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J7 versus A6 duoprop

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mike77cj

Regular Contributor
Potential new (used) boat owner here. 285 Albemarle Express with twin TAMD41P, jackshaft to 290DP drive and J7 wheels.

Factory performance of the exact setup BUT with A6 wheels is:

3,000 rpm. 27 kts
3,500 rpm. 30 kts
4,000 rpm. 34 kts

The boat Im looking at performed:

3,000 rpm. 18 kts
3,200 rpm. 20 kts
3,900 rpm. 30 kts

The hull, drive and wheels were clean, and load and conditions were somewhat similar.

I read VP replaced the Type A props with Type J props. How would my J7 compare to an A6? I believe the J7 has a bit more pitch, which would explain the 100 rpm WOT loss, but shouldnt more pitch increase cruise speed?

The VP duoprop selection / propeller sizing is either a PhD lesson, or prop voodoo.

Could my performance quagmire be an improper prop slection?

Any help here would be appreciated!
 
mike77cj said:
Potential new (used) boat owner here. 285 Albemarle Express with twin TAMD41P, jackshaft tvo 290DP drive and J7 wheels.

Factory performance of the exact setup BUT with A6 wheels is:

3,000 rpm. 27 kts
3,500 rpm. 30 kts
4,000 rpm. 34 kts

The boat Im looking at performed:

3,000 rpm. 18 kts
3,200 rpm. 20 kts
3,900 rpm. 30 kts

The hull, drive and wheels were clean, and load and conditions were somewhat similar.

I read VP replaced the Type A props with Type J props. How would my J7 compare to an A6? I believe the J7 has a bit more pitch, which would explain the 100 rpm WOT loss, but shouldnt more pitch increase cruise speed?

The VP duoprop selection / propeller sizing is either a PhD lesson, or prop voodoo.

Could my performance quagmire be an improper prop slection?

Any help here would be appreciated!
Click to expand...
I have had the same problem with my new boat in 2004 a 268 ALBEMARLE Express with kamd300, the props had too much pitch for a Diesel engine. Black smoke often witch meant too much of a load on the engine with the wrong pitch props. I couldn't make 3900 which is what my set up was supposed to yield. They specked it out for a gas engine. If after a good long run you shouldn't have black soot on your transom. Call the engine builder and state your specifications to him and he will tell you what props should be on your boat. Boat builders are notorious for installing the wrong ones
 
supposedly A6 = J6. That will give you your missing WOT rpm's I expect but not the speeds. Best guess is that if you weighed your boat it is a fair bit heavier than the factory test one if everything was clean when you did your tests. Either that or you had the boat really badly trimmed at the the different speeds.
 
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