Masterclasses > CERVIA Tug Build

TUG CERVIA HULL BUILD by Bluebird

<< < (2/13) > >>

John W E:
Once the hull has been finished these ‘legs’ as we will call them, will be cut off.   I only trace one side of the frame – and then flip the tracing paper over and repeat the operation for the next frame on the opposite side to the first frame I have drawn.   Again, drawing in my centre line first of all followed by my building board line; each half frame is drawn out individually along with its own centre line and own building board.

When we get towards the stern we have to take note because we have to measure up and begin to put a centre line in our frames which corresponds with the centre line of the propeller shaft.   Once we have all of our tracings of the frame, we then move on to trace the bow, the keel and the stern profile.   

On this particular model, you will note that she has what is known as a ‘plating rebate line’.  She also has a square keel which runs all the way around the centre.   I have marked this with two arrows on the photograph.   When I trace my keel profile, I use the inner line as the keel.  Also note; I do not trace the rudder or the propeller frame; I then move on (once I have traced the bow and so forth) to copying our tracings onto the material.   As I have said, the material I am using is modelling plywood of five-ply 6mm thickness. Or, if you are ‘old fashioned’ just like me a good old ¼ of an inch.   The procedure I use to copy trace onto the material is:

First of all – begin by drawing a centre line; adequate distance in from both edges of the plywood material to take the full frame.  Then, place the tracing so that the centre line on the tracing lines up with the centre line you have drawn.   Secure the tracing paper down so it does not move and ensure that the two centre lines do not move apart.  After checking; draw over the line on your tracing which represents the baseboard.    Now trace one side of your frame in, then flip the tracing paper over so that the centre line lines up again, along with your baseboard line and also checking that the half frame traced on your tracing paper matches up with the lines you have traced on the plywood.   Trace the other side of the frame ensuring that you have not omitted to trace the support legs in.   Most of all NUMBER IT so you will know what frame it is.

John W E:
Carry on with this procedure until you have done all the frames, as per photograph.   Once we are happy and we have all the frames traced, trace your keel onto the plywood; ensuring that you mark where the frames actually locate onto the keel.  At this stage you will note that the keel has no thickness, no depth, the frames have no notches drawn on for where they locate onto the keel or where there are any deck stringers.  This is done at the next stage.   

To determine the thickness of the keel or how tall the keel is, will depend on what size model you are building.    We don’t want the keel too thin, where it will not stand us applying pressure when we plank without it snapping, or, we do not want it too thick/tall shall we say that it starts to interfere with the centre of gravity when we put batteries in etc., when we have finished the model.

On this particular model, with it only being 28 inches long, I have made the average height/thickness of the keel ¾ inch or roughly 19mm.

So, we will begin marking the notches into the keel; for the frames.    Normally, if the keel is 19mm thick/tall, we will make the notch 10mm deep, or, half the height of the keel.  The other thing to remember is where we have marked our positions of the keel – half the frames will face towards the bow and half the frames will face towards the stern.   The meaning of this is we draw the position of the frame on the keel and the thickness of the frame faces the bow; right up until the centre frame and in our case it is frame number 3 and when we come to frame number 4 the frame thickness faces the stern of the vessel.  The reason for this will be explained and shown later on.

Now, as we come towards the stern we have to consider two things as we are drawing the keel these are:-

a)   the position of the motor
b)   coupling
c)   propeller tube

What I normally do is lay the motor or gear box in my case; and the coupling all joined together along with a brass tube the same diameter as my propeller shaft onto the drawing of the keel.  This gives me the chance to move it around and find the best position; I then mark on the keel a rough approximation of the motor mounts and also the stern tube.

I then mark in the thickness of the stern tube on the keel plan.  Eventually this is where we will cut the keel through and insert the tube.

Once we are happy with the notches that we have marked into the keel, we move on and mark the corresponding notches onto the frames that we have drawn.

When we have finished drawing the notches for the keel we draw in our deck edge notches; this is going to accommodate the stringer which is going to run right around the deck edge and give support to the deck and also the side of the hull.

After we have finished that, we have to sit and work out about removing the centre of the frames.  Some frames we will be able to leave solid and some we will have to remove the centres from.  As a side note, if we were going to turn this into a plug mould for fibre glassing, we would not need to remove any of the centres of frames.

As a rough guide, as to how thick to leave the frame wall thickness, you have several things to consider.  The frame must be wide enough not to flex when we put a reasonable amount of pressure on the edge, and, yet it must be thin enough for us to install internal items into the hull such as batteries, speed controllers and so forth.  Also, since this is plank on frame and we will be using brass pins, it must be wide enough so that the brass pin we use does not protrude onto the inside of the frame.

