Now that the prop-shaft, propeller, flexible coupling and motor mounting bracket have arrived it’s time to think about the motor position and fabricating a support block for the motor mount.

First, I ensured that the shaft and propeller protrusion from the keel was correct in relation to the rudder water pickup tube positions and temporarily fixed the tube in place. Note that the pickup tube has yet to be bent and formed!

I find that the easiest way to ensure perfect alignment of the motor and shaft is to use a rigid coupling to hold the motor and attached mount while measurement and angles can be taken for the mounting block.

With that angle established I was able to shape a piece of hardwood, in this case beech, to the correct size, shape and angle for the motor mount to fix to.
A test fit of the block and motor proved that all was well with the shaft alignment.

The beech mounting block was then drilled and fixed to the deck with some brass screws that go down through the deck floor into the keel and the motor mount was then fixed with four screws to the mounting block. With everything fixed in place and the rigid motor coupling replaced with the flexible coupling the motor turns the shaft very freely with no binding or movement.

When finally assembled the shaft will be greased in the usual manner but for now, I will remove all of these drive components so that I can finish the decks internally and the hull externally.

It's at this point in July 2022 that I had to put the Marlin build aside to concentrate on moving house and building a new workshop in the garage to be able to continue with the build.

Old workshop seen in the last picture.

See you again in 2025…….(so to speak!)

The last remaining piece of ply to fit to the hull is the stern skin and like the others is 1.5mm ply that is easily bent to the required curvature.

Before it’s fitted though I decided to fit an additional piece of balsa to the area that the water-cooling outlet will pass through just to provide a firm support to the tube. This was just a piece of scrap balsa glued in place and then the whole stern area was sanded back using my sanding plate to ensure that the area was completely flat in the vertical plane and the required curve in the horizontal.

The ply skin was then glued and pinned in place with some additional clamps and packing tape to pull the skin firmly into place, and when dry the skin was trimmed and sanded to meet the side and bottom skins. Before fitting the skin I took the precaution of marking the position of the extra balsa block on skin for when I eventually fit the water cooling outlet.

The external keel can now be fitted. This consists of three parts in 6 mm ply that are glued together as a single piece and glued and pinned, using some long brass pins, along the internal keel as far as the exit of the prop shaft.

The brass pins were punched below the surface and some filler used in the holes and then it just needed some sanding to slightly round off the edges along its length.

As my order for the prop-shaft, coupling and motor has now arrived I can think about the motor position and fabricate a suitable mount which I'll cover in the next part.

With the bottom and side skins fitted and trimmed the bow can be finished with the balsa blocks supplied in the kit.

The bow has to be made this way because ply skins would could never be made to fit, as ply cannot be bent in two planes to form the required compound curve, and balsa is an ideal material for this purpose as it is easily carved and sanded to the required curvatures.

This can be quite tricky to do well so it deserves to be done slowly and with great care as I didn’t want to resort to the use of any filler to correct any mistakes on my part.

The balsa blocks supplied with my prototype kit needed to be arranged and laminating to produce a single block that would fill the area required together.
It’s likely the production kits will have one-piece blocks to simplify this stage.

The largest piece was placed in the bow and marked with a pencil line so that a basic cut could be made with a coping saw, and then the smaller pieces and offcuts arranged to fill the required area, and then all were glued together in the bow.
When the aliphatic glue had set some more basic cuts were made with a fine saw and then further refined with a coarse file to the rough profiles required.

The final shaping was done very gradually and carefully with fines files and sanding blocks to produce the final shape, taking great care to extend the line of the lower chine along the block to meet with the front keel former.

I used a card template (not shown) to make sure that both blocks were symmetrical and checking with ‘mark-1 eyeball’ as well.

I also temporarily attached the false keel to check that it would conform and fit to the blocks as seen in the last picture.

With great relief I decided that it was as good as I could make it and it only remains to fit the ply skin to the stern to finish the hull.

Both of the side skins are in two pieces of 1.5mm ply, a short bow piece and a longer second piece that extends from F2 to the stern.
As with the bottom skins I used my heat gun to bend and shape these skins so that they fit the contours of the hull but I did find a problem with the dimensions of the larger skins after test fitting them.

It was apparent that the skins had been made slightly too short by about 10mm to span from former F2 to the stern allowing for a small overlap for trimming. Not a major problem, and to overcome it I simply made up some ply doublers from some scrap ply and added them to each of the F2B formers that would allow me to set the skins back sufficiently to reach the stern correctly.

The kit of parts that I am assembling is a pre-production prototype and my build is primarily to discover any construction snags such as this. I understand that this has now been ‘designed out’ of the later production kits of the Marlin.

