Prop question

[sasqman]

hey guys,

I have a 27 foot semi displacement hull that is powered with a 115 outboard. Right cuise speed is about 7.5 knots, and WOT is about 12-14 knots. I get full range with the 13p x 13.75" 3 blade aluminum prop on there now, and my question is, would a 13p x 14" - 4 blade stainless give me more speed or just torque?

Thanks

 

[zurk]

4 blades are more drag than 3 blades. doubtful it will give you more speed.

 

[Pineapple Express]

Is your current prop getting you the motor's rated WOT RPM range?

Aluminum props usually have thicker blades (bad for efficiency) and also flex more than stainless steel. So a heavily loaded aluminum prop actually "loses" pitch as the blades flex a bit.

A four-blade prop will probably give you more blade area which will help maneuvering. If you're getting the rated WOT RPM now I doubt you'd get much more speed with a different prop.

 

[sasqman]

Is your current prop getting you the motor's rated WOT RPM range?

Aluminum props usually have thicker blades (bad for efficiency) and also flex more than stainless steel. So a heavily loaded aluminum prop actually "loses" pitch as the blades flex a bit.

A four-blade prop will probably give you more blade area which will help maneuvering. If you're getting the rated WOT RPM now I doubt you'd get much more speed with a different prop.

Yes, I'm getting full range up to 6000rpm with the current prop. (Don't really use WOT though)

 

[Pineapple Express]

Yes, I'm getting full range up to 6000rpm with the current prop. (Don't really use WOT though)

That's not sitting on the rev-limiter is it?

You need to know that you get the right WOT revs to confirm that the prop is sized right and you're not lugging the engine so even if you don't go WOT much it's still an important measurement.

What are the specific propeller models that you have and are considering? Comparing props is like comparing drill bits...you need different ones for different applications.

 

[sasqman]

That's not sitting on the rev-limiter is it?

You need to know that you get the right WOT revs to confirm that the prop is sized right and you're not lugging the engine so even if you don't go WOT much it's still an important measurement.

What are the specific propeller models that you have and are considering? Comparing props is like comparing drill bits...you need different ones for different applications.

Thanks for your help on this.
I will post the info on the props later.

As far as WOT goes, (I'm not too sure I understand) the mercury book says WOT is 6000rpm which I am getting from my dashboard guage. I get full propulsion at this range. I used to have a 15p 3 blade on there that would free spin at about 4200 rpm. Does that make sense or answer your question.

Thanks

 

[Pineapple Express]

Usually the manufacturer says WOT RPM should be 5000-6000RPM (for example). So for a planing hull you typically aim to be in the upper half of that range with a full load (fuel, beer, gear, etc). For your boat I'd say you want to aim to be roughly in the middle of the range, so approx 5,50oRPM (in this hypothetical example). Note also that WOT means wide-open-throttle (aka full throttle)

What is the exact model of engine you've got?

When you say "free spin at about 4200RPM" you mean at WOT in forward gear? What model of prop was this?

 

[sasqman]

Usually the manufacturer says WOT RPM should be 5000-6000RPM (for example). So for a planing hull you typically aim to be in the upper half of that range with a full load (fuel, beer, gear, etc). For your boat I'd say you want to aim to be roughly in the middle of the range, so approx 5,50oRPM (in this hypothetical example). Note also that WOT means wide-open-throttle (aka full throttle)

What is the exact model of engine you've got?

When you say "free spin at about 4200RPM" you mean at WOT in forward gear? What model of prop was this?

2005 Mercury 115 4 stroke EFI - About 600 hours
Displacement of the boat loaded with fuel and such is about 6500lbs

Free spin meant that the prop/motor could not push the boat anymore past 4200 rpm. It would like free spin/turn but not propel the boat faster.
That's why I went down to a different pitch, and now get full range of RPM

 

[zurk]

was it ventilating ? ventilation can be solved with dole fins. try your old prop with a piece of 1/4" aluminum sheet strapped horizontally to the motor leg 1" below the waterline and see if it will go full RPMS with the old prop.

 

[Pineapple Express]

was it ventilating ? ventilation can be solved with dole fins. try your old prop with a piece of 1/4" aluminum sheet strapped horizontally to the motor leg 1" below the waterline and see if it will go full RPMS with the old prop.

If it was ventilating it would be over-revving, not lugging down at 4,200

 

[G-Auto]

Both of these videos will help describe what Pineapple is on about.

 

[sasqman]

Both of these videos will help describe what Pineapple is on about.

A couple great videos. You guys are awesome for information. After watching........ I believe I was experiencing prop slip, not free spin on my aluminum 13p 3 blade. (Not a 15p) Just a couple updates on my first post. I am actually running an 11p prop right now. This is the one. No slipping........ as in as the rpms go up, the boat keeps moving faster.




This is the stainless one that I do have and want to try.






Should I try this 4 blade, or try a 12p 3 blade stainless to gain a few more knots?

 

[sasqman]

I also punched this into a prop calculator. Does this seem right for prop slip?

 

[sasqman]

Here are a couple pictures of the hull and also of video of the wake doing about 13 knots at about 5500 rpm. (Which we don't run at too often)

 

[G-Auto]

A couple great videos. You guys are awesome for information. After watching........ I believe I was experiencing prop slip, not free spin on my aluminum 13p 3 blade. (Not a 15p) Just a couple updates on my first post. I am actually running an 11p prop right now. This is the one. No slipping........ as in as the rpms go up, the boat keeps moving faster.

View attachment 55260


This is the stainless one that I do have and want to try.



View attachment 55261


Should I try this 4 blade, or try a 12p 3 blade stainless to gain a few more knots?

The question is do the RPM's stop on their own? When you get to the max rpm for the engine does it stop climbing or does it go a couple hundred rpm higher and hit a rev limiter or over rev the engine?

 

[G-Auto]

Here are a couple pictures of the hull and also of video of the wake doing about 13 knots at about 5500 rpm. (Which we don't run at too often)
View attachment 55263 View attachment 55264

You are beside me in Cresent beach...

 

[sasqman]

The question is do the RPM's stop on their own? When you get to the max rpm for the engine does it stop climbing or does it go a couple hundred rpm higher and hit a rev limiter or over rev the engine?

RPM kept going, just no propulsion. Like the prop was spinning, but the boat was too heavy to push faster. But remember, this was my old 13p prop. (Which is for sale lol) The new aluminum 11p 3 blade prop on there now gives me full range of throttle.

 

[sasqman]

You are beside me in Cresent beach...

Yes sir!! Lol. Small world

 

[G-Auto]

RPM kept going, just no propulsion. Like the prop was spinning, but the boat was too heavy to push faster. But remember, this was my old 13p prop. (Which is for sale lol) The new aluminum 11p 3 blade prop on there now gives me full range of throttle.

That is backwards from the way props work. it is possible the rubber hub in that old prop is damaged. You "proped down" which means you should be able to achieve a higher range or RPM now. The fact you could achieve a high rpm with a steeper prop would make me think its slipping as well.

 

[sasqman]

It was about 3 years ago that I had the 13p on the boat. Maybe my memory isn't correct. Maybe the RPMs did stop. I know when I put the 11p on it was because of it not being able to achieve full rpm. Now it does. Here is the stainless 4 blade I have. Should I expect any difference? Or should I try a 12p to try and gain a bit of speed? Or should I bite the bullet and stick a 200hp on it.........lol?