John V

On the subject of water tanks I thought you might be interested in this photo (from the Ormelite website) This is a picture of the lower half of a Ormelite hull, the water tank can be seen forward and thwartships. It is pretty large, I don't know the capacity but full to empty accounts for about 2" of draft forward. It sits under the Vee berths in the forward cabin Picture by Danny Orme from Ormelite cruisers website

I once had a very obscure fault that gave similar symptoms so it might not be your problem. .............................................however................................................... On one boat I had there was a fault that showed up as a lack of power at high rev's when reversing, it didn't seem to be any worse at low/med revs but soon as you started to go full astern there was no apparent increase in power. it gradually got worse and there then was a noticeable delay before the ahead power became available when changing from astern to ahead. The fault was a sheared woodruff key on the prop. When going astern the prop was slackening slightly on the taper and slipping on the shaft, it only being noticeable at higher revs as at slower speeds there was enough friction for it to still "drive". When you then went ahead the thrust from the prop drove it back onto the taper. It was only when the wear increased that It became noticeable enough for me to work out what was happening.