There was another small problem 😮 with the upper chines in that they were not wide enough at the stern to meet the edges of the formers but again a simple solution was to add some scrap balsa to bring them out to the correct position. Once sanded back they were just fine to meet the top edges of the side skins. Again, this will be rectified in the production versions.

Some sanding sealer was applied to the internal faces of the skins, avoiding the glue areas that meet the chines and formers, something that is not strictly necessary but to me it seems like a reasonable precaution to protect the internal faces of the skins from damage by any (unlikely) water ingress.

The skins were held in place by the usual assortment of clamps and pins but in addition I also use brown packing tape pulled tightly across the hull to really pull the skins down tightly along the joint with the bottom skins. The packing tape has extremely high tensile strength and is ideal for this purpose and leaves no residue when removed.

The smaller skins at the bow were then heat formed and sealed internally before glueing, pinning and clamping into place and once all of the glue had set the clamps, pins & tape was removed and I then had the very satisfying job of planning and sanding back all of the skins along the top edges and the meeting edges of the side and bottom skins. There's a small gap in the skins as seen in the last picture that will need a bit of filler after the bow blocks are fitted and shaped, this is because the skins were moved further back to get the overlap at the stern, again a small revision required for the production models.

I feel that I'm now finally making some some good progress with this build 🤗 but there's a long way to go yet 🤔 and hopefully those following this blog aren't totally bored yet 😴. Let me know?

The next part will cover the fitting and shaping of the balsa bow blocks 😎.

In response to a recent forum thread on 'tools' I have concluded that one of the most essential tools to have when building model boats is the humble clamp.
You can never have too many of them in all sizes and types!

I employed a small selection from my collection of clamps in the fitting process of the bottom skins along with a number of brass panel pins and a few ‘push pins’ to hold the skins in place while the aliphatic glue sets.
The clamps come in a variety of sizes and types and are usually quite inexpensive. I used to buy a lot of them at the 'tool' trade stands at the big model shows (most shows are long gone now ☹️) but they also often crop up in ‘pound stores’ and the like.

Once the first skin has set I fitted the other side in the same fashion although I have easily enough clamps and pins to do both at the same time!

When all were set after a few hours I used my small hand plane and sanding plate to trim back the excess ply to the strakes in preparation for fitting the side skins.

That’s when I noticed a minor issue with the side skins, which I’ll cover later.

BTW. Does anybody know where I can buy a metal version of the plastic push pins that many of us seem to use?

The plastic ones work extremely well but the heads on them seem to break rather too easily in use, especially when gripped and pulled out using pliers 😠.

I have found some metal headed ones on the interweb that are sold in artist supplies stores and some for glazing uses but they are very expensive in any useful quantity.

There are also metallic silver finished plastic ones out there which will, of course, be no better!

Can anyone suggest a supplier?

Before I can fit the bottom ply skins, I need to work out the rudder position relative to the water pickup and in turn, the propeller, and then drill the keel for both.

However, the rudder needs to be modified so that it has a lower profile and this is done by turning the blade by 90 degrees.
I had done this previously when I built the VMW Thames Police Launch so I knew it would be a simple operation.

The blade of the rudder is held in the slotted shaft by two brass rivets and these are easily removed with a punch, then the blade can be rotated about one of the holes and a new hole drilled for the repositioned rivet which is then peened over to fix the blade position.

After cleaning up the parts, I applied some flux and soft soldered the parts together for good measure.
The empty rivet hole was also filled with a small offcut of brass rod, soldered in place and then flatted with a file and emery paper.
After a polish with some fine wire wool, it looked bright and shiny although I will probably re-polish it and apply some clear lacquer as part on the finishing process.

Now I could use the rudder to determine the correct position in the keel and also the water pickup tube location.
My new prop-shaft has not arrived at this time so I used a substitute to check position and clearances.
The holes were then drilled ensuring that they were vertical and the rudder and water pickup tube tried for fit.
I still need to bend and shape the pickup so for now the 6mm brass tube is pushed through the keel for fit.

On with the skins now!

This is probably one on my favourite parts of a boat building project.

Fitting the skins marks a transition between the skeletal form of the hull and the something that starts to resemble a boat.

My sanding plate made a good job of flattening the keel, formers and chines and so I pinned the 1.5mm ply skins roughly in place so see how well they fitted. I put a slight chamfer on the meeting edges on the keel to ensure that they butted together without any gaps.

I added some extra balsa formers either side of the keel just to add some extra support to the bottom skins and prevent resonance, and these also help to locate the styrene ‘conduits’ for the wiring and cooling circuits.

One trick I like to use is to pre-form the skins to conform to the curvatures of the hull using a heat gun, in this case a re-purposed paint stripping gun.
Used carefully and sparingly the heat from the gun is sufficient to relax the glue bond between the ply layers to allow them to be formed by hand to the hull curvatures.
I generally over bend the ply so that when the skins cool and the glue bond restored the skins relax back a bit and into the desired curve such that the skin can lay on the hull formers requiring very little pinning or clamping.

