Finding underfloor fuel tank to fit

[all finny]

Just picked up a Four Seasons 505. Stessco.
It has a seperate poly fuel Tank 45litre at the rear of the boat under the transom. 65hp Yamaha on back.
Ive removed the flooring and see that there is an area in the middle that is set up to hold a fuel tank. However it’s not very deep.
850 wide x 600 long x 190 deep. Probably still handle 75 litres but it’s the depth that would be an issue.
Trying to work out what type of tank was originally designed to go into it, and how the inputs/outputs where designed.
Not sure how to find a replacement and if they are stock item (which would be great) or need to be fabricated. I assume it will be alum or stainless.
Will get a photo up next time.

[Dignity]

Look up Sant Marine, I'm sure there will be one that's going to fit in that space.

[Noelm]

You will find they made their own tanks, but as mentioned, Sant would be my first stop, they have an online catalogue that has dimensions.

[blacklab]

Just went thru the exercise myself..
Those dimensions would give you about 90 ltrs ( maybe I added up wrong..)
Sant marine tanks is a good option, but it really was difficult finding one with my cavity from them, mine was shorter in height as well.
I ended up getting a custom aluminium tank made, for the cavity, which ended up around 145 ltrs, closest size that would fit from Sant marine, was
only 120 litres, because of my cavity size...
So you have options either way...

Col

[gazza2006au]

All Finny i have a 5.7m glass boat my fuel tank cavity is only 230mm so there should be options out there for u

45L is ok for a 5m hull 65hp unless u love to run the engine flat out or do 100km day trips that 45L should be ok for the average boater

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[sjp2]

as blacklab says ,to much mucking around with a non custom made tank,that way you can work out filler and vent placement to suit ,just make sure an experienced engineering shop does it ,i had to pull mine back out twice because of leaks and in the end took it to a different place and got third time lucky .

[all finny]

Thanks for all the feedback. Looked at the Sant Marine tanks and they look good. However none that fit the cavity I have. They are all longer than the 850 that I have. ☹️ I’ve asked it they have one that will fit, but it’s not in their current list.
I’ll look at custom made, but have also heard the negative stories, and not sure who on Northside Brisbane has a good reputation.

[gazza2006au]

Just ask for a tig welded tank without the welds ground down, when i was looking for a tank the local ally guy i have used in the past said he can mig or tig a tank its up to me which one i want

Some may grind the welds i donno if this is for astetics or poor workman ship cover up but a decent tig welder with a steady hand should do a A grade tank

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[EdBerg]

TIG welding a tank would be better and the tank would need to be pressure tested, MIG has a tendency to leave pinholes and cold joints which leak, It is certainly faster to build a tank using MIG but the welds are much better with the TIG process.

I just built a spare (about 60L) ally tank for my boat as I am heading up to the 1770 meet and greet and so want more reserve fuel as we are hope to go offshore. It doubles up as a footrest in front of the passenger seat. My large MIG welder can weld Ally but it isn't any good for thin 3mm sheet which is what I made my tank out of and even with my welding the tank with TIG, there were still a couple of pinholes that needed fixing hence the reason for pressure testing. My TIG welds were ground down as I don't do enough TIG welding, 99.99% percent of my welding is with MIG so don't get enough practice on TIG to make it look good. But if you get someone who does TIG for a living and is a good welder, the welds could end up being a work of art and you would not want them to be ground down anyway. To pressure test you only need about 4PSI but that also depends on the shape and design of the tank, I did mine to about 8-9PSI.

Another issue is that not all welding places are certified to do petrol fuel tanks and so you would need to check.

[all finny]

Looks like i need to get a tank custom made. have sent dimensions off to two places so far, and will see what i can get and at what cost.
Was looking to get the filler installed in the gunnel but not sure if that's a good idea, I'd also then need a fuel gauge, but wasn't sure how accurate they were. My last boat was under the floor but had a lift up panel and I could put a measure stick into to check the fuel level.
How reliable and accurate are fuel gauges on boats ?

[gazza2006au]

AF there are new modern fuel gauges gone are the days where u had to measure the ohms and cut the rod to suit your tank

Now you just measure your tanks depth ie 6inch and buy a 6inch sener unit they are calibrated to register 4 quarters of your tank 1/4, 1/2, 3/4, 4/4 no calibration needed just screw in plug in and away u go

For me i know my tank measures 59L so i will use the modern vesselview to register my fuel use reset it when i fill up and use 50L before refilling

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[Fed]

They are reliable & accurate but they only display the depth of the float inside the tank so that means you have to "Know Your Fuel Tank".
When the sender is located right in the centre of the tank they don't suffer too much from the fuel moving around.

There is NO Ohms measuring but you will need to cut the swinging arm at the right length to suit your tank depth.

[all finny]

Thanks again.
I've had an aluminium tank made to suit the space. Looks good but lots of fine tuning to get hoses and recutting and filing to get hole sizes correct, fillers to fit and the tank to be supported from movement and wear.
Almost finished and going to connect to the original Yamaha gauge. Should give me around 100 litres, but more importantly it frees up more storage space were the old poly tank was sitting.