How many twin jet drive Vee's have you seen??
#31
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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.
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.
I don't see I/O's at the drags either. I see V-drives.
In terms of losing thrust when coming out of the water, worst to best:
1) Jet
2) Surface drive
3) High X-dimension I/O
4) Standard X-dimension I/O
I think most people would say the surface drive would make for the fastest offshore boat drive, yet it is one of the worst for losing thrust when the boat comes out of the water. Second fastest would be the high X-diminsion I/O, yet it also loses thrust easier than a standard X-dimension I/O. Clearly there are other factors involved.
I noticed that there is a huge bias on this forum towards drive systems that are black, have an "M" on them, and hang off the back. If it doesn't meet those three criteria, it is shouted down as "cr@p". I guess it's human nature to be suspicious of something you are not familiar with. It is nice to see a few people think a outside the box once in a while, though, instead of just following the herd.
Michael
Last edited by Michael1; 06-01-2012 at 12:08 PM.
#33
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#34
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Cant believe I'm going to type this but....
On the Hi Perf O/B side, 99% of the market owns Merc, (Wait a minute. For the first time in my life I AM a 1%er!) but there is a reason. You know who else has built a Hi Perf O/B for the last 30 yrs?? No one.
Exact same scenario for the I/O's. I sold boats back in the late '80's when OMC attempted to w/their "King Cobra's". But typical OMC crap. No home work, only stickers and they died a gruesome, premature death. Then in the early/mid '90's Volvo stepped to the plate w/the exact same result as OMC and for the same reason.
In the mean time Merc continued building and evolving their drives. Hi Perf and actual race versions. No one else is doing or has done that. Period.
As to a Hi Perf hull, an O/B is impossible to beat for efficiency up to a certain size/weight and then the I/O's run away and hide.
Yes there is a ton of parasitic loss getting from the end of a crank to the prop of an I/O but again compared to what??
Here's one of the biggest limiting factors to everything else also. Trim.
A V-drive can handle more power but w/a best case prop angle of negative 7* (I think. Maybe 5?) that's like taking your Hi Perf O/B or I/O and burying the trim and putting your foot on the floor! That's 20 - 30 MPH in my junk. Screw that!
A jet w/trim helps but when you dial in up trim in one of those you're now taking 90% of your thrust and throwing it in the air where it does zero. (if your thrust is not in the water, pushing backwards you are wasting it) Watch a true race boat at speed. Their rooster tails are long and low. That's why. The exception being drag boats but that's because they are accelerating and intitialy the prop has huge slip #'s. Watch at the end and it will lay down.
Once you really understand going fast you realize that a tall rooster tail is a complete waste. Look all the pix's of the jets w/mile high rooster tails. Now look at how much of the hull is dragging in the water. Most of those guys are going 50 or less.
Don't get me wrong, if a guy wants a jet, have at it. Want to improve a design...., better yet.
But going fast in a small to mid size boat w/massive power you only have two options. I/O and surface drive.
Lastly, as to the surface drive unloading in chop you need to also understand that even an I/O is running in a surfacing condition so half their prop is out of the water at speed as well. That is a large part of why they and O/B's run so well.
Last edited by Twin O/B Sonic; 06-01-2012 at 04:28 PM.
#35
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A jet w/trim helps but when you dial in up trim in one of those you're now taking 90% of your thrust and throwing it in the air where it does zero. (if your thrust is not in the water, pushing backwards you are wasting it) Watch a true race boat at speed. Their rooster tails are long and low. That's why.
Jets have a reputation as being inefficient, and under some conditions they deserve this reputation, but not at the speeds most people think of. Check out this graph on Propeller and Jet Efficiency vs. Speed
It turns out jets really lose efficiency at low speeds, not high speeds. Reviewing tests of comparable Jet and I/O boats on boattest.com confirms this finding. Jet boats are not what you want if you are the type to putt around harbors to see and be seen as some people do. Also jet boats are not the best boats if you think you are frequently getting into sudden, poor water conditions where you have to go long distances very slowly. You might run out of gas before you get back! Interestingly, surface drives are not particularly efficient at low speeds either.
Michael
Last edited by Michael1; 06-01-2012 at 10:45 PM.
#36
RIPSTA Racing Team
VIP Member
56', 4- 850hp detroits with 4 Hamilton jets is what I drive at work. It's no speed boat but it ain't no lake boat neither. Top speed 40 mph which isn't bad for a 40 ton+ vessel. Shallow V bottom and has the water intakes for the jets set back at the transom just ahead of each jet. This is an ocean capeable vessel and I've had zero issues with losing thrust in rough water. My point is that although I wouldn't recommend a multipul jet drive for a power boat of size, I am sure that if there was a market for it they would work the bugs out. I can't describe how well this boat handles and responds to controls and manuevers. I wish my cat handled half as fast and accurate. besides whos power boat has thrust while catching some air? Are those props moving air and pushing forward while airborn? The jets work in the same manner. While airborn throttle back and on re-entry drop the hammer. So whats the problem? How many drives have been popped on re-entry? Wouldn't have that issue with a jet I don't think?
#37
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Thread Starter
Good points.
I don't know what your definition of massive power is (>700hp I would presume), but you better have a huge wallet, and a boat big enough that can support a massive I/O or Surface drive. Jet drives are very inexpensive in comparison, even 1/10th the cost, and, inexpensive to fix (rarely needed). In fact, the prop alone would likely cost more than an entire jet pump. They are small and lightweight, too.
Michael
Michael
Very true on the big power I/O's too. Every time I think about going to I/O's from my O/B's I do just enough research to scare myself silly
firepilot, 100% agreed but way different environment. When I was still boating on Lake Erie most of the island ferries were jet drive. Don't enough about them to answer why but I'd guess manueverbility and lack of maintenance.
Point I'm trying to make in regards to jets in a twin engine, Vee application, in rough water is how do you keep the intakes in the water at their angle of attack?
In the picture bellow both intakes would be exposed meaning cavitation and zero thrust. In that set up I can run WFO through enough chop that the hull would be clearing water but props would still be submerged and same would be true for I/O's.
Or try turning the boat at speed.
#38
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Thread Starter
Here's the one I saw. Ends up being an Exaclibur which is kind of interesting.
The owner just listed it on Craigslist. Probably not worth anything but clearly a very rare bird.
http://treasure.craigslist.org/boa/3168820477.html
The owner just listed it on Craigslist. Probably not worth anything but clearly a very rare bird.
http://treasure.craigslist.org/boa/3168820477.html