ballast system for the robbe type 7

[cougar]

hi guys
I am going to be building a robbe type 7. and i want to put a ballast tank in it. basicly i am a big fan of pump boats. as my la never sank. once i got it working right. ran it now for a year. and never once lost it. so after looking at some websites. and talking to some poeple. I am not confused enough to asked for help.
Most people are using the engle 750 piston tank to bring the boat under. i talked to a friend and he said to use a 500 ml iv bag. and then another said to use a 2 litre ballast tank, i was thinking that using the subtech pump, i could build a 750ml or a 1 litre ballast tank. just run the air intake out the aft bulkhead and run it to a connector and then run some fuel tubing up into the sail. maybe right behind the aft scope?
so the question i have is has anybody tried this? and how big should my tank be
thanks in advance

[JWLaRue]

If you are looking to

If you are looking to be able to both run a fully submerged and fully surfaced (e.g. at the correct waterline), then the robbe U-47 really needs a one liter ballast system. A 750ml system comes close, but you end up with a somewhat lower waterline.

-Jeff

[bob_eissler]

I am going to convert

I am going to convert my Type 7 from RCABS to a water pump system like Subculture likes to suggest. As you have found out, the Robbe is not a good boat for RCABS since the WTC is so big & the deck needs to be so far out of the water. I am going to use an electric fuel pump model airplane guys use- it has a 2 way switch but I will change for one easier to switch with the servo I have been using for RCABS. Why do you need the outlet under the sail? as long as it is in the water I don't see it mattering. Size of the tank-depends on the weight of the material you want out of the water, 750 ml might be a little small.

[cougar]

bob
my la has a hose

bob
my la has a hose attached to the periscope. this meens i can drag air into the ballast tank before i become deskawashed. the pick-up line is at the bottom of my la. and it always in the water. (btw i use a model airplane fuel pump as the pump for the ballast tank. i just hook it up to a 3 way toggle switch to reverse the pump) what i am thinking with the 7. is to attach a pick-up line to the bottom of the aft section on the boat. then bring the line into the bottom of the aft bulkhead. then to the pump , from the pump to the tank. the top of the tank would have a hose leaving and going to the aft bulkhead and out. to a tube. from the tube. i would run some fuel line along the top of the wtc to the sail. and up to the floor of the wintergarden. this would be just to pick up the air. when i need to seperate the boat. all i would need to do would be to unplug the lines.
but thanks for the input on the size. 1 litre it is. now can anybody tell me how fast the subtech pump will fill that 1 liter? using 12 volts. the hobbico fuel pump will do it in 30 sec.