makes the boat think it's longer, by moving the center of gravity back (less boat in the water) =faster

makes the boat think it's longer, by moving the center of gravity back (less boat in the water) =faster

Well put !

makes the boat think it's longer, by moving the center of gravity back (less boat in the water) =faster

Also, it gets the propulsion point away from the rear of the boat in "clean water", away from the wash of the hull, the ride plates purpose is to give the nozzle something to push against and makes the hull "adjustable".

Think about a setback plate on an outboard, works exactly the same way.

Thats the part that always confuses me  .  If you set the pump back, say 6" than add plates (always longer than 6") how is the nozzen going to be in "clean water", away from the wash of the hull ?  Didnt you just make the nozzel closer to the back edge of the hull by adding the plates ?

 The plates makeing the hull adjustable I understand, The ride plate angles up to create an adjustable amount of rocker, giveing the boat more bow lift helping it to air-out and be faster.  Or down would add a hook, slowing the boat and planting the bow down.  Right ??

Exactly, but you are forgetting something, if you look at the jetpump as an assembly, the shaft is at a downward angle to the hull, so if the pump is further back it is automatically putting the thrust point lower in relation to the bottom of the hull..Does that make any sense?

So technically and to a point, the farther back the pump, the lower the thrust angle, the more need for some wedge and bigger demand for a plate.

 
  OK, now I'm getting really confused.
The way I see it,  Looking at the pump as an assembly with a downward angled shaft, it wouldn't make any difference where you set the pump (forward or back). the thrust point will always be at the same height (compared to the keel). Unless you make the nozzle end of the pump itself longer or set the intake lower in the hull or at a different angle, or used a droop of course.

 In my humble and sometimes ignorant opinion  .  The reason to set the pump back would be increase the leverage the nozzle/thrust point has on the hull and to be able to make the ride plate long enough to be an affective adjustable hull extension .  

I think I see where your confusion starts, if you set the pump back, the intake hole goes back with the pump, so the more boat you can get out of the water without ventilating...its hard to explain but the pump angle stays the same, the actual height stays the same but the "leverage" we're talking about comes from the change in distance from the transom. I drew a picture but somehow does not reflect what I'm getting at.

The blue lines are the hull.
The red line is the shaft centerline.
The green line is turbulent water (wash).
The violet is the intake area.
The thin black line indicates wetted hull area.



You can see from the shitty drawing that there is a lot of wetted area in front of the intake on the setback pump.

You can also see the entry area of the pump to the water is technically lower in the water do to boat attitude (they only sit "flat" on the trailer).

I hope this helps and does'nt confuse further. I'm sure Josh (JBP) can explain it more than me, we played with pump setbacks a lot on the aluminum boat (which with a plasma torch and a TIG welder can be done in about 4 hours) The difference is unbelievable.

10-4, In some hulls it just won't do a damn thing, maybe do to weight or length, or even just a horrible starting center of gravity (bow plow).

Thanks, and your very welcome, I'm like you, didn't get anything done either, ran out of parts...and money. So just been working on a gate for my side yard. Oh well, next week...

Thanks GT.  I've got the '75 Miller and I guess I'll just leave the pump where it is and add in the droop and see what it does.  No need for a lot of work that may not do anything at all.  May need less down angle on the Place Diverter to get up on plane faster. 

Besides ....ya can pull the local plant life out of the jet without burning yourself on the headers!!

Good point, plus you won't sink the boat with the hand hole cover off.