Hi Guys, I purchased a 96 Sea-Doo XP. Ive heard a lot about the XP's porpoising problem and have experienced it my self. I talk with a guy who owns a 95 XP 800. and he recommended aTrim tabs for quicker planing and eliminating porpoising. Should buy trim tabs and/or a ride plate? If So, what types would you recommend? I have seen several manufactures: R&D, Inland Jet, Aggresser, GT Manufacturing, Ultrac, Westcoast..
Tom, trim tabs will help cure most of your porpoising with a top loading intake grate. And your XP will plane faster and be more stable in rough water. I would also recommend Aftermarket sponsons for the complete handling package. Those brands are all pretty good.
PWC hulls (as well as most recreational jetboat hulls) are what is known as "planing" hulls, meaning that with sufficient power, they rise to the surface of the water and actually skim or "plane". How? Physics!! As a hull moves through the water, it forces the water it comes into tact with downward. That's the action. The reaction? The water pushes upwards against the hull, creating lift.
At low speeds, your pwc acts like a "displacement" hull, plowing through the water supported by the weight of the water supported by the weight of the water it is displacing. This is refferred to as static water pressure. Pick up that speed, and your Jetski is designed to rise to the water's surface using lift, where it skims along with much less drag, supported by the water's velocity pressure.
Jetski Hull Terminology
The V of hull refers simply to the hull's angle when viewed from the bow, or the front of the boat. Curving in from each side of the rub rail and joining at the bottom center-line, or "keel" most all hull shapes form a "V". If the hull sides meet at a sharp point at keel, the design is referred to as a Full V, and will typically offer a smoother rough-water ride due to its ability to "slice" through the water. It may however, sacrifice some stability due to its tendency to roll with the waves.
That same deep V can also produce drag, leading many PWC manufactures to choose a modified V, a design that flattens as it moves aft, from the bow to the stern, or the rear of the boat. The goal? to retain some of the benefits of a V hull, while improving stability (and often speed) by placing a flatter surface in the water.
The angle of the hulls "V" is refered to as the deadrise, and is measured in degrees. Simply put, deadrise is the angle at which the hull sides curve away from the horizontal.
As you might expect, A shallower hull angle will typically produce better speed, planning easier than a deeper angle. These sharper deadrise hulls, however, will typically offer better handling due to their ability to slice more easily through the water. They will, however, sacrifice some stability in the process.
Yes, Most hull designs feature a deadrise that changes as you travel away from the bow torward the stern, resulting in a sharper degree deadrise around the bow and amidships area that flattens as you move backwards. The goal? To combine aggressive handling with a stable ride.
Follow the hull sides downward from the bondline and you'll notice a point where the shape angles rather dynamically, a point where the hulls more vertical sides and the hulls angled bottom actually meet. This is whats known as chine.
Chines determine a great deal about a hulls handling. ability, as well as its directional stability. Softer chines (those with a more gental angle) will result in hull that rolls more in a turn. Most Jet skis feature sharper chines that will produce a greater hook to the turns, as their greater angle provides a better "bite" into the water.
a chine's position will also affect the sharpness of a turn. typically, the lower the chine is placed on the hull,the quicker turn it will help produce.
Often thought of as "rails" running length wise along the hull bottom, strakes are raised "lines" in the hull that produce lift. How so? when accelerating from a dead stop in the water, water is forced outward from the keel and is pushed against the strakes, where its force then lifts the boat and helps it to plain. Once the craft has planed,strakes help the hull to ride on less of its surface area, improving overall speed.
Typically the greater number of strakes used on the hull, the lighter and flatter the hull will run in the water. To much lift, however, and stability suffers, as to little of the hull area remains in contact with the water. Sharper, larger strakes can also prevent the hull from sliding out in the turns, but again, a proper balance is the key.
All the rage in offshore powerboats, steps are cut into the hulls of some PWC to create additional points of lift, and reduce the "wetted" area of the hull.
as water flows along the hull, its lift is greatest at the hulls leading edge, then lessens as it flows toward the stern. Eventfully the force of the water becomes drag. By placing steps into a hull, designers in effect create additional leading edges, making hull plane quiker.
The "fins" or "rudders" typically attached to the hull sides at the stern of your boat or Ski, sponsons can significantly improve the handling performance of a watercraft by keeping the stern hooked to the watercraft by keeping the stern hooked to th water during turns. Today, sponsons primarily come in two shapes:
A hooked fin or a paddle type rudder. In general, the larger and deeper the design cuts into the water, the more aggressive the ride becomes.
A PWCs ride plate is the piece of metal or composite material that covers the jet pump area (pump cover) of the stern of your Jet Ski. But, more then just a protective plate, a ride plate can determine a great deal about your ride, as its the lone spot on your machine that is almost always in contact with the water.
Designs can add "hook" to your jet Ski, forcing the bow deeper into the water, or aid in tracking with multiple grooves. They can be reduced in angle to improve top speed.
an area that is becoming increasingly important is the intake portion of your hull design, the pocket where water is sucked up into the jet pump.
Enter the intake grate, in esscence a protective set of bars to shield the intake from sticks and rocks.
Intake grates can do more, than simply protect your impeller or pump. They can help load it with water, typically by adding a "scoop" to the grate that protrudes lower into the water. But the grate scoop creats drag, lowering your speed.
