Help with Prop specs

Help please ... need help to increase RPMS into recommended range. Have a 2600 Pursuit inboard with 385 single inboard Crusader 8.1L 2008 gas engine. Vessel has gear of 1.56 to 1 made by Liberty Velvet drive tranny. straight shaft to prop is 1 3/8 diameter shaft; two small gland bearings to prop.

runs great, comes out of the hole to plane quick and light, top end speed is excellent too at 36.8 mph on the GPS with 3/4 full fuel and full water with 2 men on board.

Only problem is the RPM is a little light ... Crusader says the optimum WOT should be at 4400-4800 RPM with the sweetest spot at 4600 RPM.

Vessel feels and runs great with a new 16.35 x18 Michigan DQX 4 blade nibral Nibral prop, but WOT RPM is shy and light hitting only 4250 RPM.

Don't want to lose the top end or cruising speed we have as performance feels fine and we are checking to make sure our tach is correct ... Anyone with any ideas would be helpful???? Thanks

Allen

Help please ... need help to increase RPMS into recommended range. Have a 2600 Pursuit inboard with 385 single inboard Crusader 8.1L 2008 gas engine. Vessel has gear of 1.56 to 1 made by Liberty Velvet drive tranny. straight shaft to prop is 1 3/8 diameter shaft; two small gland bearings to prop.

runs great, comes out of the hole to plane quick and light, top end speed is excellent too at 36.8 mph on the GPS with 3/4 full fuel and full water with 2 men on board.

Only problem is the RPM is a little light ... Crusader says the optimum WOT should be at 4400-4800 RPM with the sweetest spot at 4600 RPM.

Vessel feels and runs great with a new 16.35 x18 Michigan DQX 4 blade nibral Nibral prop, but WOT RPM is shy and light hitting only 4250 RPM.

Don't want to lose the top end or cruising speed we have as performance feels fine and we are checking to make sure our tach is correct ... Anyone with any ideas would be helpful???? Thanks

Allen

Help please ... need help to increase RPMS into recommended range. Have a 2600 Pursuit inboard with 385 single inboard Crusader 8.1L 2008 gas engine. Vessel has gear of 1.56 to 1 made by Liberty Velvet drive tranny. straight shaft to prop is 1 3/8 diameter shaft; two small gland bearings to prop.

runs great, comes out of the hole to plane quick and light, top end speed is excellent too at 36.8 mph on the GPS with 3/4 full fuel and full water with 2 men on board.

Only problem is the RPM is a little light ... Crusader says the optimum WOT should be at 4400-4800 RPM with the sweetest spot at 4600 RPM.

Vessel feels and runs great with a new 16.35 x18 Michigan DQX 4 blade nibral Nibral prop, # 4 cup ... but WOT RPM is shy and light hitting only 4250 RPM.

Don't want to lose the top end or cruising speed we have as performance feels fine and we are checking to make sure our tach is correct ... Anyone with any ideas would be helpful???? Thanks

Allen

Help please ... need help to increase RPMS into recommended range. Have a 2600 Pursuit inboard with 385 single inboard Crusader 8.1L 2008 gas engine. Vessel has gear of 1.56 to 1 made by Liberty Velvet drive tranny. straight shaft to prop is 1 3/8 diameter shaft; two small gland bearings to prop.

runs great, comes out of the hole to plane quick and light, top end speed is excellent too at 36.8 mph on the GPS with 3/4 full fuel and full water with 2 men on board.

Only problem is the RPM is a little light ... Crusader says the optimum WOT should be at 4400-4800 RPM with the sweetest spot at 4600 RPM.

Vessel feels and runs great with a new 16.35 x18 Michigan DQX 4 blade nibral Nibral prop with #4 cup, but WOT RPM is shy and light hitting only 4250 RPM.

Don't want to lose the top end or cruising speed we have as performance feels fine and we are checking to make sure our tach is correct ... Anyone with any ideas would be helpful???? Thanks

Allen

Help please ... need help to increase RPMS into recommended range. Have a 2600 Pursuit inboard with 385 single inboard Crusader 8.1L 2008 gas engine. Vessel has gear of 1.56 to 1 made by Liberty Velvet drive tranny. straight shaft to prop is 1 3/8 diameter shaft; two small gland bearings to prop.

runs great, comes out of the hole to plane quick and light, top end speed is excellent too at 36.8 mph on the GPS with 3/4 full fuel and full water with 2 men on board.

Only problem is the RPM is a little light ... Crusader says the optimum WOT should be at 4400-4800 RPM with the sweetest spot at 4600 RPM.

Vessel feels and runs great with a new 16.35 x18 Michigan DQX 4 blade nibral Nibral prop, #4 cup but WOT RPM is shy and light hitting only 4250 RPM.

Don't want to lose the top end or cruising speed we have as performance feels fine and we are checking to make sure our tach is correct ... Anyone with any ideas would be helpful????

Thanks

Allen

 

Change to prop of same diameter and slightly less pitch if available.

 

Or reduce cup slightly

 

you may use the same diameter and use slightly less pitch you may submit boat hydrostatics to calculate the right pitch

 

reducing pitch is solution.

 

get in touch with Rick W
he's in here somewhere
seems to be the prop pitch guru
B

 

Two inches of pitch will increase your RPM by about 300. It should also give you a higher top speed.

 

i'd check your tach to be sure that it's reading correctly, since you're very close the the recommended RPM level. if tach is accurate, and you really want to increase your RPM that little bit to "max", then slightly smaller pitch will increase your RPM. you may have difficulty getting only 1" less pitch, however (even 1" less may be too much) and more than 1" reduction will put your RPM's too high (not good). sounds like performance you're getting now is good, though. There are other things that you could do to your boat setup that could result in performance improvements.

 

Try polishing your prop, how's the leading and trailing edge looking?

Boat bottom clean? Trim correct? Can you move some of your load

forward? You're so close and on the correct side for max RPM. Play

with your variables before you start messing with your prop geometry,

in my opinion. How's your gas? Oil correct viscosity? Tried "Molyslip"?

Tom

 

Engine tuning up to snuff? Plugs okay? Etc., etc...

 

Damn good advice from Submarine Tom. If the motor feels close to adequately tuned CHECK THE BOTTOM, then everything else.

 

Allen here ... boat bottom is prefect as is all underwater hardware in working order. Engine is new with only 100 hours on it and running perfect Tach is spot-on as checked with an RPM gun on the fly wheel. So it seems I am indeed left with the prop and going to try for the 1 inch pitch reduction. Thank you all so much. I will report back to the forum once we have altered the prop.
Allen B.

 

One inch of pitch is too much I think.

Can you shallow out the cup a little instead?

You're sooooo close to what you're after.

Be careful and good luck.

Tom

 

Allen,you need to remove 2" of pitch to get your motor to 4,800 RPM.


H