anyone gain mph's by switching to SS prop?

[mango1229]

i have an 18' boat that has a '98 force by merc 120.(its been a great running motor) runs 41-42mph @ 51-5300 rpm's prop is 13.25x19. i'm considering a 20 or 21 pitch ss prop. ive spent hours reading all about stainless steel props but i'd like to hear from guys who have actually ran both on their force o/b. Did your boat/motor feel any more efficient? did you actually add a few mph's to your top speed?..Most importantly, did you feel the ss prop was worth the extra expense?
thanks!

 

[SkiDad]

i have both for my 125 engine in 15 pitch - the 15 pitch stainless runs 1-2 mph slower at WOT but has a better hole shot and mid range. I've run it with both of my lower units and it runs 300 rpm less than the aluminum with both It's hard to compare b/c while they are both michigan props the blade design is different and the SS has some cupping - the AL does not. The AL is 13x15 and the SS is 13.75x15 I think if you are looking for more speed you want to stay with a 19" pitch SS - going to 21 will drop your RPM out of the max horsepower range. Since your lower unit is a mercury you can run any of the mercury props that fit that size gear case. at the end of the day you might see 2 mph increase - your speed is already pretty good. I hear that turbo 1 and stiletto are good - you might even consider a 17 pitch in the Stiletto b/c I hear it is very raked for it's pitch. I have also hear that the Powertech RED3 is a good prop and you can get it in any pitch from 12-23 i think in 1" increments. Ken at Propgods could help you.

 

[mango1229]

thanks for the good info! just out of curiosity what is your top speed? i'm surprised your losing rpm's with the stainless. I would think you'd gain 2-300 rpms with ss in the same pitch-and- With cupping, drop back down to where you started RPM wise..i'm happy with my motors performance but after seeing people claim they hit 45-47 mph with their 120's, i was thinking maybe my aluminum prop is hindering some performance.. Now i'm also reading conflicting info about stainless props being easier for the motor to turn due to thinner blades. And some state that aluminum is easier to turn due to being lighter in weight?! does any of this have an effect on fuel consumption as well?

 

[Frank Acampora]

It really depends upon the stainless prop. IF you buy the same brand and pitch stainless, it will run about 200 RPM faster than the aluminum. HOWEVER, all bets are off with different brands. Because of different design, blade area, effective pitch, and rake, different brands will run differently than the aluminum prop you have on now.

Personally, I do not run anything other than stainless props--oh, occasionally a bronze old school prop-- but basically, nothing but stainless.

Again, personally, I like the Ballistic. While it does run slower RPM than the aluminum, it does give the same top speed Disclaimer--ON MY ENGINE/BOAT

AND, with the 98 Force, I don't think you will need to make any modifications to run the high rake Ballistic prop. Ballistic is a fully cupped prop. do not buy one in a higher pitch than what you have now.

 

[Jiggz]

I ran a 13.25X19 SS prop with my '89-125HP Force. The boat is a 19.5' Bayliner Ski Boat with a top speed of 42~45 MPH at 5100 RPM with two people onboard, on a sea state of 1 and virtually zero winds. As Frank mentions, I do not believe any higher pitched SS prop will do you any good. However, a comparatively pitched SS might. And we are talking maybe no more than 2 mph in top speed improvement. So the question, is that worth the hundreds of $$$ for SS prop? I know one thing for sure about SS prop, the do look sexy! Lol

 

[slx12001]

Jiggz
Is that 42-45 mph on gps? I run a 1989 1804 bass trophy 18' 125hp force and cant turn a 19p more then 4200rpm @ 40mph gps. I run 17p aluminum about 5200rpm and 39mph gps calm water. Motor compression is 140+ psi all 4 cylinders and tuned well. I owned a 1985 16' open bow Capri with an 85hp force as well. 17p 5200rpm 38mph gps. What hull are you running and prop? Thanks, Doug.

