Tohatsu 5-HP: Gasoline or Propane

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
bikerGus
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Tohatsu 5-HP: Gasoline or Propane

Postby bikerGus » Mon Jan 15, 2018 12:51 am

I'm undecided on which TOHATSU 5-HP engine to buy for my 2006 Montauk: gasoline or propane fuel.

The local Tohatsu dealer says the advantage of propane for an emergency engine application is there is no carburetor maintenance (because there is no carburetor); thus there is never any starting reliability worries due to infrequent use. Also, propane runs cleaner. And the small 11-lbs composite propane boat tanks are light; they take-up about as much room as a fire extinguisher.

[What is your] opinion?

jimh
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Mon Jan 15, 2018 9:35 am

The obvious problem with operating an outboard engine on liquid propane fuel: where can you re-fuel? It certainly will limit your opportunity to re-fuel on the water. Also, how do you determine how much fuel is remaining in a tank?

I think the dealer's representation of the engine as having "no carburetor" is a bit misleading. The propane fuel must be introduced into the air stream going into the combustion chamber. And there must be some method of throttle control. I doubt the little 5-HP engine has direct-injection and computer control. It probably has a carburetor, but just a slightly different one than would be used for gasoline.

bikerGus
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby bikerGus » Mon Jan 15, 2018 10:55 pm

It will be interesting to see if Tohatsu sells many of these [propane fuel 5-HP engines].

The Viking composite propane tank has a window showing fuel level and can be refilled at any propane vendor.

pederj
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby pederj » Tue Jan 16, 2018 5:59 am

I watched a video on youtube about this. There is not a carburetor on this, but there is a "fuel/air mixer", which looks like a carburetor.

Jefecinco
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby Jefecinco » Tue Jan 16, 2018 9:57 am

Is the Tohatsu engine for trolling? If so, and if the engine will accept them, it may be convenient to use Coleman disposable propane bottles. We have a lantern and a grill that use the disposable bottles. Then simply screw into the regulator. You could easily store a few in the RPS of your Montauk.
Butch

jimh
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Tue Jan 16, 2018 12:16 pm

pederj wrote:There is not a carburetor on [the Tohatsu 5-HP propane fuel engine], but there is a "fuel/air mixer", which looks like a carburetor.


A carburetor is a fuel-air mixer. If the Tohatsu engine has a device that mixes the propane fuel with air, then it sounds like a carburetor to me. If it looks like a carburetor and does the same function as a carburetor, why aren't we calling it a carburetor?

For the dealer or his salesman to insist the engine has "no carburetor" is either due to lack of knowledge or intentional disingenuity. The fuel has to be mixed with the air in some manner. In the absence of direct-injection of the fuel into the combustion chamber by a specialized direct-injection fuel injector and computerized control and metering of the amount of fuel, there must be some sort of fuel-air mixing device external to the combustion chamber.

There are "how-to-do-it" instructions for modifying a gasoline carburetor to work with liquid propane fuel by changing the size of the orifices and adding a flow-control mechanism.

Unless the Tohatsu 5-HP engine has direct-injection and computer control of volume, it sounds to me like it has a carburetor. Now the specially-adapted carburetor and use of liquid propane fuel may reduce some problems compared to using a standard carburetor and gasoline fuel--and that could be a selling point. But the engine still has some device mixing fuel and air and controlling engine speed, probably with a throttle plate.

jimh
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Tue Jan 16, 2018 1:49 pm

GUS--thanks for the added information that a propane fuel tank with some sort of sight-gauge of fuel level is available. That solves the problem of determining how much fuel remains in the tank.

As for the abundance of locations to get a propane tank refilled, I can agree there are many of them on the highway. But not nearly as many as there are retail gasoline stations. And they are not self-service. And it probably will take 15-minutes or more to get the tank re-filled. And you are not going to find a re-fill station at most fuel docks on the water.

I am also curious about the recommended installation of the propane tank. Generally when propane fuel is used on a boat it has been for cooking. The fuel is typically kept in a special enclosure with a shut-off valve in the fuel line. All that to prevent any leaking of propane vapor into low parts of the boat and hull, where it might accumulate and become a risk for fire or explosion.

What does Tohatsu say about mounting of the fuel tank? Do they provide any specific warnings, cautions, or recommendations?

Also, what is the relative cost of the fuel compared to gasoline? That may not be important if the planned use for the engine is only as an emergency or contingency engine if the main engine fails. But to know the relative cost of the fuel might be helpful in making a decision.

Ridge Runner
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby Ridge Runner » Thu Jan 18, 2018 8:49 am

According to Tohatsu:

    "The MFS5C LPG is based on the MFS4/5/6C engine platform with some key changes to allow it to use LPG or propane. The MFS5C LPG comes from an existing reliable, proven and durable platform. The major component changes to the propane-powered version are the fuel regulator, the fuel shut off valve, the cylinder head, the mixer (replaces the carburetor) and the LPG fuel connector.

    "During operation, the LPG enters the engine through the fuel connector and travels to the fuel regulator first. It takes the high pressure LPG coming from the LPG tank and steps it down in two stages from high pressure first to 6psi, and then from this midpressure down to 0.6psi.

    "This process in the regulator brings the fuel pressure to a safe and usable pressure for the outboard. The fuel then travels to the fuel shut off valve. The shut off valve is controlled by a negative pressure supplied from the engine as the engine rotates. As soon as the engine ceases its rotation, the negative pressure stops and this immediately shuts the fuel flow off to the engine. The fuel shut off is normally in a closed position and is only opened with the engine rotating. This feature makes the MFS5C LPG safe, by only having low pressure LPG flow when the engine is operating. The fuel then flows from the fuel shut off valve to the mixer. The mixer does the same function as a carburetor, by mixing the fuel with air to send an atomized fuel/air charge into the combustion chamber. The updated cylinder head optimizes the combustion to offer a clean and reliable burn."

