Propane outboards

[njwilliams449]

I see that Tohatsu has recently brought out a 5hp propane outboard, and there is talk of their release in Australia. I am interested to compare it with the 5 hp Lehr propane outboard that has been around for a few years, if anyone has experience of either or both please? I am looking to power a 3m inflatable tender for general cruising yacht use, eg runs of up to 2-3 miles, some fishing etc. Advice most welcome.

[HopCar]

Hi NJ, I sold Lehr Outboards before I retired. I really like the idea of propane fuel for small Outboards. This is especially true if the engine isn't used frequently. The carburetor doesn't gum up.

One difference between the Tohatsu and the Lehr is that the Lehr can be run on a 1 pound propane bottle screwed into the power head. This is not a big deal to me as you can only get five horsepower for about half an hour on a pound of propane.

The Lehr is a couple of pounds lighter but not enough to make a difference.

The Lehr has what to me is a chintzy plastic cover. I don't know but I bet the Tahatsu is fiberglass.

I understand that Tahatsu offers it with an extra long shaft making it suitable to use as an auxiliary engine.

I've never seen the Tahatsu but I'd love to get a look at one.

[Cheechako]

I just have trouble with thinking propane as an outboard fuel. First, getting propane in Timbuktu often requires a taxi trip, finding the propane depot closed, returning, going next day, adaptors that vary throughout the world, getting filled, etc. Heck I even had that problem in Florida! It was a $20 Taxi trip to a place that was closed for the day! Not to mention either having an unstable tank in the bottom of the boat or a tiny one on the engine. A 4-5 HP gas outboard with the built in tank is sooo much easier if you don't want a boat tank in the dink.
I'll give you the very clean burning of propane though!

[a64pilot]

The other issue is the energy density of propane is less than what gasoline is, meaning that it takes twice as much propane volume wise.
A 10 lb propane tank hold about 2gl of propane, that equals about 1gl of gasoline, roughly.
Not a deal breaker, just think of how much gas you use, and double that for propane.

However if this motor is actually designed to run on propane, it can be made way more efficient than the average engine converted to run on propane, propane has I think about 115 Octane and most gas engines are designed for 87 Octane, the much higher Octane will allow much higher compression and ignition timing, making a much more powerful engine per cu in and or increase fuel efficiency.

IF it’s designed to run on propane, I know of no engines that have been though. Be really neat if it is.

[Plangstraat]

I have a 2.5 hp LEHR that I use an my Highfield UL260 dinghy. I've had it for three years now and it has been entirely reliable. I appreciate the ability to use the same propane bottles on the BBQ and the outboard. One less thing to carry aboard.
My only complaint is the same as HopCar. The plastic cover is chintzy. Worse are the cheap clips that hold down the cover. I keep having to replace them as they break.
Other than that, I'm sold on propane as an environmentally friendly fuel. The other advantage is that, since the LEHR gets sporadic use, there is no gummed up carb to worry about.

[HopCar]

A64 it's not that bad. Gasoline produces about 114000 BTU per gallon.
Propane produces about 91000 BTU per gallon.
So propane has about 80% of the power of gasoline. Not 50%. Given that a properly designed propane engine is more efficient than a gasoline engine, the difference in volume used is not a lot more, but yes it is more.

[kmacdonald]

Why in the world would you want a propane engine? The only advantage is propane doesn't go bad but you can treat gas to last 2 years so that is easily solved. I don't think I'd take one if they were giving them away.

[a64pilot]

Originally Posted by HopCar

A64 it's not that bad. Gasoline produces about 114000 BTU per gallon.

Propane produces about 91000 BTU per gallon.

So propane has about 80% of the power of gasoline. Not 50%. Given that a properly designed propane engine is more efficient than a gasoline engine, the difference in volume used is not a lot more, but yes it is more.

