Sea-Jet Evolution

Sea-Jet Evolution

TERRY SMALL reviews the Robbe kit

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After many successful sales years for the Robbe Sea-Jet fun boat, the time was ripe for a successor and this is the Sea-Jet Evolution. This new design has been both visually and technically overhauled by learning from competition racing craft, so its appearance is significantly racier than its predecessor, plus it has an improved roll stability. Thus, the hull has been made wider and lower and to give the Evolution the power it deserves, Robbe have equipped it with two electric brushless motors. The two propeller shafts are coupled directly to the motors, therefore there are no gearboxes or flexible couplings to absorb any of the power. A centrally mounted rudder and the new hull shape guarantee high speed and manoeuvrability. Length is 685mm, beam 240mm, height 370mm and total displacement 2100gm.

The kit

Robbe are never short on quality or quantity and have produced a very comprehensive kit, Photo 1. This includes two good quality brushless motors, a two piece hull and seat section in black ABS vac-formings, all requiring no painting, which is fine as long as you like black! The driver is a vac-forming that does need painting after assembly. An ABS boat stand, CNC cut plastic sheet parts and running hardware etc. are included, so all that needs to be purchased are batteries and a basic two channel radio set with one servo and two brushless motor speed controllers. These last items can be a significant cost, ranging from low to high prices. Instructions including exploded diagrams are all in the usual Robbe format with section numbering representing general assembly sequence and cross referenced to the part number. A separate sheet gives a clear understanding of the profiled parts including an electrical wiring diagram.

Construction

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The vac-formed parts required just a little trimming and a light sanding to smooth the machine cut edges. The vac-formed stand supplied can be strengthened at its base by bonding a 4mm piece of plywood, cut to the stated dimensions, to the underside of it. This is not supplied, although I found it more than strong enough for the task without. Two very nice sheets of thick white CNC cut plastic parts require each part to be identified before removing, best done with a sharp Stanley knife or similar, Photo 2.

Each building step is numbered so there really is no room for error and the recommended adhesives are indicated in the instructions. The first step is to assemble the inner mounting frame which all easily slots together gluing the parts that need to be, and avoiding the ones that don’t, all described in the step by step text. The motors are bolt to the cross member so as to be easily removable for routine maintenance. As I wrote earlier, the brushless motors and all relevant running gear hardware is included in the kit, but not the speed controllers. The internal framework is tailored to fit exactly in place on the hull bottom.

It may be best to insert the drive shafts and couplings, Photo 3, and also add the framework for the battery tray and the rear frame for the receiver and rudder servo, Photo 4, before gluing to ensure perfect alignment. Please note the servo cut-out is for a standard size unit, but it will need a little trimming for other size servos. I also checked to make sure the drive batteries fitted properly as once the top section of the hull is in place there would be no way of modifying the interior. The rudder comes ready made and requires fitting using the inner framework and lower hole in the hull for location purposes whilst also making sure that it sits perfectly upright, Photo 5. This is most important to ensure good handling at speed. Once satisfied that all is square, glue the inner framework to the hull using a small amount of five minute epoxy glue. It is wise to ensure the joining surfaces are all well keyed using rough or medium grade sandpaper before gluing. The drive shafts and rudder tube are then also glued firmly to the bottom of the hull, creating a good solid watertight tube to hull joint. A tip here is to key the outer shaft tubes using a rough file to ensure a better bond. The drive shafts need to be parallel with each other and at the same angle. Having rigid shaft to motor connections makes this easier and to be honest you would be hard put to get it wrong with this kit.

Once the rudder is properly positioned and the servo mounted, making the linkage is simplicity itself. The linkage wire is cut to size 148mm long as instructed, and a 90 degree bend formed at one end using pliers. The tiller and screw ball-link clevis which goes on to the threaded end of the linkage were installed next, at the same time making sure that the servo is set to its neutral position before joining to the servo horn.

