4 blade prop

[handy69]

i have a 1987 Sea swirl topaz sport with an OMC cobra 5.7 liter motor 300 hp and have been using a 14 1/2 x 19 3 blade prop top end rpm is 5400. should i use the same in a 4 blade prop?

 

[alldodge]

top end rpm is 5400

Get a new Tach because your motor is not turning that fast.

Going from a 3 blade to a 4 blade with the same pitch your going to drop RPM around 200

 

[QBhoy]

Hi. Think a few details are off. The HP number and the rpm number are both surely wrong. Hopefully the members can help, once the figures are good.

 

[handy69]

Get a new Tach because your motor is not turning that fast.

Going from a 3 blade to a 4 blade with the same pitch your going to drop RPM around 200

Get a new Tach because your motor is not turning that fast.

Going from a 3 blade to a 4 blade with the same pitch your going to drop RPM around 200

your right i meant 4400

 

[QBhoy]

Get a new Tach because your motor is not turning that fast.

Going from a 3 blade to a 4 blade with the same pitch your going to drop RPM around 200

Think that’s generally good advice. But some exceptions for sure AD. I actually found that going from a Merc 4 blade ally 21” to a 3 blade tempest plus 19” dropped the rpm by at least 50-100 rpm (because the 4 blade was on the limiter at 4900/4950) to about 4850 and increased speed by around 5 mph. Just clearly had less slip at the top end

 

[handy69]

Hi. Think a few details are off. The HP number and the rpm number are both surely wrong. Hopefully the members can help, once the figures are good.

i should have mentioned the motor had been changed to a 300 hp 350 gm about 6 years ago and i will confirm rpm tomorrow at the lake. thank you for in put.

 

[alldodge]

your right i meant 4400

ok, then 17P 4 blade

 

[jth877]

Weight of boat? I found the merc/quicksilver prop calculators very accurate. My boat is 21ft, 220hp 5.0L. 3000lb plus gas/meat/gear. The calculator chose a 4 blade aluminum 19p, 14.5" diameter OR a 3 blade 18p aluminum 16" diameter. I opted for the 4 blade.

At a loaded weight of 3500lb, wot was 4400rpm at 47mph. That's the boat speedo not gps.