How fast can we go?
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.