I did a small research
on the topic during my total rebuild
of my OE 36.
I did replace my Stainless ( 316 grade) 250l diesel
tank with a Polyetylen of 200 l.
I started reading Mr Ferenc Mate´s book " from a bare hull" where he describes the usable materials ( best-worst)
3 Black Iron
4 Stainless Steel
He argues against Stainless due to pinholing as a result of SS not being stainless with oxygen starvation that occurs with stagnant waterin the bottom of a diesel
tank and also possibble reaction with sulphur in the Dieselfuel.
I then read about Polyetylen and understand it will be as durable as Monel at a fraction of the cost if correctly instaled.
I would like to challange the fire risks of these tanks
v.s metal tanks
First if you have not tried it I would like you to boil Water
in a paper mug over a candle. It seems like an impossible task but the truth is that the water
constantly cools the paper and will not catch fire until the water has boiled away and the temperture can rise to the papers ignition temperature.
The same thing would be applicable for the polyetylene tank.
The real threat comes from the hoses that connect the tank where small amouts of fuel
or no fuel
at all ( vents) will get a fire to burn throug in no time.
From my old profession as an Airline captain
I do have had my share of thoughts about fire onboard my yacht.(It is really only one place where fire is worse and that is in an aircraft.)
When it comes to the engine compartment and the fueltank I have installed a fire bulkhead so the tank area and engine compartment are separated. This bulkhead is covered ( just as the inside of the engine compartement with sound dampening multilayer foam that is not combustable.
I have tried the foam before installing it with a propane torch and It will definately not burn. The accoustic aspect should also be taken nto consideration since the tank will act as a resonance box if no bulkhead and foam is present.
I do have one automatic fire exting. and one fixed mount manual that can be released from the cocpit. I do have One " engine hot warning with sound ( trigging 65deg C) and one fire warning trigging 95 deg C.
I do of course have my engine fire checklist due to the fact that you do not get a second chance to do it correctly , that is the proper actions in the correct order.
On the subject fuel tanks and installation I did read an article from a lifeboat skipper that had noticed that they never have had a tow in of a yacht with fuel related problems when the engine suction point is in the tanks lowest part!
He said that the very common construction with the suction pipe ending about one inch from the bottom aggrevated the formation of sludge / water / dieselbug that collected in the bottom only to be stirred up when you got in some heavier weather and of course then blocked the filters or worse destroyed the injection punp.
He argued that it was far better to constantly get the dirt and water out of the tank and keep on bleeding water from the water separating filter and change the filters on a regular basis.
I adopted this philosophy and did also install a simple fuel polishing system with an electric fuelpunp and a water separating fuelfilter that normally just sucks the fuel from the low point and after filtration / waterseparation returns the clean fuel into the tank. ( this is what takes place when you run your normal diesel with a fuel return line but my VP MD11 C do not have a fuel return.)
This electric fuelpump can with the switch of a valve substitute the engines lift pump if it fails.
So I did go with polyetylene and adopted suggestions that I did think made good sense when it came to the installation.
Good luck in your change of both tank and engine!