View from the boat­yard

Mike Pick­les talks us through the process in­volved in retrofitting a bow thruster

Practical Boat Owner - - Contents -

Retrofitting a bow thruster

We’re not great fans of bow thrusters here on the river. You sim­ply don’t need them when you’ve got a 2-knot flow one way or the other to work with. So when we hear that un­mis­tak­able sound we usu­ally drop tools and watch who­ever this lat­est vis­i­tor is, try­ing to move his ves­sel side­ways into the tide with of­ten hi­lar­i­ous ef­fect.

That said, in static wa­ter, such as a locked ma­rina, we ac­cept that they are very handy. A few years ago I spot­ted a scal­loped bow thruster tube on a Southerly and thought, yeah, that looks like a good idea. I could see there would be a re­duc­tion in drag (and prob­a­bly noise) com­pared to the usual ‘eye­brow’ while sail­ing, es­pe­cially when heeled. So, along came an op­por­tu­nity to try out our own de­sign.

We first looked into a retractable thruster as there’s no doubt that these have the least neg­a­tive ef­fect on per­for­mance. The is­sue is that many yachts sim­ply don’t have the room or a suit­able lay­out to be able to in­stall one – be­sides which, retractable thrusters aren’t cheap. So the de­ci­sion was made to try out our de­sign on a Southerly 38; and after the suc­cess of that, we fol­lowed up with a slightly more tech­ni­cal ver­sion on an alu­minium Ovni 38.

The per­for­mance loss when sail­ing on these two boats was neg­li­gi­ble, maybe 0.2 knots com­pared to the usual ¾ to 1 knot we usu­ally saw from the con­ven­tional in­stal­la­tions with eye­brows. Yes, the in­stal­la­tion is a lot more com­pli­cated and does re­quire me to cut two dirty gashes down each side of your pride and joy, but the re­duc­tion in drag and the re­sult­ing ac­ci­den­tal in­crease in strength in this im­por­tant pound­ing make it well worth it.

The length of the orig­i­nal scal­lop on the Southerly I saw was, in my opin­ion, a lit­tle too short. I’ve al­ways used a rule of thumb that an ob­sta­cle will cast a dis­tur­bance shadow of at least three times its size at the kinds of speed we sail at. So, if I have a 200mm hole, I need at least 600mm aft for the flow to set­tle down again. One day when I’m rich I’ll do the CFD to back up the the­ory, but at least the prac­tice seems to sup­port it.

Be­fore we start, I should point out that there are lots of ways of do­ing the same thing, es­pe­cially when it comes to work­ing on boats. The meth­ods I use are those I feel most com­fort­able with, given that it isn’t my boat and that if it goes wrong it’s go­ing to be very ex­pen­sive. This pretty much fol­lows for all of our work at the yard: we’re gen­er­ally risk averse as fail­ure in part or to­tal is not a good long-term busi­ness strat­egy, es­pe­cially as our busi­ness is largely gen­er­ated by word-of-mouth rec­om­men­da­tion and re­turn­ing happy cus­tomers. So, while the fol­low­ing is a run-of-the-mill yard job, in iso­la­tion it in­volves pretty ma­jor surgery – with dras­tic con­se­quences if it fails. You have been warned.

Suit­able lo­ca­tion

For a bow thruster to work ef­fec­tively, it needs to be po­si­tioned as far for­ward as pos­si­ble but also needs to be a tube di­am­e­ter be­low the wa­ter­line so as not to cav­i­tate in use. Then we come up against the phys­i­cal lim­its in terms of avail­able space for the tube and the mo­tor, and the nec­es­sary ac­cess so that the tube can be safely glassed to the hull.

Look­ing at the hull from the in­side can give you a good idea of a suit­able lo­ca­tion, but it’s of­ten tricky to iden­tify that lo­ca­tion from the out­side. There­fore, you need to find a da­tum point that can be viewed from both in­side and out that is as close as pos­si­ble to the pro­posed

lo­ca­tion. Luck­ily for us, this Ni­chol­son 39 has a through-hull depth/speed trans­ducer on the cen­tre line around

2ft aft of the pro­posed thruster lo­ca­tion. From this point, it is rel­a­tively easy to project lines mark­ing the bulk­head po­si­tion (ad­ding a bit for luck) on the out­side of the hull.

