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.
1984 Fast/Nicholson 345 "FastForward"
".. 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)