Help me understand prop selection and fuel economy

[Cwazy1 89]

In an effort to understand everything on the build spec, there is one thing that is important but I'm missing some understanding on- and that is prop selection.

 

The build spec says +70 dollars for the high altitude prop from factory... not a terrible upgrade (if it can be considered an upgrade).

 

My question is really... if running the same ballasted weight, the same speed, the same altitude, will the stock prop or will the high altitude prop give the better fuel economy? I'll be running 11.4mph, zero elevation, and will be running an upgraded bags listed setup. I rarely go much faster than 25mph so max top end speed is not a priority.

 

What I understand is that a high alt prop will give higher RPM at surf speed. It will also give lower top end speed, but higher acceleration during hull shot. Does the higher RPM at surf speed translate into more fuel usage? Or does it actually do the opposite and fuel consumption is decreased as the engine will have to work less hard?

 

One thing to consider is that I'm trying to understand this from the 1:1.72 tranny ratio and not the old 1:1.4

 

My goal is the best fuel economy. Summertime moorage for me is restricted to no fueling on the dock, which means sneaking in 5gal containers.. currently on my '15 malibu, I need to make 2 fuel runs as it sucks up 20g per 3-4h set. My old merc black scorp only did 10g per same time period.

Edited September 23, 2016 by Cwazy1

[DrNate 426]

The high altitude prop has a lower pitch, meaning that it takes a more aggresive "bite" into the water when it turns. This translates to a better hole shot, lower RPM's at surf speeds, lower gas usage, and lower top end speed. And, yes, it is worth every penny.

 

I will put a caveat in here though and say "at least on my boat" it does. At surf speeds, the more aggressive prop allows me to run at lower RPM's thus saving gas.

 

So the rule of thumb is actually just backwards of what I said. If you do some reading on the internet, if you lower the prop pitch it should raise your RPM's (and therefore use more gas). But that is at wide open throttle, and I never do that. Someone may correct me if I'm wrong but it seems at the lower speeds we are running, lower prop pitch actually equates to less RPM's

 

From the web:

 

Propeller pitch is the theoretical forward movement of a propeller for one revolution, assuming that there is no prop slip. For example, a 21 pitch propeller will theoretically move 21 inches for every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. More aggressively and efficiently designed propellers will slip less.

When selecting a propeller pitch for your boat, it is important that the propeller runs at the upper end of your engines wide-open-throttle RPM range. If you want your RPM's to increase, go down in pitch. To decrease RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will change approximately 400 RPM's.

For water sports or extra people on board, you should generally drop 2" of pitch to help compensate for the added weight and drag on your boat. It makes a noticeable difference in your boat's hole shot, fuel efficiency, RPM's, and overall performance. You should ALWAYS carry a spare propeller on board, and if you're into water sports or occasionally load the boat with extra people, a spare prop with a lesser pitch is a good idea. When thinking a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.

Edited September 23, 2016 by DrNate

[Race City Marine 60]

 

The high altitude prop has a lower pitch, meaning that it takes a more aggresive "bite" into the water when it turns. This translates to a better hole shot, lower RPM's at surf speeds, lower gas usage, and lower top end speed. And, yes, it is worth every penny.

 

I will put a caveat in here though and say "at least on my boat" it does. At surf speeds, the more aggressive prop allows me to run at lower RPM's thus saving gas.

 

So the rule of thumb is actually just backwards of what I said. If you do some reading on the internet, if you lower the prop pitch it should raise your RPM's (and therefore use more gas). But that is at wide open throttle, and I never do that. Someone may correct me if I'm wrong but it seems at the lower speeds we are running, lower prop pitch actually equates to less RPM's

 

From the web:

 

Propeller pitch is the theoretical forward movement of a propeller for one revolution, assuming that there is no prop slip. For example, a 21 pitch propeller will theoretically move 21 inches for every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. More aggressively and efficiently designed propellers will slip less.

