TROUBLESHOOTER COM­MENT

Passage Maker - - Crosstalk -

Thank you for yet an­other re­ally good ar­ti­cle (“The Fuel Burn,” Pas­sage­Maker, March 2018). May I be so bold as to add some fac­tors to think about when run­ning a twin-en­gine boat on just one en­gine?

A twin-screw boat has one en­gine on ei­ther side of its cen­tre­line and is there­fore driven in a straight line. Stop­ping one en­gine will in­tro­duce asym­met­ric thrust which in turn will want to turn the boat. To counter this, op­po­site wheel needs to be ap­plied. So straight­away, we have ad­di­tional drag caused by rud­der de­flec­tion as well as the boat be­ing forced to “crab.” Both in­crease the drag. And more drag = more fuel burn at the same speed through the wa­ter.

Be­yond this, you need to con­sider the ad­di­tional force on the rud­der and whether the rud­der stock is built to with­stand this force for sus­tained pe­ri­ods of time.

For the dead en­gine, I ab­so­lutely agree that keep­ing the shaft and prop spinning freely is cor­rect and yes, you need to know if the shaft seal (stuff­ing box) re­quires lu­bri­ca­tion. Fur­ther­more, you need to know the gear­box man­u­fac­turer’s de­tails about free­wheel­ing and whether any lim­i­ta­tions ap­ply.

So, ad­di­tional con­sid­er­a­tion needs to be given to the asym­met­ric thrust, the rud­der stocks, shaft seals, and gear­box lim­i­ta­tions. For me, the only ben­e­fit of do­ing this on my boat is to keep en­gine hours down. Is it worth it, given the above? Not in my mind. Piers du Pré Flem­ing 55, Play d’eau Guernsey, Bri­tish Isles

Thank you for your well-in­formed let­ter. I agree with all of your com­ments. I am not ad­vo­cat­ing that boat own­ers run their twin en­gine boats on one en­gine. My point is that do­ing so rarely re­sults in any­thing other than min­i­mal dif­fer­ences in fuel econ­omy and in­tro­duces a host of other prob­lems (as you also pointed out).

If the dead en­gine prop can­not free­wheel (due to trans­mis­sion con­cerns), then you are ef­fec­tively drag­ging an an­chor with the fixed prop and fuel econ­omy will suf­fer sub­stan­tially. Ad­di­tion­ally, run­ning on one en­gine can im­part stresses to the drive train not nor­mally ex­pe­ri­enced, rais­ing the risks of cre­at­ing a new prob­lem. —SZ

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