Prop change - a little surprised at the results

TexMonty

Seaman
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Dec 26, 2014
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Well I needed a spare prop. I have a 5.0 MPI in a Monterey 234FS and was running a stock 14.25 x 19 aluminum - 1.62 ratio. Performance was pretty good - 48-49 MPH at 5000 with gear, 1/2 tank gas and two adults. I wanted a SS so I went to my old faithfull boat prop shop and they said I should run the Solas NS3 14 x 19 prop. I got out on the boat this weekend with the new SS prop an with just me in the boat with all the same other conditions all I can get is 46 MPH at 4700-4800 rpm. It planes well but I lost a few MPH! Is this normal? It also seems I can trim out farther with the SS. Maybe it is the cupping? Not a big deal, but just curious.
 

steelespike

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Apr 26, 2002
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A ss prop will sometimes load the motor more.You could have a prop shop remove some of the cup.I've seen this improve rpm and speed.
I wonder if the prop isn't getting the boat up on the water as much as the al prop at 5000.
You might go back to the al prop just to see if something else has changed.
If it is an entry level ss prop it may not perform any better than the al prop.
 

SkiDad

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Jul 18, 2010
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i have the same deal- my 15p Aluminum runs 5600/36 mph my 15p Stainless runs 5300/35 mph - both are Michigan but stainless is cuppped.
 

gm280

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Jun 26, 2011
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Both condition doesn't sound unusual when you take into consideration that the SS props are definitely heavier then the aluminum props. So the engine sees a little more load with them turning. And the reduced RPMs is what dropped the MPH speed... Aluminum props will give a little and with their lighter weight will spin up a little higher then the same SS version. SS props won't flex as much and are certainly a lot more durable to hits and such. So what you gain in being a more solid durable prop, you loose in the RPMs area... I'd still take the SS over the aluminum as my personal opinion...
 

alldodge

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Aluminum starts flatting out as rpm increases causing a reduction in pitch. The stainless prop does not flatten out. Go back to your prop guy and see if you can try another manufacture. Everyone of the manufactures change things. Maybe a Mirage or Viper
 

TexMonty

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Dec 26, 2014
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Thanks all - what everyone is saying makes sense - more cupping, stiffer prop. etc. I just don't ever remember noticing it in the past and I have always started with an AL and bought the SS for everyday and kept AL for spare. I will stop by the prop shop where I bought it and see what they say. They are good guys and I know they will adjust it if I want. Thanks for the input.
 

rengnath

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Feb 15, 2015
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In my experience, when switching from an aluminum prop to a stainless, I need to drop 2" of pitch to obtain the same RPMs. Without dropping any pitch, I'll be about 350-400 RPM below on the stainless prop.

When switching from a 3 blade to a 4 blade, I also needed to drop 2" of pitch. Most people will tell you that you at least need to drop 1" of pitch for a 4 blade.

I have gone between my factory 3-blade 21" pitch (AL), 4-Blade Solas Amita 19" pitch (stainless), 4-Blade Solas HR Titan 19" pitch (stainless), and a 4-Blade Solas Rubex HR 17" pitch (stainless). All of the props hit 4800-4850 except for the HR Titan, which only hit 4500 when fully trimmed.

IMO, stainless is the only way to go unless you are often going in to shallows, stumpy areas, or unknown territory. You do risk shredding your gear housing with stainless if you hit something. Then again, when you run in to a sandy/slightly rocky bottom or hit a stick, you don't nick or bend the prop. Stainless seems smoother and the boat tracks much better at slow speeds.
 

Frank Acampora

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Jan 19, 2007
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Props by different manufacturers and even the same prop by the same manufacturer in a different material are all over the place performance wise. For example: on my Force outboards it is always true that the stock Michigan stainless will run about 200-400 RPM more than the same Michigan aluminum prop. A Michigan stainless Ballistic will however run at least 400 RPM slower than the stock aluminum prop.

What I am saying is that every boat/engine combo is different and every prop is different. You simply must experiment.
 

steelespike

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Apr 26, 2002
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19,069
The weight of the props doesn't enter into the equation.The exception is when shifting gears the heavier ss prop
may induce a tiny bit of increased wear on the shifting dogs. With the correct idle and smooth shifting technique
it really is of no consequence.
 

