howtobuild

Drilling and cutting fibreglass - How to fit rudders and propeller shafts in your GRP hull.

One aspect of modelbuilding that seems to worry a few customers, is cutting and drilling fibreglass. This is the material that all Kingston Mouldings hulls are made from, but it really isn't such a difficult material to drill, file or saw. As a general rule, you should use metal-working tools with the sort of tooth size that you’d use on a soft metal like aluminium, anything finer will tend to clog the teeth too easily.

You probably won't need to do very much sawing with GRP laminates, but when you do, nothing more elaborate than a hacksaw or coping saw is needed. Most coping saw blades are a bit coarse for thin mouldings like most GRP hulls, the teeth can jam in the sawcut, and to overcome this, you need to hold the saw at an angle to the surface of the material being sawn, so that more teeth make contact with the fibreglass as you cut your way through it. Power tools are really not a good idea for this sort of thing, in fact for most modelling generally in my opinion, it’s all too easy for them to cut too fast or run away with you and damage whatever you’re working on.

Some people find coping saws difficult to use where a straight line is required, it's all too easy to saw in a series of zig-zags, so if you need to make a straight cut, a hacksaw blade is often better. Ideally, you want something called a padsaw, a handle which uses sections of standard hacksaw blades. Saw blades are graded in TPI or teeth per inch, and 24TPI is about right for most GRP cutting jobs.

Files and saws are designed to cut in just one direction, nearly always as they are pushed away from the user, and you should always cut from the outer or gel side of a moulding. If you try to do it from the rough side, you'll break small chips of gel off the smooth or gelcoat side.

Occasionally, usually because of difficult access to the area concerned, you’ll have to file or saw from the rough inside of a moulding. The way to avoid breaking chips from the edges on the other side of the moulding is simple, you just reverse the file or saw blade so that it is cutting the material as you pull it towards you through the hole.

You don't usually need to use much pressure as you file or saw, and if you don't try to cut too quickly, you're less likely to make mistakes. Always remember that it's quite easy to remove material from a GRP moulding, but it’s a lot more difficult to correct mistakes or put it back if you go too far, so take your time and work slowly.

Sometimes, you may need to make neat holes in a hull, maybe to make the slot for a keel in a yacht hull, or to cut freeing ports or similar apertures in a scale hull like a tug. There’s more than one way of doing this, and some people favour removing as much of the waste material as they can by drilling a series of holes, and then filing through the holes to connect them, finishing off and smoothing the edges with more filing.

There’s nothing wrong with this method, though I don't like it much myself. I usually prefer to drill just one hole not too close to the edge, large enough to poke an Abrafile through. These are small round-section files that are designed to be held in a coping saw frame, and as they have teeth all round, they can be used to cut in any direction.

For the sort of small holes that are usually needed in GRP hulls, I often find it easier to cut these files in half and to work with a short length of Abrafile held in my fingers. If the hole is large enough to make this worthwhile, I use a length of file to enlarge the original drilled hole into a slot long enough to poke a hacksaw blade through.

In most cases I stick pieces of masking tape all around the edges of the planned hole to mark them, rubbed well down with a fingernail. When you are filing or sawing GRP, the edge of a piece of tape is much easier to see and work to than a thin line that has just been drawn or scratched on the surface.

GRP isn't difficult to saw, and it isn't very difficult to drill either, but drill bits do need to be either fairly new or properly sharpened. If the drill is blunt you'll have to use too much pressure, which can overheat the material. You won’t get a clean hole, and there is a considerable risk of the drill slipping. It’s usually better to use hand drills rather than power drills, especially for small holes, as it's much easier to control them.

Whichever kind of drill you are using, you should always drill from the smooth or gel side of the moulding to avoid chipping the surface, and if you stick a piece of masking tape where you want to drill, this will help to stop the drill from wandering away from your marks as it starts to bite. If you need to drill a large hole, say six millimetres or a quarter or an inch or larger, it's usually best to start with a small hole and then follow that with a larger one. There is an advantage here in that if the first hole is out of line, this can easily be corrected with a small round file before you run the larger drill through it.

One common problem when drilling holes through thin GRP is that the drill can snatch at the moment it breaks through the moulding, and this often results in a slightly ragged hole that isn't perfectly circular. To help prevent this, you should ease off the pressure at this point, and this is another reason why I often use a slightly undersize drill and finish the job off neatly and accurately with a round file.

If you need to fix anything to a GRP hull, items like rubbing strips for example, it’s nearly always best to use adhesives alone. If you prepare all the surfaces properly and use a suitable glue in the way that the adhesive manufacturer intended, you should end up with a bonded joint that is almost as strong as the materials being joined.

