Just bought a 18ft semi V boat the other day and finally got the mpg. It is a 12jg berkley with B impellar, 4 degrees wedge and jetavator with se loader. With 3 grown men, ice chest and 20 gallons of fuel in the tank (tank in the bow of boat) I gps it at 62mph it had a little left but not much. But I pulled 5800rpm and then 6000rpm after losing passengers but didn't get a GPS reading that time. My question is should I try out an A or AB impellar or go with a ss loader to see were that gets me before I spend the money on a impellar. I want to GPS 70mph.
Motor was just assembled mild 454
9:1 CP
540 lift 238 248 D @ .050 114 LBSP
049 larger ovals small port work 3 angle valve job
Weiand high rise tunnel ram with 2 600 carter competition
Howard roller rockers 1.7 ratio
Msd ignition with 6AL box
Cam operation rpm is 3000 to 6000.
Pictures of the boat, from the side, while making a pass would help. When you are doing your testing, try to keep everything consistent. Lose all the ice chest, passengers, and unnecessary weight. Keeping the fuel level constant would also make things easier for you.
Your RPM should not change with different weights in the boat. The only way the RPM will change is with cavitation or by sucking air (pseudo cavitation).
I'm assuming you have an aluminum impeller. It sounds like the impeller is right where it is supposed to be, compared to your cam RPM range, assuming that there are no underlying issues loading your pump (slipping the impeller).
If the trailing edges of your impeller are "properly" detailed, it will drop your RPM by about 300 and your MPH might pick up. But then again, your RPM might not be reaching peak horsepower.
I would start with a 2nd 4-degree wedge, getting your trim and attitude sorted out. The two wedges can be 'clocked' to adjust the nozzle angle. You didn't mention a Place Diverter. If you were feeling spendy, it's worth it because you will ultimately end up with one anyway. Although, you can still test with wedges.
Take a straight edge under your boat, holding it flush with the keel. Check to see if your loader ramps are hanging below keel. You didn't mention a shoe, so more aggressive loader ramps might be necessary to keep the pump loaded, at the cost of drag. Maybe, shorter loader ramps could help you achieve greater MPH. Try to hook up with some friends who might let you borrow a different loader for testing.
Lastly, make sure that you are actually getting full-throttle, because it happens.
There is a lot you can do with just adjusting hardware.
Cheers,
Joe
