ASK THE EX­PERTS

White smoke when start­ing; sterng­land re-pack­ing; black­ing the hull us­ing a roller; fit­ting a wa­ter pres­sure re­duc­ing valve; the right way to wire up a bat­tery bank; rais­ing up bal­last for ven­ti­la­tion; and prob­lems start­ing a Yan­mar en­gine

Canal Boat - - CONTENTS -

QI have a 58 ft nar­row­boat and at the last dock­ing in 2015 the ma­rina pointed our that the pro­pel­ler was rather too close to the end point of the swim.

Is there an op­ti­mum dis­tance for the prop boss from the end point of the swim? I have an 18” x 13” Ax­iom pro­pel­ler and at the last dock­ing fit­ted a stain­less weed cut­ter disc on the shaft for which there was just enough space.

We are hav­ing it blacked at the same ma­rina next month April and I wanted to ex­tend or fit new shaft to fix the prob­lem. MICHAEL MARSH

ATONY REPLIES: You say you have a prob­lem, yet do not tell me what it is and it seems the yard gave you no rea­son for their ad­vice so I can only guess.

The prob­lem with hav­ing the prop too close to the stern post is that the stern post masks the wa­ter flow into the prop but your boat ap­pears to have an ex­cep­tion­ally fine stern swim and ef­fec­tively no stern post at all. The aft shaft bear­ing is even in an ex­ten­sion to the stern tube that brings the prop even fur­ther back from the stern post. Be aware that mov­ing the prop back might in­crease tiller vi­bra­tion.

I do not think there is a prob­lem on your par­tic­u­lar boat and if there is one it is more likely to be prop-re­lated.

For canal work I would want the min­i­mum un­sup­ported shaft over­hang com­men­su­rate with en­sur­ing easy wa­ter flow into the prop. Your en­gine will move for­wards and back­wards on its rub­ber mounts when you go into ahead and astern so un­less you have thrust bear­ing fit­ted you need a min­i­mum clear­ance/ shaft length be­tween the prop boss or weed cut­ter and the end of your stern tube of 5 to 10mm with a max­i­mum shaft over­hang of about 1 shaft di­am­e­ter.

See http://www.prop­pro­tec­tor.co.uk/in­for­ma­tion. php? in­fo_ id=2&o sC sid=5f62c 99 a8ce 700 de 444 cafd 34 ac 67 a 14

In your case be­cause of that very fine swim end you will prob­a­bly get away with the lower fig­ure.

Be­fore spend­ing money on this, my ad­vice is to get the yard to give you a writ­ten guar­an­tee that the work they rec­om­mend will make a mea­sur­able and no­tice­able dif­fer­ence to the boat’s han­dling and per­for­mance.

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