hi am new to subs and i would like to build a simple model that uses a pump to dive. i would prefer that it is not more that 60 cm(24 inches) long though. can someone tell me what type of motors, material,batteries taht i could uses. i do not have any specif design i just want it to work and to be less than 24 inches long
thanx

Well, I'm no sub expert but I do have one - and a whole pile of boats. Since you haven't gotten any other responses I'll try to help. Most if not all small subs use "dynamic" diving system, ie they have dive fins that are servo operated and dive under power. The sub is weighted & balanced to float at something around mid depth on the hull & level. On these subs forward motion is required to dive & maintain depth. If the motor quits the slowly float back to the surface. Working ballast tanks are (a) expensive and (b) require more room than is available in small subs. When using a working ballast tank system it is EXTREEMLY important to have a failsafe to blow the ballast in the event of radio failure or the sub will sink to the bottom & die there! You can make a simple, cheep, dynamic dive sub from PVC pipe with shaped nose & tail cone. Use brass plates for the forward & aft dive planes. Make a water-tight radio/motor box - you could use tupperware - another watertight compartment forward for the battery. The rest of the hull needs holes so it can fill with water - make sure there is nowher that air can be trapped or it won't flaot leve. lUse a 500 size motor. Make sure the boxes remain water tight! Remember that the water will be UNDER PRESSURE depending on the depth. Unless your diving in very clear water you won't be able to see it!

try this website : www.subcomitte.com for more info on model submarine dynamics.

Its actually pretty simple to get to work, though finding a pump that will work a vacuum first is a little difficult but not very expensive. I would suggest looking in your local Aquarium supplies store. Many of the newer dive systems use a simple impeller pump and a middle chamber, also if you are not worried about acutally seeing how it is working then check if domestic water pipes are small/big enough to fit.

Accesories to allow propeller and servo transmission to pass the end caps of your tube are easily (if not expensively) available.

There are plenty of sites describing how people have done this, spend 30 minutes having a search...you will learn a lot more than can be typed here in a few minutes!

HTH

Andrew.