Us­ing too much throt­tle is ac­tu­ally point­less – and here’s why

Canal Boat - - Letters -

I read Kevin Blick’s ar­ti­cle ( CB, October) with great in­ter­est, and hope that I might be al­lowed to of­fer some thoughts of my own. My own per­spec­tive for this is a lit­tle un­usual – my wife and I have been clock­ing up four weeks a year aboard hire boats for over 30 years now, and for most of that time I have de­ployed a GPS which ac­cu­rately dis­plays speeds ac­cu­rate to 0.1 mph, so my views are based on sound facts rather than guess­work.

So let’s get down to the fun­da­men­tals. A boat’s prop works by draw­ing wa­ter through it­self. This wa­ter usu­ally has to come either from the sides or be­neath, as the prop is gen­er­ally too close to the hull for much wa­ter to be drawn from above. If the prop is un­able to pull wa­ter through it­self freely, as is the case, for in­stance, when the cut is either nar­row or shal­low, then no amount of ex­tra throt­tle will change that fact.

Think, for in­stance, how all nar­row­boats labour when leav­ing locks, as well as when ne­go­ti­at­ing nar­row bridge holes or aque­ducts. Thus the max­i­mum speed at­tain­able by any boat is usu­ally de­ter­mined by the pro­file of the cut it­self. In­ci­den­tally, ev­ery­one who has ever taken a boat onto a nav­i­ga­ble river will know that, wind and cur­rent aside, any boat feels far more lively on a river than on a canal.

In a few places – Duke’s Cut springs to mind here – you can ac­tu­ally feel the change as the boat makes the tran­si­tion from ‘wide and deep’ to ‘nar­rower and shal­lower’, or in­deed vice versa. Con­versely, I sus­pect that no one has ever achieved a true 4mph any­where on the Mac­cles­field !

All this means that for the vast ma­jor­ity of the canals, the ex­ist­ing 4mph limit is in fact no more than an as­pi­ra­tion. Even at­tain­ing this speed is usu­ally im­pos­si­ble, and any thoughts of main­tain­ing an av­er­age of 4mph over an hour or more re­main mere fan­tasy, es­pe­cially as you will in­vari­ably be slow­ing down fre­quently for moored boats.

I use my own be­spoke soft­ware for plan­ning my trips, and I never plan for more than 3mph for UK canals, and for our forth­com­ing trip on the South­ern Ox­ford, I have as­sumed an av­er­age of no more than 2.5mph.

Some stretches of, for in­stance, the Grand Union or the Trent & Mersey do per­mit slightly higher speeds, as would the Cale­do­nian Canal, though sadly I have no per­sonal ex­pe­ri­ence of boat­ing on the lat­ter.

Now let us con­sider the mat­ter of pass­ing moored boats. There are ac­tu­ally two sep­a­rate is­sues at play here, as both speed and throt­tle-set­ting are rel­e­vant. Hap­pily, most nar­row­boats these days have ef­fi­cient bows which pro­duce rel­a­tively lit­tle wash and very few boats have the square sterns that pro­duce no­tice­able stern wash – in­deed the stern wash pro­duced by most boats is pretty neg­li­gi­ble these days.

My ex­pe­ri­ence is that the thing that peeves most boaters, my­self in­cluded, is that of hav­ing the boat pulled away from the bank by pass­ing traf­fic.

The im­por­tant thing to un­der­stand here is that the pull is not be­ing pro­duced by bow or stern wash, it’s that pesky prop try­ing to suck loads of wa­ter through, as the steerer is us­ing too much throt­tle. It doesn’t mat­ter how slowly that boat is mov­ing, it’s the use of ex­ces­sive throt­tle that will do the dam­age. In France we had the in­ter­est­ing ex­pe­ri­ence of try­ing to moor a boat with a heav­ily loaded peniche pass­ing us in the op­po­site di­rec­tion.

This boat was only do­ing, maybe 1.5mph, but be­cause the skip­per hadn’t throt­tled back, the pull on the ropes was so se­vere that it was all that we could do to suc­ceed in hold­ing on to just one of the ropes, such was the pull on them as the peniche passed.

I would con­clude by whole­heart­edly agree­ing with KB’s as­ser­tion that the blame for ex­ces­sive speed­ing is equally shared by both own­ers and hir­ers. I be­lieve it’s about 50/50, and has been for many years.

CHRIS DODGE, via email

It doesn’t take much to up­set things

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