View from the boatyard
Mike Pickles talks us through the process involved in retrofitting a bow thruster
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.
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