GRP just doesn't stand up at all well to localised stress points of the kind that result from screw or rivet fixings. These will usually cause minor surface deformations in the fibreglass that are difficult to hide completely by filling and painting over them, and rather more seriously, stress cracks will often begin spreading out from screw or rivet heads over time. Such damage is difficult to repair, and apart from looking unsightly, in bad cases such cracks can allow moisture to penetrate the moulding.

The only exception to this ‘adhesives only’ rule might be where you wanted to attach items like bilge keels or a rudder support to the bottom of a hull. If the glueing area isn't very large, adhesive its own might not be strong enough to withstand knocks, and a few small screws driven into the keel from inside the hull to strengthen the glue joint are often helpful.

If you are screwing from the inside of the hull, glue a thin strip of metal inside the hull first, and screw through that. You could use plywood instead, and then you’ll need washers under the screw heads to spread the load properly. Remember that GRP doesn't like localised high-stress points, and that's what you are trying to avoid.

Something that can cause problems with building models using fibreglass hulls is the job of drilling holes for rudders and propeller shafts, neatly and in the right place. When models are built from wood, hulls are constructed around a framework of bulkheads and formers all slotted and glued together, the propeller shafts and rudder tubes, are normally built into the structure as it is assembled.

With a GRP or fibreglass hull on the other hand, the hull skin is all there is to begin with, so in this case you have to work in the opposite direction, and build any internal framework inside the finished GRP hull skin that you start with. The most important dimensions are the vertical alignment of the shafts, their height above the hull baseline, and the position on the centre line, or on either side of the centre line if there are two shafts, so your first job is to mark an accurate centre line on the hull. If your hull was produced in a two piece mould, don't just assume that the split line is exactly in the centre, measure the hull to check.

I find it helps to stick strips of masking tape on all the relevant areas on the hull first, and then to draw your marks on the tape in pencil, most marker pens aren’t really accurate enough for this. If your hull has a moulded keel though, you can take this as your centre line. Mark out the prop shaft position, and do the same for the rudder. Where there is more than one prop shaft, don't assume that the rudder centres are exactly in line with the propellers, they often aren’t.

Marking the rudder position is simple, you can take all the dimensions off the plan, but don't drill the hole yet. Once you've drawn a neat line to mark the axis of the prop shaft, next you have to find the exact place on that line where the shaft enters the hull. If you don't install shafts at the correct height, you'll probably run into problems later with propeller clearances, and remember that prop shafts and the hull baseline often don't run parallel.

The best way I’ve found of getting a really accurate fit is to start with small drilled holes at right angles to the hull surface, and then to enlarge these as necessary with a suitably shaped file, checking the fit at regular intervals. Don’t try to drill holes at an angle, you’ll rarely get it right. If any holes are noticeably out of position or off centre, use a file to correct this. All you need in the beginning is a hole that's large enough to poke a small round file through, so that you can enlarge each hole to the exact shape and size required.

If you need to install a propeller shaft where the it emerges from a moulded keel, the shaft is often as thick as the keel section it has to pass through, and this means that some care is needed. Drill a small hole at first, the gelcoat will be thicker in places like this, and it's often quite easy to break large chunks of it off by careless drilling. There is much less chance of doing any damage to your precious hull if you use files or some abrasive paper wrapped around a length of dowel to enlarge a small drilled hole to the required final dimensions.

Have the prop shaft or rudder tube handy to check progress as you sand away, so that you can achieve an exact fit. If the hole needs to be at an angle, file or sand carefully away at the top and bottom of each hole, until you arrive at the correct angle. Work slowly, and keep checking to make sure you aren't enlarging any of the holes too much.

Once you've cut accurate holes in the hull in all the right places, there isn't really very much more to it. The correct procedures for fixing prop shafts in to hulls is covered elsewhere on this site, you can use either epoxy or polyester adhesive for these joints. Although a slow-setting epoxy glue should provide the strongest bond between the prop shaft tube and the fibreglass hull, as long as you are careful about cleaning all the surfaces before applying any sticky stuff, I've found that shafts glued in with polyester adhesive, such as a filler paste like P38 or Plastic Padding, are perfectly satisfactory.

Whichever glue you use, you must abrade metal surfaces thoroughly, and then clean off any dust by wiping with solvent immediately before you apply any adhesive. After leaving everything to set hard, any gaps on the outside between shaft and hull, can easily be filled with filler paste. You can also add more filler around the shaft on the inside to strengthen and neaten the joint.

What are described here are the techniques that I've used over the years, not necessarily the only way of doing the job successfully, just what I’ve found through experience to be the simplest and most foolproof method for me. The most important bit is marking everything out accurately on the surface of the hull before you begin. You should check everything very carefully indeed before you cut or drill hole. Measure at least twice but cut once as the old saying goes.