Yes Propane is about 20% less power density, but I believe in converted engines it is more than 20% less efficient because they don’t have the compression ratio and spark timing to utilize Propane efficiently.
Similar to alcohol, if you design an engine to run on alcohol, phenomenal power can be made, it’s an excellent racing fuel, but regular cars don’t run well off of it as they are not designed for it.
It was very popular to convert automobiles to Propane in the 70’s, but as you have noticed they are very rare now, reason is even if you don’t pay road tax on Propane, it’s still way more expensive and the range is poor.
Many engines like forklifts that are used inside run propane because the exhaust is less obnoxious.
I have torn apart Propane engines that have many hours on them, and until they start burning oil, they are clean as a whistle, when they burn oil of course the oil leaves carbon.
The oil on a propane engine stays clean too, I wouldn’t mind running the same good synthetic oil for years in a Propane engine.

What is Key is, is this engine designed for Propane and has the compression, cam timing and spark lead to run it, or did they just convert a gas motor?

[IslandHopper]

If they were dual fuel capable I might consider one....

[Sailmonkey]

Propane for an outboard makes sense if you cruise in a single locale, IE near home, and long periods elapse between outboard uses.

For voyaging further afield, propane makes very little sense, however your using your outboard daily so fuel gumming isn’t a problem.

So depending on your expected use propane may or may not make sense for you.

[HopCar]

Originally Posted by a64pilot

What is Key is, is this engine designed for Propane and has the compression, cam timing and spark lead to run it, or did they just convert a gas motor?

Lehr Outboards can not be easily converted to gasoline. Their compression ratio is too high. They are designed to run on propane. I imagine Tohatsu is the same.

I wish I had a propane fueled chain saw. When this last hurricane hit Miami I needed my chainsaw to clean up the yard. Of course I rarely use it and it wouldn't start.

My generator which will run on propane or gasoline started right up. It hadn't been started in more than two years. It had never been run on gasoline. It ran at least twelve hours a day for ten days. Since it is multi-fuel it is lower compression than one designed for just propane. This genny is rated 6kw on gasoline and 5.5 kw on propane.

I also have a Honda 2000i which have a great reputation for reliability. It wouldn't start until I replaced the carb.

Where propane shines as a fuel is on infrequently used equipment and if you are concerned about the environment.

[ranger58sb]

Originally Posted by kmacdonald

Why in the world would you want a propane engine? The only advantage is propane doesn't go bad but you can treat gas to last 2 years so that is easily solved.

I never had that kind of luck with our local ethanol gas for our previous 2-stroke outboard. Probably 2 months max, and I tried all manner of stabilizers... so I usually recycled our dinghy gas into a car after about 3-4 weeks.

-Chris

[SailFastTri]

Originally Posted by ranger42c

I never had that kind of luck with our local ethanol gas for our previous 2-stroke outboard. Probably 2 months max, and I tried all manner of stabilizers... so I usually recycled our dinghy gas into a car after about 3-4 weeks.

-Chris

Similar experience here. Even with stabilizers, ethanol is unstable and is a magnet for atmospheric water vapor that condenses into the fuel as the tank breathes. Phase separation is a problem.
The stale fuel in this image had been stabilized upon purchase, but left to sit in this tank, in the sun, for about 2 months.

[CalmSeasQuest]

Instead of propane, I'm hoping for a dinghy-sized (20HP~25HP) version of this...

https://newatlas.com/yanmar-dtorque-...utboard/51675/

To me; this ticks off all the wants...diesel, longevity, fuel economy, torque... The current 50HP version is still too heavy at 175KG, but hoping a smaller version could come in lighter.

[thinwater]

Originally Posted by SailFastTri

Similar experience here. Even with stabilizers, ethanol is unstable and is a magnet for atmospheric water vapor that condenses into the fuel as the tank breathes. Phase separation is a problem.
The stale fuel in this image had been stabilized upon purchase, but left to sit in this tank, in the sun, for about 2 months.

Only if you leave the vent open. Close it when not in use. If you are getting phase separation you are either leaving the bent open or the lid leaks.

Was the vent open or closed on the aged fuel? What was the stabilizer (many are snake oil)? If it was truly sealed, air tight, that is very unusual. The sun does not help (there is no reason to do that). I know that samples stored air-tight in the dark are generally stable for at least several years. The military has specs on this and I have done the tests.

So something is not right.