The moment of truth!
Weather has improved, decided to try the unfinished hull with an open water test.

The home made reverse & steering nozzle performance was disappointing. They function in theory and move significant amounts of water. The dimensions of the model though restrict the range of movements necessary to ensure full steering and reverse operation.
These restrictions cause significant disruptions to flow impeding performance and manoeuverability.

JBKIWI offered some words of advice and suggestions following his experience with waterjets. Spent some time trying to figure out how they could be incorporated, but the size restrictions still restrict performance unacceptably.

Have decided to install bucket style nozzles to get some testing in over the summer. Might resurrect the original nozzles next winter and experiment with the necessary modifications.

The flow is much better with these nozzles and a plane can be easily achieved. The hull sits level at rest, but at speed the bow lifts excessively. The performance is good.

Considering the forces generated by a waterjet. The input flow tends to pull the stern down, lifting the bow.
The bow on this vessel is higher than the stern, probably due to its military heritage. This is exaggerated in the last picture, as does the perspective. Think it needs to be lower though.
See pictures

Going to experiment with some simple, fixed transom flaps to increase the upwards force and level the hull

The hull is now fair and prime painted. Decided to try it in a water tank to test for leaks, check ballast, waterjet impeller and reverse operation.

The usual adjustments quickly concluded the ballast and jet rotation questions. There was a couple of leaks around one jet mounting. Easily resolved with silicon seal.

The clamshell nozzles do not to give as much reverse thrust as the bucket style.
As the test tank is very limited in size and shallow, not sure how they will operate in open water.
The ice is now retreating on the Lake Ontario so hope to find out shortly.
The inner jet outlets are also directed towards the centre of the transom, reducing their effectiveness.
At least am now forewarned of the problem and can fit bucket style reverser units if required.

Decided a hull of about 30” O/L should make a nice model, based upon my Visby experience.
Had plans printed using drawings from the web and, although somewhat pixelated, they are usable. Made the hull as a typical frame and strip style with a thin plywood skin.
It presented no unusual problems, made up using various plywood thicknesses and basswood strip.

The deck edge support strip was made up of ¼” square basswood up to the area when the bow curve begins. Have done this before, not usually very well.
Decided to try to laminate this forward section using a 1/32 bass wood strip and, whilst the wood still requires bending, it turned out superior.
Did most of the bending by saturating the wood in ammonia. Shaping around a curved pattern, followed by drying with a heat gun. Once the shape was achieved laminated the next strip to it with CA glue. This proved much easier than trying to bend a full section strip.
The finished result has the correct curvature and is robust.
Find the ultimate test of bow curvature is to turn the hull over and see if both sides fit the plan. To my relief they did!

Once the outer skins were fitted and blended into a smooth shape, was unhappy with the chine.
This vessel has a chine strip onto which the bottom and upper skins fit. The strip has a land (about 3/16” wide) on the outer underside, the outer corner of which blends into the upper skin.

This is similar to the one on my Daman 4207 model. Seems to be a development of the traditional chine strip. However, it is difficult to obtain a sharp corner as it is easy to round off the edge with the repetitive sanding needed to make a clean land.

Decided to try another approach. Once the skins were fitted and the land established, CA glued a thin metal wire onto the lower corner of the upper skin.
Could then blend the wire into the bottom skin using epoxy, leaving a clean corner on the upper skin. This has the added benefit of truing up the corner. The pictures show the general idea.
Not sure how robust the wire edge will prove to be, but it is nice and sharp.
So far, has survived much emery cloth smoothing without any separation!

The Interceptor and VIP vessels have slightly different chines. The Interceptor does not have one, but there in a small one on the VIP version.
As have decided to build that version, added a thin styrene strip to inside of the wire chine edge to reproduce it.

Have found several water jets with steering and reverse capability, using buckets. Have not found one, in this size range, that offers “clamshell” type reversers with steering.

The clamshells are mounted separately from the steering nozzles, unlike the bucket style. So, when the nozzle is steered with the clamshells lowered, the water is directed towards that side of the nozzle. This can give the vessel good manoeuverability.
Decided to try see how these could be replicated.

After the usual scratting around with offcuts of plastic pipe realized, using regular copper plumbing fittings, the basic layout could be achieved. When the excess copper was cut away, the shape was replicated. The remaining pieces were then soldered together to form the clamshell unit.

Mounted this to the waterjet nozzle, which had earlier been fitted with a swivelling copper pipe reducer to form the steering nozzle. All the functions were then replicated.

The final picture shows the complete unit on a dummy half transom. Still needs some fettling and the linkages working out. The general idea is evident though.
Having made all my mistakes on this first unit, making the second was a pleasure!

Once the hull is made, will fit two of these units to the stern. Can then apply the final touches, install linkages and all should work - Inshallah!
If it does not, will revert to the bucket style as have spare units.

Readers of my HSwMS Visby blog will be aware the model is complete and operating well.

Was quite happy spending last Summer just sailing models; that was until JBKIWI wrote a forum entry introducing the Swedish CB90 class of waterjet propelled fast assault class! Sparked my imagination.

After some research, found the base hull has been developed into a number of forms, including attractive “Interceptor” and VIP vessels.

The hull is of a hard chine style and should not present any unusual challenges. The waterjets are not the same as the Visby, in that the reverse/steering function is different. From experience with the Visby, consider a reverse capability essential.

Decided to use the same base 20mm waterjet units as the Visby. They power a hull in the 30 – 40” O/L range nicely. My preferred size. They also work well and do not leak. Problems friends have encountered with other units.

This offered two approaches, one use “modellers license” and fit the same bucket style units as on the Visby. The other was to use the base waterjet unit, but make up the correct Kamewa “clamshell” style reverse /steering units. These were not available through the jet supplier.