Boat propped incorrectly, need to change to a smaller pitch

Roj115

Petty Officer 2nd Class
Joined
Aug 21, 2004
Messages
102
I just put my 2010 Glastron GT180 in the water and tested it for the first time.

It has a Evinrude ETEC 150 HO on it. Boat weighs about 2000 lbs with engine.

Current prop is a stock BRP Aluminum 14.5" x 19P.

With just me and two medium sized dogs in the boat and nearly a full load of fuel, the boat did 5000 rpm at WOT, trimmed up, Speedo said 50 mph FWIW.

Hole shot for waters-skiing is okay on that boat but a little less than I had expected it to have for that much horsepower.

Max rpm at WOT should be around 5800 for that engine.

The prop is too big which would also explain the slightly lower than expected hole shot.

I want to switch to a stainless steel prop anyways. Given the numbers achieved with the current prop, I'm thinking 17 pitch would be the most likely size to go to in order to get ~5,800 rpm at WOT.

Agree/disagree?

I want a good all around performing prop. I mostly use the boat for water-skiing and a bit for day cruising.

Thinking about either the Solas Rubex NS3 14.25" x 17" or maybe the Michigan Wheel Ballistic A 14.75" x 17".

Anyone have an opinion on these props for my intended use?

Could also look at a BRP Viper which I think has a bit more rake to it than the other two I'm considering. ​I don't really have an opportunity to water test the third party props, only the Viper.


Thanks!
 
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steelespike

Supreme Mariner
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Apr 26, 2002
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19,069
GT180 base 150 etec 52.6 wot 5700, 0 to 30 5.3 seconds.Don't know what prop.Also wonder if it may have been a 150HO.
Your numbers don't work. A 19 can't do 50 at 5000. using 10% slip indicates about 5700 rpm for 50.
 

gbrowne9

Petty Officer 2nd Class
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Jan 1, 2010
Messages
102
I would look into a turning point 14x17. I have a 23P on my 115 merc and it runs better than a 22p stainless.just my opinion.
 

Roj115

Petty Officer 2nd Class
Joined
Aug 21, 2004
Messages
102
GT180 base 150 etec 52.6 wot 5700, 0 to 30 5.3 seconds.Don't know what prop.Also wonder if it may have been a 150HO.
Your numbers don't work. A 19 can't do 50 at 5000. using 10% slip indicates about 5700 rpm for 50.

What calculator are you using?

I just plugged in the numbers here:

http://www.go-fast.com/Prop_Slip_Calculator.htm

And for a gear ratio of 1.86:1 5000 rpm with 19 pitch, it gives me an 11% slip for 50 mph and a theoretical speed of 56.

That sounds reasonable if I did everything correctly.

At any rate, I've never trusted boat speedo readings very much.

I think the second time I tested it at WOT, it only showed about 48 mph. According to the calculator I used, that would be 14% slippage which seems too high. My guess is the error is in the speedometer, which seems to be quite affected by the smoothness or roughness of the water the boat is going through.

I didn't have a GPS to confirm my actual speed which is too bad.

Of the two gauges, I would guess that the tach is likely to be the more trustworthy and accurate. Agreed?

And if I'm only getting 5000 rpm at WOT, then prop is much too big.

In talking to a local prop shop, he is recommending a 4 blade 14.5" x 15" stainless Solas Rubex for that boat. Worth trying?
 
Last edited:

steelespike

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Apr 26, 2002
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I use www.rbbi.com Its a green page. 5000,1.86,19" prop 50 mph It produces negative slip.Impossible.
If I leave out rpm, use 10% slip and other numbers to calculate for rpm I get 5700 rpm.
15" sounds really low for s 2000 lb boat. I stumbled on a 3100 lb boat that used a 14 or 15" pitch.
If you do a search at the Etec site you will find 50 is highly unlikely at 5000.Think about it
You have a boat that in tests a 150 produces speeds around 50 at wot indicated rpm 5700.
You can't do 50 @ 5000 with your setup.
As far as the tach goes historically the tach and on board speedometer are the weak links in calculations.
If we use 5000 and 10% slip I get 43.5 mph. If a gps gets 43 mph then the tach is accurate.
The gps is the only source of accurate speed time after time. Even then you need to be aware of currents , tides and wind.
You can use the gps to determine if there are currents of any consequence.
Before any prop change you need to confirm the rpm or get a gps speed.Most cell phones have a gps app.
 

