The power of the prop
I have a 57ft x12ft widebeam weighing about 30 tons fitted with an 18in x 12in four-blade prop. The tip of the blade is only an inch clear of the swim, will this cause loss of power cruising down the River Trent?
RAY RUDGE, via email TONY REPLIES... Reduced prop tip clearance is more likely to result in extra noise and a greater propensity for something to jam between the tip and hull or skeg than to reduce the thrust in any meaningful way.
Down the Trent you will need less power because you will be going with the current and the tide; power becomes critical when you make a turn to enter one of the tidal locks below Cromwell, and, even then, I think that the correct matching of your prop to the engine, gearbox reduction and boat will be far more important than tip clearance.
I estimate your waterline length at about 55ft, and your maximum revs a bit over 3,000rpm with a gearbox 2:1 reduction ratio. So keeping 18in as the maximum prop diameter, my on-line prop calculator gives 17.71 x 12.16 for a three-bladed prop with a 55% DAR. Your four blades should be better at pushing the boat but would not make much difference to the maximum speed. The calculator gave a maximum speed of 7.56kt. A different prop will not alter this much because it is based on hull length for displacement.
You should be fine on the river, but just make sure that if you are on the tidal section and want to turn into Torksey, West Stockwith or Keadby you time your arrival to coincide with the tide slackening off.