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What was the depth of water where you conducted your 'speed trial'. It is quite possible that the increase in revs may have resulted in 'dragging' the stern deeper in the water and therefore friction between the bottom of the boat and the canal bed could have come into play.
I tried to increase engine revs fully as suggested by David and Dot but as I moved throttle forward to above 1300 very little increase in revs if any and no boat speed increase but engine clearly working much much harder. Was propably at about 3/4 throttle but did not push it further as did not want to damage engine or gearbox. This was done on Trent and Mersey canal between Willington and Findern. Also spoke again with Piper staff and they said my boat was actually fitted with a 17 x 10 in their notes. Having done much reading on matching engine HP and prop size it seems to me that in order to be able to cruise at 4 on canals and if u were to go on rivers say 6, the engine needs to be able to release its full HP range. The 28 HP beta engine I have has a max of 3600rpm. Also I'm concerned if I were to go on rivers I may not be able to cope with the river currents and loose control. Please note I'm a responsible boater and not looking for silly speeds that damage the canals. I just need to feel confident I'm in control of the boat. Thank you Dot and David and another for replying. Hope you can help further. John