Making adjustments to the Jetski carburetor air/fuel mix is pretty easy. nearly all watercraft carburetors are fitted with high speed (30-100 percent throttle) and low speed 90-30 percent throttle fuel mix adjustment screws. In a stock application the screw closest to the mounting plate of the carb is always the low speed screw; The one nearest the air intake is the high speed adjuster. turning them out (from bottom closed position) will richin the fuel mix.
Pop Off Pressure
The relationship between pop-off pressure and low speed jets. " those two circuits overlap" although the low-speed jet continues past 1/4 throttle engine operation, whereas pop-off pressure has little to no effect due to increasing air velocity in the carb. also, one can make up for slight inadequacies in the other.
"For example, if your pop-off pressure is a bit high, you can compensate by increasing the size of the low speed jet, The opposite is true, too: a slightly undersized low speed jet can be compensated for by using less pop-off pressure."
When does it become necessary to adjust the pop-off?
"Pop-off pressure refers to the pressurization of the needle valve with compressed air, through the fuel inlet," "this is a reference to the amount of pressure needed to open the needle valve-- or 'pop' it off its seat. The higher the pop off the more pressure required.
"When personal watercraft or Jetboats come from the factory they have fairly high pop-off, due to the fact that they also have somewhat restrictive air intake systems that cause the engine to generate very high manifold pressures. The higher the manifold pressures, the higher the pop off pressure required to properly regulate fuel delivery to the engine"
As you modify or change your flame arresters to a less re-strictive type ( meaning more flow), you are lowering this manifold pressure--- and are inadvertently creating a 'lean' condition in your engine. Changing the pop-off pressure--- which is nothing more then switching the Needle Valve Arm Spring in the regulator to a spring with a softer compression (gram) rating. "sometimes in order to achieve the desired pop-off pressure it's necessary to also change the needle valve size. Just remember its always better to use the smallest needle valve size to obtain the correct pop-off"
Tools Use Pop-Off Gage
Carb Performance Trick
Remove the choke and replace its function with a primer kit. Though most PWC enthusiasts tend to regard the use of a carburetor primer as "band aid" for an ill tempered engine or comp device, it offers benefits for virtually any watercraft.The single biggest limitations to a carburetor operation is airflow; anything you can do to improve the flow (and velocity) of air through the carb will usually result in increased performance. Choke assemblies, which incorporate regulatory devices similar in appearance to throttle plates (but located at the top of the carb) are an obvious impediment to air flow.
Basic Mikuni Super BN Calibration
There have been a number of diffrent carburetors used to for feeding PWC over the years, primarily by mikuni, with the end result being the popular mikuni super BN carb. It is designed specificaly for watercraft -- meaning it is essentially a sealed unit that canot vent its fuel chamber to the atmosphere (which would allow fuel to leak into the hull in rollover situations).
Though accurately tuning this (or any) carb for your application is limited by other possible weal linmks in the engine, Use this procedure for calibrating tune-able circuits during testing -
1 - Low Speed Adjusters: to adjust for smooth idle
2 - Pop-Off Pressure: just off idle to 1/4 throttle ( in conjunction with low-speed jet).
3 - Low Speed Jet: just off-idleto 1/3 throttle
4 - High-Speed Jet: 1/3 -to 3/4 throttle
5 - High-Speed Adjusters: 3/4 to wide open throttle
I would like to modify the impeller in my 1998 Sea Doo GTX limited. Im interested in getting more top end speed. The only modification so far is a high compression head and intake grate. I'm planning to put on flame arresters in the spring. Ive spoken to a couple of dealers for modification parts and I am not getting very clear answers. Can you guys help. Thank You Jim
Hi Jim, Your stock impeller is a stainless steel 14-21 progressive pitch. (14 is bottom-end, 21 is high speed). We do offer a impeller performance modification that will give you a little more speed for $135 which includes rebuild.
We have a 1992 Yamaha WaveRunner III that has recently developed an anoying problem. It starts fine when cold or hot, but after operating for about three minutes the engine RPM is limited to about half throttle or less. If you throttle it more then about half throttle the engine dies. We have replaced the fuel filter, installed new plugs, and replaced the cdi unit with a MSD enhancer unit under assumption that the overtemp circuity had failed in the stock unit. The MSD unit didnt correct the problem. We also ran the CDI with the thermoswitch electronically disconnected with the same results. To add to the confusion the engine does not show any other signs of overheating, such as being hot to the touch or crackling noise from thermal expansion. If the engine is turned of and restarted after it goes into this RPM limit phase, The engine seems to run at full rpm for another few minutes. seems like the problem is in the rev limiter. what do you think?
T.B. I would recommend looking into a fuel system as possible problem. Check the carburetor diaphragm and make sure it has not dried out or stiff. Also there is a small filter in carb that you should check to make sure it is not clogged. Also check your reeds to make sure they are not worn out. The fuel system should pressurize so make sure your gas cap seal is in place and the gas cap does not have any cracks. When open the gas cap you should he pressure release. Good Luck
I purchased a 1998 Kawasaki 1100ZXI recently. I have read different arcticles on this watercraft about top speed. Could you please you please give me the performance data on this model? Its my understanding that the newer units are a little slower. One post stated that the 1998 Kawasaki 1100ZXI Jet Ski finally hit the 60mph mark on your radar gun., Then another arcticle stated 58mph. The watercraft gives me a great ride and it's fun to ride. I have been riding watercraft for over 8 yrs. Thanks for your help.
R.J. We radared the 1100ZXI at 56.1 mph. Kawasaki claimed a top speed of about 54-55 mph on the ZXI, and it is very unlikely that any stock ZXI has reached 60mph.
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