Prop diameter help

[kal4438]

I am new to this site, with this being my first post. From what I have read this site is filled with very useful information and people that seem like they really know what they are talking about. To start off I just want to say thanks for all the help and information posted!!

I am running a 1982 Evinrude 35hp long shaft on an aluminum 14' Lowe fishing boat. This boat is heavier that most 14' aluminum boats because of the front and rear casting decks, livewell, and console. I would estimate my boat to be between 600-700 pounds. The prop i had been running for years was a 10 1/2 x 11 aluminum. I recently tried a 10x14 ss prop on my boat which seemed to work just fine. Its hard to test and know the difference because of how chewed up the original aluminum prop is. Im looking to purchase a new ss prop for my boat but I am having some trouble in deciding what to go with. A gentleman I spoke with told me the 10 inch diameter would normally be best for this motor, but because it is on a heavier boat that the 10 1/2 inch diameter would be best for my rig. My question is, which diameter, the 10 or 10 1/2 inch, would cause my motor to turn more rpm's and I would like some facts or opinions on which diameter ss prop to go with. Any and all help is greatly appriciated.

Thanks to all!!

 

[Bondo]

Re: Prop diameter help

My question is, which diameter, the 10 or 10 1/2 inch, would cause my motor to turn more rpm's and I would like some facts or opinions on which diameter ss prop to go with.

Ayuh,... Welcome Aboard,....

Forget about the Diameter....

Shop by Pitch, 'n diameter takes care of itself....

Diameter is a function of blade design, shape, 'n pitch....

 

[Japie]

Re: Prop diameter help

Generally the smaller the diameter the higher the rpms, pitch also plays a great part, I have a similar boat to yours but with a 50hp on the back, that motor hit the rev limiter because the prop was too small, I kept the same diameter but moved up in pitch from 10x11 to a 10x13, problem solved. The test is to see how quickly your boat gets on the plane, if it is sluggish you might need a smaller prop and sacrifice speed but too small a prop and it will over rev.

 

[steelespike]

Re: Prop diameter help

Without your present max rpm and speed there is no way to know if your operating within your motors rated rpm.Its very important for the health of the motor that it doesn't over rev or lug. I would be surprised if the 14" is allowing full revs.

 

[1980Coronado]

Re: Prop diameter help

Let me preface this with...I'm not an expert in prop selection and I'm not an outboard motor guy...that said there are some basics regarding props I can share.

You changed the prop from aluminum to SS. You reduced the diameter by 1/2" and increased pitch by 3". Reducing the diameter would have increased your rpm, but increasing the pitch by 3" would have had a much larger effect in reducing the rpm. Overall you should have seen a reduction in rpm at wide open throttle with the larger pitch SS prop and possibly a slower response out of the hole. Now....you state that it worked fine, but without knowing what your wide open throttle (WOT) rpm was before and after the change it's impossible to say that it's the right prop for your setup. Typically a stainless prop will flex less than an aluminum prop at WOT, but this is usually more pronounced on higher horsepower engines. On a 35 hp engine, I wouldn't expect to see much difference due to flex in the prop. There is a rule of thumb regarding propeller pitch. For every 2" change in pitch, you will see a corresponding 400 rpm change. A 2" increase in pitch should reduce rpm by 400, given that the props are of the same diameter and mfg. Changing the diam or mfg. (blade design) of the prop will change this rule of thumb.

Running a SS prop on this motor isn't going to yield a performance increase IMO. On larger motors there is an advantage due to prop flex in the aluminum. The SS prop is stronger and will last longer, but they are more expensive....it's a personal preference thing in your case. I just don't think it's going to give you any performance difference over an identical aluminum prop.

In order to figure out what prop is "best" you need to first check your WOT rpm with the boat loaded they way you normally want to use it. You'll want to know what the max rpm for your engine is and select a prop that puts you close to that max w/o going over. If you are normally the only one in the boat you may use a different prop than if you normally have several people in the boat. With a heavier load you'll use a smaller pitch prop to increase the rpm to get up on plane. With just you in the boat a larger pitch prop may be what you want. It's just something you have to experiment with to see what works best for your particular setup.

Think of a prop as a gear, like on a bicycle. A lower pitch is like a lower gear on the bike. It's easy to peddle and get moving, but your legs are going like crazy. A higher pitch is like a higher gear on a bike. Starting out in a higher gear takes a lot of effort, but once your going your legs are moving at a more comfortable pace. Your boat only has one prop, so only one gear. You have to find the one that is the right balance of rpm at WOT for the load your pushing. That may be over simplified, but you get the picture.

 

[kal4438]

Re: Prop diameter help

Thanks to all for the great advice! Looks like I will be doing some more research on the specs for my outboard and installing a tach to get the best performance.

Mark,
I figured that with the smaller motor there would not be much difference between the aluminum and the SS props besides over $100!! I live in south Louisiana and do a lot of marsh running. I'm not sure how familiar you are with that, but if I put another aluminum prop, in 2 years I will be back either replacing the prop of running with a chewed up prop loosing performance. It's a tough area but a 14" marsh bass will fight harder than most 4 lb lake bass! I just can't stay out.

 

[Silvertip]

Re: Prop diameter help

Had you read the "sticky" at the very top of this forum you would have learned what is needed in order to make a stab at prop selection. Understand that a prop has many variables, not just diameter and pitch. Material, rake, cupping, blade shape, blade thickness, are just a few more. Two props of the same diameter and pitch from two different manufacturers can have very different performance characteristics. And an increase or decrease of 1/2 inch of pitch will have a near zero effect on rpm. Props designed for any engine fall into a range of diameters for the simple reason that larger diameter props simply won't clear the torque tab or AV plate. So the trick is to know what the current wide open throttle rpm is with what YOU consider an average load. You then shop for a prop for your specific engine. A range of designs and pitches will be listed. If your engine cannot reach the manufacturers WOT receommendation you need less pitch. If the engine over revs, you need more pitch. You select from the chart and then compensate a bit for three blade vs four blade, aluminum or stainless etc. If you get the idea prop selection is best done by testing you would be right. Otherwise it is a crap shoot.

 

[1980Coronado]

Re: Prop diameter help

Thanks to all for the great advice! Looks like I will be doing some more research on the specs for my outboard and installing a tach to get the best performance.

Mark,
I figured that with the smaller motor there would not be much difference between the aluminum and the SS props besides over $100!! I live in south Louisiana and do a lot of marsh running. I'm not sure how familiar you are with that, but if I put another aluminum prop, in 2 years I will be back either replacing the prop of running with a chewed up prop loosing performance. It's a tough area but a 14" marsh bass will fight harder than most 4 lb lake bass! I just can't stay out.

I understand! One thing you may want to consider....a SS prop is stronger, but if you strike something hard you may end up damaging more than just the prop due to the increased strength. Aluminum props are easier and cheeper to recondition than a SS prop, and the SS prop may not be repairable at all if bent. There are limits. The SS has it's place but it may not be a good choice for you and the type of use.