school me on jet drives

[PowerAddict]

I don't know alot about jet drives on larger boats say a 21 footer or so. Are they any good?

 

[The-Machinist]

Re: school me on jet drives

I have 2 friends that run big block jet boats, one a 426 hemi, and the other a 455 olds. they both swear by berkley pumps. They say they wouldnt run anything else, EVER. I do know that neither have ever had any problems with the berkleys.
Hope this helps.

 

[triumphrick]

Re: school me on jet drives

Obviously, all PWC use jet drives, but on larger boats my understanding is the drawback is loss of efficiency. There has been a big push in some Florida waters to restrict boats to jet drives due to the damage to the manatees. Lots of people raised holy he** and used all the formulas for how inefficient they were. Lots or maybe all of the outboards have jet drives as an option.
I used a Yamaha 16' with twin 135's and that thing would fly! It would be hard to convince me they were not a viable alternative.

 

[tashasdaddy]

Re: school me on jet drives

berkely are one of the best jet pumps. the problem with the majority jets, is you loose +-30% of the engines output in the jet pump. you have 100 hp on a propped motor, you get about 70hp out of the jet pump. so you are burning 30% more fuel, to get less power. jets have there place in the shallows, and rocky rivers, where i prop would not survive.

 

[1976 Slickcraft]

Re: school me on jet drives

Yeah, I agree, it takes alot of power to turn a jet.
That said, there is nothing cooler than a big long rooster tail!!
Jesse

 

[PowerAddict]

Re: school me on jet drives

aahhh.

So you're saying it takes a lot to turn the pump or there's a greater slip factor with a jet pump?

You don't have to trim with a jet pump do you?

 

[uscboy]

Re: school me on jet drives

Jet drives on my 21' Yamaha are great. Lots of pros, boat barely rises when
getting on plane, it planes in around 3 seconds, does 0-30 in a little under 7
seconds... the hole shot on these boats is amazing.

It does take a bit to get used to at low speeds. CobraJet steering fins help,
but it's still differnet. Technically more control than other boats, but since
it's so different it takes a while to get used to and until you do you can feel
like you have less control.

And these boats are near the top of their class in fuel efficiency in online
test and I have no complaints in real life experience either... pretty good for
having two engines.

Plus, these things are maintenance free just about. Oil changes, oil filters,
and occasionally a new set of plugs. No transmissions on them, power is
always in one directions, gates just drop in front of the jets to redirect the
flow and give you reverse.

Anyway, every boat has it's pros and cons, but after learning about these
Yamahas about 4 years ago I've wanted one since and finally bought a new
one this year... love having the jets, if for nothing else than the amazing
integrated swim platform with a lack of props back there.

 

[fdmsiv]

Re: school me on jet drives

Saying that Jets are less efficient than props does need some notes.

At low speed, jets are less efficient than props for smaller boats.

At higher speeds, jets are more efficient than a submerged prop.

The drive unit of the jet is more efficient than an open water prop. Due to the tip of the blades being close to the tunnel and some other factors you get an efficiency of 60 ~ 70%. A submerged prop maxes out around 60 % and that is on a good day. You lose quite a bit of power in the transfer from the engine to the jet drive. Quite of bit of this power is needed to lift the water for the drive, its only 6" or so, but it is a huge amount of water.

Submerged props begin to cavitate and lose efficiency at high rpms, jets thrive on the higher rpms.

In the end it comes down to personal preference, efficiency is relative to what you want to do with the boat.