Does a jet drive need all that hp??

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ChetsJug

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« on: October 27, 2017, 02:46:06 PM »
my baby brother started with the boat in the family. 18' & Berkeley behind a  454. He had a guy build the motor to 400+ HP and wanted more when he had the money.

I'm a little lost. I could see more HP as you put on bigger and bigger props. I cant see that much difference in impellers. Once you reach 3,000 rpm's the pump is going to pump the same CFM isn't it? Why couldn't a 4 cyl Pinto motor do the same thing and save gobbs of gas?

Im thinking a Rotary Wankel or electric motor... No, I'm not using a Pinto :p

Seriously, what HP can you drop down to before you loose top end. I'm not looking to go 70. Skiing speed would be fine maybe in the 40-45 range.



FordLover

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« Reply #1 on: October 27, 2017, 07:24:51 PM »
You are so freaking cute!!! Like I giggled reading it! Hold on.....I need to read it again. But typically torque is an awesome thing, from a V8 and horse power is another good friend of pushing your float around. More awesome people will.chime in. 👍

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76Challenger

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« Reply #2 on: October 27, 2017, 08:11:50 PM »
You got to be kidding...he can't be serious? This is no pinto...just use Google you will find plenty of examples of how you could not be more wrong...




1977 Challenger

GT Jets

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« Reply #3 on: October 27, 2017, 10:05:02 PM »
my baby brother started with the boat in the family. 18' & Berkeley behind a  454. He had a guy build the motor to 400+ HP and wanted more when he had the money.

I'm a little lost. I could see more HP as you put on bigger and bigger props. I cant see that much difference in impellers. Once you reach 3,000 rpm's the pump is going to pump the same CFM isn't it? Why couldn't a 4 cyl Pinto motor do the same thing and save gobbs of gas?

Im thinking a Rotary Wankel or electric motor... No, I'm not using a Pinto :p

Seriously, what HP can you drop down to before you loose top end. I'm not looking to go 70. Skiing speed would be fine maybe in the 40-45 range.

There are three big time problems..


Problem one. The torque curve on a jet pump is completely and 100% linear. This basically means the faster the engine spins, the more torque is needed for acceleration. This means that smaller displacement engines will need to spin a bazillion RPM against a gear box to get the torque back....


Problem two. The hull is able to move only because of jet thrust and the speed is determined by the thrust velocity, hull drag and ultimately volume. The more volume and velocity, the more flow, pick any one and it requires more power, otherwise when one goes up, the others go down....


Problem Three. With the torque curve being linear, the engine needs almost no horsepower to turn 3000 RPM, that said, the hp and torque to get the impeller turning to 6,000 (or double) is about 10 times greater. So using basic math, that Pinto or Chevy II won't hang.....




Hope this helps...


GT
« Last Edit: October 28, 2017, 09:50:46 AM by GT Jets »
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

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« Reply #4 on: October 27, 2017, 10:26:21 PM »
Damn it Glenn! That is what I said! You just made it sound way smarter and cooler. Lol

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aquaholic72

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« Reply #5 on: October 28, 2017, 02:14:04 AM »
Or in lamens terms that we all understand.  Gobs of torque gets you moving..  Horsepower keeps you moving.. 

your friendly local v-drive owner

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« Reply #6 on: October 28, 2017, 09:52:15 AM »
Or in lamens terms that we all understand.  Gobs of torque gets you moving..  Horsepower keeps you moving.. 

your friendly local v-drive owner


But........ In exactly the opposite way that theory works in a car. In a car, the torque comes in first. On a jet, it comes in last.

GT
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

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« Reply #7 on: October 28, 2017, 06:28:56 PM »
Time to face reality.  Baby bro's been bitten by the bug.  It won't be long before he's thinking, "Man, I spray more horsepower now than my first engine made!"

So, in a word, YES, he does need every bit of that horsepower.  It's one of those things where you 'get it' or you never will.

Jet boats aren't for everyone.  They don't produce a nice wake for skiing or wakeboarding.  If you want to cruise around the lake in economy, then maybe a whacker is better suited to your needs.   To each their own.  I have had a lot of fun in every type of boat.

Cheers,

Joe
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

ChetsJug

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« Reply #8 on: October 29, 2017, 05:08:51 PM »
... The torque curve on a jet pump is completely and 100% linear. This basically means the faster the engine spins, the more torque is needed for acceleration. This means that smaller displacement engines will need to spin a bazillion RPM against a gear box to get the torque back....

GT
[p]
Thank you GT for being the only one that read my entire post without skipping to the soapbox biblethumping. I am well read in torque charts and I can see your points. I am a diesel mechanic and 35 year truck driver. I have always thirsted for knowledge of how to operate my beast. I'm not stupid nor am I here to be cute.

