GTM Hydro Construction Guide

The following measurements represent the latest set up criteria:

Rudder from transom (rearward) 35mm (this should not be critical left or right side your choice), Rear end of strut from transom 25mm, Motor from inside transom 80mm, cells as far forwards as they will go!!

Before you start - read the instructions - any problems call or email us!! All measurements are from the rear looking forwards unless otherwise stated.

The hull is supplied as 2 moulds, hull and deck. Looking from the rear the RX goes to the left and the servo (micro gear ) to the right. The cells will be in three parallel sticks of two with one on top to the left (from the rear) Actual location depends on the cell that you use. 

1. Trim the hull sections round the inside of the stepped lip. 

2. Decide where you want to mount your turn fin (inside or on the rear of the right sponson), make and fit a light-ply doubler for this area. Rough up with 240 grit wet or dry, and epoxy in place.

3. The functional hydro need a stiffener across the sponsons to stop them bending. I made mine from 2 layers of 2mm light-ply ½ an inch wide and the length was the same as the width of the sponsons. Shape it so that it fits as far forward as possible. Dry fit it making adjustments until the top and bottom hull meet without pressure. Block the base up on a flat surface so that the bottom of the sponsons are in line and flat to the water level. I glued my stiffener with epoxy to the centre and the sponson sides but you could glue them to a pre-fitted plate on the sides. Watch the weight though as on these small models weight is critical to success.

4. Some people prefer to mount the water pickup in the left sponson going from the bottom and coming out the back. If this is your preference then fit it now - if not wait for the rear mounted version below. Plate the bottom of the sponson with styrene first… and fit a doubler to the rear - then drill a hole diagonally through the rear and bottom of the sponson so that they are both about 1/4" from the rear edge. Fit the aluminium tube so that it is 1/16" proud if the sponson bottom and ½" out the rear. The angle needs to be 30-45 degrees to the horizontal. Glue it in place and then trim the bottom flush. Now file a lead in on the front edge. 

5. Clip the body halves together and dribble liquid poly in to the join from the inside. On the sponson sections you will have to do this carefully from the outside.

6. When set cut the 'lid' out along the recessed lines - you might like to extend this forwards further to get better access. Fill the hull with water and make sure it does not leak. Fix any leaks now as they will be near impossible to do later.

7. As with the right sponson double the bottom of the transom with 2mm light-ply to support the running hardware.

8. Draw a line down the centre of the transom, and along the rear bottom of the tub. Decide which side the rudder will go (mines on the left but Ernie's is on the right!). Line up your hardware so that the prop runs on the centre line and mount the bracket etc as appropriate and on the opposite side to the rudder!!. Ideally the prop needs to trail the tub by 25-30mm. 

9. The motor needs to be as far forward as possible, but remember you need to get the cells out. With the lid cut on the lines provided that means that the boss of the motor will be approximately 75mm forward of the transom. Fit the motor cooling coil and make a mount out of light-ply, screw the motor to it using spacers to provide an air path to the cooling holes in the motor. The mount will lean rearwards so the motor is at an angle. The angle needs to be sufficient to make the route of the flexi as smooth as possible. I.e. 15 to 20 degrees.
10. With the motor mount made you need to fit the prop hardware and cut the necessary hole. Mine starts at about 27mm and runs to about 40mm from the transom - but I have a brass tube over my Teflon… it could be smaller with just Teflon. You need a gentle curve from the motor to the prop - which is angled slightly down (2-3 degrees) to get some lift out of the x431 platic prop. A cut down 1735 or 2135 might work just as well but have not been tried. 

11. Make and fit the servo mount from light-ply so that the servo fits on the same side as your rudder, use a small rubber bellows (e.g. the Graupner one) to seal the linkage. To fit line everything up roughly and drill a small hole (1/8th inch) then open out the hole so that the bellows fits and so that the linkage is centred on it. Get everything in place and glue the servo mount permanently in position with epoxy.

12. The receiver goes on the opposite side as far back as possible and is held in place with velcro. A full size one will just fit but it's tight… and there is a weight penalty!!

13. I built a water pick up from a 30mm length of bent aluminium tube (I made it from a 3 inch length , it has about a 70 degree bend in it and the end is ct at an angle to aid water pickup. Some people prefer to mount the water pickup in the left sponson going from the bottom and coming out the back. You need to decide how to route and and small (1mm or 40 thou) brass rod. The rod is formed round the pipe two turns then removed and tightened up slightly so that it is a screw fit. The other end of the rod is secured to one of the mounting bolts. The water is routed using silicon tube through the upper rear of the hull and exits the same way. There is a small bit of aluminium tube in the outlet to stop the tube being pulled inside the hull.

14. Fit small strips of styrene round the opening so that the lid removed above sits on them

15. Fit your preferred aerial and mount.

16. Plate the sponsons if you have not done it already! Same size as the bottom currently in 1mm styrene but make sure the edges are shape!

17. fit the turn fin or its mount with small self tap screws - remove them and dribble superglue in the hole - when it's dry screw the screws in again.

18. Remove all the hardware and paint it…. 240 grit wet and dry first with a touch of washing up liquid and air dry or pat dry - don't rub it as static will build up - paint to scheme and re-install all hardware. A touch of silicon sealant around the screw entry holes berore tightening will ensure that they are water tight. 

19. Have fun!!

If you have any suggestion to improve the set up or these instructions please let us know!