For those of you who are unfamiliar with my latest build thread i have been messing about with the use of centrifugal pumps as means of propulsion, you can see it here if you wish:

http://www.modelboatmayhem.co.uk/forum/index.php?topic=36072.0So while I'm waiting for a new motor to come from far away lands for that I'm going to do a little side project along similar lines:

This toy jet ski i found on *bay i thought would probably make a nice conversion project. Its small by model boat standards at 8 inches long.

Its powered by a small toy motor you know those flat sided ones from a 1.5v c battery driving a 3 bladed 19mm impeller:

With only 1.5v driving it the performance was feeble and so promptly removed the gubbins with the hope of fitting a full brushless/lipo conversing inside. But before i go nuts first i had to find out what weight parameters i have to work in, so heres how i did it.

To work out how much weight i needed before the boat sank i needed to do a displacement test. This test will work on large models too as long as the insides are water proof

First up was to block off with bluetack the holes for the motor and rudder shafts

I then floated the bottom half of the hull in the kitchen sink, the water must be deep enough for the boat to fully submerge

next you take a jug of water (or bucket) fill it up so that there is more water in the jug than what will fill the hull and weigh it.

While swmbo wasnt watching i used her kitchen scales. The smallest scale of the dial is 25g, the jug of water in this case weighs 925g

you then fill the boat slowly till the boat sinks:

take the jug and whats left of the water in it and weigh it again:

the jug now weighs 600g. To find out how much water went into the boat to sink it you take the first weight (925g) and subtract the second weight (600g). In this case it takes 325 grams to sink the hull.

I need to add the top and rider to the hull so ill have to weigh them:

Those come to 125g. Adding them to hull means i now only have 200g of rc stuff i can add before the hull sinks.

Because i dont want the hull to sink i need to find the desired waterline. I refilled the jug till it weighed 925g again then filled the hull with the water till i got to the desired waterline:

looks about right, after this pic i added the rider on as well, which made it go a little lower.

i reweighed what was left in the jug:

800gs are left.

To get the hull to sit at the desired water line i took the original weight again of 925g and subtracted the second weight 800g. total 125g of weight needed to get to desired waterline, and 200g to completely sink the boat. The lipo i have got this boat is 50g thats only 75g left to keep to the waterline, i then need to add a servo, receiver, motor and esc. The tolerances are going to be very tight. At least i have 75g of head room before the boat completely sinks, but i want to try and avoid being over.