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Author Topic: Prop question  (Read 1560 times)

fish/hunt4ever

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Prop question
« on: May 15, 2023, 12:20 PM »
On the new boat it has a 13x24 prop and gets me out of the hole quick, just seems like the 175 does not hit the speeds I am thinking it should.  Just wondering what everyone else thinks of the prop that is installed on the boat.  Thanks for all and any help.

Mac Attack

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Re: Prop question
« Reply #1 on: May 15, 2023, 01:25 PM »
Need to look at RPM of the engine at full speed.
Compare that for the manufactured published allowable rpm specs of your motor.
There’s a rule of thumb often used that says each inch of pitch is generally worth 150 – 200 rpm.
Lowering the pitch increases rpm and vice versa.
So, for instance, if you were to lower from a 23-pitch to a 21-pitch propeller, you’ll increase engine RPM by about 400 revolutions.

4 blade props give you a better hole shot but you sacrifice top end.
Stainless props are stiffer than aluminum, so they resist flexing.
A thinner stainless steel prop spins up quicker and will attain a higher rpm vs an aluminum because of this.
So a stainless steel prop will out-perform an aluminum prop both out of the hole and at top end.
However, there's tons of guys that won't use a stainless prop for fear of damaging the lower unit or motor if they hit something with the prop.
All things to consider.

Good luck

Dorado

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Re: Prop question
« Reply #2 on: May 15, 2023, 03:48 PM »
Need to look at RPM of the engine at full speed.
Compare that for the manufactured published allowable rpm specs of your motor.
There’s a rule of thumb often used that says each inch of pitch is generally worth 150 – 200 rpm.
Lowering the pitch increases rpm and vice versa.
So, for instance, if you were to lower from a 23-pitch to a 21-pitch propeller, you’ll increase engine RPM by about 400 revolutions.

4 blade props give you a better hole shot but you sacrifice top end.
Stainless props are stiffer than aluminum, so they resist flexing.
A thinner stainless steel prop spins up quicker and will attain a higher rpm vs an aluminum because of this.
So a stainless steel prop will out-perform an aluminum prop both out of the hole and at top end.
However, there's tons of guys that won't use a stainless prop for fear of damaging the lower unit or motor if they hit something with the prop.
All things to consider.

Good luck

Great advice.  I have found that here in WY, you need to go down several sizes in the pitch of the propellers that come on boats set up to run at lower elevations than we typically have here.  Boysen is around 4,800 feet.  The lakes I go on the most range from 6,400-8,000, you really notice that difference! 

I would go out with your boat loaded as you typically use it (not empty of people, gear and gas) and note your maximum RPM at full throttle.  You want to be able to achieve close to max RPM at full throttle.  Your manual you got with the new boat should tell you what the max RPM is, looks like 5,500-6,000 RPM.  I would guess you are going to want a 21" pitch, but you won't know until you get out and test it. 

I am in the process of doing the same thing on my new (to me) boat.  It has a 19" x14.5" and I am thinking I will need to go with a 15".  Your hole shot will noticeably improve with the correct prop!  Plus, as I have read, it is better on the engine to not overload it with a too large propellor that does not allow max RPM...

fish/hunt4ever

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Re: Prop question
« Reply #3 on: May 15, 2023, 08:56 PM »
RPM on the motor when I trim it out, fully loaded with 5 people and my gear and dog, I get the 6000 rpm that they say I should be getting. The boat gets on plane right now, pushes you back in the seat. Just seems like everyone I talked to said they usually get about 45 mph out of their boats, I know not mine, but figured it was worth a question, plus I always learn something new from these types of questions.

As I am one of those guys that tend to stay away from stainless props but now understand the reason that some really like to run them, I just seem to hit things from time to time and would hate to kill my lower unit and then have to pay to repair it, but I guess hitting something might do that anyways.

TT

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Re: Prop question
« Reply #4 on: May 16, 2023, 01:02 AM »
What is your speed at 6000rpm loaded?

Mac Attack

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Re: Prop question
« Reply #5 on: May 16, 2023, 07:38 AM »
Lifting and lowering the motor on the transom a hole or two also comes into play.
This is why guys use jack plates.
Too low and it creates too much drag and slows down the boat.
Too high and it cavitates and it will "blow out" on sharp turns.


Lastly, don't believe other guys on the speed of their boat.
Like the size and numbers of the fish they catch, they will lie.
 ;)

Use a tachometer and your GPS.
Not the speedometer on your boat if it has one.
Boat speedos are notoriously inaccurate.
They use a pitot tube and water pressure thru a long plastic tube connected to it and the speedo to extrapolate a rough speed.
The GPS is far more accurate.

Dorado

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Re: Prop question
« Reply #6 on: May 16, 2023, 10:04 AM »
RPM on the motor when I trim it out, fully loaded with 5 people and my gear and dog, I get the 6000 rpm that they say I should be getting. The boat gets on plane right now, pushes you back in the seat. Just seems like everyone I talked to said they usually get about 45 mph out of their boats, I know not mine, but figured it was worth a question, plus I always learn something new from these types of questions.

As I am one of those guys that tend to stay away from stainless props but now understand the reason that some really like to run them, I just seem to hit things from time to time and would hate to kill my lower unit and then have to pay to repair it, but I guess hitting something might do that anyways.

Sounds like you have the right prop for your needs!  SS prop definitely going to give better performance.  I have heard mixed opinions about the claim that using a SS prop you will blow out your lower unit if you hit something.  With the general point being that if you hit something hard enough to smash up an aluminum prop, you are probably going to damage the lower unit anyways.  Nicking something with an SS prop is not going to damage the lower unit. 

fish/hunt4ever

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Re: Prop question
« Reply #7 on: May 16, 2023, 05:14 PM »
GPS speed on my fishfinder is say 36 mph. But just got through the 2 hour break in period and starting on the 8 hour one that I can run full throttle for 5 minutes before I back out of it. Have the tilt and trim on the motor and use it like my old boat. I got 28 to 31 out of the 90hp on an almost 18 foot boat, granit that it was a side console and this one has the full windsheild and is almost 20 foot long. Like I said just seems a bit off for top end for me. Thanks so much for all the great information everyone.

Dorado

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Re: Prop question
« Reply #8 on: May 17, 2023, 09:49 AM »
GPS speed on my fishfinder is say 36 mph. But just got through the 2 hour break in period and starting on the 8 hour one that I can run full throttle for 5 minutes before I back out of it. Have the tilt and trim on the motor and use it like my old boat. I got 28 to 31 out of the 90hp on an almost 18 foot boat, granit that it was a side console and this one has the full windsheild and is almost 20 foot long. Like I said just seems a bit off for top end for me. Thanks so much for all the great information everyone.

That does seem low.  I was getting over 40 mph on my 18' 10" Ranger with a 150 and a 19" prop at 7,500 feet.  A heavy fiberglass boat.  But it only achieved around 5500 RPMs and has a max of 6200, so I am definitely over-propped.  I wonder if you are under-propped?  The engines today are rev-limited by the computer, so you may still have something left that a bigger prop might be able to exploit?  I would contact a professional to get the proper set up dialed in...

fish/hunt4ever

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Re: Prop question
« Reply #9 on: Jun 08, 2023, 07:22 AM »
Well was looking at the prop this weekend as I took what I thought was for the prop off the lower unit but this looks like a 14 pitch prop.




Might have to go up as I

 



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