Been trying to find an interesting model for my next build and was intrigued by the articles on a semi-scale waterjet powered Finnish stealth corvette in the Model Boats magazine. Thought that a similar model would be an interesting introduction to the world of waterjets.

Quickly discovered that to make a scale model with a manufactured glass fibre hull (my preferred construction), would have to find a similar vessel as nobody offered a GF hull for that ship. The author of the article had made his semi-scale model by adapting another hull.

After searching around, discovered then decided on the Swedish Visby class. This was an early version of a stealth corvette and has had excellent reviews in the various defence forums. Was waterjet powered and of similar dimensions, meeting my requirements.

Concluded this would make a nice project, so started to accumulate the necessary parts.
Researched the class in more detail and found Swedish naval vessels are also know as “H.M.S.”, but in Swedish! The Swedes have a proud naval tradition and use a similar nomenclature to the British. They are referred to as “H.Sw.M.S. XXX” amongst the Naval fraternity.

Decided the Visby class would make an ideal project so started to accumulate the necessary bits.
Another modeller donated a pair of used KMB 28mm waterjets, but missing the reverse / steering nozzles. After this stroke of luck, turned to obtaining the missing nozzles. Thought getting a pair would be easy!

Made innumerable attempts to make contact through the KMB website. Then E mails and even telephone calls from Canada to place an order, got absolutely no response.
Finally, in desperation, thought as “RN in Munich” lived in the same country and speaks German, perhaps he would be able to make contact locally. Doug gave his usual generous support.
After many attempts - nothing (thanks again Doug – now I know what Fleet Admirals are for!). Fortunately, we both gave up.

By now had obtained some scale GA drawings and printed them off at the appropriate size. Discovered that the KMB jets were too big to fit in the stern recess anyway!
Searched for alternatives and found there are many different sizes available.
Through AliExpress, found a 22mm jet unit made by MSQ. With some trepidation, ordered a pair.
These arrived quite quickly and were nicely manufactured. Think they are 3D printed, but not sure. Anyway, they look good and robust. With minor modifications, they will be fine. The KMB are the black and the MSQ white in the photo.
With the wisdom of hindsight, it might be easier to install an even smaller unit, but too late. Anyway, the 22 mm should move more water than a smaller one.
As the original vessel could do some 35 knots, this should make max. speed more realistic.
The challenges of squeezing these units into the available space do not look insurmountable. Rather too late anyway as now have them.

Original idea was to try and combine the twin nozzle reverse and steering linkages to avoid drilling too many holes in the transom. The MSQ units also have a water-cooling tapping, but in the outlet nozzle, making another set of transom holes necessary.

Decided that as the waterjet installation would be congested, making up a simple jig to resemble the transom mounting face would help. This would also help trial the installation of control linkages and the cooling circuits. Cut a plywood sheet to the same dimensions as the scale transom and fitted the jet nozzles into it.

Many of the earliest installation ideas were then quickly dumped! The top link on the nozzle reversing bucket also fouled the underside of the deck above the waterjets. Removed this linkage and determined a straight pull linkage required very little extra force. This linkage installation could be further simplified if a flexible joint was introduced.

Thoughts to combine the two nozzle reversing linkages were shelved. Much simpler to run them separately through the transom.
It does not look feasible to combine the two steering linkages either. Will need to leave them separate through the transom.

Spent several hours trying to mock-up the nozzle layout to experiment with various control arrangements. Concluded that the permutations in working in 3D, along with vagaries in the drawings, have decided best to wait until have an actual hull to work with.

Once the waterjets are fully installed, will investigate combining these two sets of linkages, but closer to the operating servos inside the hull.
That will be sometime in the future though.

Had a stroke of luck! During the vessel research found the Australian Group “Task Force 72”. This has two members who have built this model. They are a font of knowledge and very willing to share it. Will incorporate their experiences and advice into the build.

Started to assemble the other major components.
The use of a 3D printed bow thruster was suggested on this website. Purchased one from Shapeways, it duly arrived and looks good.
Was apprehensive when it was shipped by UPS that would be stung with an extra “Brokerage fee”, as is levied by DHL. Fortunately, not.
Just need to obtain the motor and gears now. The requisite items are usefully listed by Shapeways.

Accumulating the necessary servos, fuse block, Rx, BEC items etc. so these are ready when required. One of the problems with buying from the Orient is extended delivery times. The impact can be minimized by planning and purchasing ahead.

Placed an order for the hull so, apart from getting fully prepared for the project, am planning to hold off building until all my other projects are complete.

Will restart this blog once building commences.
Until then, a Merry Christmas and Happy New Year to all my readers.

The hull is in transit, have also found a set of drawings that show the stern details more accurately. There is a recess in the stern between the nozzles and had hoped to be able to fit linkages through this, above the waterline. Did not previously know the dimensions.

The drawings enabled taking the wooden former had previously mounted the jet nozzles on and add horizontal top pieces straddling the recess. Now had the principal items of the installation. Was apparent these waterjets could only just be squeezed in, so made the top pieces out of Perspex to observe the various layouts.