Tohatsu is also clearly marketing the LPG motor to sailors and boaters that have propane onboard for either heat or cooking fuel. Having a propane-powered outboard in such situations eliminates the need to carry gasoline. http://www.tohatsu.com/outboards/5_4st_lpg.html
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TransFueler
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby TransFueler » Thu Jan 18, 2018 11:32 pm

Propane offers at least one very good advantage: the carburetor never gets mucked up with old ethanol fuel. Considering that is a major problem with outboards that aren't run very day, it should be a huge benefit.

My house generator runs on propane, and even after a year of not running, starts right up on propane.

Reading all the many posts on the [problems] of winterizing and otherwise prepping outboards for storage, I think a propane-fueled engnie would be absolutely terrific. Not much goes wrong with these small outboards other than fuel-related problems. Why not totally eliminate that with propane?
1987 17 Super Sport Limited
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TransFueler
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby TransFueler » Thu Jan 18, 2018 11:33 pm

Many marinas offer propane because most boat cooking stoves are propane powered.
1987 17 Super Sport Limited
2003 Johnson/Suzuki 90 Four Stroke EFI

Ridge Runner
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby Ridge Runner » Fri Jan 19, 2018 4:32 am

Tohatsu did not invent the term "Mixer". It is a standard term used in the NG/LPG industry, typically used in large industrial systems. The term "mixer" and "carburetor" have been both been used as LPG fuel has been adapted for use in consumer products. Here is an example of a large LPG Mixer: https://www.youtube.com/watch?v=XJhy3G9uW64

Image of the Tohatsu MFS5CLPG Mixer:

Mixer 2.JPG
Mixer 2.JPG (64.82 KiB) Viewed 12754 times


Tohatsu provides excellent part schematics and catalogs for all its outboards. They are available at: https://www.tohatsuoutboardparts.com/Parts-Books.html
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2005 170 Montauk, 2010 E-TEC 115 H.O.
2016 210 Montauk, 2017 E-TEC G2 200 H.O.

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jimh
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Fri Jan 19, 2018 9:23 am

RIDGE--thanks for the drawing. The linked video was boring. I gave up after about 15-seconds.

I have a standby generator. Rather than use propane I opted for natural gas. The generator starts itself every two weeks for exercise, then emails me a report about it.

I use propane for cooking on the boat, but the fuel is in small canisters. You cannot refill them. I am not familiar with refilling propane tanks.

I am surprised that so many marinas are claimed to offer the service of refilling propane tanks. I cannot recall ever seeing that being offered at any marina fuel dock I have visited here in the Great Lakes, based on not seeing any sort of sign or any mechanism at the fuel dock for accomplishing it.

I am sure that if you hiked into the typical small town that we visit by boat you could find propane available, but whether or not the supplier would be able to re-fill a particular type of tank you owned would seem questionable.

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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Fri Jan 19, 2018 9:32 am

What is the weight of the fuel tank that would be used with a Tohatsu 5-HP engine when the tank is empty? When it is full?

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Re: Tohatsu 5-HP: Gasoline or Propane

Postby Jefecinco » Fri Jan 19, 2018 9:45 am

I spent about a year running an 18 megawatt diesel power plant with six dual-fuel engines. Under normal operating conditions the engines operated on 10% diesel (pilot fuel) and 90% natural gas. We were occasionally required to operate at 100% diesel and the ratio was continuously variable. Those engines were incredibly clean burning. Oil was never changed even though the plant was continuously operating. Oil was tested every 300 hours and the samples always looked like new oil. We periodically inspected the combustion chambers with a fiber optics camera. The tops of the pistons beneath the fuel injector ports looked unused except for small ash deposits. In commercial applications maintenance costs are considerably less when engines are fired with natural gas.
Butch

floris van den berg
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby floris van den berg » Sat Jan 20, 2018 5:15 am

Go with a gasoline motor in combination with akylate fuel.

fallguy1000
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby fallguy1000 » Sun Jan 21, 2018 5:49 pm

Operating temperature ranges? Propane is horrible in cold weather. Of course, most boats operate above freezing, but I would not trust propane for temps below 50F.

padrefigure
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby padrefigure » Mon Feb 05, 2018 9:30 pm

And one difference between a carburetor and a "mixer"--the carburetor also converts the state of gasoline from liquid to a fine mist so that it can combine readily with air. Droplet size is a very big deal. The LPG (or CNG for that matter) "mixer" only meters the fuel flow and combines with air for combustion. I would guess that mechanically, getting two gasses to combine uniformly is simpler than transforming a liquid into a nearly gaseous state and then combining it with a gas for combustion.

jimh
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Re: Tohatsu 5-HP: Gasoline or Propane

Postby jimh » Tue Feb 06, 2018 8:29 am

PADRE'--many thanks for that very cogent analysis that propane is already a gas at normal temperatures. I had not considered that characteristic of propane.

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Re: Tohatsu 5-HP: Gasoline or Propane

Postby Dutchman » Thu Feb 08, 2018 10:31 am

fallguy1000 wrote:Operating temperature ranges? Propane is horrible in cold weather. Of course, most boats operate above freezing, but I would not trust propane for temps below 50F.


I would trust it completely and with a small Coleman green tank I would always have a reserve for my small OB and I could use it on the grill too.
My L(C)NG fired 4 stroke combustion engine in my car in the seventies never had a problem firing up in below freezing temperatures in Europe and I assume 40 years later they would have improved on this technology for either natural and/or propane gas.
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