sound has tides so that might not be a fair comparison. You do after all have to "get" some where. I plan all Sound Trips to somehow use the tides if range is a factor. With out Tides, displacement speed, hull speed, always wins.

Formula Jr
08-02-2005, 07:49 PM
Jeff and Phil, you are on the money there with, "Know Thy Boat," and its range. You also have to include the environment you're working with. If you've got a steady 35 mile/hour wind against you, or you're riding against a 7 mph tide, then its a time thingy, not just a mpg thingy.
Water is fluid, it is moveing under you at its own pace. The thinking of all these factors is the beauty of boats.

...

KFLB143
08-10-2005, 10:44 AM
:moped:
My marina was closed for gas and my backup marina had a fuel spill
and closed down while I was waiting.
Question, when you are running out of gas, is it better to go 1100 rpm slow or 2800 rpm on plane ?
How do you go further, fast or slow? I thought keep the carb on two barrels not four.
I ran out about a mile away, at 2800 and now I wonder?
http://www.donzi.net/forums/attachment.php?attachmentid=13571
Minimum, idle speed, as slow as the engine will remain running will get you the farthest in the longest period of time.

IMO

LD

MOP
08-10-2005, 12:10 PM
:moped:
Minimum, idle speed, as slow as the engine will remain running will get you the farthest in the longest period of time.

IMO

LD

I disagree, just enough throttle to raise the bow enough to get the fuel to stay at the back of the tank. I died at idle and will never do it again, I still had 4 gallons in the tank.

Phil

Formula Jr
08-10-2005, 05:12 PM
Ok I am copying this out of a BOATING mag I have sitting here on my desk. These are the #'s they got testing a 22 Classic with 496HO.

RPM_____MPG_____MPH
1000 ----- 2.2 ------- 6.7
1500 ----- 1.6 ------- 8.5
2000 ----- 2.9 ------- 19.9
2500 ----- 3.3 ------- 33.4
3000 ----- 3.1 ------- 41
3500 ----- 2.8 ------- 49.8
4000 ----- 2.2 ------- 57.3
4500 ----- 1.9 ------- 64.8
5000 ----- 1.9 ------- 70.5


Looks like I would be best running at 2500 to make sure I made it to the fuel doc. :yes:

ChromeGorilla, I don't doubt that Boating mag published those numbers, but there is something wrong with them. The 1000 to 2000 RPM numbers don't make sense. They may have missed a very narrow RPM range, 1200, or so that would have been better than 3.3. If they had done 100 rpm intervals of measurement this should have shown up. I'll see if I can find the formula for displacement speed.
Here we go:

Displacement speed would be
1.34 X the Square root of the LWL. Water line length.
Can't find the LWL of a 22 but I'll guess at 20 feet.
This gives 6.9 or 7mph after converting from knots.
We can work backwards to find the engine speed.

((??/1.5)*25)/1056 = 7

this works out to 443 engine rpm. There would be a huge slip number here though. But since they started at 1000 RPM, Boating my have completely missed this displacement speed which may have been at 800 RPM or at some interval they didn't measure at. You'll note that the MPH numbers are already going down from 1000 to 1500. I just don't think Boating was looking for or found this magic RPM where the MPG spikes very early on and before planing.
Using Boating 41% slip number at 1000 RPM, that magic spike in MPG should have been somewhere around 624 RPM using my non-slip 443 number.
Just past engine idle if thats at 550-580 rpm for that engine. I'm thinking they started measuring after the boat was fighting its displacement speed.