Surprising Prop Test Results - can you guess which prop worked best?

[Roj115]

I recently tested 3 propellers to replace the current propeller on my 2010 17' Glastron 180GT powered by an Evinrude E-Tech 150 HO. The results were not what I expected and really help illustrate how difficult it is to predict which propeller will be best for a given boat/motor combination given the vast number of variables involved.

The propeller that was on the boat when I bought it was a stock BRP 19 pitch, 3 blade, 14.5" diameter aluminum prop. At WOT the propeller gave 5050 rpm and around 47 - 48 mph (according to a handheld Garmin GPS).

For the ETec, the max rpm is 5,850 and the gear ratio is 1.86:1 so my goal was to get a prop that gave me as close to 5,850 at WOT without going over it.

Here are the 3 props that I tested.

Please note all of the following propellers were Solas brand and were all Stainless Steel.

Prop 1:

17 pitch, 3 blade, 14.25" diameter.

Prop 2:

17 pitch, 4 blade, 14.25" diameter. Blades are more cupped than the 3 blade version

Prop 3:

4 blade, 14.5" diameter. Again, blades are highly cupped on the Solas 4 blade props.

There was a clear winner. Care to guess which one came out on top? ;-)

 

[GA_Boater]

The one you left behind the prop nut. :smile:

 

[David Young]

I'll guess........The 17 pitch 3 blade 14.25 diameter. Just a guess

 

[82rude]

door number 2

 

[steelespike]

3
2
1

 

[64osby]

You didn't state the pitch on prop #3.

I'll tag on with steelespike.

 

[Fed]

One of your spark plug leads must have fallen off.
http://www.usboattest.com/boat-tests/bowrider/2012/glastron/gt-180/817/2012-glastron-gt-180.html

 

[Roj115]

You didn't state the pitch on prop #3.

I'll tag on with steelespike.

Whoops, my bad! Prop 3 was a 15 pitch, 14.5" diameter 4 blade stainless Solas prop.

 

[Sea Rider]

The one that remained there after testing all 3, LOL!! Would be interesting to know which one filled the 350 rpm gap or close to it to achieve max wot 5850 rpm.

Happy Boating

 

[Roj115]

It was actually an 800 rpm gap. Original rpm with the stock BRP19 pitch, 3 blade 14.5" diameter aluminum prop was 5050 rpm.

Surprisingly, Prop #1, the 3 blade 17 pitch 14.25" diameter stainless steel Solas prop brought the rpm up to 5800 - 5900 at WOT and gave me a top end speed of 50 - 51 mph according to the GPS. This result completely surprised me as the rule of thumb is that you gain maybe 300 - 400 rpm for a 2" drop in pitch. I figured going from Aluminum to a stiffer stainless would reduce the rpm gain a bit but that didn't seem to make any difference. On the other hand, I thought the quarter inch drop in diameter would gain me a few rpm but I didn't expect it would make a significant difference to the amount of rpm gained.

As I expected, Prop 2, the 17 pitch 4 blade stainless Solas, was too much prop. I think I got about 5,400 rpm max and top speed was down around 46 or 47 mph.

The prop I figured would be the winner was Prop 3, the 15 pitch, 4 blade, 14.5" diameter Solas came in second but was a bit too small despite the amount of cupping Solas puts into the blades of their 4 blade props. It delivered 6,000 rpm at WOT but a top end speed of only 46 or 47, if I recall correctly.

So the surprise winner was the 3 blade 17 pitch. I never would have guessed that it would deliver 800 more rpm with only a 2 inch pitch drop but the smaller diameter and probably some changes in some other variables must have had a significant bearing on the final result. It just goes to show you what an inexact science propeller selection is and how difficult it is to predict how a given boat/motor combination will perform with a given prop on it.

 

[David Young]

Nice test.

 

[82rude]

Wonder how a 4 blade aluminum solas would of fared?At least on my boat the diff between a 4 and 3 blade was zero in all respects except I can keep the boat on plane at a lower rpm with the 4 blade vs the 3.

