4 blade to 3 blade advice

[mpsyamaha]

i purchased a boat that has the wrong prop on it. its a 4 blade 21 pitch stainless prop. with a light load the boat only hit 4600 rpms trimmed out.

i want to purchase a 3 blade SS prop for it and im debating on whether i should get a 19p or a 17p. what do you guys think?

 

[QC]

Re: 4 blade to 3 blade advice

Generally speaking, you will pick up some RPM just going to a three blade with no pitch change. However, we really need some more info . . . take a look at the Read This First sticky above ^^^^

 

[mpsyamaha]

Re: 4 blade to 3 blade advice

sorry about that....

the boat is a 1987 21' Offshore CC, probably weighs 2000-2500lbs, with a 98 200hp evinrude Ocean Pro. i think i should be shooting for 5500-6000 rpms at wot correct?

my goal is a combination of a fair to good hole-shot, with a high top speed. this is a fishing boat that may take long trips at times, not going to be pulling skiers.

heres a pic

 

[QC]

Re: 4 blade to 3 blade advice

Nice boat! Do you have a speed number at the 4600 RPM? Preferable GPS.

 

[mpsyamaha]

Re: 4 blade to 3 blade advice

thanks QC, the interior was fully restored and pretty much everything is new including Ttop, leaning post, wiring, fuel lines, etc. the exterior gel coat is original and in beautiful shape.

about the speed... no, i wish i did bring my handheld GPS on the test drive, but i forgot. i wont have a chance to get out on it until sunday.

my old boat would hit 39mph on gps, this one is definitely faster even though the rpms are low. im guessing its mid 40s with the current prop.

 

[QC]

Re: 4 blade to 3 blade advice

Well I'm thinking 41 - 42 MPH with around 15% slip assuming the numbers you show are right (1.86 gear ratio). Any faster and I am going to wonder about the tach . . . Based on your hole shot and speed compromise goal a 4 blade at 17 or 19 may be just right, but I'd want to confirm all data first. I always suspect tachs and gotta have confirmed speed before I would make any changes. How's the hole shot now?

 

[mpsyamaha]

Re: 4 blade to 3 blade advice

hole shot now is ok, not super fast but decent. you can feel the motor has more in it though. the 21p 4 blade is lugging it. i was thinking a 3 blade because they are better for top end speed.

my snap-on timing light has a digital tach so i can check the tach in the boat. ill test it out on sunday.

 

[hwsiii]

Re: 4 blade to 3 blade advice

You also might try reading this for a better understanding of changing props for better performance, and the second one for what the major differences are between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=366160

http://forums.iboats.com/showthread.php?t=365653


Mps, QC is right as always about the 17" pitch prop in the exact same make and model of 4 blade prop you have right now. Anytime you change even the model of prop from the same manufacturer the geometry of the props changes and it makes it much harder to interpolate the difference in RPM that occurs. You can possibly get another 2 to 3 MPH by changing to a SS prop but it very easily could require testing 2 or 3 props to attain this. By changing to the 17" pitch 4 blade your hole shot will improve dramatically and you should not lose more than 1 MPH, if any. Here are my numbers to show where I get the data from and your RPM should increase to about 5,700 and you will not be lugging the motor anymore and your fuel efficiency will increase as well.

Prop Pith hange

Prop Slip

H

 

[skargo]

Re: 4 blade to 3 blade advice

Too bad our props aren't interchangeable, I have a 19p 3 blade stainless, and I need to drop 300 rpms or so.

 

[mpsyamaha]

Re: 4 blade to 3 blade advice

thanks for the info guys

i called the prop shop that my company uses today. they said i could try another pitch prop if i didnt like the first one, so i decided to go pick one up. not so worried about making the wrong choice now! they gave me a great price too.

i decided on a stainless steel power tech 3 blade 17p. it was between that and the apollo ss prop, but the power tech looks to be of better quality. if the rpms are too high then ill go to the 19p of the same prop. ill let you guys know the results.

 

[junior1113]

Re: 4 blade to 3 blade advice

i have same setup(hydrasport 20' vector) and same motor (93 200). see aviator pick. i run a 17" turbo ultima 4 blade , with a jack plate 48 mph @5900rpm. tryied many props and this is the best by far. i would shoot for 5800-6000 to insure long engine life

 

[mpsyamaha]

Re: 4 blade to 3 blade advice

well i got to try out the power tech 14.25" x 17p 3 blade stainless prop today. there was about a 1ft chop in the intercoastal and it hit 43mph(gps) at 5500 rpms. this was with a fairly heavy load... 4 adults, almost half a tank of gas, and maybe an extra 100lbs of gear and stuff. it is much more responsive than with the old 21p 4 blade, and it planes off real quick.

i figure that should be good right? the normal load i will carry will be only 2 people and less gear.

 

[junior1113]

Re: 4 blade to 3 blade advice

sound good! way better than it did. nice boat