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Old 23-02-2010, 20:15   #16
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come to think of it, most lawnmowers are carburated and gravity fed
Scott
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Old 23-02-2010, 20:47   #17
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Scott,

The issue has to do with the pressure to the carburetor. Carbs don't like to see more than a few psi when running and don't like pressure when not running. If you have ever seen someone put a fuel pump made for EFI on an engine with a carb, you will know that the extra pressure doesn't work.

You are correct that most small gas engines use gravity feed but they use very little head height so that the pressure is extremely low.
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Old 23-02-2010, 20:50   #18
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Lawn mowers have pretty small tanks and if they leak it runs out on the ground. If they catch fire you can run. Gas is just to dangerous on a boat (in my way of thinking) to have an outlet at the bottom of a tank. Not worth the risk.
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Old 23-02-2010, 21:23   #19
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OK.. I'll agree that carburators don't like a lot of pressure. but with a 1/4" or 3/8" fuel line, you are going to need to a lot of elevation to get any serious pressure.


I can see where a fuel leak would be a problem as it would continue leaking if unnoticed until the tank was drained. Which is another good reason for the shut off valve at the tank.

Scott
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Old 23-02-2010, 23:29   #20
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There are a lot of sailboats out there with gasoline engines that have a fuel tank higher than the engine.

I honestly don't know where the paranoia from gasoline comes from....

Most every explosion I have heard of is from improper fueling or some Darwin Award nominee trying to pump out a tank with a drill pump or a wet-vac.

as an aside.....doesn't everbody shut their fuel off as well as their thru hulls when they leave the boat?
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Old 23-02-2010, 23:33   #21
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Scott,

the diameter of the fuel line is not a factor for the pressure. Just the specific gravity of the liquid and the height of the tank. As was demonstrated in the thread, a height of 6' doesn't pose any trouble for a diesel engine. I am confident that up to 7 psi is okay for every diesel engine because I researched that when I designed our fuel system (which includes 7 psi pressure pumps).

cheers,
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Old 25-02-2010, 01:15   #22
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Thanks Nick,
I was trying to work that out in my head. I know with fire hoses the amount of pressure gained going down hill is different for 2 1/2 inch hose than it is for 1 1/2 inch. but that could be because of the friction in the lines.

I do remember that 1 psi is the same no matter the size of the container. what changes is the square inches. That is why a 3 inch hydraulic cylinder is stronger than a 1 inch cylinder with the same psi opperating pressure.

Scott
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Old 25-02-2010, 07:39   #23
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Scott,

I believe what is mixing you up is restrictions (friction) in the line as you guessed. When there is no fluid movement, the restrictions do not matter and pressure is constant in the horizontal plane. As you introduce flow, the restrictions lead to a change in pressure along the line. For fuel line, the flow is relatively low compared to the line size so the pressure drop is low but it is present. For fire hose, the pressure drop is significant different in 1.5 and 2.5" hoses since the hose diameter is relatively small compared to the flow rate so the losses are much greater for 1.5" where there is much more restriction.
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