Expert: Ray Wilkinson - 3/25/2006

Question
Ray

I want to be able to pull a 40ft cold air blimp behind a boat traveling at approximately 15 to 20 MPH. I would like to attach the front of the blimp to a line going up to a para sail. I need to design the blimp in a manner that keeps this air ship level and directly behind the boat in a variety of wind conditions. Can you help me solve this design problem or point me to an engineer that can work with me.

Answer
Bob

This is a tricky one, and not strictly within my area of expertise, as it's aerodynamics and control. I'll try, though. The best solution depends on how accurate you want to be in following the boat. I don't see a major problem with keeping the blimp horizontal - it's just a case of making sure the tether lines are in line with the centre of pressure (for starters, call it the centre of the blimp), although it may take a bit of fiddling around to fine-tune this. However, having the blimp stay directly behind the boat in different wind conditions is far from straightforward. If you put vertical tail fins on the blimp, it will tend to turn into wind at low forward speeds, and will not sit directly behind the boat - again, some playing around may get an acceptable result.

If you want it to follow the boat more accurately, you could put rudders at the back, which are turned as the tether moves out of line, either by a simple mechanism or a detection/correction system. This will tend to steer the blimp back towards the boat. The difficulty is getting the right amount of corrective movement in the rudders - too little and the effect will be too small, too much and the blimp will oscillate wildly about the correct line. Either way, this works by trying to correct out an error, so there will always be some offset, which may be quite large in adverse conditions (low forward speed with high crosswind speed).

If this isn't acceptable, the only way is to use some sort of active system, detecting the position of the blimp relative to the boat and generating steering commands for the blimp.

I hope this helps. I realise I haven't offered you a step-by-step solution, but it can get complicated as you should now see.

Regards

Ray