1tricky1:
I would have to agree with Mandelon. Actually I would suspect the load would be less in your typical 18-20 ft jet application. I believe the motor spins more freely in a jet vs car application. Think about it, when you stab the throttle in a jet, it revs very quick, alot quicker than in a car, Depending on the pump set up and its' tolerances, it will rev up quicker due to a certain level of cavitation. Revs up great on the trailer out of the water right? Won't pull the same RPM's across the water now will it? No. The faster the motor runs, the faster the impeller is turning trying to move more water through the bowl which doesn't change due to rpm. The more water that is "waiting" to go through the pump as the motor gets up to the top RPM it will pull in the water, the greater the load. If you were running some type of variable nozzle out of your pump, then perhaps the load would remain constant, however, on a standard pump, like Blown said, the higher the RPM, the more load on the motor.
P.S. I'm sure there are more technical terms that relate to the limitations of the pump and the physics that go along with it, but I'm not that concerned. Think of it as pouring an increasing amount of sand, or even water, through a funnel. Only a certain amount will flow easily through until it starts to back up in the neck of the funnel. That backup is what creates the load on the motor. Capeiche?
This is a tricky thing and I think both thoughts are correct,depending on the exact application it will differ a bit,As in impeller size,loader,boat lift,etc.A jet has not a thing hanging under the boat but a bit of loader.Run a 100 and let off see how fast the boat stops.Same thing take a car run a 100. What stops faster boat.Alot more drag in the water so the engine will have to work harder to keep it up and going. Just my thoughts on this.
Badboat1
how about the fact that an engine in a jet doesnt see nearly as much... uhh.. run time as an engine in a car...
a car see's a lot more driving every single day, then a jet boat does, what twice a week, for 8 month's of a year, if that...
so, even if it'll be making more stress in hte boat, it wont be making it as often as if it were in the car... huh?!
just stirring the pot
duke
Badboat1:
What stops faster boat.Alot more drag in the water so the engine will have to work harder to keep it up and going. Just my thoughts on this. I think if you broke it all down, you'd find out it is far trickier than ever imagined. The drag on the hull and the "stickiness" of the water are definately a factor, granted the initial drag of the hull are overcome to a certain effect based upon the amount of wetted area that the hull is running based upon the speed across the water. All of the inertia of the car remains seemingly constant. Of course the boat will slow much faster when you let off the accelerator but that doesn't play much on the load of the motor as that is an ever changing variable.
As far as Duke's comment, it seems that while the boats don't see as much use, the amount of use they see based upon load and RPM use, most in the 2500-3000+ range would easily quadruple the water time v. time on the road.
jordanpaulk:
[QUOTE]Originally posted by Badboat1:
[qb]As far as Duke's comment, it seems that while the boats don't see as much use, the amount of use they see based upon load and RPM use, most in the 2500-3000+ range would easily quadruple the water time v. time on the road. Do you really think that an engine in a jet works that much harder? just curious, not trying to question your comment.
how about if the engine were in a truck, some towing here and there,
what if you lived on a hill like i do, going up hill and down hill. or going over the grapevine.
Do the engines in jet's, like everyone's on here, run in the same temp. level's as everyday engine's in the world? i'd guess that a jet engine runs cooler then most cars/trucks.. and of course the heat put's more stress on the car/truck right?
just stirring the pot, i kinda got off topic but thats ok..
duke
When you are driving a car down the road at 75mph, 2000rpm, "part throttle".....you are only letting a small amount of air/fuel into the engine. Cam wants to let alot more in, yet the "part throttle" is only letting X amount of fuel/air in. This is not making much load on the engine.
When you want to go faster in a boat...you open the throttle more, letting more fuel/air in leading to more cylinder filling power to be able to increase the speed of your boat....hence the load get's greater.
In a boat depending on how fast you want the impeller/prop to turn (both have different realms that they work in).....it takes HP to turn it. Low in the rev range you have ALOT of torque.
A jet drive will go easily through this "range" rather quickly to where the HP starts to become more than the torque. Unfortunatley, that's where it starts taking ALOT more HP to continue the gain of higher rpm's that everyone is looking for w/ a jet boat (including me wink ).
A prop boat will/can blow or cavitate if you have alot of hull weight with alot of torque (the engine is turning the prop faster than the hull speed). Also, if you have a cam in a prop boat that is too big for the engine (no torque) then you have a sluggish out of the hole boat.
Turbo/superchargers, more CID or Nitrous are good ways for adding more fuel/air to the engine to create MORE POWER wink !
Hope this helps smile_sp .
You can drive around in your car all day at 60 mph in 3rd gear turning 4000 rpm getting 12 miles to the gal and only using maybe 50 HP to do it. You drive around in your jet boat at 4000 rpm and its going to take maybe 200+ HP @ 3 mpg to do it. The speed part comes with the hull design and set up.
You can't compare a jet with a car. Its not the same thing. A jet needs power to make rpms, Thats why most jet boats are big blocks. and yes a jet boat is much harder on an engine than a car would be.
As the speed of the boat increases, the amount water is forced into the impeller assembly at a greater rate. Depending on your grate and loader setup.....seems to me that would offset part of the power needed to simply turn the impeller.
I'm no engineer here guys, just trying to understand. Plus, I like to argue!
If the load on the motor increases with speed, then is there a variable percentage of power attributable to the load above what is used for porpulsion??
Well, I guess that answers that....
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