Prop Question

Alright guys, 1542 mon ark boat. Put some custom floors and tossed a 50hp Johnson on the rear. This is a 97’ with the 3 3/8 gearcase. I am running a 17 pitch prop and need to bump up to a 19. But... I cannot find one. Would getting a prop with more surface area in the same pitch and diameter bring my RPMs down and speed up a bit? I fear I may be getting a ton of slip with the style prop I have also. This thing is on redline, planed out in 3-5 seconds on the hole shot.

A 19" pitch will lower the RPMs.

Have you contacted any prop rebuilding shops to see if they can repitch it for you into a 19?

A 4 blade will bring your rpms down as will a stainless one. I wouldn't expect a 4 blade to bring the speed up any assuming it was the same pitch. What kind of speed are you getting? That is a really big motor for a 1542, especially considering you don't seem to have it weighed down with large decks and other stuff. If you dont really care about speed or at least adding in more, a 4 blade stainless prop would def get your rpms down and would give you more holeshot.

wmk0002 said:

A 4 blade will bring your rpms down as will a stainless one. I wouldn't expect a 4 blade to bring the speed up any assuming it was the same pitch. What kind of speed are you getting? That is a really big motor for a 1542, especially considering you don't seem to have it weighed down with large decks and other stuff. If you dont really care about speed or at least adding in more, a 4 blade stainless prop would def get your rpms down and would give you more holeshot.

Click to expand...

Please explain how a stainless prop brings RPM down ( assuming the prop is of the same pitch and # of blades).. not trying to bust your balls - trying to learn.

My understanding is that stainless props give more RPM because the blades are thinner and cut the water better than aluminum.
Also, my understanding of 4 blade props is that they give better holeshot and ability to stay on plane at lower RPM.. kind of like 4WD for a boat with a sacrifice of top speed because your trying to turn 33% more propeller.

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Correct.

GYPSY400 said:

wmk0002 said:

A 4 blade will bring your rpms down as will a stainless one. I wouldn't expect a 4 blade to bring the speed up any assuming it was the same pitch. What kind of speed are you getting? That is a really big motor for a 1542, especially considering you don't seem to have it weighed down with large decks and other stuff. If you dont really care about speed or at least adding in more, a 4 blade stainless prop would def get your rpms down and would give you more holeshot.

Click to expand...

Please explain how a stainless prop brings RPM down ( assuming the prop is of the same pitch and # of blades).. not trying to bust your balls - trying to learn.

My understanding is that stainless props give more RPM because the blades are thinner and cut the water better than aluminum.
Also, my understanding of 4 blade props is that they give better holeshot and ability to stay on plane at lower RPM.. kind of like 4WD for a boat with a sacrifice of top speed because your trying to turn 33% more propeller.

Sent from my SM-G903W using Tapatalk

Click to expand...

That's a good question about stainless and I don't know the exact answer. I don't have a lot of personal experience with SS props because I usually have underpowered boats and can't afford to play around with pitches in SS props. The general rule of thumb I have always heard though is that if you straight swap from AL to SS, you drop a pitch. I would guess part of the equation to that would be that, despite being thinner blades, steel is roughly 2.8 times the density of AL so taking into account thinner blades the SS prop may be 2X heavier. Not sure how much weight plays into it but on trucks tires of same size but with different ply ratings usually see different MPGs and a common mod to outboards is a lightened flywheel so it does matter to some degree. SS props also typically have cupping which will drop rpms if fully submerged...but when run with the motor up high can help keep the prop locked up while losing water resistance thus bringing rpms without prop slip.

What is your current RPM with the 17"? What is the max RPM for that engine?

To answer your questions, sometimes but not always. All props are made differently. A Solas new saturn has nice thick big ear blades and will usually bring RPM down if comparing it to the same pitch Powertech SRA3. Another option is to have your current 17" sent off and have it reworked. Sometimes adding some cupping will bring it down as well as increase the speed AND let you run it higher on the transom in choppy water before it blows out.

wmk0002 said:

GYPSY400 said:

wmk0002 said:

A 4 blade will bring your rpms down as will a stainless one. I wouldn't expect a 4 blade to bring the speed up any assuming it was the same pitch. What kind of speed are you getting? That is a really big motor for a 1542, especially considering you don't seem to have it weighed down with large decks and other stuff. If you dont really care about speed or at least adding in more, a 4 blade stainless prop would def get your rpms down and would give you more holeshot.