Tail_Gunner

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Jan 13, 2006
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The prop you went to is a big eared pusher its noted for pushing heavy loads lets take a look at slip...aka a pontoon prop

1.62@5000@19 yeilds 56mph in therory and 48.5 actual which is 13%
1.62@4750@19 yeilds 53mph in therory and 46 actual which again is 13%

Does your hull have bottom paint on it or growth...is it in seawater that is the first question....
 

dazk14

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Jul 22, 2008
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966
Both condition doesn't sound unusual when you take into consideration that the SS props are definitely heavier then the aluminum props. So the engine sees a little more load with them turning. And the reduced RPMs is what dropped the MPH speed... Aluminum props will give a little and with their lighter weight will spin up a little higher then the same SS version. SS props won't flex as much and are certainly a lot more durable to hits and such. So what you gain in being a more solid durable prop, you loose in the RPMs area... I'd still take the SS over the aluminum as my personal opinion...

Sorry, but just full of incorrect information. No disrespect intended.

THe OP should have been more specific....and not call the prop a Solas "NS3"...which I'll presume is a Solas "New Saturn" 3 blade...

+1 with tailgunner... The Solas "New Saturn" is NOT a go fast prop when(ever) there are other options and there are a ton of options...

What other props do they have for you to try? A properly fitted stainless prop should beat your current aluminum in every way.
 

TexMonty

Seaman
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Dec 26, 2014
Messages
65
I am sorry tail-gunner and DAZK14 - I missed any followup posts past steelespike on 3/17. No paint and no growth - bottom is waxed and smooth as a baby's behind. Fresh water. My al performance was very close to what TG says - I was right close to 50 on GPS at 5000 rpm. I still have the box off the SS prop and will check exactly what one it is. I will post it up tomorrow. If they sold me a big eared pusher they can take it back - even though I have been going to this prop shop since the 70's and have seen them do some great work on problem applications. Thanks and sorry for missing posts
 

TexMonty

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Dec 26, 2014
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It's an NS3 P# 9531-140-19 and yes it says big eared. It did not look much different than al. I guess I should have paid more attention but like I said I used this prop shop for years so I didn't ask a lot of questions. Based on my specs about boat any suggestion about a better prop/brand? Slip at 13% is about normal? Boat is as said 234fs 4600 #. The al performance looks perfect just lookin for a spare and always went ss as primary prop.
 

TexMonty

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Dec 26, 2014
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Another I realize I did not buy a go fast boat even though I'm a go fast guy. 50ish is good enough for now and it cruises with either prop nicely at 30 ish at about 3100 to 3200
 

Tail_Gunner

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Jan 13, 2006
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What do you want to improve mid range or top end..actually that 19 alum is doing well. One very big issue here is tach accuracy...it might be turning 5200 with the 19 in that case there are some good possibity's. Again what are you going to pleased with there are a few prop's that should should make you smile on midrange performance but i dont think there's a 3-4 mph top end gain going on here.

The Stilletto Baypro 2 in a 18 would lower planing speed increase midrange pop and be much smoother.....I just looked at them here....Really...hmmm very proud of that prop now arent we....:faint2:
 
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TexMonty

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Dec 26, 2014
Messages
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Thanks TG - I agree the AL prop is doing well, and yes I thought about the tach too since we know it probably isn't 100% accurate. I am not unhappy with the AL performance, and really can't tell any difference in the SS from the AL until I go to WOT. It seems I can trim out more with the SS but it certainly does not show it on the GPS. I will take a look at the one you mentioned and just play around with props and see what I come up with. Now that I have a better understanding I am good. Thanks everyone for your help.
 

Outsider

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Apr 24, 2007
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The OP changed diameter, material, manufacturer, but not pitch (which isn't consistent from prop to prop). Tack is the same and any 'error' should be consistent. Don't know what he was expecting, but it's not surprising he didn't get it. Now, he's back to the starting point, just with a different prop ... :cold:
 

Tail_Gunner

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Diameter means little unless high HP and heavy loads are present....think 5000lbs at least and a flat hull.
 
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