Once the cen­tre point of the tube is found, a small hole is drilled through both sides and a 5mm-di­am­e­ter rod is in­serted through to check align­ment both in­side and out. When happy with this, it’s time to open the small hole out to 12mm, which al­lows us to use a length of M12 threaded bar as a cen­tre line rod that won’t bend dur­ing the mark­ing process.

We have a special tool for mark­ing the hole, but as with most things in a busy yard it has gone walkies, so I made a very quick and easy tool which ac­tu­ally worked just as well as the ‘special one’. I still pre­fer to stitch-drill rather than use a 200mm hole cut­ter: yes, it takes longer, but it is far more con­trol­lable. Leav­ing the cen­tre line bar in place gives you a guide so you drill at the cor­rect an­gle. I use an 8mm drill for the stitch holes, and usu­ally have to sharpen the drill bit a few times on a bench grinder by the end.

Once the holes have been drilled, I use a jig­saw to cut from hole to hole. Un­lock­ing and loos­en­ing the base screw on the jig­saw al­lows you to keep the blade at the cor­rect an­gle. Don’t be tempted to try and cut the en­tire hole with a jig­saw un­less you are 100% con­fi­dent that you won’t wan­der off line. I don’t have such a level of con­fi­dence, so I stick to stitch-drilling.

With the cen­tres re­moved, it’s an easy if dusty job of re­mov­ing the rough drill slots us­ing a sanding drum with 40- or 60-grit pa­per. A few words on the dust: it’s par­tic­u­larly per­ni­cious, in­tru­sive, smelly and def­i­nitely anti-so­cial stuff. You need an FFP3 mask and a dis­pos­able hooded over­all over your over­all. In­side, you need to re­move ev­ery­thing from the for­ward cabin and seal the doors. Even then, you will find ev­i­dence that you missed a bit. Out­side the hull, it’s much harder to con­trol so it’s a case of dam­age lim­i­ta­tion. We evac­u­ate that shed and the guys move to one of the oth­ers, grum­bling un­der their breath, un­til I’ve fin­ished and fired up the vac­uum.


Holes in boats are never a good thing. Huge great holes in a pound­ing area of the hull are an even less good idea, so I em­ploy a belt-and-braces ap­proach and en­sure I bond the tube ex­ter­nally as well as in­ter­nally. To do this I cham­fer the out­side of the holes so I can get a good 15mm glass thick­ness around the ex­ter­nal joint. Ini­tially I cut 50 or 60mm back from the main hole, but this will be in­creased when I get to glass­ing.

While the nom­i­nal di­am­e­ter of the tube is 200mm, be­cause it’s cut at an an­gle the largest dis­tance from one edge to the other is 270mm, so the pro­posed scal­lops will need to start at 270mm wide and ex­tend at least 700mm aft. Look­ing from the front, I can see I’ll need a scal­lop depth of 45mm.

The next task is to make a mould for the two scal­lops. The ini­tial layup of the scal­lops needn’t be too thick as ad­di­tional lay­ers will be added later when bond­ing it to the hull for both in­side and out. The first scal­lop hole cut into the hull is al­ways a lit­tle scary as the hole is BIG and you need to be sure you have it in the right place,

which on a boat with con­stant curves is not easy (although your eye is pretty good at align­ment and of­ten bet­ter than a tape mea­sure, as many hulls are not as sym­met­ri­cal as you might think).

Grind­ing this scal­lop hole pro­duces prodi­gious amounts of dust, so sheet­ing off the area is im­por­tant – and cov­er­ing ev­ery inch of your body in pro­tec­tive gear is vi­tal. Even­tu­ally it’s a case of grind a bit, test dry-fit and grind a bit more, un­til it all sits nicely to­gether.

While in the early stages we as­sessed whether there would be suf­fi­cient ac­cess to glass the tube and the scal­lop to the in­side of the hull, now we have a dry fit we need to look more closely at ex­actly how we pro­pose to glass it all in. Ob­vi­ously the un­der­side of the tube and scal­lop will be the hard­est to get to, so I grabbed a brush and roller and ran a test to check I could get to all the joints. As a re­sult, a lit­tle more bulk­head had to be cut out of the way and will be re­placed later.