Photo # 1 shows the stern side elevation; # 1 is the stern face, # 2 the top horizontal piece and # 3 inclined stern face. Photo # 2 shows the nozzles mounted on the wood stern face and Photo #3 with the Perspex top pieces added.

The steering linkages look straightforward and expect to be able to link each nozzle to opposite sides of a single servo crank. The servo will be fitted forward inside the hull. The links will be above the waterline and sealed simply. Photo # 3 mocks this up.

The reversing mechanism is another matter! Whatever link arrangement is fitted it must allow adequate lateral movement to allow the nozzles to swing easily for steerage.

The crank mechanism on top of the nozzle originally fouled the underside of the horizontal piece. Had already decided the crank could be dispensed with by using a direct acting link. Unfortunately, there is not enough space for this link without drilling holes very close to the waterline, leaving insufficient space for an effective seal.

Next idea was to mount inverted, individual servos on top of the horizontal piece. The shafts would protrude through so could fit a servo arm that acts on the short link to each nozzle.
Not enough space for this. The nozzles fit too closely to the underside surface to allow an arm to be installed and to also move freely. This can be discerned in the closest (Port) nozzle on Photo 3.

Only straightforward idea, as these nozzles are longer than the original, is to lift the crank rod and drill through the rear face, mounting a servo on the inside. Rather like the outboard (Stbd) nozzle in Photo 3.
This would add a visible external link, not to scale. It would considerably simplify the installation and maintenance.
ce though, also lift the linkage holes so they are well clear of the waterline. The links will also be under the stern, hidden from most observers.

There are several permutations on each of these ideas, but until can try on the hull will not modify anything.

Has anybody any other suggestions?

The hull is now in transit, also found a set of drawings that show the stern details more accurately. There is a recess in the stern between the nozzles and had hoped to be able to fit linkages through this, above the waterline.

The drawings enabled taking the wooden former previously used to mount the jet nozzles and add horizontal top pieces straddling the recess. This gives the principal items of the installation. Was apparent the waterjets could only just be squeezed in, so made the top pieces out of clear perspex to observe the various layouts.

Photo # 1 shows the stern side elevation; # 1 is the stern face, # 2 the top horizontal piece and # 3 inclined stern face. Photo # 2 shows the nozzles mounted on the wood stern face and Photo #3 with the clear top pieces added.

The steering linkages look straightforward and expect to be able to link each nozzle to opposite sides of a single servo crank. The servo will be fitted forward inside the hull. The links will be above the waterline and sealed simply. Photo # 3 mocks this up.

The reversing mechanism is another matter! Whatever link arrangement is fitted it must allow adequate lateral movement to allow the nozzles to swing easily for steerage.

Had three basic ideas to do this:
1) The crank mechanism on top of the nozzle originally fouled the underside of the horizontal piece. Had already decided the crank could be dispensed with by using a direct acting link. Unfortunately, there is not enough space for this link without drilling holes very close to the waterline, leaving insufficient space for an effective seal.

2) Next idea was to mount inverted, individual servos on top of the horizontal piece. The shafts would protrude through so could fit a servo arm that acted on the short link to each nozzle. Not enough space for this. The nozzles fit too closely to the underside surface to allow an arm to be installed and to then move freely. This can be discerned in the closest (Port) nozzle on Photo 3.

3) Only simple idea, as these nozzles are longer than the original, is to lift the crank rod and drill through the rear face, mounting a servo on the inside. Rather like the outboard (Stbd) nozzle in Photo 3. This would add a visible external link and not be scale. It would considerably simplify the installation and maintenance though, also would lift the linkage holes so they are well clear of the waterline. The links would also be under the stern, hidden from most observers.

There are several permutations on each of these ideas, but until can try on the hull will not modify anything.

Has anybody any other ideas?

Happy days - hull arrived, looks good.

Can now fully explore the waterjet installation. Had decided to buy waterjets that were fully functional with both steering and reverse to emulate the real vessel.

The smallest could find was 22mm dia. For some reason was thinking as 28 mm jets would fit, 22 would also fit. Quickly established that as 28mm will not fit a 1:72 scale Visby class hull, neither will a 22 mm!

So, could either sacrifice full functionality and buy smaller jets without the reverse function, or modify the hull to accommodate the 22mm jets already purchased.

After reviewing the jets and comparing them to the hull, decided that with some courage to tackle the hull mutilation, 22 mm jets could be fitted. The body of the jet unit is fitted under and forward of the stern, so it would not be readily apparent.
This class of vessel also has a recess in the nozzle mounting bulkhead forward of the stern. By enlarging this, the jet and associated linkages looked as if they could be squeezed in. The side profile would be unaffected, hiding these modifications.

Carefully marked the hull to determine the areas that required cutting out. As this became apparent, became more confident in the planned modifications.
Using a Dremel tool started cutting, well inside the perimeter of the area marked.
After each cut offered the jets up to the hull, to ensure the cuts would be extended only by the minimum.

