Wet spot on hull around strut

[AdamHowie]

Launch is in about a month and today we discovered a wet spot on the hull around the shaft strut on Puissance. A 1980 Tanzer 26. It is about the size of a car tire and there was no evidence of a drip from the topsides. It has an inboard Yanmar 1gm 8 hp and a 1" shaft. The diesel fuel tank is located just aft of the engine, directly inline with the wet spot, also the water tank is further aft but was drained completely in the fall. Noted in the bildge was what smelled like diesel but there was also water in there too which is normal for a 32 year old boat.

What could this be from? A leaky tank? It was filled 3/4 of the way in the fall and is showing 3/4 now on the indicator gauge

There is no way of accesing it through the cockpit locker, the only way would be to cut the cockpit sole, or remove the engine to inspect. It was not a factory install and was repowered at some point in its life. tank is metal.

[Faster]

Hard to imagine any leakage from inside the boat actually permeating the hull structure to show on the outside... Certainly diesel in the bilge is an indication of a problem but for that to show on the outside???

I'd look for something else first...