Roj115

Petty Officer 2nd Class
Joined
Aug 21, 2004
Messages
102
I use www.rbbi.com Its a green page. 5000,1.86,19" prop 50 mph It produces negative slip.Impossible.
If I leave out rpm, use 10% slip and other numbers to calculate for rpm I get 5700 rpm.
15" sounds really low for s 2000 lb boat. I stumbled on a 3100 lb boat that used a 14 or 15" pitch.
If you do a search at the Etec site you will find 50 is highly unlikely at 5000.Think about it
You have a boat that in tests a 150 produces speeds around 50 at wot indicated rpm 5700.
You can't do 50 @ 5000 with your setup.
As far as the tach goes historically the tach and on board speedometer are the weak links in calculations.
If we use 5000 and 10% slip I get 43.5 mph. If a gps gets 43 mph then the tach is accurate.
The gps is the only source of accurate speed time after time. Even then you need to be aware of currents , tides and wind.
You can use the gps to determine if there are currents of any consequence.
Before any prop change you need to confirm the rpm or get a gps speed.Most cell phones have a gps app.


It was on a calm lake so currents and wind were negligible. The question is, was the speedo out by about 15% or was the tach out by about 14% or were they both out by a bunch and the error compounded?

I've never trusted boat speedometers so my instinct tells me that the speedo is more likely the one that was out and the tach was more likely telling the truth but I have no way to verify that. It could be the opposite way around or that both were out a bit.

The problem is, the boat's out of the water right now and I am 400 miles from it and won't be back there for a month. Based on the data that I have, I can't make a prop selection until I figure out where the error came from and get some more accurate data.

Thanks for your help and information!
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
When you get back there get a gps speed. An induction tach will get you a comparison rpm or perhaps you can borrow a shop tach.
In desperation guess 10 or 12% slip and use ratio,prop size,speed calculate for rpm.
 

Roj115

Petty Officer 2nd Class
Joined
Aug 21, 2004
Messages
102
I downloaded a bunch of the ETEC 150 and 150 HO performance reports for various runabouts today and started plugging their data into the rbbi.com prop calculator to see how much slip they were getting at various rpms on the various boats. I assume they had accurate instrumentation aboard those boats when they did their tests.

I got some very interesting results.

Basically, slippage on Aluminum props at low rpms before the boat planes can range 50 - 60%. Once planing, it drops to about 20% @ 3000 rpm then keeps dropping to about 4.2% at 4500 - 5800 rpm.

Stainless steel slips less at lower rpms but still gets up as high as 55%. At mid range, slip is about the same as Aluminum but at the higher rpm range, it seems to be as little as 2.5%.

Of course, the validity of this data depends on whether or not they had accurate measurements of speed and rpms when they did these tests. Hopefully, they didn't just go off the boat instruments but they don't specifically state what equipment was used to produce the data in the reports. They do report the water and wind conditions though.

If their data is accurate, it would appear to indicate that assuming 10% slip at WOT may be overly conservative and that it might be more accurate to assume something more in the range of 3 - 5% at WOT, depending on the material the prop is made of.

I'm thinking it might be interesting to start a new thread and post the Evinrude performance test data with the resulting slippage calculations so people can see how the propeller slip varies over the range of rpms for various propeller materials and types.
 
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Frank Acampora

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Jan 19, 2007
Messages
12,004
Ballistic is a great prop, BUT it has lots of surface area, has very high rake, and is fully cupped. I forget if that is bow lifting or stern lifting. On my Force engines the Ballistic turned the same top speed with a 400-600 lower RPM compared to a stock Force prop.
 
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