4 blade prop

[xltier]

does a 4 blade prop cut down on rpms in the same size as a aluminum.if so is there a rule of thumb on how much??? thank you

 

[fishrdan]

Yes, rule of thumb is to drop 1" of pitch going from 3-blade aluminum to 4-blade aluminum (or SS to SS).

Exception is Mercury Spitfire 4-blade props, they are supposed to interchange with 3-blade aluminum props of the same pitch.

 

[xltier]

so if I am running way to high rpm and I use a 4blade of the same pitch it will lower my rpm.

 

[dingbat]

so if I am running way to high rpm and I use a 4blade of the same pitch it will lower my rpm.

In theory yes, in practice generally.

Not all props are equal. Some are designed for specitfc needs like lifting the stern, lifting the bow, etc. Each design will behave differently on each boat/motor combo. Usually takes more than one try to get it right

 

[xltier]

In theory yes, in practice generally.

Not all props are equal. Some are designed for specitfc needs like lifting the stern, lifting the bow, etc. Each design will behave differently on each boat/motor combo. Usually takes more than one try to get it right

Thank you again.I am familiar with 3 blades but not 4.i really should have a different gear ratio.like a 1.50

 

[Bob_VT]

Well a 4 blade has an additional 33% of blade surface as compared to a 3 blade. Dropping an inch in pitch is common so it will be close to a 3 blade.

Seriously it it easier to play with props then trying to change gear ratios.

Are you speaking of the boat in your profile picture? A 4 blade helps heavy boats and adds some control but you need to decide the goal -

Water sports?
Trolling?
Cruising?

Please don't say top speed because the boat in the profile picture is not a high speed runner.

 

[xltier]

I replac e the the 4cyl with a 1994 4.3 v-6 and gen2 outdrive and transom assembly.so it has way to much rpm.witch I figured on anyway. its a fishing boat in the rivers -mouths of the river and stuff like that.

 

[Bob_VT]

Hopefully you have the 1.81 gear ratio now that you have swapped and I bet that motor/outdrive combo will spin a 23P nicely What is your current prop?

 

[xltier]

i do have the 1;8. I am taking off a 12 1/2 x19p to try a 12 1/4 x 21 this weekend.its borrowed.i just installed a new tach today so I can see where I am exactly.then ill see about another pitch change.maybe I can borrow a 23 somewhere.

 

[fishrdan]

I am taking off a 12 1/2 x19p to try a 12 1/4 x 21 this weekend.

Diameter of those props sounds small, should be in the 14"+- range.

More diameter will lower RPM, less slip. May still need a different pitch prop though.

 

[xltier]

really,where could I find that info.

 

[fishrdan]

Bring up iboat's prop finder and look for 4.3L Alpha props. low pitch props are in the 15" range, mid range 14" diameter, then going into 13" diameter in the higher pitches. Diameter is a function of the prop's design so don't get too hung up on selecting the diameter, but with the 12"+- diameter, sounds like those are outboard props.....

 

[xltier]

I am sorry for wasting your time. that 12 is actualy a 14.on both. 14 1/2 x19 going to a 14 1/4 x21. again I apologize.!!

 

[fishrdan]

Ah, no problem.

You're on the right path trying the 21P prop, might need more pitch too if it's still overshooting WOT RPM.

With a heavy rig like that, a 4-blade prop will bring it up onto plane quicker (as Bob_VT mentioned). I'd figure out the pitch to bring down the RPM's first with used or borrowing props, then buy a 4-blade based off your 3-blade prop tests, if you're interested in getting a 4-blade prop.

 

[xltier]

I am going to be installing a set of Bennett self leveling trim tabs today also..so that may effect it also.that islander likes its bow up all the time.