Prop selection

[JustinW]

purchased a new prop in a pinch while on a trip, just for a small boat, 35hp outboard. I usually run a 10x13 three blade but in the pinch I bought a 4 blade with a lower pitch. I want to go back to my 10x13 pitch because I have more top end power and the boat is plenty light enough to handle a 10x13. When I got my current prop the owner of the small marine shop told me he only carries 4 blade props. I didn't get a clearly defined set of reasons on why he likes the 4 blades (for any boat). Those of you out there, what are your opinions on prop selection?

 

[CON8IV]

Hi Justin,

In my experience, a 3-blade would probably be the best option for a 35hp. The dealer you mention likely gets a better deal on the 4-blade models. I can't remember doing business anywhere that DIDN'T carry 3-blade props also. Here's a link where you should be able to get your question answered by a real authority http://www.maxrules.com/index.html I have gotten several ignition questions answered here, just do a search or you can contact them directly. I like a prop that will let my motor run at just a smidge higher than max. recommended RPM when running light. Note I said "running light" because I often pull waterskiiers or have a boat full of people and can achieve max RPM when loaded. This is probably not a concern for you w/ a 35hp though. Check your manual for your motor and go with what it recommends.

Safe Boating!

 

[cobaltblue]

Propping is a matter of two things. RPMs and speed. The differences in three, four and five blade props are these. More blades usually equals more bite, you can go with a smaller diameter prop and still keep the RPMs down. A smaller diameter will also at times allow more pitch which could equal more speed. More blades can also mean more lift which can change running characters of the vessel.

With a 35 hp motor, I would stay with a three blade as running a little more RPM will be less harmful than constantly lugging it turning it too slow.

On another note, you need to be careful if your motor is still under warranty as exceeding the manufacturers RPM rating will void the warranty.

 

[matt_unique]

W.O.T. RPM is the number 1 consideration - make sure your engine is running in the recommended RPM range from your owners manual. An RPM too high or too low will damage or shorten the life of your engine and definitely void warranty.

--Matt

 

[tribaltim]

I have no idea what you just said but I have a 25hp Suzuki engine and it has 3 blade's. In the ocean it run's like a champ.

 

[matt_unique]

I have no idea what you just said but I have a 25hp Suzuki engine and it has 3 blade's. In the ocean it run's like a champ.

Ha ha - too many acronyms in boating and diving. Wide Open Throttle....make sure you're planed off, normally loaded, etc., etc. and use that as your measure. A prop that increases or decreases your RPM out of the manufacturers suggested range will damage your engine.

--Matt

 

[JustinW]

WOT = wide open throttle
RPM = Revs per minute

I figure the 10 x 13 prop that was on it has been there for a long time with light and heavy loads and has worked like a champ, so I ordered another 10 x 13 today from Cabela's. Its just an outboard so I don't have a tach on it, but it sounds like its in the right range at WOT, not wound out or bogged down.

I found that the number of blades is a compromise between efficiency and smoothness. A 2 blade prop being the most efficient but more vibration, the more blades the smoother it will run. It least as i understood it.