Numbers Crunched?

[FAST FRED]

Our current boat is a lobster style house built on a Navy 50 ft Launch.

Since 1967 it probably had a half dozen engines .

Built with a 6-71 of 235hp/2100 and a 1.5 to one tranny it worked great for Uncle Sam.

Unfortunatly in its last repower (before we purchased her) a very newish Grey Marine 6-71 was installed with a 3 to 1 tranny.

The orig prop was 26 X 17 (with orifgtranny),
with only E bay the best the owner could find was a 30 X 26 to use with the 3-1 tranny.

The engine easily loads this prop and never break a sweat.In fact its so lightly loaded at 1500engine 500shaft that it still smokes a bit of white , sure sign of underloading.
As a loading help I installed HN60 (instead of HN90's) which reduces the HP to 180at 2000rpm.

At the boat yard a fellow has a 32X30 that has a reasonable price ($600 , used excellent cond).

2 inches more dia and 4 more pitch sounds great!

Does anyone have a sugestion on the power required to turn this prop at 150rpm to 700rpm shaft speed?

My goal is to load the engine a bit more at 1500 rpm (normal cruise)and perhaps go from 7K to 8K at this rpm.
Hopefully the top rpm would still be avilable for emergencys , but at 10 to 12 GPH ,high cruise wont be often!!

Engine loading is so light now that only 3.5 GPH are burned at 7K -1500rpm.


A second question , Does anyone have a DOD or other website that might be able to locate an operators manual for this 50ft Navy Personelle Launch?

Have been quite unsucessful with Google search.

Thanks all,

FAST FRED

[FAST FRED]

OR if someone has a web site I can plug into??

My Skeens is still over a month away,

FAST FRED

[MikeJohns]

Best to get Dave Gerrs Propellor Handbook if you can