The power of the prop

Canal Boat - - Back Cabin: -

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.

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.