Running a two stroke from a large static tank? How to deal with mix?

[Cheechako]

Originally Posted by Franziska

Thanks. We are talking of fifty gallon. Occasionally this tank might sit for a number of weeks unused, which might lead to separation of oil and petrol. You can't simply stir it then, hence the question.


The external system should essentially create the Premix on the go, injecting the oil into the fuel line between tank and engine.

We usually give the small outboard tank on our dinghy a short shake before starting up.

Still, I'm not talking about the dinghy out boards above..

I would say the gas in your carbureator will get you out of the anchorage/marina and by then the boat motion has mixed the 50 gallon tank adequately. Gas powered powerboats with one or 2 engines just start and go and have done so for probably 75 years. I think you are overthinking this.

A boat in a slip or at anchor also has boat motion/action daily.

[Jim Cate]

Originally Posted by Kettlewell

This one YT video indicates no separation after 6 weeks. I've left 6-gallon cans sitting for more than 6 months with both 2-stroke and Stabil in them and the motors fire right up in the spring and run perfectly. Back in the day lots of fishing boats used 2-stroke motors along with 100-gallons or more of fuel in internal tanks.

Ahaa! This (admittedly not really controlled) demonstration lends credence to my previously expressed doubts about separation.

When you consider that petrol is itself a mixture of several organic compounds, and that they don't separate in storage (although some may flash off from a vented tank), why would you think that adding an additional miscible mixture of organic compounds would act differently? I'm pleased to see some evidence of the stability of the mixture... it is certainly what I have experienced in my usage of outboard fuel.

Jim

[OutOfControl]

Originally Posted by Cheechako

I would say the gas in your carbureator will get you out of the anchorage/marina and by then the boat motion has mixed the 50 gallon tank adequately. Gas powered powerboats with one or 2 engines just start and go and have done so for probably 75 years. I think you are overthinking this.

A boat in a slip or at anchor also has boat motion/action daily.

I have run two strokes for many years and Cheechako is right. The boats motion should be more than enough to keep the fuel properly mixed.

[Don C L]

You may have the answer you need, but if it were me I'd mix the oil I knew I was going to need with some gas in a jug for gas first, pour that in, and then add the rest of the gas needed, just to be sure the the oil was already dissolved in some gas first. Then I would not think about it again until its time to refill.

[RaymondR]

They have to boil the crude oil in a distilation tower to get the gas portion to separate so I doubt that the mix will separate.

[Franziska]

So, looks like was overthinking this and it might be a non issue in reality :-)



Thanks everyone.

[catsketcher]

Gday Fran

I don't get why you would consider going two stroke. The old Tohatsu 9.8 is just a 6/8 hp block with different carby jets. I don't think you will get the low down grunt you need for a slow boat from the two stroke. Also the reduction on the gearbox is 2-1 not the 3-1 of the Yammie hi thrust. Then there is the prop size issue. A cat with lots of windage (even your cat) needs lots of grip on the road - and that is prop size.

For 20 years our 38ft cat had a single 25 hp four stroke. At first we had a Honda with a 9 inch prop. It was a super nice three cylinder engine but it had a small 9 inch prop. It did not work well in reverse and the boat would slow down markedly in headwinds. Then we switched to a Yammie 25 with a 12 inch prop and 2.4 to 1 reduction (instead if 2 to 1). It was night and day. Although the motor is rougher (twin cylinder rather than 3) it is such a better engine for a cat - it has proper reverse, heaps of prop area, and very reliable.

I would not worry about the reliability of the 9.9. Get it serviced before you go foreign. Change the oil and be nice to it by keeping it free of salt inside the cover. My older engine has had nothing done to it apart from oil changes and regular services over its 16 year life. It is just a trooper and I would much rather it than a simple carby two stroke, although there may be an E tec 2 stroke that could be okay - but then you have the complication of e tec.

Keep the Yammies

[jamhass]

One of the concerns you expressed was the availability of parts in isolated regions. It is reasonable, even recommended to carry those bits which can immobilize the engine but are relatively easy to replace. Were it me, I would carry a spare ignition unit, carb, impeller, etc, and appropriate gaskets.

