Fuel Line Length

[Interest]

Can anyone tell me how far from the motor I can have the fuel tank it is a 8hp 4 stroke yamaha I would like to mount it up the front of the boat

[robert graham]

I can't say exactly/technically, but if I had a long fuel line run of say 8 or 10 feet or more I'd go to at least the next larger diameter of fuel line to help reduce any restrictions.....I'm sure there's a hydraulic formula for this....

[99yam40]

that small motor is not pulling much fuel, so I do not think it would matter much how long the line was.
The more fuel you move the line length and diameter may make a difference.

you can always hook(tee into the line) up a vacuum gauge to measure the vacuum being pulled on a long line to see what difference there is between a short and a long line or different diameter lines

[DennisG01]

I would think that the issue might be one of "weight". Meaning the more fuel that is in the line, the heavier it might be. That being said, I have no idea what the fuel system on the 8hp (or even my 250HP) is capable of in terms of fuel weight that it can pull. However, if the spot that the tank is located at is high enough, then I would suppose that gravity can help, at least to some extent.

Might be a good question for someone at Yamaha. Otherwise, fuel line is pretty cheap - try it. If it doesn't work, it's easy enough to fix on the fly.

'Course... I'm just thinking out loud here and could be way off base!

[Jason2tpa]

First off, what's the length of your boat? That will be the determinant factor on whether or not the length of fuel line will exceed the vacuum affect provided by the fuel pump.

Any length of a fluid filled tube has a specific resistance factor. Double the length of that fluid filled tube and guess what, it doubles the resistance factor. Doubling the resistance factor results in one-half the flow through the tube.

As you extend the fuel line to a certain length your fuel pump will fail to deliver the appropriate amount of fuel to maintain combustion. At what length will this occur?? That's a physics equation beyond my friday evening skills.

Extend the fuel tank to the point where the engine starves for fuel. Then cut off a foot or two of line, reinstall fuel connector Boom.

[cpostis]

Personally, I don't think you'd have a problem running 25 foot or more of 5/16" fuel line. Unless your diaphragm fuel pump cannot handle the increase vacuum in the line (I highly doubt it couldn't). Just make sure you have no sharp turns in your fuel line as not to restrict flow further and possibly kink a line.

Don't know Yamaha's fuel consumption for an 8hp 4 stroke, but Tohatso's is published at .9 gallons per hour @ 6,000 rpm. So, if yours is close to the same, you'll be drawing about less than 2 oz per minute.

[ausnoelm]

It's just a simple fact of life that you get a "pressure" drop in fuel lines (any liquid for that matter) the longer the hose, the bigger it needs to be, however, with an outboard, the method of moving that liquid through the line (fuel pump) is constant, to a point that increasing the line too much will result in not enough fuel being delivered for higher speeds (regardless of HP) most standard fuel lines have a pretty fair degree of tolerance built in, so, probably an increase in length of around 25% would see little difference.

[cleddau1]

The resistance to flow of a fluid through a tube is directly proportional to the length and inversely proportional to the 4th power of the radius of the tube. Thus doubling the length doubles the resistance but the resistance decreases exponentially as the bore is increased. Thus eg a 4mm tube has approx 3 times the resistance of a 6mm tube. So if your new fuel line is, say, 3 times longer than the old one then using a tube with a bore 50% bigger gives you approx the same resistance........

[robert graham]

The resistance to flow of a fluid through a tube is directly proportional to the length and inversely proportional to the 4th power of the radius of the tube. Thus doubling the length doubles the resistance but the resistance decreases exponentially as the bore is increased. Thus eg a 4mm tube has approx 3 times the resistance of a 6mm tube. So if your new fuel line is, say, 3 times longer than the old one then using a tube with a bore 50% bigger gives you approx the same resistance........

I believe this is the correct technical answer....so go bigger on your hose size diameter....

[DennisG01]

Is the physical weight of the fuel an issue? I could be mis-thinking it, but I'm imagining a small drinking straw compared to a larger diameter one. The larger one requires more vacuum (PSI, or whatever your favorite unit of measure is) to lift the soda, right?

[cleddau1]

The extra force required to lift a fluid against gravity (additional to overcoming the resistance to flow through a tube of circular xsection as described previously) is, counterintuitively, unaffected by the diameter of the tube, but is related to the density of the fluid. Easier to visualise the other way round, so, a vertical column of 32ft of water or 760mm of mercury both exert a pressure of one atmosphere, irrespective of the width, shape or squiggleyness of the column. In the case of the original question if the tank and carb are the same height any up part of the fuel line will be cancelled out by the down parts. If tank is lower than carb more force is required and vice versa.

[exeter27]

I would think the real issue is the height the liquid must be lifted. Even if the tank were at the front, siphoning type action will bring it to the back. The syphoning effectwill following the hydraulic formula previously offered. But, if your tank were set on a seat in the front, the fuel would be at a higher level than if the tank were on the floor in the rear. In that case, the tank at the front likely offers less vacuum resistance than the one in the rear.

I'm sure going with larger size will eliminate any doubt. But, if it were me, I'd just go with the regular diameter hose, even if the tank is on the floor. That engine just doesn't flow enough fuel so that the current diameter will pose enough of a restrictive action IMO.

[cleddau1]

Oops! Schoolboy Howler! I should not have used the word force but used (negative) pressure instead as clearly a greater force is required to lift a column of fluid a yard wide than one an inch wide but the negative or vacuum pressure required would be the same...

[throrope]

I suggest conidering 3/8 soft anealed copper tubing with short lenths of hose at the tank and outboard. Dirt cheap compared to ethanol rated hose and lasts forever.

[mudjon09]

Gasline length iguessyour taking about a 8 hp

I had a 8hp yam 2 strk new,I called yami service and he said 16 ft most. On a5/16 line.tank on front seat Of 14 ft Jon.
Ran great had a hell of a time using 3 gal a day..
Mudjon