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There is a removable hatch which is held down by screws over the battery and motor compartment. All the screw hole positions are clearly marked and just need to be drilled. Over this fits a removable seat which requires a framework inside it, assembled and glued in much the same way as that in the hull. Also, to the rear top part of the hull there is a one piece vac-forming which requires an internal strengthening frame, again all slotted together and glued in place and which should be set back 1.5mm from its front edge. At the front of the hull there are two pieces with locating notches which line up with the seat frame and help lock it in place. These are glued together then fixed to the rearward sloping face at the front of the removable black access hatch on the hull top section and this can be just seen in Photo 6 on the left hand side. Once the glue has set, these were sanded to follow the same curve and angle of the fairing. Underside of the hull hatch there is a retainer plate and a nut to be fitted to hold the seat down when a bolt is passed through to it. It is a simple job to press the nut into the hole in the ABS plate and then glue the whole thing in place.

Next up is the seat with its supporting frame. A bit of trimming may be needed to achieve a good close fit, at the same time ensuring the retaining bolt is in the correct position. I always just lightly ‘tack’ such framework parts in position with superglue before applying epoxy glue in any quantity. It is thus an easy matter to break the joint at this stage rather than later when loads of glue has been applied. Photo 7 is an underside view of the seat with its internal framework.

Radio control

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A standard two channel radio set is required so a Futaba T2ER (or similar) r/c set with one servo for rudder movement is all that is required. The two brushless motors will require their own brushless motor speed controllers, which have three connecting leads instead of the two for conventional d.c. motors. As I wrote earlier, it is surprising that speed controllers are not included to match the motors, but I guess that might make the kit price prohibitive, although you are going to need them anyway which is rather puzzling.

Robbe recommend their own Roxxy BL Control 730 Part No. 8644 for their motors, Photo 8. These can be used with either NiMh or the increasingly fashionable LiPo batteries. Using the correct gauge of wire and appropriate connectors is very important, especially when large quantities of amps are being used. ‘A Model World’, were able to supply all the necessary items by return of post.

Both controllers will need to be connected to the same receiver channel via an r/c Y lead. Some soldering will be required on the controller’s output and input power leads using the right gender plugs and sockets following the clear wiring diagram. You will note that in Photo 9, I have used heat shrink sleeving to doubly secure the connections and insulate them.

A single 8.4v Sub-C NiMH battery supplied by the Component Shop is the power source, website: www.component-shop.co.uk, tel: 08451 309967. A Y lead from this power pack to each of the two controllers will also be required. A LiPo battery could be used instead, but I have no experience of using these. I do know that care has to be exercised when charging, plus that they would normally be charged away from the model.

As mentioned earlier, Robbe supply the wiring diagram which notes that if two esc’s are being used with a BEC system (Battery Eliminator Circuit), then the positive receiver connection of one of the esc’s will need to be disabled or both controllers will try to power the receiver and rudder servo, Photo 10.

NiMh batteries respond well to a regular charging and discharge cycle and if you can afford it, I would recommend the purchase of a good slow and fast charger that can be used for a number of different types of cell and work both from the mains or a 12 volt battery source. I currently use a Graupner Ultramat 10, Part No. 6410.

Hull and deck joint

As you will have gathered, the easiest way to build this model is in two halves, top and bottom. So, once all the internal assembly is complete including r/c installation, the two halves can be joined. As before, key the mating surfaces thoroughly before gluing. For this special task, the glue is supplied in the kit. It would appear to be actually a black silicone sealant of the type used around sinks and baths in our homes, but the product information is in German so I cannot be 100% sure. Suffice to say, the application of it, clamping together, wiping away excess and leaving overnight, joined the two halves splendidly, Photo 11.

Next up was to fit the U shaped rubber fender which fits over the joining lip. For this, superglue gel (slow set) was squeezed into the bottom of the U shaped rubber (in sections) and then it was pushed into the required position, slightly stretched and held with bulldog clips awaiting the glue to set, Photo 12. This whole task takes no more than 30 minutes or so as you work around the hull.

Driver

This is a nice large detailed vac-formed figure supplied as a body trunk with separate arms and legs. The first job is to remove any moulding marks, Photo 13, especially at the joining surfaces. Gluing the legs and arms to the body is simply achieved by gently roughing up the required areas where the glue is to be applied and stick! Setting the right angle of each limb has been made easy as they all have their own interlocking square keyways, Photo 14. Superglue can be used, but I opted for five minute epoxy glue and held the trunk and limbs together with sticky tape until fully cured.