When happy with the hole on one side, it’s an easy process to make a card­board tem­plate to mark out the other side. Then it’s a case of rinse and re­peat.

A good key

Mov­ing in­side, the worst job is grind­ing back the in­te­rior glass to pro­vide a good key. I aim for a border of clean glass layup, at least 6 to 8in, fol­lowed by scrupu­lous clean­ing of the dust with a high-power vac then a good blast with an air line, fin­ish­ing off with a thor­ough wipe-down with ace­tone. The bond­ing area of the tube and the scal­lops get the same treat­ment be­fore fi­nal po­si­tion­ing.

We are us­ing polyester resin and layup as op­posed to epoxy. It’s very much cheaper and faster to lay up, and the thick­ness of layup will be im­mensely strong, the ad­di­tional weight not be­ing a fac­tor on this boat. The tube and scal­lops are bonded into place us­ing bond­ing paste, which is the same stuff most decks are bonded to the hull with these days.

The layup is time-con­sum­ing, es­pe­cially the tricky glass­ing un­der the tube. I use a cou­ple of mir­rors and a long-han­dled brush and roller. It takes a bit of prac­tice work­ing up­side down us­ing a mir­ror, but it’s ob­vi­ously im­per­a­tive that you get a good, con­sol­i­dated layup.

When glass­ing the tube, it’s worth spend­ing time cut­ting the glass for each side be­fore­hand and lay­ing them on top of each other in or­der of use. This is an easy way of en­sur­ing you get an even num­ber of lay­ers on each side as it’s very easy to lose track of how many you have done.

Due to the shapes and an­gles in­volved you have to use short pieces of cloth around a foot long and a max­i­mum of 150mm wide. The first lay­ers are chopped strand as they pro­vide a bet­ter grip to the old glass than wo­ven cloth. Then I move onto 100mm-wide bi­ax­ial tape and then 450g 150mm-wide bi­ax­ial tape.

A top tip here is to wet out the glass on a scrap strip of hard­board first and then care­fully trans­fer it to the tube/hull joint. which en­sures you don’t get any dry ar­eas. At the same time, it’s a lot eas­ier to wet out us­ing the min­i­mum amount of resin: a very com­mon fault of the ama­teur lam­i­na­tor is us­ing too much resin, which will re­sult in a brit­tle and less strong lam­i­nate. It takes me at least three full days to get all the lam­i­nat­ing done – it’s not some­thing that can be rushed.

One of the trick­i­est jobs is lo­cat­ing the po­si­tion of the gear­box in the tube: you only need to be a gnat’s out and the prop will foul on the tube. So, by ad­ding a few strips of mask­ing tape you can mark up the po­si­tion ac­cu­rately, tak­ing into ac­count that for the prop to be in the cen­tre of the tube the mo­tor has to be off­set to one side or the other.

Lew­mar (bless them) pro­vide a tem­plate page in the in­stal­la­tion man­ual to aid fit­ting. They even call the page ‘Cut­ting tem­plates’. They then ruin all this work by wa­ter­mark­ing the page ‘not to scale’! Sure enough, they aren’t to scale, but frus­trat­ingly, not by much at all. Com­pletely bonkers. So, ig­nor­ing the man­ual, I use the ac­tual sad­dle as a tem­plate, and mark and cut out the re­quired holes.

The owner had de­cided to lo­cate the thruster bat­tery next to the tube, so once the tube and scal­lops have been glassed, and the gear­box and sad­dle po­si­tion set­tled, the next job is to glass in some shelf sup­ports and then make a tem­plate for the shelf. For this job, gash ply­wood cut into strips and a glue gun are your friends – a fast and ac­cu­rate way to tem­plate any panel on a boat. The shelf is made from 19mm marine ply and is glass sheathed. To fin­ish, I ap­plied a few coats of gel­coat and topped it off with gel­coat with added wax in styrene, oth­er­wise know as flow­coat or topcoat gel­coat.

This job eats 40-grit sand­pa­per discs: you can eas­ily use a full box of 50 on a boat of this size

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