If the return is to the tank it stands to reason that it will cool as it is introduced to the fuel in the tank. It's probably moot point with engines commonly used in yachts
I asked lots of people including Yanmar mechanics and just found it interesting that there was never really one answer to setup, how much fuel is returned or fuel temp.
Say your fuel is 5? C warmer how much power are you loosing? If its 10? C warmer are you loosing twice as much power?
Also how many people, including myself,never really thought about it all.
Edited and the degrees symbol still comes out as a ?.
Add to that cooler air is also better, so the air pick up should be as low in the bilge as possible ( issues with sucking water ) or at least not in the hot engine bay, but then a lot of engine noise comes from the air intake. There's a lot of ways to squeeze a bit more hp from a motor, but unless motor racing ????
ps I've found the degree sign didn't work here and settled for the star
Select to expand quotewoko said..
Add to that cooler air is also better, so the air pick up should be as low in the bilge as possible ( issues with sucking water ) or at least not in the hot engine bay, but then a lot of engine noise comes from the air intake. There's a lot of ways to squeeze a bit more hp from a motor, but unless motor racing ????
ps I've found the degree sign didn't work here and settled for the star
Cool air has a definite effect on hp but cool fuel has a far greater effect than air temperature. I found this out in my race car hence the fuel cooler! Also something to consider with diesel is the flashpoint. Normal road diesel we all use now is safe but years ago I used diesel from an ethanol garage and it smelt sweet like cane sugar and I suspect the flashpoint was much lower. The Gardner main engine ran cold and the diesel return line was always dry so I was confident it was safe. The Gardiner air intake went down to just above the bilge water. According to the handbook it was for cold air and the water collected the dust.
Plenty of stuff online about hot fuel.
"Pre-electronic engines metered fuel by volume and not mass, whether gasoline engines with a carburetor or mechanical fuel injection or Diesel engines with mechanical fuel injection. For anything beyond a carburetor, the mechanical fuel system required some amount of fuel to cool the fuel system. That fuel is returned to the fuel tank along with waste heat. The bottom line is as the fuel tank level dropped, the fuel would get hotter. In the late '80's, the Ford V-8 gasoline ambulances had return fuel back to the tank that was so hot it exceeded the vapor point of gasoline. If the ambulance was idling, a common practice, the fuel tank cap would vent gasoline vapor that would hit an ignition source and the result was exploding ambulances became all too common. It is the reason ambulances now are all diesel powered. Diesels have the same waste heat going back to the fuel tank. Diesel has a higher vapor point than gasoline but as the fuel gets hotter, it would get less dense and result in a loss of power. As Diesel engines switched to higher pressure fuel systems for emissions purposes, the waste heat in the return fuel to the tank got worse and many vehicles had to implement fuel coolers. Full electronic engines have fuel temperature sensors to compensate for increased fuel temperature in order to prevent power loss at high fuel temperatures. The fuel cooler is still required as even Diesel engines can heat the return fuel up to the vapor point. I am aware of one pickup truck program where a prototype vehicle caught fire because of excessive fuel temperature at low fuel condition."
I've just installed a D1 30 Volvo and they won't commission it unless the fuel return line goes back into the tank. I had it to the filter, but not good enough as the hot fuel causes problems the new fuel
coming in to heat up too much. re the tank cleaning. I had the professionals take my tank away and clean it ($500 for the 150 litre tank). Lots of black gunk came out. The bosse's comment was not to use additives of any sort as the additive does not get burnt off in the motor but just build up and itself becomes an issue. Their advice was to clean the tank every few years.
Diesel fuel conditioner is OK, just use as the manufacturer recommends. More is not better.
One of the problems with fuel conditioners is that they allow water to become an emulsion with fuel so it can pass through your engines fuelling system .This can cause corrosion on parts and cause steam to form with in the injector which leads to erosion of injector components.
Talking with a mate yesterday who has a trawler, he had problems with water and bacteria in a previous trawler that had the fuel pickup 100mm off the bottom of the tank. His current boat draws fuel rught off the bottom of the tank, he has had no issues with dirty fuel on the current boat his theory being water cannot accumulate in the bottom therefore leaving no environment for bacteria to grow. Make sense?
Select to expand quotesailorsilas said..
