Speeding boats—facts and figures

HAVING read several ‘statements' about speeding boats, the wash and damage they cause and the various ‘solutions' offered, I thought it time to throw in some reality checks, writes Orph Mable.

 

Yes, a speeding narrowboat does cause a wash. Yes, this causes damage to the bank and yes it does have a sometimes disastrous effect on moored boats when the moored boat is pulled from its moorings or the occupants toppled by the sudden movement. No argument from me on these points.

Jet propelled

It has been suggested within the pages of narrowboatworld recently (maybe tongue in cheek) that by fitting a ‘jet propulsion' system down the centre of the boat this would greatly reduce the effect of a speeding boat. But I believe that this would have little effect.

Whilst not having a degree in hydro-dynamics I do understand what causes the type of wash being so clearly demonstrated by the Napton Boat Grace in the recent picture.

Wikipedia

This is caused by hull speed of the boat. A useful description is given in Wikipedia:

Hull speed or displacement speed is the speed at which the wavelength of the boat's bow wave (in displacement mode—a narrowboat has a displacement hull—Orph) is equal to the boat length. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest to trough dimension (height) increases as well. When hull speed is reached, a boat in pure displacement mode will appear trapped in a trough behind its very large bow wave.

From a technical perspective, at hull speed, the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Though the term 'hull speed' seems to suggest that it is some sort of 'speed limit' for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.

Modifications

By looking up ‘hull-speed' much more detail can be found. Several narrowboat shell builders have studied the effect of this and added shape modifications to their designs over the years with varying degrees of improvement success.

I agree with Victor that reducing the prop size on certain boats would reduce the craft's ability to reach the required speed to cause this effect, but the down-side of this is that the engine would not be operating with its optimum loading, at lower speeds, thereby increasing fuel costs and wear.