Re: Islander 30 fuel tank size
It's entirely possible that there were options.. but not all that common - usually the option was an second tank for more capacity. It's equally possible that someone retrofitted a newer tank - often such tanks are necessarily smaller because you can't slide in as big a tank as original for clearance/access issues (the originals are installed before they put the deck on the boat)
Also, for boats primarily daysailed a smaller tank lessens the risk of stale fuel as the entire contents are recycled more frequently.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"