I've just installed a D1 30 Volvo and they won't commission it unless the fuel return line goes back into the tank. I had it to the filter, but not good enough as the hot fuel causes problems the new fuel
coming in to heat up too much. re the tank cleaning. I had the professionals take my tank away and clean it ($500 for the 150 litre tank). Lots of black gunk came out. The bosse's comment was not to use additives of any sort as the additive does not get burnt off in the motor but just build up and itself becomes an issue. Their advice was to clean the tank every few years.
Would have to question the comment about additives, made by the tank cleaning company 
. Large diesel users such as many in the farm sector, the heavy transport sector and rail operators use products such as fuel doctor without any of the claimed problems suggested by the tank cleaning company, I have used it for about 10 years and the engine in my boat starts straight up with no fuss.
Personally I get the feeling that the recleaning every couple of years is a piece of salesmanship. 
Select to expand quotecammd said..
Talking with a mate yesterday who has a trawler, he had problems with water and bacteria in a previous trawler that had the fuel pickup 100mm off the bottom of the tank. His current boat draws fuel rught off the bottom of the tank, he has had no issues with dirty fuel on the current boat his theory being water cannot accumulate in the bottom therefore leaving no environment for bacteria to grow. Make sense?
A reasonable presumption, my tanks gravity feed the engine, and as such have no riser in the tank at all nor filter / screen. The cav filter / water separator doing the work..
a while ago I discovered some black sorta stringy substance in the post tank - pre filter fuel line, that come from the lowest point of the system. Could be from accumulation of water and the onset of giraffe
I did have a situation where an additive caused problems. After an algal blockage in my early "learning" days, I looked around and decided "Fueltreat" biocide was the solution. It did suppress algal growth, but formed high-viscosity "bobs" in the tank which on two occasions blocked the 1/4" tank outlet (yes I know, stupid design, but not of my doing). Since switching to the stuff they sell at WW's about 5 years ago, I've had no problems whatsoever. A couple of tank internal inspections revealed no water and no grunge.
Fueltreat I think is really aimed at the big-engine market, where the outlets and lines are much bigger than my baby's weeny 1/4" ones. It's recommended by Caterpillar.
Cheers, Graeme
I couldn't give the diesel away, no one wanted it so I have been pulling out a couple jerry cans at a time and taking it home and running it through a diposable filter then a chinese "racor" and giving it a squirt of fuel doctor. After that its been going into the Pajero, I put in 40lts a go and top off the tank at the 7/11
Hearing rumours of fuel rationing about to start, could probably sell a jerry can for $50+ each soon except for my son has got in on the action now and full jerry cans keep turning up empty.
Reminds me of the time a thief was syphoning fuel out the trucks at work, turned out he was sucking all the filth of the bottom of the tanks his common rail hilux couldn't digest it and major troubles followed. Be sure not to mix up the contaminated fuel containers with the polished stuff
Yep, not just circulating it, its going from one jerry can into another via the filters. Its been coming out of the keel tank pretty clean so far which surprised me.
I spoke with the original owner/builder and he advise me he built a 10ltr sump into the bottom of keel tank directly below the fuel feed outlet. I haven't managed to find it yet by sticking a copper pipe down there but when the tank is empty I will get a cheapy inspection camera that connects onto a phone to check out the inside. I am thinking that sump will be full of filth.
Select to expand quotecammd said..
Yep, not just circulating it, its going from one jerry can into another via the filters. Its been coming out of the keel tank pretty clean so far which surprised me.
I spoke with the original owner/builder and he advise me he built a 10ltr sump into the bottom of keel tank directly below the fuel feed outlet. I haven't managed to find it yet by sticking a copper pipe down there but when the tank is empty I will get a cheapy inspection camera that connects onto a phone to check out the inside. I am thinking that sump will be full of filth.
Fair guess that you don't want to find it until most of the contents are out then.
Last 30 litres came out yesterday, might save that to use to flush the tank. Ordered an inspection camera off ebay to plug into the phone so I can look inside. Thinking of using some electrical conduit with a cap on the end and drill a series of holes all around to make a sort of wand to stick down the tank filler pipe (50mm entry) and pump fuel through it to try an flush any gunk off the walls . Does the brains trust have any better ideas, will most likely be impossible to get it all with baffles in the way and such limited access.
ps: There is no sump as previously reported, that was confused with the sump for the bilge. The last 200 litres of fuel coming out has been very clean and I have had the boat out in some rough Moreton Bay chop only half full, I would have expected that to stir some gunk up so will interesting to see what's inside, I will post some pics when I get the inspection camera.