The final picture shows the modified hull. The flat panel above the nozzle apertures is now removeable and gives access so the linkages can be maintained and adjusted from above. The rear bulkhead, where the nozzles will be fitted remained much as it was, so buoyancy and stability are not be affected.

The moment of truth!
The first of what will prove to be several trial installations. Will not bore readers with a description of all, but jump to the final version.

The hull mods described previously both allow the jets to operate in steering and direction; they also provide access to the operating linkages through the removable cover panel. This proved to be a godsend as can now make linkage adjustments without jet removal.

First step was to locate the water jets onto the stern bulkhead. Because of the size of the jets experimented to see where they would fit. Due to the hull and linkage layouts the range of choices was limited. They very much fit where they fit!
Fortunately, this is very close to where the drawings show them.

Had been advised the jet units should be fitted to the bottom of the hull first, then into the stern bulkhead. Once located, the bulkhead can be drilled to accept the nozzle. The mounting hardware holes in the bulkhead and the hull can now be drilled.

Drilled the holes, located the jets and bolted them in position. Continued to install more bolts in the hull and stern until the jets were firmly located.

Fitted the steering and reversing actuating linkages next.
Started with the steering, as shown on the attached picture. None of the rods supplied with the jets were long enough for my installation. Used scrap bicycle spokes, suitably reworked, instead.
A used sail winch servo was considered to have enough torque to handle both steering nozzles, was installed between them.

Used a couple of smaller servos, one to operate each reversing bucket. These were wired in parallel, so could use one Rx channel to control both. Again, used modified bicycle spokes as actuating rods.

The bucket must remain capable of being moved by the steering control. To allow this, cut the reversing bucket spoke and inserted both ends into a flexible nylon sleeve. Movement for steering and reversing can now take place simultaneously.

After observing a number of dry runs, concluded the linkages work with the minimum of deflection and can control both functions.
Used the Tx “End point” range limiting function to ensure all the servos “bottomed” electrically, not by mechanical interference.

The pictures show the installation.

Now confirmed the waterjet controls operate properly.
Removed all components to reinstall the waterjets with sealant.

Fitted a flexible pipe to direct cooling water from each nozzle fitting into the rear of the hull. This flexible tube can be seen at each side of the nozzles and then through the stern bulkhead and into the ESC. The coolant then continues to each respective motor.

Have not puzzled out the outboard discharge yet. There are several discharge points available on this model, am hoping to use one per side. If the coolant flow is inadequate can incorporate a pump if needed.

This would be the last opportunity to ensure all the supporting structure was fitted properly and to install the wiring and electrical system. Much of this can be removed if required in the future.

Plans are to get the hull into the water this summer. The planned test runs should reveal any issues which can then be resolved before the finish is applied.
Will then park the hull to become my next winter project.

As this is my first venture into waterjets, am impatient to see the model on the water.
The attached picture shows our favourite sailing location. Easy to appreciate that the next blog instalments may be some time off!

My wife noticed me trying to check for leaks using a wallpaper tray to partially immerse the hull. Our indoor test tank had been replaced with a shower recently.
Was offered the use of a large, plastic underbed storage box. This is deep enough to allow the waterjets to be properly immersed to check for leaks. Horror! Water entered in an almost continuous bead from the joint between one jet intake and hull.

Originally had built up a bed of silicon around each jet to hull joint. Was hoping the silicon would cure around the jets to create the seal. Suspect the retention screws had relaxed as the silicon set, slightly releasing the joint. Water then seeped through.

Did not want to remove the jet to reseal it, so searched my local auto store for ideas. Permatex make a “Flowable” thin, clear, silicon sealant that is designed to run into joints by capillary action, cure and seal them. It is really intended for windscreen and similar applications. Hoped it might work here.

A bead was run around all the hull / jet joints, inside and out, and left to cure. On retest, the major leak was solved, so now searched for any lesser ones. Water was seeping up some of the retaining bolt threads and coming out around the head washers. Treated them with thread sealant and retightened. Almost no leakage.

Decided to retighten the nuts a final time and retest. Water now seeped in from one of the jet bodies!
Closely examined the installation and discovered a hairline crack in this 3D printed item. The body has right angled surfaces, which mount to the stern and hull bottom.
Think my tightening induced a bending moment, thus the crack.
Did not consider the fasteners were overtightened, but perhaps they were. Wonder if 3D items are not as robust as more traditional components and need careful installation. Thoughts anybody.

Removed the nozzle and worked flexible silicone sealer into and around the crack- it sealed. Relaxed the fasteners on both waterjets and retested. Overall seepage is minimal.
Think will watch what happens and, if necessary, change the damaged jet body next winter when the project will be completed.

Feel confident now starting the open water program once the lakes open up.
Also noticed the jets move considerable amounts of water and that the cooling system works. Do not need an extra pump and can plumb directly overboard.
The hull rides stern heavy and bow light, all within “trim balancing” expectations.

This experience reinforces a decision made after an earlier model had sunk on the maiden voyage. ALWAYS fit a water alarm until confident the model performs safely.
Not a bad idea to leave it fitted permanently either.

To be continued when open water is available.