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

[Franziska]

Hi,



how would you run a set of two two stroke engines from a large built in 50 gallon tank, which obviously can't get easily be shaken by hand to get the 1:50 mixed properly before start?

Ideal would be to run directly from the large reservoir as it avoids situations where a day tank would run empty when you are in a bad weather situation.



Is there some (reasonably priced) equipment for this which can "inject" the oil from a separate tank as needed? Other solutions?

If you suggest equipment I appreciate a link to suitable manufacturers.



Please don't suggest getting a 4stroke instead 🙂

[thinwater]

Add the oil before your start filling. The filling will mix it in. You could add it in several increments if you wanted to.



Who shakes even a 5-gallon can?

[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..

[Lepke]

There are various injection systems for injecting oil or water. Check industrial supply. Not Cheap.

[Munoz]

If you really insist on going the oil injection route Suzuki built a series of oil-injected two strokes in the late 80's. Might be cheaper/easier to find a couple of those 2nd hand than installing a dedicated oil mixing system.

If it where me, I'd just put the oil in the main tank as suggested., you could always rock the boat a bit or find another way to mix it if you're really paranoid.......

[catsketcher]

Seems like a strange system - did you get rid of your twin 9.9 four strokes? If it was me I would not mix the two stroke. I found that leaving premix for a while caused the fuel to produce strange products - a varnish like thing in carbys is the main one.
Maybe use a 20 litre plastic fuel tank for each outboard. Use a pump to pull fuel from the large tank to fill them, you could have a fuel flow meter on an electric pump and add the correct amount of oil (I wouldn't, I would just check the sides of the small tank and work out how much went in and do the maths). I don't have large tanks on my 38ft cat. I still just use the two 20 litre fuel tanks and fill them with jerry cans. I may go months without refueling but using a two stroke will take more fuel - so maybe an electric pump to fill the smaller in use tanks would be good - but why 2 stroke? The Yammies four stroke 9.9s are so good. I have never had an issue with running out of fuel (say in bad weather) in 24 years of owning the boat - so using the small tanks would be a good first step . Easy to check fuel levels too.

[Franziska]

Originally Posted by catsketcher

Seems like a strange system - did you get rid of your twin 9.9 four strokes? If it was me I would not mix the two stroke. I found that leaving premix for a while caused the fuel to produce strange products - a varnish like thing in carbys is the main one.
Maybe use a 20 litre plastic fuel tank for each outboard. Use a pump to pull fuel from the large tank to fill them, you could have a fuel flow meter on an electric pump and add the correct amount of oil (I wouldn't, I would just check the sides of the small tank and work out how much went in and do the maths). I don't have large tanks on my 38ft cat. I still just use the two 20 litre fuel tanks and fill them with jerry cans. I may go months without refueling but using a two stroke will take more fuel - so maybe an electric pump to fill the smaller in use tanks would be good - but why 2 stroke? The Yammies four stroke 9.9s are so good. I have never had an issue with running out of fuel (say in bad weather) in 24 years of owning the boat - so using the small tanks would be a good first step . Easy to check fuel levels too.

Just checking for now.



We might have a chance to get two new Tohatsu 9.8 two strokes. Hailed by many as one of the most robust engines around. Simpler than the Yamahas, easier to fix or get fixed in remote places.



I just learned that it's very difficult to get parts like the CDI unit for the Yamahas in some non first World countries. Takes a lot of money and even more time to get such.

Thankfully we had a second engine (in some locations you need to motor a few miles before you can sail).

The other is that mechanics in this type of places are much more familiar with the less complicated two strokes.





Much lighter than the Yamahas.

We could even get a third one for the dinghy. So three identical engines on the boat. One fails and you still would have the dinghy one which you can mount in its place.



Still there is the inconvenience with the mix.

Regarding letting premix sit, that's exactly what I would like to avoid. Have the main tank as normal and mix in the oil thereafter.



Maybe you are right and we need some sort of daytank for each of the two engines. The stupid thing is the fuel transfer would nessecitate it to install a big pipe to be reasonable fast.



No saying we go down the two stroke route, but its worth a thought.

[Franziska]

Originally Posted by Munoz

If you really insist on going the oil injection route Suzuki built a series of oil-injected two strokes in the late 80's. Might be cheaper/easier to find a couple of those 2nd hand than installing a dedicated oil mixing system.

If it where me, I'd just put the oil in the main tank as suggested., you could always rock the boat a bit or find another way to mix it if you're really paranoid.......

Rocking the boat is not very successful on a multihull 🙂
As to the Suzuki option, we could get brand new Tohatsu 9.8 for a reasonable price, so no need to buy old motors.

[Jim Cate]

Do you have any authoritative info that says the mix will separate whilst sitting? I don't know what would actually cause such an action.

But rather than elaborate tankage mods to accommodate the problem if it is real, how about adding a small circulating pump in the tank to keep it stirred up? Seems a lot simpler to me!

Jim

[Benz]

Given the short lifespan of pre-mixed fuel, I'd add a daytank large enough to re-fill while running, say, when it was down to two gallons, dump some more oil in and open the valve from the big tank to fill up. I'd put a filter between the big tank and the daytank.
It will be hard enough to keep 50 gallons of gas fresh (unless you motor a lot), that's not pre-mixed. If you go to dodgy places like Central America, where there's already a good bit of water in the gas, it gets worse.

[Franziska]

Originally Posted by Jim Cate

Do you have any authoritative info that says the mix will separate whilst sitting? I don't know what would actually cause such an action.

But rather than elaborate tankage mods to accommodate the problem if it is real, how about adding a small circulating pump in the tank to keep it stirred up? Seems a lot simpler to me!

Jim

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.

[Pete7]

Mariner used to do a 50:1 mixing widget. Plastic tank with oil in which the fuel tank connected to and after mixing at the right ratio went on to the engine.

However, they weren't particularly reliable and today probably very difficult to find.

Worth noting that the RNLI always removed the engine oil mixing system from the Evinrude 60 and 70hp engines because that is just one less thing to go wrong.

If I was mixing a larger quantity from containers, put the oil in the container, add fuel and give a good shake. then transport to the yacht and fill the main tank.

Add petrol fuel stabilizer too because modern day petrol goes off faster than milk.

[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.

https://www.youtube.com/watch?v=MyWpN_bhuFQ

[NorthCoastJoe]

If your tank is a drum, do you have any unused bungs in the top. If so remove one insert a flat stick and stir it up a bit. Use a good wood for it. If you really want to mix it up use a dowel or rod and cut a slit in one end. Then slide in a piece of sheet metal in the slot and secure it in place. If you size the dowel/rod right you can spin it with a drill motor 3/8- 1/2 inch or 10 to 13 mm.

I have used this method for honing cylinder bores.

[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.