HOW TO: TURN A BOAT ON ONE EN­GINE ONLY

Motorboat & Yachting - - CONTENTS -

If one en­gine goes down, would you know how to manouevre your boat?

There are two main rea­sons for buy­ing a twin-engined boat, the first is re­dun­dancy (if one breaks, the sec­ond can get you home) the other is ma­noeu­vra­bil­ity (two en­gines give bet­ter con­trol at low speed). That’s all well and good un­til one en­gine does break down. You may still be able to get home but berthing it has just got a whole lot trick­ier, so it pays to know how your boat han­dles on one en­gine.

As with any emer­gency it’s best to have a go at this in ad­vance, prefer­ably on a calm day with plenty of space to play in. The best way to do this is by learn­ing how to turn your boat on one en­gine. If you can mas­ter this you are half way to get­ting it into a berth.

Ob­vi­ously, there are lots of sin­gle-engined boats which turn just fine with one en­gine, but it’s mounted on the cen­tre line with a sin­gle large rud­der that steers equally well in both direc­tions. Twin-engined boats have off­set en­gines and two smaller rud­ders, so if one en­gine breaks it’s al­ways eas­ier to turn in one di­rec­tion rather than the other. If there is enough space to turn around us­ing ahead only then turn in the di­rec­tion it wants to go, with the en­gine on the out­side. That is also the case with steer­able drives such as twin outboards or stern­drives, even in re­stricted spa­ces. How­ever, with a shaft­drive boat and only lim­ited room to ma­noeu­vre, it’s eas­ier if you steer the boat against the work­ing en­gine. This is be­cause there are three fac­tors that af­fect the steer­ing of a twin-shaft boat: prop off­set (the fur­ther the pro­pel­ler is from the cen­tre­line, the greater the steer­ing ef­fect), prop walk (the side­ways travel all pro­pel­lers pro­duce, par­tic­u­larly in astern) and the rud­ders (more ef­fec­tive in ahead than astern due to in­creased wa­ter flow from the pro­pel­ler thrust). Know­ing th­ese dy­nam­ics will help you use them to best ef­fect.

If your port en­gine is dead, leav­ing just your star­board en­gine run­ning, and you turn the boat to port (the way it wants to) you will have all three fac­tors work­ing for you – prop off­set, prop walk and rud­ders. The mo­ment you run out of space and go astern, they all start to work against you – the off­set is on the wrong side, prop walk is push­ing you the wrong way and the rud­der is all but use­less, so you’ll end up back where you started.

The bet­ter op­tion is to turn to star­board, against the work­ing en­gine and use the rud­der to over­whelm the other two. Go­ing ahead, the off­set is against you and so is the prop walk but you can over­come them with the help of ex­tra revs to in­crease wa­ter flow over the rud­ders.

Once you have turned through the first 50º or 60º, take the boat out of gear. Now there is no off­set or prop walk to fight against but the flow of wa­ter over the rud­ders means the boat keeps turn­ing. When you run out of room and en­gage astern, the prop off­set and prop walk are now work­ing for you. You don’t even need to ad­just the rud­ders as you won’t be go­ing astern quickly enough for them to have an im­pact. Th­ese two fac­tors alone will drag the stern through the re­main­der of the turn, com­plet­ing the 180º spin in a con­trolled man­ner. In the next is­sue we’ll cover how to get into a berth us­ing only one en­gine.

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