Hello Will. I too have been searching for different types of ballast pumps for my 32nd Parallel type 7 U-boat , with little luck ! It seems for this boat that the SHURFLOW pump is the best, as long as you put a very good filter inline with the water intake side of the pump! In 4 years I have gone through 3 sets ( total 6 pumps ! ) due to sand getting into the pump and chewing the insides to bits. This pump does not do well with dirty water over time, the lake I sail in has alot of sand and other sediment that gets drawn into the ballast system. Like i said , if you use a good filtering system you should have no problems with this pump.
The pump 32nd P. sold in recent years was SHURFLOW model # 100-000-20 and had a pumping rate of 1.0 gallons per minute. If you can and this is what I now have in my boat is an older model of the SHURFLOW that 32nd P. sold many years ago , model # 120 type 1 , this has a great pump rate of 1.5 GPM ! , but is larger and weighs one pound ! If you can find this pump ,get it !! Go to SHURFLOW\'S web page and look for their pumps , but you will only find the 1.0 GPM model for sale nowadays
I also tried SUBTECH\'s ballast pump and tee setup . SUBTECH has some great stuff and I use all of Skips electronics in my boat . But after alot of experimenting with this setup I removed it and went back to the SHURFLOW\'S , why ? The pumps are great , no problems with them ,but they were made for a boat half the size of our type 7\'s and had a LOOOOONG dive time of over a minute and a half !!!!!! No good, our ballast tanks are Too big for this small pump. I think it is rated at best for one half gallon per minute only.
You could use a pump made in Germany found in most hobby stores and used for R/C airplanes, it is used for fueling the planes. It is a good pump ( 12 V ) and has a pump rate of 1 GPM , if you cannot get a SHURFLOW this is the next best thing .
You may also try car windshield washer motors also , but they work best at 12V , if you run on 6 V it will work much less and it has a very small outlet port. You want a pump with the biggest ports possible to move the water and not restrict it.
If you like you can email me for further details and other info on this model as I have two of them, one is being converted right now into an 8 foot long type 7 D minelayer. If you want to try SUBTECH\'S pump I can tell you how I set this up in my boat as some modification was neccessary .