For painting, he had an all over white primer coat from the Halfords range of car spray aerosols, then the colour coat was from the Revell/Humbrol enamel tinlet range, Photo 15. The driver is held in position on the seat using a suitable self-tapping screw in his posterior. Velcro is supplied, but the screw is more permanent – he may not float if he falls off in the middle of the lake! The seat is supplied black, but I sprayed it in grey primer. All the painted surfaces were then left for four days before having a single airbrushed coat of Humbrol satin varnish to seal them.

Decals

These are self-adhesive and require cutting out with scissors and/or a sharp knife. The various box lid pictures show where each sticker needs to go. Application is easy by having a small bowl of very mild soapy water handy. Remove the backing paper, dip the decal in the water and place on the model. The idea of the soapy water is that it allows the sticker to be positioned and slid more easily, then by smoothing out the sticker and starting from its centre, this squeezes out the air and water allowing it to adhere permanently, Photo 16. The tricky decals to apply are around the foot treads, Photo 17.

Setting up

It is best to apply a small amount of silicone grease or equivalent on all the shaft and rudder post moving surfaces to lubricate and make them fully watertight. Trimming the model and setting up the radio gear is most important, this second task easily being done whilst the model is on its stand. Ensure the Tx control sticks are at neutral and the trims likewise, turn on transmitter, connect up the drive battery pack and all being well, the electronic speed controllers will start to self-set and once this is complete after just a few seconds, it should be all be ready for operation! An initial check by applying a little throttle and both motors should start, but contra-rotate, with the handed propellers turning in opposite directions. This should be anti-clockwise for the port and clockwise for the starboard when viewing from the stern. If all is okay, the rudder throw can then be checked and if necessary adjusted by altering the length of the linkage in the boat or using the transmitter trim setting.

On the water testing

A calm day was chosen for the first test run as after having a major mishap with another model using brushless motors I was a little worried to say the least. These motors are fast, powerful and very efficient, BUT you do not get instant reverse with the reversing types of brushless esc, and this model has non-reversing controllers anyway! So, thinking ahead of your model’s direction of travel and speed is a very important consideration.

The usual bank side tests took place making sure before the ‘off’ that the hatches were secured and the seat firmly on. No weed in the water or objects that could cause problems were noted, so all was ready to go. The model was placed in the water, it balanced and floated okay and gentle power was applied with the throttle stick. Up came the front and Sea-Jet Evolution was off and with a gentle application of rudder, this indicated a reasonable turning circle. Full power was applied generating more lift in the hull with more loads more speed and yes, it was moving fast! Turning at speed was very responsive and controlled and once throttled back, the speed soon fell away, but this is not instantaneous with a momentary delay measured in fractions of a second. A quick check inside the model at the river bank and there was no water inside and the motors were hardly warm. Running time from the battery pack at high speed was around nine minutes, depending on the charge and state of battery pack. As soon as there is a noticeable drop in speed indicating the battery is starting to ‘dump’, that’s the best time to bring the boat back to the bank. You will notice from the running photos that I have not fitted the figure. The simple reason is that I forgot him on the day – sorry! Please also note that there is a small additional rudder inspection hatch at the stern, fitted for personal convenience and held down with double sided tape.

Conclusion

The Robbe Sea-Jet Evolution is a very easy model to assemble and by following the straightforward instructions, the builder cannot go far wrong. The motors and drive shafts due to the positive slot together framework fit and align perfectly, but do properly key and glue the framework to the hull. Mind you, that does not mean you should pour in loads of glue as it can add unnecessary weight and do grease the drive train periodically. It goes well, it looks great, it’s fun on the water (with or without the figure!) and it goes fast, but remember it’s a model that goes forwards only, and brushless motors can be unforgiving in an emergency ‘stop’ mode, so care is needed on a busy lake. However, it is a shame that the appropriate dedicated esc’s are not included and are extra to the basic kit which was supplied by ‘A Model World’, tel: 01606 891999, website: www.amodelworld.co.uk Current kit price is approx. £184.50.

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May 2024

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