Reply With QuoteGenerally speaking most boats seem to run fastest with the diverter shooting about a 3-4 rooster tail. I would buy a hand held gps and make back to back to back passes in the same direction testing different height settings with the diverter.
I have a 18ft. Sterling jet. 468 bb and jg pump. Cover says A impeller. Has jetovator unit on it. Originally had 650 quik fuel vac. secondary carb and little 3" air arrestor. So far I put 750 mech secondary holley and a 10" arrestor. Jets a little lean on secondarys so I'm going richer next. First pass boat went 60 on gps, second pass seemed to be faster with the nozzle up starting to shoot rooster tail, but did not have gps on. Want to get to 75 mph
What changes can I make to the boat to pik up speed. My first jet boat, not really fimiliar with the different set ups and parts available.
Generally speaking most boats seem to run fastest with the diverter shooting about a 3-4 rooster tail. I would buy a hand held gps and make back to back to back passes in the same direction testing different height settings with the diverter.
You may want to add a shoe/rideplate if you do not already have one. If you do, try adusting the ride plate to about 3 degrees up from the keel. Also a droop snoot is a popular add on with your pump. I would concentate on the pump before spending too much money on horsepower. All the horsepower in the world will not make a jetboat fast without a properly setup pump.
I was looking at the carb and the pedal to see if it was open all the way and it wasnt it was only 1/2 way so I need a differnt style .I found one that will work on my set up so it sould be here next week .I am then going to make another run next weekend on gps .Then will see what I need from there.
Had the boat out this weekend. Water smooth as glass. Boat topped out at 61 mph on gps at 4800 rpm. On way back, there was a light chopp and the boat ran 65. I would like to go 75, would the ride plate ,shoe or drop snout help with the speed? Is there an intake problem? Is that why the boat is faster with a light chop? The boat has a flat pad about 10" wide all the way to the back, my friends boat has a semi round pad. Is there any advantage or disadvantage?
Had the boat out this weekend. Water smooth as glass. Boat topped out at 61 mph on gps at 4800 rpm. On way back, there was a light chopp and the boat ran 65. I would like to go 75, would the ride plate ,shoe or drop snout help with the speed? Is there an intake problem? Is that why the boat is faster with a light chop? The boat has a flat pad about 10" wide all the way to the back, my friends boat has a semi round pad. Is there any advantage or disadvantage?
The light chop helps lift the boat out of the water more, reducing drag, therefore gaining speed.
Without seeing your boat it's hard to tell but honestly I'm thinkin' you need more horsepower/RPM to start. The only way to run 75 at the RPM you're spinning you'd need a much bigger impeller and big horsepower to spin it to your current RPM. My boat is an 18' copy of a Tahiti, and with only a loader, no shoe, ride plate or droop, my boat runs 76mph...but I'm having to spin an AB impeller at 5400RPM in order to do that.
How long has it been since the pump has been rebuilt? What size impeller? Shouldered wear ring? Loader?
GSP's my ride at 74mph last weekend , running a AA stainless at 4800 RPM in light chop. Place diverter up at about a 150' rooster. She's alot slower with the place diverter even with the water. The side trim tabs help imensely though to keep the boat stable.
GSP's my ride at 74mph last weekend , running a AA at 4800 RPM in light chop. Place diverter up at about a 150' rooster. She's alot slower with the place diverter even with the water. The side trim tabs help imensely though to keep the boat stable.
Exactly. Bigger impeller and enough power to spin that bigger impeller to the same RPM he's turning now.
Exactly. Bigger impeller and enough power to spin that bigger impeller to the same RPM he's turning now.
A bigger impeller doesn't always make your boat faster. Its more of a matching game, matching the engine's output with the RPM's the impeller allows the engine to spin.
So, if your engine makes its power somewhere around 5500 RPM it wouldn't make much sense to stick an impeller in it that would stop the engine at 4600 RPM. Your not getting the gusto out of the engine that way and leaving out some untapped HP.
There also comes a point of diminishing return with hull design and how much fluid you can flow through the pump. Unlike a prop which is more like a screw going through the water, (i.e, the faster you spin it generally the faster you go) pumps can only flow so much. The rest is in the hull design, hull condition, weight, etc.
Some hulls were not built with much lift in them and no amount of impeller or engine would make those things cook in the top end.
How dry is your boat riding? How does your rooster tail read? You can read the roost and tell whether or not the pump is loading correctly. If it isn't loading correctly then a loader may benefit you. Where are your tanks located. If you have a bow tank thats full chances are your running pretty wet. Take WOT passes at a near empty tank.
My guess is your lack of speed is in the hull.
~Brian
A bigger impeller doesn't always make your boat faster. Its more of a matching game, matching the engine's output with the RPM's the impeller allows the engine to spin.
So, if your engine makes its power somewhere around 5500 RPM it wouldn't make much sense to stick an impeller in it that would stop the engine at 4600 RPM. Your not getting the gusto out of the engine that way and leaving out some untapped HP.
There also comes a point of diminishing return with hull design and how much fluid you can flow through the pump. Unlike a prop which is more like a screw going through the water, (i.e, the faster you spin it generally the faster you go) pumps can only flow so much. The rest is in the hull design, hull condition, weight, etc.
Some hulls were not built with much lift in them and no amount of impeller or engine would make those things cook in the top end.
How dry is your boat riding? How does your rooster tail read? You can read the roost and tell whether or not the pump is loading correctly. If it isn't loading correctly then a loader may benefit you. Where are your tanks located. If you have a bow tank thats full chances are your running pretty wet. Take WOT passes at a near empty tank.
My guess is your lack of speed is in the hull.
~Brian
You're correct. A matching game is exactly what it is.
However, take for example a motor that spins a B impeller at 5500RPM. Install that combo in a boat. GPS it.
Now, take a motor that can spin an AA impeller to 5500RPM. Install that combo in the same boat. GPS it.
Combo 2 will make the boat go faster than combo 1 did at the same RPM. This is because the bigger impeller can move a lot more water at 5500RPM than the smaller B impeller at 5500RPM.
The drawback is that yes you have to step up the amount of power being made at 5500RPM to be able to spin the bigger impeller to 5500RPM.
The point I was trying to make is that the only way he will ever get his boat up to 75mph and only have to spin at 4800RPM is a bigger impeller and MUCH more horsepower at 4800.
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