If others had read my post thru, I said a Pinto motor for an example and clearly stated I would never do that (you'd think the Ford Lover would have had a little sympathy; HA!  ;) ). Someone said I would see "How wrong I was" Well I never made a point to be wrong about, I stated first thing I was lost and asked a question. I also said I am considering (I hate having to repeat myself  :banghead:) a Wankel 3 rotor which has proven themselves in jet boats, or an electric motor set up. That being said...

I have found some more information and enlightened my self on the high performance impellers that need more torque. Thus you chart did tell me what I need for the speed I want to cruise at. I'm also a half-assed pilot and suddenly related Aviation to this area of thought. I remember that a boat is like an airplane in that you don't stomp on it to get up to speed and then throttle back and coast. You have to stay on it a cursing speed to maintain altitude, or in this case, trim & velocity.

I totally get needing to go up to higher RPM's and the power it takes. Thank you for your guidence  8)

For the rest of you, rest assured I will never ask a question around here again. I'll walk my dog through a Sharia Law neighborhood next time I want to listen to a bunch of insults.

76Challenger

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« Reply #9 on: October 29, 2017, 06:56:43 PM »
Your post read and came off as being a smart ass not someone asking legit questions...sorry if you mis-wrote your post...

I am also pretty sure you edited your original post to be more reader friendly as I do not beileve the way it is worded now is the same as it originally was..just saying... If not excuse my mis- read of it.

1977 Challenger
« Last Edit: October 29, 2017, 07:11:40 PM by 76Challenger »

aquaholic72

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« Reply #10 on: October 29, 2017, 08:48:17 PM »
Damn!  I felt this thread was packed with good info,  Not insults!! 

your friendly local v-drive owner

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« Reply #11 on: October 29, 2017, 11:01:20 PM »
Your post read and came off as being a smart ass not someone asking legit questions...sorry if you mis-wrote your post...

I am also pretty sure you edited your original post to be more reader friendly as I do not beileve the way it is worded now is the same as it originally was..just saying... If not excuse my mis- read of it.

1977 Challenger

I'm not so sure it was edited. Maybe.



On to the Wankel, you will notice that most of the Wankel engine jets you see use a multi stage Scott pump, not a Berkeley style. And 90% of the time are turbocharged. Those set ups typically touch 85-90 MPH.

I had a customer years ago with a cool old Glaspar with a Berkeley Jet and a Chevy Straight 6 292 with side draft Weber carburetors, the boat cruised right along at about 35 mph and could pull skiers and sounded crazy... But fuel miser it was not... My small block powered Bahner did just under 60 mph and we could run it pretty much all day on a tank of gas, but we were tuning and skiing...... NOt going 60, so I guess there is value in your thinking. If you want to stay low power, go multi stage pump, but the pump will cost about what more horsepower costs... Those Hamilton pumps are built from unobtanium and your kids college funds.... LOL


GT
  • Boat #1: 1992 Carrera 20.5 Elite (I/O bitches)
  • Boat #2: 19' Bubble deck Jet BBC Berkeley
If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

76Challenger

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« Reply #12 on: October 30, 2017, 11:00:34 AM »
I agree I am not sure either...like I said if I mis-read it please excuse my error

1977 Challenger


96Laveycraft21Xt

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« Reply #13 on: October 30, 2017, 03:09:00 PM »
my baby brother started with the boat in the family. 18' & Berkeley behind a  454. He had a guy build the motor to 400+ HP and wanted more when he had the money.

I'm a little lost. I could see more HP as you put on bigger and bigger props. I cant see that much difference in impellers. Once you reach 3,000 rpm's the pump is going to pump the same CFM isn't it? Why couldn't a 4 cyl Pinto motor do the same thing and save gobbs of gas?

Im thinking a Rotary Wankel or electric motor... No, I'm not using a Pinto :p

Seriously, what HP can you drop down to before you loose top end. I'm not looking to go 70. Skiing speed would be fine maybe in the 40-45 range.


You are mostly correct in that the SAME pump with the SAME impeller with require the same HP to turn a given rpm.

That 400 hp 454 will turn an "A" impeller to roughly 5000 rpm and that boat will likely cruise somewhere around the 60 -65 MPH range. 


Example: 

Our 18 foot Tahiti has a 455 olds about 430 hp and turns 5100 rpm, boat will max out at 69 on GPS

friends 16 commander jet with 350 chevy and 260 hp will turn 4400 ish rpm and will max out at about 45 mph



HOWEVER, both boats are pretty even cruising down river at 3800 rpm, and i bet the 350 will do better on fuel.  Its all about how fast you want to go.....

 


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