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Where to find proper prop size?

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Blackbart62

New member
If I were to have an engine but no prop on it, how would I find out what prop size, pitch, # of blades, etc...to look for? :confused: There are alot of different numbers on my prop. 48 77346 (or 8) A4 19P. Then there are much larger numbers in two different sets of 4. ie: 6920 and 8285
 
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The 19P indicates it's a 19 pitch prop.

It's a little more complicated than simply looking on a chart to pick a prop.

The boat that the motor is on and how it's typically loaded is just as important as the horsepower of the motor.

Think of the prop as an extension of the gears - so just like a vehicle. The gears will be alot lower in a 3/4 ton truck than in a two seat sports car even though they both may have 250 horses under the hood....
 
try this, do a google search for "Outboard prop diameter and pitch"
There are a lot of sites avalible. Look for one that includes boat length, use, engine size and such. That no. you included starting with 48 is the merc part no.
Next time you post, please incude all the info on your motor and boat so we can better help you...........
 
Thanks guys. It is for a 90 hp 2 stroke 3 cylinder Mariner ELPT (1984 I was told) It sits on a 16' Vanguard Raider bowrider. I was told by the past owner that it is 5 hp oversized for the boat. The engine serial # is 7214500. I've looked on numerous sites (which I find confusing) because I can choose from a 3 or 4 blade prop, stainless or aluminum. I don't know which is the better application. I won't be pulling skiiers or tubes, just touring and fishing from it. I'm not sure whether it is oil injected or not? As far as prop diameter goes can't I just measure it? I can't tell what it is by all of the numbers.
 
I can give you some "rules of thumb" with regards to props -

The less number of blades, the higher top speed potential (all else being equal) but the more blades, the less vibration you will experience (and the motor) - so while a single blade might give you the best speed it would vibrate so much it would likely do damage to the seals if not the gears in the lower unit.

So if you have two props, both 17 pitch, the 3 blade will give you a hair more speed than the 4 (but you would need a pretty sensitive gps to see the difference - definately nothing you would "feel").

4 blades "bite harder" than a 3 blade - so if you are looking to "pop a skier" they are desirable.

A 4 blade will also get you on plane at a lower speed and keep your there at a lower speed all else being equal - a definate plus for a cruiser that is more interested in fuel economy than zipping accross the lake to beat the guy beside them to the prime fishing hole.

A stainless prop will flex less than aluminum which will flex less than a composite (most composites are some form of plastic).

Less flex, means a better bite on the water, so in this case a 3 blade 17p stainless should give you a little better speed than a 3 blade 17p aluminum etc - again, without finely tuned equipment it's not a difference you would ever notice.

Additionally, if you are running in salt water the stainless will withstand corrosion considerably better than aluminum but definately not as well as plastic which just doesn't corrode.

As far as cost - the composites and aluminums are often comparable in price - a stainless can cost 3 to 4 times more - but maybe you don't mind paying 400 bucks for a stainless prop.

Then you have to consider repairs should you "ding" a blade or worse yet really fold one over if you hit a rock.

The stainless will resist small impacts the best of the bunch but if you do damage it, it is the most expensive to repair.

At the other end of the scale, the composites often have replacable parts which you can repair yourself right on the water. One of my props is a 4 blade ProPulse. You can vary the pitch between 16 and 21" (but not while you are underway) - so if I'm going out "light" I can crank it around to 21 and if I'm loaded heavy or the weather is hot/humid (which restricts an engines ability to breathe properly) I can dial it down to 16 pitch (or any whole number in between).

Each blade is independant so I carry a couple spare "blades" - that way if I toast one, I can change it in about 5 minutes and be on my way - with a stainless or alumimum model, if you fold a blade over during an impact you either better have a spare prop on board, a towing plan or a couple of really big paddles.

As a guess, based on the "average boat out there" of your size kitted with that motor I would suspect you should be running a 17 pitch under normal conditions (a 15 if you load very heavy and maybe a 19 if you load very light) but in the end it's always the tachometer that tells you if you have the correct prop for your conditions and the way you load/use the boat - you can only get close with a chart which can't even hope to consider all the variables...
 
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galamb said:
I can give you some "rules of thumb" with regards to props -

The less number of blades, the higher top speed potential (all else being equal) but the more blades, the less vibration you will experience (and the motor) - so while a single blade might give you the best speed it would vibrate so much it would likely do damage to the seals if not the gears in the lower unit.

So if you have two props, both 17 pitch, the 3 blade will give you a hair more speed than the 4 (but you would need a pretty sensitive gps to see the difference - definately nothing you would "feel").

4 blades "bite harder" than a 3 blade - so if you are looking to "pop a skier" they are desirable.

A 4 blade will also get you on plane at a lower speed and keep your there at a lower speed all else being equal - a definate plus for a cruiser that is more interested in fuel economy than zipping accross the lake to beat the guy beside them to the prime fishing hole.

A stainless prop will flex less than aluminum which will flex less than a composite (most composites are some form of plastic).

Less flex, means a better bite on the water, so in this case a 3 blade 17p stainless should give you a little better speed than a 3 blade 17p aluminum etc - again, without finely tuned equipment it's not a difference you would ever notice.

Additionally, if you are running in salt water the stainless will withstand corrosion considerably better than aluminum but definately not as well as plastic which just doesn't corrode.

As far as cost - the composites and aluminums are often comparable in price - a stainless can cost 3 to 4 times more - but maybe you don't mind paying 400 bucks for a stainless prop.

Then you have to consider repairs should you "ding" a blade or worse yet really fold one over if you hit a rock.

The stainless will resist small impacts the best of the bunch but if you do damage it, it is the most expensive to repair.

At the other end of the scale, the composites often have replacable parts which you can repair yourself right on the water. One of my props is a 4 blade ProPulse. You can vary the pitch between 16 and 21" (but not while you are underway) - so if I'm going out "light" I can crank it around to 21 and if I'm loaded heavy or the weather is hot/humid (which restricts an engines ability to breathe properly) I can dial it down to 16 pitch (or any whole number in between).

Each blade is independant so I carry a couple spare "blades" - that way if I toast one, I can change it in about 5 minutes and be on my way - with a stainless or alumimum model, if you fold a blade over during an impact you either better have a spare prop on board, a towing plan or a couple of really big paddles.

As a guess, based on the "average boat out there" of your size kitted with that motor I would suspect you should be running a 17 pitch under normal conditions (a 15 if you load very heavy and maybe a 19 if you load very light) but in the end it's always the tachometer that tells you if you have the correct prop for your conditions and the way you load/use the boat - you can only get close with a chart which can't even hope to consider all the variables...
Click to expand...


wow veryvery well said thanks!
 
Thank you very much Sir, I appreciate your time and expertise. I'll not only take all of this to heart but, I'll also print it for future reference....:) Oh, and buy an extra prop.
 
If you find a dealer "worthy of your hard earned money" they should be more than willing to spend a few minutes with you to determine what prop may best suit your needs.

If they simply ask what boat/motor you have and pull out a chart find another dealer - they should ask additional questions as to how you use the boat, how many passengers you carry etc.

Make them work for the "mark up" they are charging for the product... :)
 
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