Hey there and welcome to the boards...although I'm in NorCal.
And don't be fooled by these guys...
Found
On
River
Dead
Now to get to your issue at hand here. A lot of people have pulled engines out of cars and dropped them into jet boats, only to be disappointed by the performance. That's because most car engines are made to pull a low stall torque converter to 2000 RPMs or so. However, a jet pump is more like a high stall torque converter. We'll use mine as an example. You dump the throttle and the pump "stalls" at 5400 RPMs. In other words, the pump will not allow the engine to spin any higher than 5400 RPM. However...
Each size impeller has a horsepower requirement curve. The ideal setup is to use the impeller with a hp requirement curve that closely matches the engine's HP curve. You would select an impeller that will allow the engine to swing up to its peak RPM, or the RPM where it makes the most horsepower.
Knowing this, you would build the engine so that it peaks out at the RPM you want to be able to spin. Then select the impeller that will allow the engine to spin the pump up to that RPM.
Also, you wanna build the engine so that it can withstand high RPM for long periods of time as well. Mine cruises 3500RPM @ 40mph which is a great speed to cruise at. But there will be the bursts...you just cannot stop the urge to dump it to the floor once you get comfortable with doing that.
Also, it's not ONLY how much horsepower you're making, it also matters what RPM you're making that horsepower at. Also remember that horsepower comes from torque and RPM.
Remember..all the engine is doing is spinning a water pump. The engine and pump have no idea that they're attached to a moving boat. The engine and pump could be mounted stationary and they're still gonna spin the same RPM and do the same thing as they do on a moving boat...pump water. The engine/pump combo does not see a load difference between forward/reverse or slow/fast speeds. The forward motion of the boat has little to no effect on RPM. The RPMs don't increase as the boat gains forward speed like a prop boat or a car would.
The more water it pumps through, the more the load demand goes up. Torque is key here...jets love torque. The more water you push through the pump, the harder the engine will have to spin it to increase its RPM.
Good luck with your venture.
Very well put Jetaholic!!! And BFFted, here's one of the impeller charts he was speaking of. Unfortunately, your rpms are too low to use the chart. But, it will give you an idea of what he's talking about.
But here's one that covers the entire rpm range. According to this chart, with your approximate 225 HP, you should be able to turn an A impeller to about 4000 rpm +/- a few.
Good luck.
