So... How does this thing work again??

[H8Monday]

Alright guys.... I've got this impeller to torque or HP ratio thing... but where the hell do you get the almighty MPH increases??
All of the talk is always about the RPM. Where does the GPM (gallons per minute) figure into this thing?? or is it pump PSI??
explain how speed figures into this RPM - horsepower thing.
If I spin an 'A' at 5500 rpm, does that equate to more MPH than a 'B' at the same 5500?? If I cut my wheel clear down to a double 'D' http://free.***boat.net/ubb/wink.gif and crank out 7000 revs... will I finally go faster?? When do I get to go faster?
I completely understand the method of using the impeller cut, to taylor the pump to the engine performance numbers... But when do I get to see the almighty Mile-Per-Hour goddess??
How do I seek her out after getting my pump and engine into holy one-ness??
Everybody has figured out how to turn 5500 RPM...
I WANT TO -GO- FASTER!!! http://www.pirate4x4.com/ubb/smilies/bounce.gif http://www.pirate4x4.com/ubb/smilies/bounce2.gif

[LVjetboy]

Speed is primarily a function of hull efficiency, Hp and weight...weakly influenced by impeller rpm. Although impeller size should be matched to your torque curve for best results, the primary influences on top speed are hull design, how many cajones your engine has and whether or not you haul that 300 lb date you call a girlfriend http://free.***boat.net/ubb/smile.gif
jer

[LVjetboy]

Sorry H8Monday,
Just ran that last response thought the bs filter and realized I have no idea what your date weighs...so please forgive me.

[H8Monday]

Well, since I don't date fat chicks...then the speed thing must be more related to the pump.
So... I ask again. How do you get more speed?
Is it GPM related? ( more flow )or PSI related? (more pressure)
Or as my very PETITE girlfriend ponders....
http://www.pirate4x4.com/ubb/smilies/girly.gif "Perhaps its related to the number of fades and other graphic patterns of the gelcoat? or the Square root of the total area of the anodizing." http://www.pirate4x4.com/ubb/smilies/girly.gif
Whatever... I still need a good explaination of how to make this thing go faster...WITH my topless princess along for the ride. http://www.pirate4x4.com/ubb/uploads/bigglasses.gif

[Jungle Boy]

HB, If you spin a "A" impeller at 5500 rpm, you probably spin a "B" at about 5800 rpm. An "A" at 5500 rpm will be faster than a "B" at 5500 rpm. It just depends on where your motor has the most torque. I've seen guys pull their hotrod motors from cars and put them in a boat and be real dissapointed. Jets need torque numbers more than HP. An "A" will give you better out of the hole performance than a "B". I had a American Turbine chart that gave HP ratings of different impellers at certain RPM @ sea level. You may find it on their website. As far as speed goes weight and hull design is the huge factor. I don't know if this helps but that's my $0.02 in canadian dollars.

[LVjetboy]

GPM is directly related to pressure (PSI) and nozzle area, so not an either <or> situation. The higher your pump PSI (driven by pump rpm and efficiency), the more your GPM for a given nozzle area. And since F=ma=mv^2 and v=Q/A, your thrill factor is direcly related to the square of your nozzle flow rate (adjusted for nozzle area).
WHATEVER.
You can up your flow rate and velocity (thrill factor) by turning your impeller (whatever that size may be) at a higher rpm or by increasing pump efficiency. That means more hp and more $$$...build a stronger engine or a tighter pump (MPD), period. How YOUR HULL responds to added force will determine your final speed... another issue. Although matching impeller to engine is important, changing impeller size alone will net little gain in thrill, unless your engine/impeller combo are grossly mismatched (see Jungle Boy above).
So unfortunately, as your petite topless girlfriend ponders gelcoat fades and anodized square feet, your jet responds to more hp, hull and pump tuning. More cajones. No mystery there. What happens after-hours is entirely another thing... or maybe not? http://free.***boat.net/ubb/smile.gif
Of course I could be wrong about this whole thing, but I don't really care.
BTW, any pictures to share with us?
Cheers,
jer
[This message has been edited by LVjetboy (edited September 27, 2001).]
[This message has been edited by LVjetboy (edited September 27, 2001).]

[Unchained]

I agree with LV on the hull efficiency. Put a 4' straight edge lengthwise on the bottom of the hull and see if it lays flat. If not then fill the low spots. Also did you check your intake pressure ? As Greg S. once said intake pressure on a jet boat is what going fast is all about. If a jet boat is not making intake pressure then the pump is sucking the hull down into the water and you will never go fast. To build intake pressure you need a good pump loader and shoe. The principal of intake pressure is that whatever the pump can't use spills out the front of the pump intake and creates lift.
I'm waiting for those pics.

[jetboatjake]

I TOTALLY agree with LVjetboy.....Post some pics of your petite topless girlfriend. http://www.plauder-smilies.com/person/pirate.gif ARRRR ye matey

[spectras only]

I agree with most of the opininons above.The quest for more top end speed lies in the amount of thrust [1000 pound will move the boat faster than 650,5500 GPM will move the boat faster than 4500]coming from your nozzle. More HP [over 475],bigger impeller, tuned nozzle and a straight hull that rides like a bass boat http://free.***boat.net/ubb/biggrin.gif equals higher speeds.Also ejection seats for passengers will give you an edge when racing your friends http://free.***boat.net/ubb/biggrin.gif

[rivercrazy]

Exactly. Unless your turning too large or small of impeller relative to your engines rated HP and Torque, you will not see any appreciable increase in top end.
A friend of mine, in a quest for cheap increase in top speed cut his A impeller to an AB. His top end didn't change by more than 1mph. What he lost in the process was a bit of out of the hole accel and fuel economy. He now needs to turn another few hundred rpm to cruise at the same speed the A impeller did.
Bottom line is to make sure you pump is B&B'd, you have a machined intake with ride plate and shoe (performed by a top notch shop) and a significant increase in HP and torque. Thats why my next motor will be BIG. Like a 540. That way I can still run pump gas, and turn an A or AA impeller to 5500 rpm or higher.