[christian.kautz]

Some ideas, no soulution: first i thought about some waterdistributing system used for flowers, but this will feed a fixed amount of oil. You have to measure the actual amount of fuel consumed on the line and add the right amount of oil. May be a raspberry will help. Or you pump a fixed amount of gas to a daytank and add a fixed amount of oil. This might be automated simply when the daytank empties.

[Cheechako]

My larger concern for long term storage of gas would be having alcohol gas.

[Sparx]

Like you I have a pair of Yamaha T9.9s, now with around 1200 hours on them. Cruising to remote places got me interested in spare parts. Today I carry spare sparks plugs, water pump, carburetor, fuel filters, zincs, ignition coil, and throttle cables. Total investment in spare parts is quite modest. I can see adding a spare CDI possibly, certainly without changing the economics to favor engine replacement.

Experience so far is the few issues that have come up are readily dealt with using these spares and attention to preventative maintenance.

Are you having a different experience? At what number of hours do these engines become candidates for replacement?

[Goodxcharly]

Never heard of oil and separating till I read this post.
I would add add all the oil to the first 5 of gas or add the oil in stages. It will mix whenn pouring into the tank.

[boat driver]

Originally Posted by Franziska

Regarding separation, no I have no proof of that, but as Oil and Petrol have different densities I do see a risk of that happening when at anchor for some weeks.
Especially as it's a rather tall reservoir (stainless steel drum repurposed).



If you pour some oil and petrol into a tall glass and let it sit, it's very likely to separate. Gently rocking it would not remix it. Guess heavy weather would do the trick, but we try to avoid that.

Now your idea is a clever solution. Simple and easy to implement. Thanks, I like it!


Regarding Benz's comment about water in fuel, we always use a water separating funnel when refueling and have a water removal centrifugal filter in the fuel pick up hose. So I hope that helps, besides usually using shoreside filling stations rather than marine ones. Usually they are cheaper and have cleaner petrol because they sell a lot more of it in a given time span.

It was also suggested a bit of alcohol into the tank to get rid of any water.

I’m a bit confused on this topic- quality
two stroke marine water cooled oil
50:1 does not separate in the timeframe of fuel life- and most high end oils have various stabilizing protections blended in.
I have run two strike outboards for 60 years without concern for separation.
Fuel has a life and depending the country of origin blending that life may be as little as 30 days to 1 year.
Biogas blends are shorter.
I generally carry 180 gallons of gas mixed that burns over a 6 month process.
Oil metering systems on outboards are designed for rpm metering to reduce pollution and plug fouling when at idle. Essentially they reduce the mixture automatically to 100:1 at idle.
The question of mixing- ideally oil first and add gas to tank.
Accurately account for gas to measure out oil but do not short the oil mix.

Really not sure who suggested that mixed oil/gas would separate.
Alcohol adding to system is same as ethanol blending.
It will absorb all water till saturated- and will not separate out in filter.
It will also create other corrosion internally on the pumps.

[Scubaseas]

Originally Posted by Franziska

No free access holes at the top, stirring is impractical in our case.


But looking at kettlewells answer, maybe it's really a non issue.



Btw. We seem to be lucky with our fuel, we have used petrol which was over a year in the full tank without any issues. This comment is relating to our current setup with two fourstrokes though.

Put a tee fitting on your vent line or sender housing. Get a small air pump, like fish tank or hand pump. Use a small hose into the tank (that terminates near the bottom) to force air bubbles into the tank which will mix the fuel. Moving the boat will mix it fine.

I like Benz' advice on using a day tank as the best solution for this problem.

Adding alcohol doesn't make water disappear and it won't cure the carb from corroding. Actually makes gunk in the carb more likely.

[rslifkin]

Originally Posted by Scubaseas

Put a tee fitting on your vent line or sender housing. Get a small air pump, like fish tank or hand pump. Use a small hose into the tank (that terminates near the bottom) to force air bubbles into the tank which will mix the fuel. Moving the boat will mix it fine.

NO! Forcibly pumping air through a gasoline tank is stupid and dangerous. It'll lead to pumping fumes out of the vent in much higher than normal (except when refueling) concentrations. And the extra air exposure will also lead to the fuel degrading significantly faster than normal.