Prop Sizing Question

[paul j]

Prop sizing. question. These are 6BTA- 300 in a 34′ Hatteras Convertible. 12.5 beam – 26000 lbs loaded (No Trim tabs) ZF220A 1:5-1 gears. 3100 rpm NO LOAD – Current props: 4Blade Nibral – 20 X 19.5 X 1.5 WOT= 2675 RPM – NO BLACK SMOKE UNTIL IT GETS JUST OVER THE 2600 MARK, 2675 WAS ALL SHE HAD TO GIVE. Looking to get 2950rpm.
NOTE**Started with props 20 X 21 X1.5 that gave me a WOT of 2600 RPM. The 20 X21 set up didn’t produce black smoke at just under the 2600 mark. The props have been reduced to the current numbers. The prop shop I bought the new wheels from calculated the 1.5″ pitch reduction would give me the additional 350 rpm I wanted over the 2600 mark.
Before I start changing these props any more, I would like input from others

Topic

[Tony Athens]

Paul,

These are early 300B’s and they like to spin up easy to well OVER 2900 RPM when your vessel is loaded up as you use it–Typically I say “loaded for bear”….

Basically, you cannot under prop this engine as long as you are happy with the vessel speed at cruise RPM’s in the 2300-2500 RPM range..

Tell your prop guy this—————-Reduce my pitch/diameter/Cup or ??, until the engine loads to 300 BHP at a solid 2950 RPM.. Getting these engines to spin up to 2950 will get your vessel on plane much easier an keep your engine much happier over the years..

Got a serial number?

Personally I do not agree that 1.5″ of pitch reduction will add 350 RPM..

Your call..

Tony

[Author]

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