 

[Sea Rider]

Nice to read that finally have learned that dialing an accurate rpm increase is extreme difficult. The rule of thumb that states 150-200 + revs with 1 pitch decrease ain't so.You can't predict in theory what's best for a given combo to suit best your boating application, ​whoever stated that near cast on stone rule of thumb is out of his mind, can't be applied to all combos as boat's weight, hull drag, HP has not been taken into the least consideration. Imagine optimizing a prop for a combo that achieved at wot 5600 rpm and in pursuit of max 6000 rpm going for 2 less pitch drop idiotic rule of thumb.

Happy Boating

 

[Fed]

So put the 19 back on and also the spark plug lead.
http://www.usboattest.com/boat-tests/bowrider/2012/glastron/gt-180/817/2012-glastron-gt-180.html

The Glastron GT 180 is just the kind of fun boating families need: sporty, comfortable and cost effective.
TEST BOAT ENGINE
Evinrude E-TEC 150, 158.2 cid / 2.592 L, V-6, 150-hp two-stroke with direct fuel injection, driving a 19-inch pitch three-blade aluminum propeller.

ACCELERATION (mph/sec)
0-20 / 4.1 ; 0-30 / 5.3 ; 0-40 / 7.6
TOP SPEED (rpm/mph)
5700 / 52.6
CRUISING SPEED (rpm/mph)
3500 / 30.6 ; 4000 / 36.4 ; 4500 / 40.5 ; 5000 / 47.3

And pick up another 6 MPH for free.

 

[Sea Rider]

The original stock 19 prop just achieved 5,050 rpm at wot, boater wanted going full 5850 max wot rpm which a second 17 pitch achieved, Why would he want going back to his stock 19 pitch if it was a poor performer. At 5050 revs engine is at the lugging side of the wot range, much worse if just lightly loaded while being tested at wot.

Happy Boating

 

[ttankmoran]

Your numbers don?t jive


For the 19 Pitch
5050 RPM + 47-48 MPH GPS = ~3% Slip


47-48 MPH + 10% slip = 5400-5500 RPM

For the 17 Pitch 3 blade
50-51 MPH + 10% slip = 6500-6600 RPM
5800-5900 RPM + 17 Pitch = 0 % Slip

 

[Sea Rider]

What doesn't jive max wot rpm or achieved speed for both 17 & 19 pitch props. Don't know about GPS speed but going 2 pitch down will achieve way more than the 300-400 + rpm idiotic rule of thumb. This would be the fifth example in which plain theory doesn't always match practice.

Happy Boating

 

[ttankmoran]

I'm not talking theory here, I'm talking math. Find yourself a prop slip calculator and find out why.

I can tell you with 100% Certainty your not traveling 51 MPH with a 17P prop with a 1.86:1 gear ratio at only 5900 RPM, unless the pitch is different than advertised, your RPM is much higher than indicated, the gear ratio is different, or you are operating in a current.

 

[Sea Rider]

Probably max wot rpm doesn't match stated GPS speed, could be a GPS inaccurany, but will match the huge rpm difference between a 19 and a 17 pitch prop, that's 400 + revs per pitch drop, stated 150-200 + revs only works ok when maximizing props to power real heavy combos, non recreational watercrafts.

Prop calculators are not fool proof is just a refference from where to start, doesn't take into account the following ; hull design, engine/transom height, trim, prop brand/model, water density, ambient temp, elevation.

Happy Boating

 

[Sea Rider]

Which slip & top speed in KM/H will this combo achieve at full wot.

Scenario :

-4.50 Mtr boat rated for 50 HP, max 6 passengers,
-Lightly loaded combo 400 KG, just driver.
-2 strokes 18 HP long shaft,
-Max wot rpm 5700 out of 5800 max
-Prop 3 X 9.2 X 9.1 (3 blade, alum, 9 pitch standard cupped)
-Flat calm dead sea no wind water cond ?
-Engine and prop 100% healthy.

Can that data put inside a prop calculator and give top wot speed.

Happy Boating