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Prop question for the prop guys

kaydano

kaydano

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Scenario #1: I hit 35 mph (max speed) at 6400 rpms with 4 people in the boat and a half tank of gas.

Scenario #2: When I'm by myself with a near-empty tank, I still hit 35 mph (max speed) at the same 6400 rpms.

What's the deal here? If I was hitting the rev limiter in scenario #1 at 6400 rpms, wouldn't I see 1 or 2 mph more when I hit the rev limiter in scenario #2 due to having ~400 pounds less weight?

It seems like I'm leaving something on the table here. Any recommendations on what to change my prop to? I need to get a spare prop, and wouldn't mind squeezing out a little more performance in the process (if possible).

Thoughts?

My current prop is a 3-blade Mercury Black Max 13.75 x 15p.
 
What engine you running and what is rpm range at wot by the manufacturer? I would think that your motor has lots of torque but the fact your not hitting a higher rpm with less load (cause your bouncing off the rev limiter), you probably need to go up in pitch. Each inch is close to 150ish (most places say 100 to 200 rpm) decrease or increase depending on which way you go.

Some marinas will let you prop test until you find the the right one.

Which boat are you running. Diameter too can play a big role. Most pontoon props are bigger diameter as well. My merchandise Enertia is a 15x15 on a 150hp merc Pro XS.

Good luck

Todd

By the Ben and Suzy is a prop god. Hopefully he'll chime in
 
I would go up 2" on the pitch, with perhaps an Enertia if you want to spend that much. Thats what I run and I am very pleased with it. Props can be funny, I have tried 4 different pitches(1" differences are available in this prop) and the speed differences were only about 2 mph between fastest to slowest, but the peak rpm varied from 6520 to 6250. I settled with the 17" because it actually was .5 faster than the 18", but I could feel the out of the hole difference. Verado's hit hard out of the hole, and I didn't want to give any of that up.
 
mtudb24 said:
What engine you running and what is rpm range at wot by the manufacturer? I would think that your motor has lots of torque but the fact your not hitting a higher rpm with less load (cause your bouncing off the rev limiter), you probably need to go up in pitch. Each inch is close to 150ish (most places say 100 to 200 rpm) decrease or increase depending on which way you go.

Some marinas will let you prop test until you find the the right one.

Which boat are you running. Diameter too can play a big role. Most pontoon props are bigger diameter as well. My merchandise Enertia is a 15x15 on a 150hp merc Pro XS.

Good luck

Todd

By the Ben and Suzy is a prop god. Hopefully he'll chime in
Click to expand...
It's a Mercury 115 4-stroke on the 24SSL.

6400 is the max rpm from the manual, and the tach reads right on 6400 at WOT.
 
Are you actually hitting the rev limiter and you have more forward motion on the throttle? Or is it just getting up to 6400rpm and that's all she'll go?
 
I am not 100% sure I'm hitting the rev limiter or not. Do you feel/hear something when you hit the rev limiter?

In the motor manual, 6400 is the max engine rpm. My recollection is there is some extra forward motion or play in the throttle handle at 6400, and so I'm not physically hitting something with the trottle lever that causes it to max out. The motor just slowly creeps up to 6400 and that's as far as it goes. My recollection is it takes a minute or so to go from 6000 to 6400, so it's a slow taper off to 6400 at the top end. There is nothing that's obvious or sudden that I can feel or hear that is causing it to stop there, it just feels like that's all the motor has.

Does that help?

Regardless of the rev limiter, I still don't understand why I get 35 mph at 6400 rpm with moderate weight, and the same mph and rpm at very light weight (just me in the boat). I would think at say 5000 rpms, I would maybe be going 28 MPH with moderate weight, but with just me in the boat, I would maybe hit 29-30 mph at the same 5000 rpm. I've never taken note of this at lower rpms, but I will next summer.

It just seems to me the speed should be noticibly different (albeit slight, as in 1 mph maybe) between the two different weights.
 
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When you hit a rev limiter, the motor cuts out and falls back to the max rpm. I think you probably have a inch or two of pitch you can increase in size since you can hit max rpm either light or heavy load. Prop selection is trial and error. For example, I can reach a higher rpm but less speed with a mercury mirage prop vs my Mercury Enertia. This has to do with efficiency and prop slip. The Enertia is a more pontoon efficient design then the mirage. It slips less and moves the boat forward per inch of pitch then other designs. Theoretically, a 15" pitch will move the boat 15" forward with every revolution of the property. But take into account slop and other factors, it will be less in the real world. You need to find a design that gives you the least amount of slip. This varies from boat to boat and motor to motor. It's a lot like guns. Some guns like certain bullets and will throw better or tighter groups with certain manufacturers.

