Is this picture in WA or ID?
It looks like there are a bunch of homes their and a bunch of game trails coming down the hills
I've been to the south end once driving home from ID to see that canyon, I didn't get to see as much of it as I was hoping to We did see a brown bear along the road to it though

The boat was parked on the WA side of the river, near the OR border, about 40 miles up from Lewiston.Those are cabins on the ID side of the river, where the state has been selling off its land to private owners. They are only accessible by jetboat as the road ends at Heller Bar.

So the intent is to run the motor at an rpm that develops max hp.

Since there is a formula to determine how much power is required to turn the pump a given rpm, the goal is to make that amount of power, at wot, at that rpm.

It doesn’t really matter if that’s the engines peak power or not.

As long as the pump load and the engine output are equal at wot, at the desired rpm....... that’s as high as the engine will rev.

so it would seem to me you would want a broad , not so Peaky power curve. 112ish lsa, with an intake in at 108 ish & 255ish @.050 in & ex, .600ish lift, Very mild lobe solid roller.

There is no need for a broad power curve.
The only rpm the engine will operate at during wot is whatever matches the load of the pump.

At wot the engine will always run at one rpm...... no matter how fast or slow the boat is going.

At lower engine speeds, there’s less load....... so having lots of low speed tq does nothing for you.

It’s not like a car where going up a steep grade at 2500 requires more work than going down hill at 2500.

With the jet....... you’re always going up hill....... and it’s always the same “hill”(load) for any given rpm.

Using Marks specs, at 3000rpm the load from the jet equals 146hp/256ft lbs.
It doesn’t matter that you might have 371hp/650ft lbs on tap at that rpm.
You can’t provide any more load to the motor at that rpm than 146/256.
If you apply more throttle at 3000rpm than is required to make 146/256........ the motor just revs higher.
The only way to increase the load is to turn the pump faster.

You keep adding throttle, the rpm keeps going up.....until the motor doesn’t make enough power to gain any additional rpm.

How heavily the boat is loaded, and how fast or slow it’s going has no affect on the engine speed.
The motor will go right to full rpm as soon as you go to wot.

Valid point

Basically, where the engines power curve at wot intersects with the pumps load curve....... that will be the terminal rpm....... which may or may not be where the motor makes peak power.

If a person has the time and money then the engine can be run on a dyno and the power curve overlaid on the pump curve to find the intersection. Then a person would know if the power curve is correct for the pump or not.

Most marine engine manufacturers will provide a dyno graph for their marinized crate engines, which can be overlayed with the pump or propeller power absorption curves, and then change the impeller or prop pitch to best match it up the engine. However most boaters on this river like myself, prefer to run a higher pitch impeller to move the most water through the jet for the best pump performance in class 5 rapids, which requires a powerful big block or a supercharged small block. Now after I swapped out the Hamilton Jet with the more capable Scott 912, which has a very good and broad performance envelope, I can install a "proper" cam to open up the engine's potential to best compliment the jet. Granted this Mopar with its small cam already outspins the LSA, Raptor 575, and comes close to matching the LT4, I would still like to have the extra power in reserve, that is otherwise being left on the table by the small cam.

Jet boat engines are from a very different world than any thing else we build and use , trust ,me on this.
Until you driven a jet boat and a prop driven boat it is hard to imagine the differences
The pump is trying to compress liquids, impossible to do, correct You can turn water into steam trying to do that though, it is called cavitation and very harmful to the pump parts

And don't forget about the sneaker rocks hidden under the water!

I've piloted jet and prop boats & barges of all sizes and vocations, and this is where the diesels have a distinct advantage in both applications...they are built for those long hours of continuous load. Costs of purchase and install, that's a whole other discussion for another forum.

Right, but then you've built the wrong motor if the peak hp is at much higher rpm.

Russ, now think about that for a minute.

You’re targeting 630-640ft/lbs at 4800rpm.

How would you build this BB Mopar motor to achieve that power level at that rpm, and have that be the point of peak hp?

How high would the low end tq have to be, and how sharply would the curve have to be dropping for 4800rpm to be the point of peak hp?

Your targeting 640ft lbs/585hp@4800....... and that’s going to be the highest hp reading.

The cam I would use to reach the target 640ft/lbs@4800, I’d expect to make peak hp north of 6000 with the heads, induction, exhaust the OP’s build has.

But...... it’s irrelevant....... since it will only rev as high as the load of the pump allows.

The other way to look at it is........ if the pump load limits the rpm to 4800......then, with the motor in use in the boat...... that will be the max HP.

Dwayne is 100% correct, I am targeting the engines output to turn at or near 4800 at WOT, as that is the pump's input rpm that equals the max torque output that a typical 4.15" stroke RB block is capable of producing, in naturally aspirated form, and cam the engine for that rpm target. I don't care if the pump won't allow the engine to run at peak HP, as re-configuring the jet for a lower KW rating to achieve that peak, is not the ideal configuration for pushing an 8,000lb boat up through a class 5 rapid...for a 2000 lb Sprint boat on a race course, then go ahead and tune the jet for peak motor HP. But to tune the motor to match the max horsepower rating of this jet, then not many on this forum have such an engine and I would be best saving my money for Mercruiser QC4V or GE T-58 turbine.

Let’s say you could build a motor with a perfectly flat torque curve from 2000-8000rpm, and that the tq value for that entire range was 712ft/lbs.
That’s 712ft lbs/271hp@2000....... and 712ft lbs/1084hp@8000.

Hook it up to the OP’s jet drive....... it’s going to max out at 5000rpm....... which would be 712ft lbs/678hp.

However, virtually any power curve that yielded 712ft lbs/678hp@5000........ would also max out at 5000rpm....... unless the upward trajectory of the power curve of the engine was steeper than that of the jet pump load curve, at the point at where the two curves cross.

Forgive a naive question......

If the engine could produce more torque (at WOT) then the jet required, then the engine could be run
at only part throttle. Is this a correct statement ?

If the "perfect" engine was 500 inches for this application, why could you not use a 600-incher at less throttle opening ?

If the motor made more more TQ than the jet required at its maximum input rating, it would mean that the jet would be operating at an rpm above its rating and outside its performance/efficiency envelope. Such condition was experienced with the Hamilton HJ212, the impeller absorbed the power but the jet yielded zero performance gain...the extra horsepower was wasted in cavitation and impeller reversion.

Sure you can use a 600 cid engine, 6.2 Raptor 575, 6.2 LSA, 6.2 LT4, 6.2 Hellcat, 632 Haxby Chev, etc...none of these engines exceed the 1200hp rating of the Scott 912. Running more cubes would allow more intake vacuum at a given hp, and not need the octane of a supercharged engine. If Scott comes out with more progressive impellers for their 912 jet, I would consider a swap to a 550hp Cummins QSB6.7.

I guess my lack of understanding is more fundamental. Why is it going to max out at 5000? If it's cavitation, wouldn't you put a higher output pump that had a higher operating range? Are gear reductions not used?

Because if the engine cannot produce any more torque than 712ft lbs over 5000 RPM, the jet has effectively "loaded out" the motor at that RPM. If it can produce more than 712ft lbs over 5000 rpm, say 862ft lbs @5500, then the jet will fully load the motor at 5500 rpm.

Oh yea, gear reductions are not desirable...they add weight and move the motor forward, throwing off the boats CG and increasing the chances of stuffing the bow into the "green room": https://youtu.be/BM7eXZeXRJo