Boat Modelling (Part II)
- Written by Malcolm. Blackman
Welcome to the second part of this series on model boat building. I hope that you found the first part of interest and are now looking forward to starting the build. If you have been bold enough to have already purchased a 1:24 hull for a 70´ Vosper, it should be 351/4” long. Although known as the 70´Vospers this class were in fact 70´61/2”.
Stiffening the hull.
As mentioned in the last part of this series, the hull has to be stiffened and this is where we will start this issue. This is done by putting wood round the inside of the top of the hull. For this 10mm square balsa wood is quite adequate for the main part of the hull, but as you can see I have filled the fore deck in with 6mm thick sheet balsa. You will find by sawing part way through the square balsa where it has to bend round any curves it will fit a lot easier. For the wood to hull joints use two-part epoxy or resin. On a model of this size I do not feel it necessary to put bulkheads in, as if you put a false deck in as I will describe, you should not find that there is any flexing in the finished model. This method of building also gives you something to drill into for deck fittings such as rail stanchions.
The next stage is to fit out the inside of the hull and to this end the position of the holes for the propeller shafts have to be marked out. Do this very carefully, using the transom (The back of the boat.) and the centreline as references. Measure from the drawings and mark the points where the shafts exit the hull. When you are absolutely certain you have the right points drill a small pilot hole through the hull. These M.T.B.s had 3 motors and propellers and therefore you have a choice as to how many motors you wish to use. The choices are one motor in the centre. Two motors on the outers, or three motors right across the boat. If you go for the all three motor option you may have trouble fitting the motors side by side and will therefore have to stagger them. (The real boats had a Z drive on the central engine which was actually positioned aft of the two outer engines.) My choice is to go with the two motor option, and therefore the centre shaft will be fixed and will not turn. If you go for the one motor option the two outer shafts will not turn. But which ever choice you make, to achieve the scale look of the boat when on it’s stand out of the water, there must be three sets of running gear under the back end.
The other consideration when deciding how many motors to use is that they will need a speed controller each.
In order that you can see what you are going to have to fit into the hull, this photograph shows the layout in my Fairmile `B´. The On/Off switches are at the top centre with the two speed controllers just below. The connections board is next, with the motors below and the universal joint couplings are coming from the bulkhead the motors are bolted to. The prop shafts are attached to the couplings and the battery can just be seen at the bottom of the photo located between the prop shafts. The radio receiver and rudder servo are located quite a bit further aft. Please make sure that any runs of wire are kept well clear of spinning components. As you can see, some thought has to go into the installation before the deck is put on.
Now for the prop shaft fitting. For the Vosper I purchased two 11” commercial shafts, and made the non-operational middle shaft from a 6” length of 4mm stainless steel rod. In order to stand a chance of getting the shafts and propellers aligned you will need to make a jig. This need not be elaborate, but must be very carefully measured and positioned. I made mine from bits of balsa wood from my scrap box. The same could be achieved with heavy duty cardboard. I started by positioning the centre shaft, and as can be seen, I made a short tube for this to pass through the hull. When you get your hull it is waterproof and will float, but after putting holes in it for the prop shafts to pass through, this happy state of affairs will no longer be the case. In order to rectify this problem, I use a product called `Milliput´ which is a two part epoxy putty much like `Plastercine´ to fix the shafts in place. It dries rock hard and has many applications. This class of boat as with most service launches, does not have a tube round the prop shafts from when they leave the hull to passing through either an A frame or P bracket. As can be seen from the above photo, the tubes from the 11” shafts need to be shortened and 2 of the bushes moved. The brass sleeves by the propellers are the start of the P brackets. We are fortunate here in Portugal, in as much as, there are some very fine businesses, who if given a drawing and instructions, will produce you some very acceptable running gear. However, for propellers and universal joints, get them through U.K. suppliers. It is worth mentioning here that for this model there are left and right-handed propellers, and you will need two of one and one of the other. And on this boat they will need to be of 30mm diameter. With the two motor option the motors rotate in opposite directions and thus you will need one right handed and one left handed propeller on the driven shafts.
When you have the tubes lined up and glued into place they need steadying and making waterproof. Here a good dollop of Milliput worked well in round each tube on the inside of the hull is the answer, and also a triangular support epoxied to the tube and inside of the hull is recommended. (A handy tip with Milliput is to wet your fingers whilst working it in.) This should hold everything steady and cut down on any vibration when dashing across the lake on full song. Remember, these boats were not slouches, having a top speed of 40 knots; they were some of the fastest fighting boats of the Second World War. Give each stage of the above process 24 hours to cure before moving on to the next stage. But in order not to waste time, while the cure takes place, make a mount that will hold the motors at the correct distance across the boat. And fix the motors to it. The universal joints 
should be made rigid for the lining up process.
Fit the ´P´ brackets and shafts, and when you have everything lined up and the shafts and motors turning freely, firmly fix everything in place to the hull. Again give everything 24 hours to make sure all is well set.
The next stage is to put in the deck supports and when you do this, make sure that any deck openings will fit between them. I put in an under-deck of balsa and as mentioned above, this will add to the stiffening of the hull. As can be seen, there is not a lot of room to get ones hands and electronics into the space 
available on this type of boat. (At this stage of the build not all of the above is fixed into place.) The styrene foredeck and torpedo scoops are however fixed, as there is no need to get at the space below. 
At this stage it is convenient to mention the rudders and cavitation plates, as these will need fixing before the deck is added in order that the couplings to the radio and servo can be carried out. I used brass tube and sheet to make these. These Vospers differ from many boats by the rudders being hung outside on the transom. If you look on the positive side, this method does not need a couple of holes below the
waterline that can leak to let the shafts through. However it does entail a bit more work to make it all look right. Once you have cut out and bent the parts as per the plan solder it all together making sure that the rudder shaft is a freely rotating fit in the guides. My rudder shafts are slightly over scale to allow for the cut to fit the rudder blades into, and the subsequent soldering. Remember to drill the rudder shafts to take the tiller arms, which make everything work when the internals are connected up. Also as can be seen from the
photograph I make my masts in a similar manner using brass sheet, tube and rod. Using this method with masts if you have a series of brass tubes that are a fit one inside the other you can achieve a very realistic mast for a warship, and indeed some other types of vessel.
This is a convenient place to finish this months article, so next month we will look at the deck, painting, radio and electrical installation, and also the superstructure.
If you have started a build, or are contemplating a build, good luck, and if you need to contact me I am available by E-mail on: malcolm.blackman@clix.pt
