Z-Drive for a Tug

Hi everyone,
I'm designing an ASD Harbour Tug and got stuck with the propeller thing...
I can't decide over the size and other dimensions of the z-drive.
Is there any way I can calculate the details (like propeller dia, type, nozzle size, shape etc)? does it go the conventional way? Is there any source available for the relevant formulae (like from the rule-books:GL or others)?
or will I go for what the different companies have on offer according to the power available and the thrust requirements?

Regards
Hasan

 

hi,

you're going to want to optimize your propeller/nozzle system for maximum thrust at your required design speed. The more thrust required, the larger the propeller diameter is going to be, also remembering that the larger your thrust requirement is, the more HP you'll need. Normally for harbor tugs, your design speed is going to be around 4 knots. I hope you have designed the hull to swing a large propeller, but this will depend on thrust requirements at your design speed and don't concern yourself about propeller efficiency. It's a tug and with tugs we don't care, we are only interested in maximizing the developed thrust from the propeller/nozzle system. once you have optimized the propeller/nozzle system, the forces developed by the prop/nozzle will be used to size the Z-Drive. nozzles will develop about 40% more thrust at bollard pull when compared to an open wheel. The nozzle will be attached to the Z-Drive via 3 stators, so to avoid vibrational problems, stick with a 4 bladed kaplan style blade. The stators will also act as a pre-swirl device, so knowing this, skew your blade at some 20 deg and you will recover about 4 % of lost energy!

as for formulas to size the nozzle systems, formulas do exist, but are very hard to come by.

hope this helps,
Arthur