Top Speed?

[dlandersson]

You're going to get a ton of responses.

When you get a chance, look at your anodes, external AND internal (which are often overlooked).

[Dogginit]

Thanks for all the insight everyone! Lots of great information flowing. I am running a Vortex 10x15 pitch prop and think it waaay too much prop. I understand that prop pitch defines the amount of water pushed and therefore a lower pitch means less water (per revolution) but increased power and my case hopefully a better top speed (---or even a plane). Not sure how to factor in diameter though. Any suggestions on diameter and pitch for 2008 26M and 50hp Merc four stroke?

[Inquisitor]

Thanks for all the insight everyone! Lots of great information flowing. I am running a Vortex 10x15 pitch prop and think it waaay too much prop. I understand that prop pitch defines the amount of water pushed and therefore a lower pitch means less water (per revolution) but increased power and my case hopefully a better top speed (---or even a plane). Not sure how to factor in diameter though. Any suggestions on diameter and pitch for 2008 26M and 50hp Merc four stroke?

Also have to factor in red-line (or maximum you want to run it) and also gear box ratio. Obviously its a lot easier if someone has your same motor, boat and similar mods (weight) in the boat.

For instance, I have the 60hp Merc BigFoot. It has a red-line of 6000 rpm and a lower gear ratio than yours so its made to swing a bigger prop. I currently have 14x11. Totally empty of mods, gear, water and no mast and sailing gear (just out testing the motor) reached 20 mph, but only 5220 rpm. I'll definitely test it fully loaded and rigged, but I doubt it'd exceed 5000 rpm in that condition. Maybe 14x10, but I wanted to try the plastic
Piranha Propellers but no 14x10 available. Storage of extra blades and replacing a blade versus a bent up prop sounds easier in the wild.

Here is a website calculator I tried. https://www.go2marine.com/propeller-fit-guide I found it at least bracketed what I need. I used my motor data (above), length=26 ft, weight=4000 lbs, alt=0, 3 blade aluminum. It gave me:


Maybe I need to look into reducing diameter and going up in pitch. Anyone know what the pros and cons are of doing that?

[kurz]

I have the same configuration and finally stay with the 14/10 prop. 14/11 cannot go to the max, and the 14/9 goes to early to the 6000.

So sometimes I think when just doing 5kn max a bigger prop was a better mach. But otherwise - if one prop for all - I will stay with the 14/10

[Be Free]

Ballpark, for every inch of change in prop diameter the RPM varies by about 500 RPM and every inch of pitch changes the RPM by 150 to 200 RPM.

[Inquisitor]

Ballpark, for every inch of change in prop diameter the RPM varies by about 500 RPM and every inch of pitch changes the RPM by 150 to 200 RPM.

Ballpark is good. I have Dave Gerr's, "Propeller Handbook". I didn't get far. It seemed like way too much effort and way to many WAG's just to set up the equations. I now understand that its either very trial and error based or thousands of compute hours with CFD programs at the military level... and not much in between.

I intuitively understand that reducing pitch or reducing diameter will allow me to reach rated redline at WOT.

I guess... what I'm asking, say... if two props 14x10 and 13x13 (I used your numbers above to approximate these) both could reach 6000 rpm on my boat... they are using the same horse power. But what are the characteristics or the pros and cons between these two propellers? For instance, I'm guessing:

• One might accelerate better
• One might have a higher top-speed
• One might have better economy at lower speeds
• Any other things, I might be clueless about?

[kurz]

maybe to easy... can you buy somewhere 13x13 props for our merc?

[Inquisitor]

maybe to easy... can you buy somewhere 13x13 props for our merc?

I don't know if one exists. The question I'm posing is academic in nature. If two props both fully utilize the motor's available horse power, what other decision factors about those diameters/pitches can/should be evaluated?

I merely picked 13x13 since it kind-of met Be Free's ballpark estimating.

Its the same kind of issue getting a Mercury 60Hp BigFoot versus a standard gearbox Mercury 60Hp. Our BigFoot spins the prop slower, thus allowing a larger diameter prop. That's what I was told... "it is better for my type of boat". What characteristics, are enhanced/lost compared to a standard gearbox 60 Hp?

[kurz]

interest aspect.

BUT: even with the bigger gear box I don't think that we with the merc Big foot 60HP get higher speeds... or lover gas consumption...

not the aswer you want, I know...

[Be Free]

Now you are asking the hard questions.

How a particular prop on a particular engine on a particular boat is going to work in the real world is usually easier to do by trial and error than by math and modeling. Use the math to narrow your choices down to two or three props and then try them to see which ones work best in your particular configuration. You will need to know the top RPM for your engine and the final gear ratio.

If you know that you are more interested in good fuel economy at a particular speed or best speed regardless of economy you may be able to pick one based on the specs alone.