Click to expand...

Please explain how a stainless prop brings RPM down ( assuming the prop is of the same pitch and # of blades).. not trying to bust your balls - trying to learn.

My understanding is that stainless props give more RPM because the blades are thinner and cut the water better than aluminum.
Also, my understanding of 4 blade props is that they give better holeshot and ability to stay on plane at lower RPM.. kind of like 4WD for a boat with a sacrifice of top speed because your trying to turn 33% more propeller.

Sent from my SM-G903W using Tapatalk

Click to expand...

That's a good question about stainless and I don't know the exact answer. I don't have a lot of personal experience with SS props because I usually have underpowered boats and can't afford to play around with pitches in SS props. The general rule of thumb I have always heard though is that if you straight swap from AL to SS, you drop a pitch. I would guess part of the equation to that would be that, despite being thinner blades, steel is roughly 2.8 times the density of AL so taking into account thinner blades the SS prop may be 2X heavier. Not sure how much weight plays into it but on trucks tires of same size but with different ply ratings usually see different MPGs and a common mod to outboards is a lightened flywheel so it does matter to some degree. SS props also typically have cupping which will drop rpms if fully submerged...but when run with the motor up high can help keep the prop locked up while losing water resistance thus bringing rpms without prop slip.

Click to expand...

Your idea is close, but its not the weight, its the stiffness of SS vs Alu.

The aluminum props need thicker blades over SS due to the flexibility of the alu. An alu prop usually has around 25% slip due mostly to flexing under load.

An SS prop os the same pitch and design will have roughly 10% slippage as they really dont flex much. Hence the lower RPM and higher speed.

You can (Should) have cupping on any prop but it will be more effective on an SS, again due to the lack of flex.


As to airing the prop out as you mention in your last sentence, imagine the distortion of a flexy alu prop is the blades were intermittently coming out of the water Vs a SS prop. You would want to go up in pitch with a SS prop that you want to run high enough to get this unloading effect, but unless you are racing or need max speed there are more downsides than benefits in doing this.

Quite a few generalizations there.

Some AL props will run more efficiently than some SS props. I've got one of those cases. The factory white OEM Yamaha prop works really well for what it is, and is more efficient than the turbo hotshot.

But yes as a general rule you're pretty close to being spot-on.

Unfortunately, smaller motors are many times different than what larger motors are. Most say 1" of pitch equals 300 RPM. On my 25, 1" of pitch with the same rake and cupping is about 550 RPM. On the old 15hp, it was about 600. There were big differences with just one inch of pitch change.

It's not so much that aluminum flexes under normal operation, and it does (but so does SS), it's more about blade breakage and how easily they break and bend with loading beyond their intended usage. Surfacing with aluminum? Forget it. Surfacing and surface-piercing are hard on all props, aluminum being softer it will bend and/or break a lot faster. BUT at the same time, conventional wisdom says that AL will give before the propshaft bends. Sometimes it will but not always-particularly on smaller outboards that use tiny propshafts. There is a rubber hub in the center of most modern props that will eat up some of the shock loading, which is one reason I mentioned "conventional wisdom". The hub is really designed to dampen the shock during neutral-forward-reverse shifting, but it also absorbs some shock when striking underwater objects. If you hit something hard enough, it doesnt' matter whether it's Al or SS, something's going to give.

For those reasons, the only "real" way to know is trial and error.

Once again, for the original post, what are your RPM's currently with the 17"?

Generalizations?

No, brevity. Sometimes throwing words at the page just muddies the waters.

As an example, how do you find comparing an Alu prop of one manufacturer vs an SS prop from another relevant?

Even if they are the exact size and pitch their respective designs would differ, so apples/oranges.

The basis of the OPs problem is too much motor, not enough boat.

you'll probably shoot your eye out..

But you could source a 70 or 75 short shaft gearcase, put that on the 50; that should have a lower numerical gear ratio, and will spin a 17 pitch prop faster than the current gearcase, and would lower your max rpms on that skimmer.