Outboard Fuel Consumption

Joe11688 · Oct 20, 2009

Gudenuph said:
How many gallons per hour should I expect to consume in a 9.9 HP Mercury 2-stroke outboard engine pushing an O'Day 25 at 4.5 to 5 knots in calm conditions? The boat is new-to-me and I'm trying to size an open storage locker under the stbd cockpit seat. It would be fully sealed from below to avoid fumes in the bilge. Our boating is on Puget Sound with 20 to 30 miles between gas stops and I'm wondering if the available space under the seat would be adequate. I'd sure like to get the gas can out of the cockpit. What has been your experience? I'd be most grateful to know.
George

I have an 8 hp Johnson 2 stroke outboard and I've never bothered to figure out the gasoline consumption. I've never needed anything more than my 6 gallon poly fuel tank when we went on trips of 20 to 30 miles and spent a week out on the water in the Martha's Vineyard area. Most of the time we sailed when the wind was up. Today, I carry a 6 gallon gas can in my truck that I fill up at a gas station and when my boat's fuel tank is low, I replenish it at our yacht club dock. I would build the fuel compartment to accommodate a Poly 6 gallon tank. A three gallon tank won't cut it. Also, I'd stay clear of metal gasoline tanks. They rust out after a period of time. Good luck.
Joe

Joe11688 · Oct 21, 2009

Gudenuph said:
Thanks Joe.
We have 2-three gallon poly tanks that will fit the available space quite nicely. I'll get the dimensions of a 6-gallon tank also to include as an option if possible. The 3-gallon tanks are easy to tote to the boat as a balanced load and since both have hoses and bulbs, I may even be able to switch tanks on-the-run. Our past practice has been to motor just enough to keep the batteries charged but that was with a larger alternator than the generator on the outboard. Looks like more oil lighting in store, but then life is just full of little adjustments. Keeps things interesting.
George

You know George? you're probably better off using two three gallon fuel tanks because they are not only easy to handle, but they're easy to shake up. The reason why I say this is because this Ethanol as you know, attracts water and if you can shake the gas each time before you take your boat out, you probably will never need a water/gasoline filter and you'll never have an engine problem. Not only that, but the Ethanol is said to break down the oil in two stroke engines like mine and ruin them. Like you said,--they're easier to handle and I agree with you on that even though I have one of those fold-able stainless steel tote carts that used to belong to my late pal Walter, it's still a pain in the butt to haul my 6 gal. gas can to the dock to top off my fuel tank on my boat. I always remove my tank from the fuel compartment, and top it off on the dock and it's a pain.
Years ago, when we first started sailing to Cuttyhunk Island and Martha's Vineyard, I brought an extra gasoline can full of gas, and stored it on deck. We spent a week on the water, and I never even needed that can. It was in the my way most of the time. We went back four years in a row and that extra gas can never went with us ever again. The OMC 6 gal. Dura-Tank is the only plastic tank that I've found that will fit in my fuel compartment. I don't even know if they're still making them anymore. It was the only tank that my friend was able to fit in his 1986 O'Day 26 fuel compartment also. So, I think that two 3 gal. tanks is a wise choice. You need to keep an eye on them though if you are motoring through critical areas or approaching drawbridges as I do on the river that lead to the bay. Good luck George!
Joe

Joe11688 · Oct 22, 2009

Gudenuph said:
Thanks for the quick reply Joe and especially the heads-up on ethanol. The last thing I want to deal with is a cranky engine. We are very much aware of the tides all of the time since we have to deal with their currents. Five to ten knots of current are quite common out here at the various constrictions. In fact it is fun to play the back eddies and otherwise work them like you would the wind shifts. Some whirlpools can spin you around in the blink of an eye so your point about knowing if the gas tank in use will see you through is well taken.
George

It's always fun to play the tides and currents. Fortunately for me, we don't have those whirlpools in my area, thank God. Our currents don't get much above 6 Knots, I don't think.

They have these small in-line gasoline/water separation filters that you can buy for about $70 or $80, but for the small quantities of gas that we carry on board, I don't think that it's worth spending the money on. My friend Wayne feels the same way. We add Sta-Bil to our gasoline. Wayne pulls the fuel line out of his engine and lets it die at the end of the day when he puts the boat on the mooring. I don't do this, but I always grab my tank and shake it up when I first get on my boat.

Sunbird22358 · Oct 27, 2009

I'd say less than 1-gallon per hour of full-throttle running. That is approximately how much gas our 9.9 hp Johnson 2-stroke burns pushing our 14' powerboat. The powerboat is not overly heavy, but she's no lightweight! At top speed of 15 knots (a bit less with 2 people aboard!) we throw a good wake, meaning we are really a "semi-planing" hull.
6 Knots wil lbe "hull-speed" for a 25, so 4-5 knots would be at less than full-throttle, so I'd estimate in hte neighborhood of 1/2 to 3/4 gallon per hour, more if bucking wind/waves/current. 6 gallons wouls be "enough" to go 20-30 miles...... but I'd want to carry some more as reserve, maybe 1 six gal tank and one 3-gal or 3-3 gallon tanks? Perhaps a 9 or 12 gallon permenantly installed tank would work. Be sure to install it to USCG specs, even though you have an outboard, better safe than sorry (and cheaper to insure!).

Sailornels · Oct 29, 2009

I have a Honda 8 horse on my O'Day 23. I get more out of my 3 gallon tank than i ever did with my 6 gallon tank on my previous Johnson 7.5 horse.