How fast can we go?

Canal Boat - - Back Cabin - HORNBLOWER, via the CB web­site TONY REPLIES…

QAs nar­row­boats are quite long they must have a rel­a­tively high (the­o­ret­i­cal) hull speed. How much power would be ne­c­es­sary and do other prob­lems pre­vent reach­ing the hull speed?

The web­site fron­tier­power. com/li­brary/hull­types.htm gives the hull con­stant for a barge as 1 ( WLL in feet). This gives the max­i­mum speed for a 55ft boat of about 7.4kt which is close to what my boat will achieve.

Ref­er­ences sug­gest the boat’s weight has an ef­fect and I sus­pect that nar­row­boats tend to be far more heav­ily bal­lasted than, say, a GRP cruiser or yacht. If you built a nar­row­boat with a V bot­tom and sac­ri­ficed the flat floor right to the sides then I sus­pect you would im­prove the prop ef­fi­ciency and make a whole lot lighter boat.

Web­site youboat.net/ boat­power3.aspx sug­gests that for a 55ft hull maybe 100hp or more may be re­quired.

I sus­pect the prop­shaft com­bined with the base­plate’s up­sweep at the bow on many nar­row­boats will force the boat into a bow up-po­si­tion as speed in­creases. This will limit the max­i­mum speed. Also, the prop ef­fi­ciency achiev­able with a 20in ver­ti­cal stern post plus the stern swim shape will also limit the max­i­mum thrust de­vel­oped by the prop what­ever the shaft horse­power.

A nar­row­boat is de­signed for a spe­cific job where the en­vi­ron­ment nor­mally se­verely lim­its the speed.

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