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Old 11-11-2014, 17:31   #1
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Propane Outboard Fire Potential

I thought I would give some first hand experience with the Lehr propane outboard. The engine is used to push a 18ft Marshal Cat Boat. I was helping a friend move the boat across the river for winter haul out. The river was calm and there was no wind but slight outgoing current. Halfway to our destination we noticed the engine was overheating. Just steam coming out the exhaust port. Most likely we had run the engine long enough without waster for a period of time because smoke was coming coming from under the cover and the smell of hot plastic was readily apparent. I turned the engine off. I let it cool for few minutes but the engine remained very hot to the touch. I discovered the problem was a large leaf caught behind the shaft has blocked the water inflow. Still smelling plastic I became concerned about the propane connection to the motor ( a couple attached to a flexible hose which when deployed is pushed up under the canopy through a hole and end of the propane canister garage ) I was concerned about the hose bursting as we had just installed a full canister of propane prior to departure. I called for the fire extinguisher in case the hose burst. Sure enough it did and scared the crap out of me. I could hear the propane canister emptying under pressure under the canopy. With the engine still sizzling I was sure a fire was imminent and fueled under pressure. It was several minutes before the canister emptied and luckily no fire but I was certainly in unfamiliar territory. I have plenty of experience with dripping gas or diesel and even carburetor fires but nothing like propane under pressure. Very unsettling to be sure. Apparently propane's high ignition temperature saved the day, or the low oxygen under the canopy or just plain luck. The plastic/rubber flexible tube makes attaching canister convenient but I would sure feel more comfortable with hard connection. Any comment would be appreciated.

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Old 11-11-2014, 19:36   #2
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Re: Propane Outboard Fire Potential

not familiar with the lehr but im surprised there isn't a manual fuel shut-off right at the canister, or a solenoid with handy switch. seems like a design oversight that you have to just sit there and wait for the canister to empty or jump in the water til it does. wouldn't a couple turns on the can have disconnected it from the system? just spit balling here...

luckily, the fuel has a relatively narrow flash range from about 2% to 9%, and needs over 900degF for ignition. unlike nat gas, propane is heavier than air so fanning out the boat to keep it from pooling is in order. don't think id use a electric fan though...

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Old 11-11-2014, 20:22   #3
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Re: Propane Outboard Fire Potential

Sorry about your misfortune, but the main thing I am glad you are OK!

Interesting situation however. When the hose went, do you think the propane had any cooling effect on the hot engine? Like robwilk, I would think there would be a shut off valve somewhere easy to get to.
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Old 11-11-2014, 21:02   #4
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Re: Propane Outboard Fire Potential

That must have been a very unpleasant experience. With the Lehr 2.5 or 5.0 that can be run on the one pound Coleman type bottles, the only way to stop the leak in a situation like this is to unscrew the bottle.

The procedure to unscrew the bottle is this:
Flip the quick release on the band holding the tank.
Pull the tank back about three inches.
Grasp the brass hose fitting with one hand and rotate the tank with the other.
I've never tried to do this fast but it would take at least several seconds.

I don't think there was a lot of danger but it would certainly make me pucker up a bit.

As Robwilk pointed out, the flash range of propane is pretty narrow. Actually a little narrower than gasoline vapors. It's high ignition point makes it unlikely that it would ignite unless there was an open flame present.

I think most of the propane vapors would have drained overboard, but if the wind was very calm, some might settle into the boat. It would dissipate from an open boat pretty quickly with just a small breeze. Gasoline on the other hand would fall into the bilge as a liquid and slowly vaporize keeping the gas / air mixture in the danger zone for a longer time.

I'll forward a link to this thread to the head engineer at Lehr. I'm sure he'll be interested.
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