Some engine manufactures have tools on their websites to recommend props that they make. Unfortunately I've never seen one that lists a MacGregor of any type as a possible platform. You will get pretty close by choosing a generic pontoon boat.

When you are testing the prop on your boat you should see a (close) to linear increase in speed with increase in RPM below planing speed with a similar but still close to linear increase after you start to climb over the bow wave. Unless you have a very large engine you will not see the increased speed and decreased RPM transition that you are familiar with in a power boat when you get completely in front of the bow wave.

If you are increasing RPM but the speed is not tracking in a (mostly) linear fashion you are probably experiencing prop slip. This is the aquatic equivalent to spinning your tires. You are still burning gas but you are not getting anything in return. There is always going to be some prop slip but you want to try to keep it as low as possible. You reduce prop slip by increasing prop area (more blades or larger blades). More prop area affects RPM and we start around again.

[Be Free]

maybe to easy... can you buy somewhere 13x13 props for our merc?

I don't know if one exists. The question I'm posing is academic in nature. If two props both fully utilize the motor's available horse power, what other decision factors about those diameters/pitches can/should be evaluated?

I merely picked 13x13 since it kind-of met Be Free's ballpark estimating.

Its the same kind of issue getting a Mercury 60Hp BigFoot versus a standard gearbox Mercury 60Hp. Our BigFoot spins the prop slower, thus allowing a larger diameter prop. That's what I was told... "it is better for my type of boat". What characteristics, are enhanced/lost compared to a standard gearbox 60 Hp?

The lower final gear ratio on the BigFoot will allow it to turn a prop with more surface area. That is generally a good idea for our boats. You will get better fuel economy near or below hull speed and better maneuverability while docking while sacrificing some top end speed.

In automotive terms, you've got a lower rear end gear ratio. Good for pulling stuff and drag racing but not great on the highway.

[Inquisitor]

The lower final gear ratio on the BigFoot will allow it to turn a prop with more surface area. That is generally a good idea for our boats. You will get better fuel economy near or below hull speed and better maneuverability while docking while sacrificing some top end speed.

Thanks, that is what I'd prefer to hear. Although this thread is about Top Speed (motoring), I'm more interested in mileage and thus range. Any storm where we'd feel the need to run, we're not likely to be able to drive the boat through waves to take advantage of that "speed" prop. All other times like cruising a canal or the ICW where its too tight or wind direction forces motoring, I'd rather have the range.

I found this article below very interesting in explaining slip. That way I can see if I'm over or underpropped and maybe help optimize for my preferred conditions or if you're interested in absolute top speed, it should help there also. In the article, there is a link to an Ap to do the calculations for you.

https://www.mercuryracing.com/prop-school-part-6-slip/

[Be Free]

That is an excellent article. It is one that I read when I was researching which prop to put on my own boat. It's definitely written for the boater who wants to get the most speed out of their boat but the background information is applicable regardless of what your tuning goals are.

[ris]

First run prop is 14 inch X 7 pitch (extra 850 lbs).
RPM. MPH
2000 6.8.
3000 7.8.
3500 8.6.
4000 9.5.
4500 10.7.
5000 12.6.
5500 14.6 WOT.
14 X7 without extra weight and fuel per hour
RPM - MPH - - GPH (fuel) - MPG
2000 - 6.1 --- .5 ---- 12.2
2500 - 7.3 --- .8 ---- 9.3
3000 - 8.5 --- 1.2 - --- 7.08
3500 - 10.2 - - 1.6 - ---- 6.38
4000 - 12.8 --- 2.0 ---- 6.4
4500 - 15.2 --- 2.6 - --- 5.85
5000 - 17.2 --- 3.7 - --- 4.64
5650 - 19.2 --- 5.1 --- 3.76

Did I figure the MPG correctly with knowing MPH and GPH?
Richard

[Inquisitor]

First run prop is 14 inch X 7 pitch (extra 850 lbs).
RPM. MPH
2000 6.8.
3000 7.8.
3500 8.6.
4000 9.5.
4500 10.7.
5000 12.6.
5500 14.6 WOT.
14 X7 without extra weight and fuel per hour
RPM - MPH - - GPH (fuel) - MPG
2000 - 6.1 --- .5 ---- 12.2
2500 - 7.3 --- .8 ---- 9.3
3000 - 8.5 --- 1.2 - --- 7.08
3500 - 10.2 - - 1.6 - ---- 6.38
4000 - 12.8 --- 2.0 ---- 6.4
4500 - 15.2 --- 2.6 - --- 5.85
5000 - 17.2 --- 3.7 - --- 4.64
5650 - 19.2 --- 5.1 --- 3.76

Did I figure the MPG correctly with knowing MPH and GPH?
Richard

I was going to try your numbers out in the Propeller Slip calculator to see if it suggests you have the right prop for your conditions. Most of the data, you've given here, but I recall you said you have a different foot than comes with your engine normally. Do you know your gear ratio?