Originally Posted by Adam Younger
Waterjet powered boats can be tricky to get right and often performance is not so great. The waterjet units in theirselves maybe reasonably (or better) efficient - but when you combine their effects on the hull the whole package can lead to disappointing figures..
Yep, water jets are actually very efficient units as tip losses around the blades are reduced to a minimum. Being that the impeller rotates in a tunnel. An open prop lloses out to an impeller as quite a bit if energy is spent "throwing" water out radially instead of axially.
The problem with jets comes when you try to supply the impeller with a clean flow of water. You are in effect having ti 'lift' the water up into the impeller and then propel it out the rear. As AY says, hull design is critical.
During my days offshore i used to regularly helm jet drive FRC ribs. These were mainly norwegian built aluminium Ulstein SeaBear's . Awesome boats, 8 meters long, 320hp inboard diesel jet drive but were only capably of 30 ish knots. Mind you they did weigh nearly 3 tonnes. :-).
You could drive one of those through s brick wall and not notice!
Only thing i used to hate about them was ( like a lot of jet ribs) steerage at low speed/engine rpm was dreadful. There's something to be said about having an outboard leg sticking out under the hull.
Very nice rib that Parker jet.... Seems like a very serious company indeed Parker.....In fact dare i say it on here but i have been eyeing up those Parker pilothouse fast fishers for some time now.
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