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Right size impeller

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52
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Location
Centerville, tn
Water Crafts
96 spx
#1
Hello everyone. It's been a long time since I posted anything. I installed a 717 motor from a 96 spx in a 16 ft jon boat. Which added quite a bit more weight than what is was designed for. I've been running on the stock impeller. It seems pretty strong to me but I'm gonna have to replace wear ring. (Sucked up some gravel). So I thought about rebuilding pump and possibly changing impeller size. Wondering what you guys would recommend on impeller size? And I know nothing about how their rated or what those numbers mean. I get about 36 mph right now. And it runs just as hard whether fully loaded or almost empty. My impeller doesn't look bad or bent, but it does have some dings in it. Let me know what you think
 


Messages
8,224
Likes
220
Location
Virginia Sea Level
Water Crafts
2001 Sportster LE 951 Carbed
#2
I'd judge based on the WOT RPM. If the engine wasn't bouncing off the rev limiter and was revving to normal RPM then it's a good match.

Assuming the pump is loading nearly the same as in the ski, is overall boat weight nearly the same as the ski it came from, or heavier? The engine still weighs the same but the boat might be lighter? The wetted surface is probably more but weight distribution will effect that quite a bit so if someone's been riding up front or will be, consider the wetted surface will reflect that. If the boat is appreciably heavier than the ski was but the pump loads well without cavitation or other obnoxious effects, a SLIGHTLY smaller wheel will get some lost revs back.

A little change can bring a big difference in WOT RPM, you don't want to be hitting the rev limiter.
 
Messages
52
Likes
0
Location
Centerville, tn
Water Crafts
96 spx
#4
I can honesty say I have no idea what my rpm's are. I've never checked them. It sounds reved up. The boat alone is probably 350 to 400lbs. That's without motor and gear. When I first started running boat its cavitated and porpoised alot so I had to switch to a top loader grate. Slowed boat down 2 or 3 rpm's but helped in those other areas.
 
Messages
8,224
Likes
220
Location
Virginia Sea Level
Water Crafts
2001 Sportster LE 951 Carbed
#5
You really can't judge without knowing the engine RPM, but since you had trouble loading the pump and the grate fixed that, I'm not convinced spending money on a smaller wheel will help anything.

Maybe you can find a "tiny tach" first and have a look, they're pretty cheap.
 
Messages
8,224
Likes
220
Location
Virginia Sea Level
Water Crafts
2001 Sportster LE 951 Carbed
#7
The grate is interesting though, I know some of small aluminum jet boats running ski motors run very shallow gravel and rock river beds, with gravel shields covering the aluminum bottoms, and intake grates with closely spaced tangs to keep from scooping up gravel.
 
Messages
8,224
Likes
220
Location
Virginia Sea Level
Water Crafts
2001 Sportster LE 951 Carbed
#9
That's pretty dang good IMO. Dual or single carbs? Watch for broken pipe bolts or RV cover cracks, there may be some resonance occurring at certain RPMs a re-pitch could correct for.

Is compression about 150psi?

Bill O'Neal was a wiz with tuning impellers, I found one of his interesting posts here:

720 motor, only getting 44 mph max speed
 

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