Speedline Models K7 - Pocket Rocket

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Making the Speedline PR K7 into a fast working model

by Ernie Lazenby

The Speedline Pocket Rocket is a small (pocket sized) model of Jet Propelled K7 prototype. It is produced as a static model but can be motorised fairly easily.(Ed)

The following instructions result in a model that runs with a Graupner speed 400 6volt motor

Speed Controller: ASTEC Micro 20 amp.

Cells : 7 x KAN 1050 nim cells. The URC cells are exactly the same made by Kan but are much cheaper.

Radio: HI Tec Micro Receiver, Hi Tec HS 60 Micro servo.

Drive system. Custom made flexi shaft system using .098 flexible cable.
Rudder - Custom made using stainless steel.

Prop. Graupner 2318 series hydro prop (Carbon) 29mm Diameter.

General mods required to the basic kit.

Hull

The inside of the hull where the sponson booms enter need to be strengthened on the inside. I used lite ply but plasti card would suffice. I also strengthen the sides of the hull where I made the hatch.
I removed the planing wedge as this inhibits the flow of water to the surface drive prop and is not required. The prop is then the third planing point.

Sponsons.

These are very thin and need to be strengthened on the inside around the area where the booms enter. I filled the sponsons with foam then sheeted the bottom with plasti card then very thin carbon fibre sheet.(Plasti card is fine. The width of the planing surface on the sponson needs to be increased to 21mm at the rearmost part. Make sure all the edges are kept sharp.

I used carbon fibre tubes to make the sponson booms.

I have done a lot of testing varying the width of the track i.e. distance between the sponsons. The model will run at speed with the track at scale but only on flat calm water. The track needs to be increased if the user wishes to run the model in rougher water. I have found that if the sponsons are 126mm apart measured across the front boom the boat handles quite rough conditions without flipping over.

Now a final but important point to do with the sponsons. The real K7 had quite a shallow angle of attack on the sponsons because they had a slight nose down attitude from the horizontal. The boat was not prop driven and rode on the rear planing wedge. The model is prop driven and it's quite difficult to prevent the prop lifting the rear end which in turn can push the nose down a little making the sponsons run wet. I found that getting rid of the slight down angle on the sponsons made the boat run cleaner and faster and basically no one knows any different. The top of the sponsons need to run horizontal to the water. This increases the angle of attack. I have done this with my own 1/8th scale scratch built model K7 and the converted Touchwood K7. Only the real purists know the angle is different because it's barely noticeable.

Prop position.

For maximum speed and stability the center of the prop needs to be on the same horizontal plane as the rearmost part of the sponson planning surfaces. The prop should also have a slight down angle.

Positioning of internals.

Receiver.In the nose area of the hull up against the front boom.

Cells. Front of the cell pack needs to close to but not touching the receiver.

Speed controller on top of the cell pack with an aluminum foil separating the controller from the cell pack.

Motor. Central in the hull but offset slightly to the right looking from the rear of the boat. (This helps prevent prop walk.) The motor shaft should be 3mm offset to the right from the center line. The front of the motor is 9cm from the transom.

Servo. At the rear. A small Graupner bellows is required.

To make it easy to get into the hull I decided to forgo the scale hatch and made the entire top removable starting 20mm from the back of the cockpit and 20mm forward of the transom. This hatch is taped up with clear tape. Makes a totally water proof hull.

The large fin is removable to ensure the hull can be taped up and is then fitted over the top of two small brass pins.

The end product is a boat that is fast but predictable and looks very good on the water.

I have managed to water cool my motor but this does require a pick up situated above the prop and it can spoil the look of the model when not on the water. On the water it cannot be seen because of the spray. I did this to se if it could be done and it can. The cost of the 400 motors is such that replacement is not really any problem therefore water cooling could be omitted.

Running the model.

Don't be afraid to apply full power and keep it on. The model will pop out onto the plane easily. It sits very low in the water at rest but that's not a problem.(It is if you have a hole into the hull via the jet pipe!; don't put a hole in the hull at this point.)

It is a model for running in a straight line flat out. Don't try turning it at full speed. It can be kept on the plane while turning but that requires much care and needs some experience to achieve.

Using the KAN 1050 cells and the prop suggested you will get about 5 minutes run time.

Various photos showing the position of the internals in my model will be posted on the ASTEC webb site under the heading Ernies set ups.

I have rushed this set of instructions out and may need to add to them later.

Good luck.

1. 2. 3. 4. 5. 6. 7. 8. 9.

Pocket Rocket pictures:
Number 1. General layout of the interior click on the thumbnail for a larger version:
2. as above
3. Shows the offset drive line and ride plates on the sponsons
4. Method for locating the fin
5. Rudder and prop assemblies
6. Another top view
7. Close up of ASTEC speed controler and URC drive cells
8. Rudder servo and bellows
9. Scale track

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