Model Boat Mayhem - Forum

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: toy jet ski conversion and displacment test  (Read 1645 times)

Mad_Mike

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 2,597
  • Lincolnshire
  • Location: Lincolnshire
toy jet ski conversion and displacment test
« on: May 10, 2012, 06:17:28 PM »

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=36072.0

So 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. 
Logged

Mad_Mike

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 2,597
  • Lincolnshire
  • Location: Lincolnshire
Re: toy jet ski conversion and displacment test
« Reply #1 on: May 13, 2012, 11:23:28 AM »

I received the motor servo and esc the other day:



a 2030 300kv motor, a micro servo and a 10a hobby wing esc. The 800mah 2s lipo battery and receiver i nicked from my car conversion i did. Anyway ive soldered up the components and installed the motor in the boat, then i did a tank test.



Intially i used hot melt glue to fix the motor and it worked very well. I ran the pump and the thrust was more than adequate but the problem im having at the moment is excessive leaking. Ive fixed the motor with epoxy putty and while is stopped water leaking from under the motor its now started leaking through the motor and coming out the vent holes in the top. I cant use a prop tube in this case as the motor can will fowl the lid on the boat. Ive fixed a peice of vinyl to make seal on the bottom but its still leaks.
 


I think ive got a solution. Im going to completely block off all the holes on the motor and make it totally air tight. Because the pressure inside the pump chamber is forcing the water back up the shaft and filling the motor can, i think that an air pocket in the motor itself will resist water essentially acting as a postive pressure against it. In theory. If that fails then ill embrace the water filled motor seeing as its running flooded anyway and just plumb the water out the back as cooling. I dont know why it doesnt short out but it just doesnt, it keeps going no problem.
Logged

gwa84the2nd

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 134
  • Location: east riding of yourkshire hull
Re: toy jet ski conversion and displacment test
« Reply #2 on: May 13, 2012, 01:27:00 PM »

ye you can run brushless under water as teknicaly ther are no parts touching the windings
Logged

boatmadman

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 2,721
  • Location: South Cumbria
Re: toy jet ski conversion and displacment test
« Reply #3 on: May 13, 2012, 01:33:02 PM »

I can confirm that is the case, having flipped my brushless airboat a number of times and got it back by using the aero prop as a boat prop! Just turned it back over and off she went again - great stuff this brushless!!!!
Logged
if at first you dont succeed.....have a beer.....

Mad_Mike

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 2,597
  • Lincolnshire
  • Location: Lincolnshire
Re: toy jet ski conversion and displacment test
« Reply #4 on: May 18, 2012, 01:06:58 AM »

Ive not had much chance lately to write an update so here goes. I sealed the motor completely using epoxy. I epoxied scews in the top of the motor can and out one of the holes fixed a peice of 90 deg pipe:


quite frankly a mess {:-{. You can also see the rudder post i started working on.

i tried the motor again in the water with a peice of silicone pipe pushed on the 90 deg peice. I blocked the silicone pipe with a screw and ran the motor, no water leaked out of the motor or its surroundings no more, despite being a mess it worked. I then tried it with the screw out of the silicone pipe and water flowed though the motor no problem and trickled over theside. I decided to go with this and so plumbed it to an outlet in the transom. At this stage i was on a bit of a rampage and didnt take anymore photos. I fixed the micro servo and linked up to a steering nozzle. heres some pics of the inside anyway:


the receiver in a balloon at the front


the servo, motor and the cooling pipe running from the top of the can into the transom.

I made this video of it running in the domestic test tank:

http://www.youtube.com/watch?v=VmlK9_GE3_Q

The jet of water coming out took me by suprise and had quite a distance on it bearing in mind that with the original motor it shot out no further than 3 inches! But the way the boat sailed well its just **** {:-{  The little boat did float like i had planned but the raising of the bow during motion shifted the weight to the rear, this combined with the fact that the pump was sucking out the water that the hull was floating on made the boat drag its tail and dramaticly lose speed. I tried to add weight to the front to hopefully counteract this tipping back affect, didnt work. i stuck polystyrene under the stern, that didnt work.

Thoroughly disapointed and before i went any further wasting time and money doing the finishing touches i stripped the the hull of the rc gear and chucked it in the bin {:-{ If i was to have another go at the same boat id go with a conventional prop but for me it goes against the point of why i picked this as a project in the first place. %%
Logged
Pages: [1]   Go Up