Help selecting prop. Currently no top end.

[Immo]

Hi all,

I have just bought a quintrex freedom cruiser 560 with a 2008 3.0L mercruiser inboard/outboard stern drive, 135hp.
I am having trouble with top end speed. It feels like the boat is revving high but going nowhere. Seems to get on the plane well enough and we can get riders out of the water well enough, but lack of top end is frustrating.
Current prop is the original mercury prop,19 degrees alloy that came with the boat. (It’s not in good shape either).
Wide open throttle RPM is about 4200 and I can only get 45 - 50km/h top speed with an average load.
Last boat was a Haines signature 520BR with an outboard 135hp Mercury 2stroke. It had a 19degree avenger prop that performed really well. Loved it.
I was thinking of getting the same prop for the mercruiser, but any hints on other potential props that will help with top end performance would be welcome.
We use the boat as a crossover for skiing and fishing.

 

[WesNewell]

Mercury prop finder suggest a 21p prop with a 2.0 GR sterndrive at 41 mph (66 kph) 4600 rpm with 600# load.

Worldwide Leader in Marine Propulsion

Mercury Marine is the world's leading manufacturer of recreational marine propulsion engines. Get sales, service and parts info, and find a local dealer near you.

www.mercurymarine.com

 

[alldodge]

Prop not in good shape can cause ventilation. If your tachometer is accurate (many are not) then even with a bad prop your not reaching WOT rpm (4400-4800). Your motor may have an issue and be down on power.

Do you know the motor serial number?

 

[Immo]

Prop not in good shape can cause ventilation. If your tachometer is accurate (many are not) then even with a bad prop your not reaching WOT rpm (4400-4800). Your motor may have an issue and be down on power.

Do you know the motor serial number?

Thanks. It may well rev higher but I just stopped opening it up as more revs didn’t equal more speed with the current prop.
serial no 1A314786

 

[alldodge]

Thanks for the sn, its a TKS
WOT rpm is what's needed for proper prop selection. Increase speed and trim up until rpm gets to max and speed is increasing. If trimming up starts loosing speed or does not increase speed then trim down for max with both speed and rpm. The load out of the boat also effects these numbers.

If to heavy in the bow may need to move it to the rear. Using a drive ratio of 1.98 I'm seeing prop slip numbers in the 18% area which is pretty bad for that size boat

 

[Immo]

Thanks for the sn, its a TKS
WOT rpm is what's needed for proper prop selection. Increase speed and trim up until rpm gets to max and speed is increasing. If trimming up starts loosing speed or does not increase speed then trim down for max with both speed and rpm. The load out of the boat also effects these numbers.

If to heavy in the bow may need to move it to the rear. Using a drive ratio of 1.98 I'm seeing prop slip numbers in the 18% area which is pretty bad for that size boat

Thank you. Very helpful.
Alldodge, do you know of other people using props / pitch on this model that have worked well? I’m after a couple of options.
From what I can see on the Mercury site, the Vengeance 19p is well suited, and there are a few similar props that seem to have similar specs.

 

[Immo]

Mercury prop finder suggest a 21p prop with a 2.0 GR sterndrive at 41 mph (66 kph) 4600 rpm with 600# load.

Worldwide Leader in Marine Propulsion

Mercury Marine is the world's leading manufacturer of recreational marine propulsion engines. Get sales, service and parts info, and find a local dealer near you.

www.mercurymarine.com

Hi Wes, thanks for the reply.

when I use the site I get a different answer, the Vengeance 19p.
I’ve gone through this forum and found others who have compared the vengeance to similar 3 blade props and their specs all seem relatively similar. Would the prior you’ve found (21p) be better at top end but worse for low end acceleration? We do a lot of skiing and wakeboarding with the kids so I don’t want to lose too much ability to get out of the water.

 

[alldodge]

I have no direct knowledge of Quintrex boats. Member @achris lives on your side of the pond and may have some ideas.

Many folks here really like the Enertia for top end speed.

 

[WesNewell]

here's what I ran. Different numbers will result in different output.

Your setup​

• Boat type: Runabout - Open Bow
• Usage: Overall good performance/0 ft
• Engine Manufacturer: Mercury
• Engine Type: Sterndrive
• Engine Year: 2008
• Number of Engines: One
• Engine Family: Inline 4
  Engine: 3.0L
• Drive / Gear Case: Alpha 2.0 Gear Ratio
• Details
  • Calculated Pitch: 21.00
  • Calculated Weight with engine(s): 3,529.70 lbs
  • Calculated Engine RPM: 4,600
  • Calculated Boat Speed: 41.64 mph

 

[QBhoy]

Ok. So a few things to mention from me.
First thing and most obvious for me is that your current boat should never ever be compared to your previous Haines signature boat and it’s outboard of the apparent similar power. I’m a huge fan of the Haines boats. They are blistering quick hulls and as you’ll know, outrageously quick boats and designed with water ski racing in mind. Generally considered amongst the fastest boats (pound for pound) in the world. Although the particular model you had, was a slightly tamed version of the usual Haines hull...she would still be extremely quick, compared to most boats for sure. Famous for being quick, these things.

The 3.0 135/140hp GM inboard is a great wee reliable thing. But should never be compared to a 135 V6 2 stroke on the back of a racing derived hull.
Other thing is that at 4200 rpm on your current boat, you’re under propped if the tachometer is right.
Next thing is to forget the vengeance. They are a waste of money and time.
I don’t think you are too far away on pitch. You’ll be around the 17-19” mark I’d think, if it’s the usual gear ratio for that set up.
obvious props to try and squeeze the best out her would be laser 2, black max ally and tempest plus props of a pitch between 17-19. 17” tempest plus might be an idea.

 

[Immo]

Ok. So a few things to mention from me.
First thing and most obvious for me is that your current boat should never ever be compared to your previous Haines signature boat and it’s outboard of the apparent similar power. I’m a huge fan of the Haines boats. They are blistering quick hulls and as you’ll know, outrageously quick boats and designed with water ski racing in mind. Generally considered amongst the fastest boats (pound for pound) in the world. Although the particular model you had, was a slightly tamed version of the usual Haines hull...she would still be extremely quick, compared to most boats for sure. Famous for being quick, these things.

The 3.0 135/140hp GM inboard is a great wee reliable thing. But should never be compared to a 135 V6 2 stroke on the back of a racing derived hull.
Other thing is that at 4200 rpm on your current boat, you’re under propped if the tachometer is right.
Next thing is to forget the vengeance. They are a waste of money and time.
I don’t think you are too far away on pitch. You’ll be around the 17-19” mark I’d think, if it’s the usual gear ratio for that set up.
obvious props to try and squeeze the best out her would be laser 2, black max ally and tempest plus props of a pitch between 17-19. 17” tempest plus might be an idea.

Thanks for the response. That’s helpful.

 

[achris]

Hi all,

Current prop is the original mercury prop,19 degrees alloy that came with the boat. (It’s not in good shape either).

Propeller pitch is measured in 'inches of forward motion in one revolution', assuming no slip. Not degrees.

 

[WesNewell]

immo,
The prop you want depends on what you want out of the boat. There's never one prop pitch for all use types without giving up something in different uses. For cruising around by yourself for max speed you may want a 23p prop. For a heavy load pulling skiers you may want a 17p prop. If you want just one prop for multiple uses, then you would likely want a pitch somewhere in between the two extremes.