dereknkathy

jets are good for one thing only. big rooster tails. ok two things. the prop is in a cage for pwc's to keep occupants from getting chopped into chum...although the prevailing attitude towards jet skis here most guys feel the drivers belong in a chum bucket.

Twin O/B Sonic

Bottom 1/2 of the prop is still in the water (which is all we want at speed anyhow) and two feet behind the boat.
Figure in the angle of attack where we want as little wetted bottom surface as possible (less drag) and the prop boat is still haulin ass when the jet is done.
Most inefficient drive system ever conceived.

articfriends

iTrader: (1)

Your tahiti sucks water in the middle at the deepest point, picture the jets being mounted part way up each side on a vee further from water, sucking air if it gets too high.
I have a customer who is a jet boat guy, for ten years he told me about some twin jet drive deep vee and that the company only made a few of them and how he wanted one. He finally found one and bought it, from what he says the boat is 24 ft and runs mid 50's with twin big blocks, sucks gas like no tommorrow. I wouldn't want one!

Michael1

This is old school. Performance jets today set the pump all the way to the transom, and even partially past the transom.

Most jet boats people are familiar with are not dialed in at all. They just stuck the pump in there and hoped for the best. Guys who are doing the work today are relocating the pump intake, changing the intake shape, putting on ride plates and adjusting the angle and length, changing shoe depth and angle, changing loader configuration, changing nozzles, and blue printing the pump. Some boats are seeing over 20 mph top speed increases just with tuning, without engine mods.

If your I/O boat is out of the water, you better get off the throttles, before you blow up the drive.

Michael

502ss

iTrader: (3)

Great post, thanks!!

Anyone that wants to read some facts about jet pump propulsion in all types of boats read some of these threads. Some of these guys have done it and the engineering behind it. Not just speculation.

http://www.boatdesign.net/forums/jet-drives/

Jet pump technologies are advancing everyday. As Michael said, with the right planning, the right pump, and the right location I think my speeds in my older scarab could be pretty reasonable compared to my TRS's

HTRDLNCN

only very light sub 25ft jetdrive boats with shallow V with tons of hp can break 100mph regardless how much tuning they have done.
Even those can not run over 100mph in REAL 3-4ft waves regardless of hp.
Of course with enough money ANYTHING can be made fast,
you can make an 80ft yacht run 100 with enough money,
,, doesnt mean its not a futile gesture.
Same jet boat with a simple 300hp outboard on the back would run just as fast.
On boats that NEVER see any air like big cruisers jets do work mostly because you can hook up insane horsepower to them, but we are talking about offshore speedboats not yachts or lakeboats..This is after all offshoreonly.com

HTRDLNCN

The entire boat has to be out of the water a good bit before the prop loses thrust,on a jet ANYTIME the hull leaves the water you lose thrust and by nature jets dont work well with a deep vee unless you flatten the rear of the vee to provide them water which again kills rough water thrust.. It may work in nice lake water but in real ocean running you would be literally sucking air

Michael1

Just can't be done...blah, blah, blah.

They raced these deep V jets OFFSHORE in Southern California in the 70's.

The Toad

Michael

Twin O/B Sonic

aren't they doing it now?

Why are all the drag records 50 MPH faster for the prop boats than the pump boats? In a flat bottom or a tunnel w/a pod maybe but a hull that free's up and fly's
I know guys that race jet O/B's on rivers that are 30% - 50% slower than the same rig w/a prop.
Have also been deeply involved in O/B drag racing for yrs and I can tell you that it takes a serious power adder for the pump boat to run w/the O/B. Have seen it done over and over again.
Cool rigs for a toy but for a real boat......
On I/O's coming out of the water you'd have to have the hull a foot in the air. On a pump boat all you need is one inch and your thrust is over while the I/O boat is still WFO. Which one wins the race where the hull is semi air borne the entire race?
Every few yrs someone comes out w/a new pump boat thats going to teach us all a lesson.
Ain't happened yet.

HTRDLNCN

They were brave,,
doesnt look like any kind of hull Id want to take offshore,
anyone can race anything they want,
doesnt mean its any good at that particular task...