 

[jerryjerry05]

The standard prop that comes with the Force IS a cupped prop.
Just because it doesn't have the little bend out on the end doesn't mean it's not a cupped prop.
The 21p stainless is equal to a 23p aluminum.
The reason the rpm's drop is the stainless wheel is way heavier than the aluminum.
If you have a 19p aluminum now. You'll need a 17p stainless to do a straight swap.
I changed from a 17p aluminum to a 19p aluminum and got about 3mph more.
My fuel economy went down by as much as 20%

The performance with a stainless is better as it doesn't flex like aluminum.

IF????? You MIGHT???? JUST MIGHT??? hit a sandbar, or a stump or do a lot of shallow water boating????
Then stainless is NOT a good choice.
The stainless won't give/ break like aluminum.
It's possible to break the drive shaft or the prop shaft if you use stainless.

 

[Frank Acampora]

The weight of a prop has NOTHING to do with its ultimate ability to turn a given RPM. It MAY infinitesimally affect hole shot since it will take a very little more horsepower to accelerate it to its ultimate RPM. However, a couple of pound difference certainly will not have much effect at all. ULTIMATE RPM is determined by the load the prop delivers to the engine as it passes through the water. This is determined by (primarily) diameter and pitch with other factors such as rake, cupping, and blade thickness having a lesser effect.

THUS: As I said before: If you take an aluminum prop and make the exact same one out of stainless, the stainless will generally fun about 200 RPM faster and 1-2 MPH faster. I have already proved this using the old 1979-1984 Chrysler props of 500 series made by Michigan 510-19 (stainless) and 500 series 524-19 (Aluminum). The stainless of the 500 series is always faster than the aluminum.

 

[Jiggz]

No Title

Six,

Those are speedo readings and not GPS. However, these readings were taken from two separate devices, the built in speedo and also the wheel equipped Fish/Dept Finder and both readings are within 1 MPH of each other. The ski boat is a low profile with only about 16 inches of freeboard at the stern and about 20 inches at the bow with a sequential lift hull. With four people, it can still do ~38 MPH as long as it is properly balanced and trimmed. I posted a pic for you to see the sleek design of the boat.

 

[slx12001]

Jiggz
I have the 18' bass trophy which is the same basic hull as the cobra bass boat. The 16' capri was similar to the boat that you pictured. If you get a chance can you turn your phone on with any gps app to see actual speed? My speedo in the trophy bass is about 3-4 mph fast on top. There are many free apps. Thanks, Doug.

 

[Jiggz]

I've got a Garmin on the boat and it is only a matter of taking note of the speed. Normally, I just use my speedo which is the one I always refer to. Rest I assured this coming weekend I will post photos of the speedo, Depth finder and Garmin with the Tach. I and my daughter will take the boat for the ride before picking up some Dungeness crabs this Thurs.

 

[slx12001]

Cool. Your setup sounds fast. I hope to get mine running that good. Have fun, Thanks, Doug.

 

[Jiggz]

Six,

Did the run yesterday with the Garmin automotive GPS and the two installed devices. Two people on the boat, a 20-lb dog and about 100 lbs of crabbing/fishing gear for approximate combined weight of 400 lbs. And these are the recorded speed at WOT RPM of ~5200 RPM

In-Dash Speedometer - 44 MPH
Hummiingbird Fish/Depth Finder - 42 MPH
GPS - 40 MPH

Note, after double checking my prop, it was actually a Michigan 13.25" X 17P instead of 19P as initially claimed. Hope that helps.

 

[MickLovin]

No Title

i have an 18' boat that has a '98 force by merc 120.(its been a great running motor) runs 41-42mph @ 51-5300 rpm's prop is 13.25x19. i'm considering a 20 or 21 pitch ss prop. ive spent hours reading all about stainless steel props but i'd like to hear from guys who have actually ran both on their force o/b. Did your boat/motor feel any more efficient? did you actually add a few mph's to your top speed?..Most importantly, did you feel the ss prop was worth the extra expense?
thanks!

I have run a 23" aluminium, a 21" stainless and a 17" stainless, after all the experimenting I went back to the 21" stainless, if I had of found a 19" s/s I probably would have used that. My boat was a 17' half cabin carribean with a 100hp 1979 Chrysler with 150+ psi on all cylinders. as pictured below.
Sorry I couldn't find the pic of my 23" aluminium prop which actually came with my 100hp outboard.