Hope this helps

Tosd
 
Thanks Todd. Know anything about Mercury's Spitfire 4-blade prop? Mercury's website says the Enertia is for high horsepower boats 135 HP and above. I have the 115.
 
I maybe wrong here in my thinking Kaydano, but here's my thoughts. I have a 22' boat, 3 logs w/strakes with a 150, turning a 15 x 15 Enertia @ 6100-6200 rpm to get 39 mph. You have a 24' boat, same 3 logs w/strakes with a 115, turning a 13.75 x 15p @ 6400 rpm and get 35 mph. I think you're getting a lot of slip or ventilation or cavation or something. :wacko: I would love to come for a ride and see in person but we know that's not going to happen.
 
Thanks Carl. I think I'm going to have to just try a different prop and see what happens. I was planning to get a spare anyway, and the help I got here will help improve my odds of getting it right the first time. If I go with a 3-blade prop, I may add 2 inches to the pitch. If I try the 4-blade pontoon prop, I'll probably just add 1 inch since going from 3 to 4 blades, all other things being equal, drops the max rpm by 100 or so (so I read). Whichever prop works best will be my everyday prop. The other will become the spare.

I assume I should stay as close to the same prop diameter that I have now (which is 13.75 inches)?
 
I think upping the diameter would help you more than pitch. Pontoons like diameter. More diameter = larger blades.
 
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Being a novice I sure would appreciate a simple matrix on props such as:

Too low on RPM = do this

Faster hole shot = do this

Probably asking for too much effort or perhaps it is just not that simple
 
Too low rpm=go smaller, better holeshot = larger blades or a 4 blade prop. In a nutshell. :wacko:
 
All the forums seem to love the spitfire prop. Up to 125hp, so your 115 falls in nicely. Fairly inexpensive too.

Keep us posted on what you end up doing

Todd
 
TomS said:
Here is a simple calculator of theoretically how the prop variables relate - prop pitch, gear ratio, rpm, forward speed mph, slip %. Enter any 4 values and calculate the 5th. Just use 10% slip as a starting point.

http://www.mercuryra...pcalculator.php
Click to expand...
Nice link. Thanks for posting. So, I now get what prop slip is. Sort of. It's just like the slip on an electric motor. No slip = no torque. So, slip is good. To a point. That motors class back in college actually had some value, I just wasn't seeing it at the time. ;)

However, as the physical load on the electric motor increases, the slip increases, up to the point where the motor loses synch and stalls. Not sure what a boat prop does at that extreme end, but a prop in water ain't gonna stall. So that's why I say I get it "sort of".
 
mtudb24 said:
All the forums seem to love the spitfire prop. Up to 125hp, so your 115 falls in nicely. Fairly inexpensive too.

Keep us posted on what you end up doing

Todd
Click to expand...
I will look at the spitfire some more then. Thanks for the tip. What other forums have you been hearing about it? PDB?
 
Suprisenly, even though they talk about it being a ultimate pontoon prop, lots of fishing boats are seeing big gains. Florida sportsman forum. The hull truth boating forum, etc. Some guys are seeing a 5 to 7mph increase in the top end which is amazing.
 
mtudb24 said:
Surprisingly, even though they talk about it being a ultimate pontoon prop, lots of fishing boats are seeing big gains. Florida sportsman forum. The hull truth boating forum, etc. Some guys are seeing a 5 to 7mph increase in the top end which is amazing.
Click to expand...
I talked to a Mercury dealer today, and he talked with Mercury directly to answer my question. Mercury said to get the same performance I have in the Black Max, that I should get the Spitfire in the same diameter and pitch. Now of course I'm not looking for the same performance, but I thought this was interesting. I thought a 4-blade dropped engine rpm 100 rpms or so, for the same size as a 3-blade. Apparently the Spitfire doesn't conform to that standard.

Still haven't figured out what I want to get (diameter/pitch) yet, but thought I'd mention this.
 
You have to remember he did say performance, not rpm.
 
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