As the skins form a sealed compartment I gave the two bottom skins a couple of coats of sanding sealer for good measure making sure that I left the areas of glue contact were left un-treated, I also coated the corresponding internal areas of the hull with sanding sealer.
Once happy with the fit of the skins I put them aside and drilled the prop shaft tube hole through the keel using a long 8mm dia wood bit which made a very clean exit hole.
After re-pinning the skins in position I drilled and shaped the two skins to fit around the 8mm diameter prop shaft tube, which at this time I didn’t have as it was being made for me my Steve at Model Boat Bits, so I used a length of 8mm styrene rod as a substitute.

At this point I also had to drill the holes for the rudder and a water pickup for the ESC cooling circuit. The rudder, a standard 'large' brass one, will require a bit of conversion first as I need this to determine the correct rudder position relative to the water pickup tube and the propeller.

I’ll cover that in the next part.

As with the keel, the upper and lower chines are each assembled from four different pieces. This can be done over the plan but in practice it is just as easy and accurate to do directly on the workbench as the laser cut parts are very accurately cut. The balsa pieces are simply glued together to form the two upper and two lower chines.

Once the glue had set they were fitted to the hull, some formers are slotted to accept them and they overlay some others, and they were all pinned and glued in place on both sides. The two K7 parts were then added to the front with some additional supporting pieces from some ply scraps just for added strength.

I also added some additional keel doublers, made from scrap ply trimmed and shaped to clear the wiring conduits. This was just to add some strengthening to the area that will have holes bored through for the rudder and the water pickup tube.

Some additional formers were added at the F3 position just to add some extra support to the bottom skins when they are added.

At the stern the parts F5a & F5b were added to form the curvature of the stern and bulkhead F5 was then sanded to complete the curvature using my sanding plate to ensure flatness.

With all of the bulkheads, formers and chines in place I used a small plane to shape the ply keel and then the sanding plate to flatten all of them in readiness for fitting the ply bottom and side skins.

Continuing with the construction of the hull, the remaining balsa 'F' formers were glued to the underside of the box at the marked positions and the corresponding formers added to the box sides.
All of these formers act as supports for the upper and lower chines that give the hull its final shape.
At the same time I installed the four styrene ‘conduits’ through the bottom formers.
At the stern the tubes end at the face of the rear bulkhead as this compartment will contain the rudder servo and the water-cooling pickup and exhaust tubing connections.
Two of the conduits will be used for cooling water flow and return in silicone tubing, a third for servo wiring and the fourth more for symmetry that anything else!

At the forward ends the conduits stop just underneath the motor and Electronic Speed Controller locations. The ESC will be mounted on one side of the dividing bulkhead F3 which is inside the cabin and the motor will be in a separate housing in the open part of the deck.
I also made some suitably sized holes through the keel and floor to allow for the cooling tubes and motor wiring to exit.

There are four additional balsa formers, F5A & F5B, that fit onto the rear former F5 that give the stern its curved shape, these are simply glued in place and will be sanded back to blend them all together when the chines are fitted. The ply skin of the stern will be added at a later of construction.

The next job will be to assemble and fit the upper and lower chines.

NB.
My apologies for the rather sporadic posting of these blog updates, my wife has had some recent surgery that will rather limit her mobility for a while, so domestic priorities mean that workshop and blog writing time is quite low down the list ☹️.

The keel and formers can now be fitted to the box, starting with the forward piece of the keel which was glued and fixed centrally along the box and ensuring that it is central and square and equidistant from each side.

The rear piece of the keel (K5) was also fitted leaving an 8 mm gap for the 8 mm diameter prop shaft tube and the keel doublers (K6) which I had previously machined with a groove to accommodate the styrene conduits, were added to either side.
All of the lower formers were then added to either side of the keel on the underside of the box. At this point I decided that it would be an advantage to fabricate some additional formers to fit to the underside of the box at the F3C position as they would provide some additional support for the bottom skins when they are fitted.

Two pieces of 9mm ply, F1 & F1A, are glued together to make Former F1 and I added a brass pin to reinforce the joint. I don’t see any reason that this part can’t be made out of one piece of ply instead of two, but I expect that VMW have a reason to do so?
The assembled F1 former is then glued to the forward keel, I used forward cabin floor component clamped in place to act as a temporary spacer, and then the K3 was added to the front of the box to locate and support F1.

The two halves of F5 are glued and laminated together to form a 9mm balsa former which is added to the rear of the keel, with the addition of K5 to locate and support the laminated F5.

After fitting lots of effin’ formers 😉 (with more to fit!) the hull is slowly beginning to take shape.