Sterndrive vs Jets

935HP for 30kts na thanks. Now I know why I havnt seen many. I notice it is also a semi-displacement --30kts ?? I didnt know you could get semi-dispalacemt at 30KTs at that size.

They did not say what consumption was or capaicty

However yea thanks it was a jet cat.

 

Rouge Boats quotes Hamilton Jet Drives as saying. Their best small jet is at 94% efficiency compared to a traditional prop. Hamilton is one of the best jet drive companies in bussiness.
Jets have gone from about 60 % to the about 95% since the 1960s. Usually, in any engineering field, the last 10 % is the toughest to accomplish.

The Austrialian ferry boats love Jet drives. So there is always more than 1 or 2 efficiency reasons, to be less efficient.

 

This was a HUGE problem in the '70's and '80's. Basically people started putting waterpumps designed for riverboats into sea-going vessels, and the results were worse than ordinary. Hulls that weren't designed for the waterjet, and asking a 7" diameter pump to move enough volume to propell larger and larger boats. Take that same 22' Hartley and 327 cid chev with a modified bottom and Hamilton jet HJ212 waterjet and you'll have quite a different story.

 

95% of what?-- a propellor?

So if a prop is 100 then the jet is 95%? as good as the prop.

 

That is how I read it. Allow for boat & prop selection variables. The results are close enough to allow the use of either AT WOT.

Off plane ??? Well you do have to rev up the jet to even move. So down there the gas bill MAY...MAY be higher. Old school logic of pumps with fixed angle vanes.

 

am i missing something

In the begining when dinosours roamed the land we had rivers in New Zealand that were not places for boats But then came along Mr Hamilton and up set the the apple cart and made his wee boat go over water that was rally shallow and lots a bolders and stones and branchs every where . I was lucky to have been born just after the last of the dinos went bush not to be seen again but mr Hamilton and mr Bill Dippy were a coupe of fellas that had something new and revolutionary . Shallow water and fast flowing rivers became the challenge to skip up and down seamingly effortless but with nibble skills and a quick eye . I was just a lad when i first went for a ride in Mr Dippies boat up the Whakatane river year 1954 . Had a flathead 24 stud ford Vee 8 ,all ply construction . all us kids jumped at the chance to skip over the shallow water at highspeed we used to play in .
The hunters and deer stokers saw a way to get up rivers and places they would take days to get to and shoot anything that moved , then load up and bring it back all before afternoon tea and home for dinner of fresh venison or wild pork and not have to camp over night in the cold and wet .
I do remember some where the Collorado and the grand canyon were not to miss out on all the excitement either Just to show off what jet boating was all about .

Over the years mr dippy was just one of nz pioneers that i knew and grew up knowing about , wasnt long before river racing and wasnt to years after Jet sprint reared its head and thats scary . Even as a spectator its scary .Watching a 1200 turbo'd horses unleashed to go mad as fast as possible in ankle deep water at dazzling breakneck mind blowing speeds .
Was a proving ground for a lot of things wee boats with huge horse and wasnt long before glass boat fell by the way side and aluminium is the only material to use . Hull shapes were constantly modified untill they hit on a few things that got the front of the boat to stick to the water like they should and the back was loose as

All the way through my story you should have got the drift of where jets came from and what there intended use was . If you want slow boats use a prop , if you want a fast boat use a jet , the bit in between you make up you own mind . Some even use both diesels for the props and huge gas turbines to power there jets , so they have the best of both worlds !!
Jets came from one part of boating and later got adapted to be used some where else as others jumped on the band wagon !!

Every system has advantages and disadvantages some good some not so good . Shallow water and go fast use a jet !
Persomally i am a outboard freak !!

 

I think that one of the things people forget about jets is it's power density. I gives the boat the ability to put insane amounts of horsepower through a relatively small package. Hence pleasure.
Some of the high power high speed yachts out there wouldn't even have enough beam at the transom to put surface piercing props with same output on.
It's a similar reason why the navy use jet/gas turbine booster sets on a prop driven frigate.

 

Yes most PWC jets have a very low efficiency. At lower speeds they are clearly too small and at higher speeds they choke. They are very much compromised for handling and price. Here is an example of huge performance gain (<90 mph -> 106 mph) by modifying a high performance PWC to a surface propeller: http://www.boatdesign.net/forums/bo...neffective-pwc-converted-propeller-21381.html

At lower speeds it is just about very basic propulsion physics. You need big area in order to keep the velocity behind the propulsor as low as possible, since all the velocity you create in the water is lost work.