250 SJ pump

[Roost]

No worries Kevin, Don't hold back. I appreciate your thoughts. I'm curious what your concerns are and I'm really anxious to see the pics of the motor in one of your boats.

Generalities here…
The pump is the limiting factor. Since the 250/240/200 share the same intake/housing/impeller/stator/nozzle etc… Impeller speed is the only variable. Unlike a prop, low end torque isn't important to the SJ. Thus the 250’s low end torque would be of little use for your proposed build. Actually, it’s a liability. With a shallow deadrise and raised tunneled intake, the impeller sits high in the water. The impeller can’t absorb near instant torque, thus “blows out”.
On the other end of the spectrum, the under driven SJ impeller’s perf drops off substantially above 4000 RPMs. If I remember correctly (shooting from the hip here) the 250 Power Head maxed out @ 5600 RPMs, thus the 1.15:1 under driven impeller was turning @ 4900 RPMs. But there was little difference in top speed from a blade speed of 4500-4900, so essentially you’re just burning more fuel. Also, unless it’s a finely tuned hull / tunnel design, pump loading above 45-48 MPH will be a struggle.

Thanks Kevin. Very good points. It all gives me something to think about, or worry about lol.

Stay tuned -- Roost

 

[Kevin Turner]

Clarification:
For others reading, the info above is specific to Roost's proposed build of a raised tunnel / UHMW / shallow dearise / wide botm craft / balanced center of gravity for level static drifting / SusQ river fishing craft.

If we're talking Missouri style / non raised tunnel / weight rearward craft / the SJ pump easily loads to near 60 MPH. There's much more I'd like to discuss, but darn it my keyboard time is limited.

 

[Roost]

Clarification:
For others reading, the info above is specific to Roost's proposed build of a raised tunnel / UHMW / shallow dearise / wide botm craft / balanced center of gravity for level static drifting / SusQ river fishing craft.

If we're talking Missouri style / non raised tunnel / weight rearward craft / the SJ pump easily loads to near 60 MPH. There's much more I'd like to discuss, but darn it my keyboard time is limited.

Thanks Kevin. I appreciate the conversation. Keep it rolling when you get some free time.

 

[Midwestjet]

Hey Roost...sounds like a neat project. When I was looking into a Sport Jon sled 4 years ago I followed the blogs out west where they were developing the design for a light draft heavy hauler for moose hunters. The early boats tried the 200, 240 and 250 Merc Opti Sportjets and after a lot of trial and errors ( and tons of moose haulin' ) the end result was the 240 and 250 Opti's cost more, Weighed more, burned more fuel but didn't push more ( not enough to notice ) so most everyone went with the 200 Opti as the standard power package. As a 200 Opti owner I would recommend to anyone wanting to build a raised tunnel boat. Good luck with your project! Midwestjet