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Skipper
Posts: 11
Registered: ‎10-23-2019

Propeller Selection


Warning: this post is a rather long post and has lots of assumptions ; you're basic house of cards

 

I'm looking for suggestions for prop selection for a thunderjet 185.  I will likely only be able to borrow a few props for testing, but would like an idea of what might work.  The dealer is recommending a Rebel 15.5 x 17 which seems to have too low a pitch.

 

A previous model of this boat weights 1800 lbs. My objective is to have a 50 mph boat that is reasonably fuel efficient at 30 mph cruise.  I'm assuming that a 3 blade would be the best for my application.

 

I have a performance report on this boat by a Mercury 150 4 stroke. This motor has a 1.92 gear ratio and the propeller they used for the report was a 14.5 x 17 - but I have idea about the prop design.  

 

RPM Speed (MPH)
1000       8
2000     10
3000     24
3500     30
4000     35
4500     41
5000     46
5500     49
6000     51

 

The engine I have in mind is the 25" Evinrude G2 150 HO that has a 2.17 gear ratio. If I've done my math right, I believe that a 14.5 x 19.2 would give me roughly the same RPM to MPH when taking gearing into account.  I'll assume that with a 14.5 x 19 WOT would be roughly 6040 RPM ( based on 200 rpm / 1" pitch), so I'd think a starting point could be a 14.5 x 19 or 14.5 x 20.  In looking at the performance reports for similar boats, the most I saw was 15 x 21.

 

Not taking slip into consideration, and calculating volume for one revolution:

 

Prop               Line        Volume

14.5   x 19        RX3          3137

14.75 x 19        RX3          3246

14.5   x 20        RX3          3302

14.75 x 20        RX3          3417

15      x 19        Rebel        3357

15.25 x 19        Rebel        3417

15      x 20        Rebel        3534

15      x 21        Rebel        3711

 

Does this seem to be a reasonable short list?  Should I consider a 4 blade prop?

 

Thanks

 

Rob

 

 

Highlighted
Skipper
Posts: 15
Registered: ‎10-16-2013

Re: Propeller Selection

[ Edited ]

For flat out testicles to the wall WOT top speed in a helm forward weight forward design, need a strong bow lifter prop to improve front half hydrodynamics drag in the water.  Less boat in water =s faster.  Don't be scared to trim it out.

 

Raker HO in 22P should give about 5500 rpms WOT full load.  Any more than 5500 rpms WOT on these G2 V6s leaves some top end speed and fuel economy on the show room floor.  May even be able to swing a 24P Raker HO to 5500.
Absolute minimum WOT rpms on these G2s for long engine life is 5300 rpms loaded.  Overpropped Lugging kills outboards, overly hot alum pistons, rings, things start melting.

 

Adjust the vent holes for the hole shot you like.

RX3 will give you better mpg in most cases.

 

Merc 150 4 smoke is a huge motor, 3L vs 2.7L

Highlighted
Skipper
Posts: 11
Registered: ‎10-23-2019

Re: Propeller Selection

Interesting.  I would never had thought about a Raker HO.  From my assumptions in my original post, and making a huge assumption that the prop design of the merc was similar, a 14.5 x 22 should give about 5600 rpm.  Thats a big assumption but the raker is an option I'd not entertained.  I would think that a 22" would make it move well faster than 50mph (maybe 55/60?) and I'm thinking that my hull is not really designed to be for that fast.  I'll probably still try to optimize around a top end at or around 50mph

 

In any event, looks like I have alot of potential options out here...

 

I'l have to read up on the vent holes for the hole shot to understand how that works

 

Thanks

 

Rob

Highlighted
Skipper
Posts: 11
Registered: ‎10-23-2019

Re: Propeller Selection

What attribute of a propeller will give best fuel economy?  Is it the diameter?  If so, wouldn't the rebel line tend to be more fuel efficient than the rx3?

Highlighted
Admiral
Posts: 8,844
Registered: ‎07-14-2011

Re: Propeller Selection

 

 

The RX3 replaces the Rebel series and is a bit faster and more fuel efficient depending on the boat hull design.

 

 

 


"There is never just one thing wrong with a boat";
                    -- Travis McGee, main character in a book series by John D. McDonald 


 



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