That will give you some guidelines of how thick or how wide to leave the frames.  A lot of it comes from experience.  What I normally do now when I have decided roughly how much of the frame I am going remove, I draw in with pencil first, the area I am going to remove.  Go over the pencil line with ball-point pen and then replace the tracing I made of the frame over the top; and I then trace around the area I have just drawn in.   When I have completed this, I flip the tracing paper over so it is on the opposite side of the frame and then transfer the shape onto the plywood.   This keeps both sides equal and the same.    I do this to the selected frames that require the middle removed and once I have finished this, I double check all of the drawing I have done on the plywood. 

John W E:
NOW THE EXCITING BIT  :) when we are happy it is now time to cut the frames and keel out.   Every modeller has his preference of the tools used.  There are numerous saws which can be used to cut the frames out, you could use a jig-saw with a fine blade, a scroll saw, a coping saw – but, I prefer to use a Hobby’s Fretsaw – have a look at my photograph included.   There is no great advantage of using a Fretsaw, it is just a tool that I have been brought up with and used all the time.  Mind, one golden rule with any saw LET THE SAW DO THE WORK, DO NOT FORCE THE SAW INTO THE MATERIAL and always cut on the good side of the line.   It is a lot easier to remove material than it is to stick it back on.  ;D

Now then, what I do, when cutting any frames out; normally the material is roughly 4 foot x 1 foot which we have drawn the frames and keel on.   This is a bit awkward when it comes to cutting it out – so I cut it into manageable chunks around the lines – yes make sure that you don’t cut through any you need – yes I have done that before myself.

Once we have cut them into manageable chunks, and then proceed to cut around each frame, leaving about ½ mm on for us to sand off.  When we have finished cutting our frames to size, remember DO NOT CUT OUT ANY NOTCHES AT THIS TIME OR DO NOT CUT THE MIDDLES OUT sand so that you leave a trace of the line you have drawn on the edge.  At this stage, try and keep the edge of the frame nice and square with the face of the frame.

John W E:
When sanding, also, it is time to level off our support legs; again, sand them so you just leave traces of the marking line on.   Stand them up on a flat surface and use a set square to check that the centre line drawn through the frame is at right-angles to the base of the support legs.

Sometimes, you may find you will have to sand more off one leg, than you do off the other to accomplish this, but when you have finished all of this; it is now time to remove the centres out of the frames, with your chosen method of saw.

You may notice that on some of the frames I have cut circular holes towards the bottom edge of the frame.  The reason I do this on the finished model it allows passage of cables, electrical cables, servo wires through the frame rather than going over the top.   Also, it does keep the cables neat inside the hull.

When we have finished removing all the centres and we have sanded them, we can now move onwards to the next stage which is to remove the keel notches.  Some of the notches in the frames towards the stern of this model are very long.  To aid a square cut of the saw, I first use a steel rule and a scalpel to score down at least two layers of the laminate in the ply.  As I say, this helps to keep the saw running straight towards the side of the notch.   I normally do one pair of notches at the time, one on the frame and the corresponding one on the keel.  This way it helps me keep in check that I am cutting them all square and if they are slightly off square I can rectify them with a flat file and some sand paper.

John W E:
When I have finished all of the notches, I do a dry run and assemble all of the frames onto the keel.  This is to ensure that they all fit exactly where they are supposed to fit and there are no frames sticking too high up; or, sticking out to one side.  Once we are happy, and on this particular model I am incorporating the motor bed in with the frame assembly, also the rudder servo tray.  I have included a photograph to explain this.

Our next stage of the build then, includes a little bit of ‘hardware’ the propeller shaft.   This I have manufactured from a shop bought prop shafting tube.  The original length of the prop shaft tube was 6 inches and I only required a prop shaft tube 3⅛ inches overall length – to obtain the length required I reduced the overall length using a plumber’s pipe cutting tool.  I then proceeded to remove the inner burr on the piece that I required with a rounded file.   I gently tapped out the bush from the redundant piece of propeller tubing ensuring that I did no damage to the inner centre or the diameter of the bush.  I dropped this bush into the freezer for at least one and a half hours.  Not telling the lady of the house  :o after the bush had been in the freezer for that length of time, I removed it from the freezer next to the gas cooker where I had previously heated up the piece of propeller tubing I was going to use.

Quickly placing the bush into the heated end of the tube with a slight tap, the bush went into position i.e. home, and, it was left and allowed to cool down.   So, that is how I made the short prop tube.

The next stage is to fit it actually into the keel.   The first operation, where I had marked the tube onto the keel, I cut this shaded area out – see photograph also, I made two side cheeks from 6mm ply – now the keel is 6mm thick but, the prop tube is 8mm thick – so, what I did was add on two pieces of 1mm plywood to each side cheek, leaving a gap so that the tube would go through.

The first stage in the operation I glued one cheek piece in the correct position on the keel.  I allowed this to dry; once it had dried I mixed an epoxy resin up; and I pasted this around the propeller tube placing it in position, clamping the other side cheek and the top half of the keel in place – then set it aside to harden thus ensuring that the tube was in line with the keel.

Navigation

[0] Message Index

[#] Next page

[*] Previous page