When selecting a propeller pitch for your boat, it is important that the propeller runs at the upper end of your engines wide-open-throttle RPM range. If you want your RPM's to increase, go down in pitch. To decrease RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will change approximately 400 RPM's.

For water sports or extra people on board, you should generally drop 2" of pitch to help compensate for the added weight and drag on your boat. It makes a noticeable difference in your boat's hole shot, fuel efficiency, RPM's, and overall performance. You should ALWAYS carry a spare propeller on board, and if you're into water sports or occasionally load the boat with extra people, a spare prop with a lesser pitch is a good idea. When thinking a propeller pitch, compare it to a gear on car - lower gear, higher RPM's.

 

 

I'd agree except for the part about lowering rpm's by running a more aggressive prop. Typically a more "aggressive" prop means a lower pitch. That will make hole shot faster and easier with ballast. It will reduce top end speed generally. It will generally cause higher RPM's and more fuel to be used for a give speed, EXCEPT if you are struggling to get on plane...then that's bad for economy and the engine for other reasons.

 

Like they said...an easy way to think of it is that a boat with too high of a pitch is like running a car up a hill in 5th gear. It will struggle up the hill but get great economy when going on flat ground. I believe my family killed an outboard motor back in the day by running a prop with too high of pitch...the boat went really fast but was running at too low of rpm at WOT for too long, so it got smoked after awhile. A boat with a real low pitch is like running that car up the hill in 1st gear.

 

Economy is somewhat related to RPM's but moreso load on the engine. A car can be running at 5000rpm's down a hill, but using next to no gas, because there is no load.

[Cwazy1 89]

This is the million dollar question, despite the increase in rpm, does the decrease load on the engine result in better economy?

 

 

Economy is somewhat related to RPM's but moreso load on the engine. A car can be running at 5000rpm's down a hill, but using next to no gas, because there is no load.

Edited September 23, 2016 by Cwazy1

[Race City Marine 60]

This is the million dollar question, despite the increase in rpm, does the decrease load on the engine result in better economy?

 

Not likely as long as the engine is capable of smoothly pulling the load either way. It's a tough case to make that higher rpm's get better economy on the lake. Likely it would be worse.

 

A larger prop may run more efficiently (larger diameter with lower pitch) than a smaller one based on personal experience, but I'm not going to stake my reputation on it.

 

One of the direct injection motors should get you running back to the pump less though.

[Race City Marine 60]

This is the million dollar question, despite the increase in rpm, does the decrease load on the engine result in better economy?

 

 

I think that idea may have come from wakeboarding. If you can get your rig up on to plane in 10 seconds instead of 40 each time, that will yield savings, even though you'll burn a little more when cruising. Also a lower pitch prop will hold speed better and cause the engine to hunt and over compensate less, which could hurt economy.

[Cwazy1 89]

Since I'm more interested in surfing, which doesn't require planing, it sounds like unless I'm having a hard time getting to speed I should go with the standard prop.

[h20king 794]

Since I'm more interested in surfing, which doesn't require planing, it sounds like unless I'm having a hard time getting to speed I should go with the standard prop.

 

no go with the high altitude prop. Im averaging 3.8 gph with the standard engine high altitude prop and 1:72 to 1 gear ratio. You do want to plane when surfing if not the wake does not completely form.

[h20king 794]

This is the million dollar question, despite the increase in rpm, does the decrease load on the engine result in better economy?

 

EXACTLY when we first got our 238 with the new drive ratio and high altitude prop I was surprised how high the RPM's were compared to our 14 226 with the 1:23 ratio. With the New radio we are averaging 3700 rpm's compared to our 14 at 3100'rpm's and expected that it was sucking fuel. To my surprise this is not the case. By keeping the engine in the power band it actually decreases fuel consumption. This is our largest boat so far and it gets better fuel consumption than any boat I have owned.

[Cwazy1 89]

In an effort to test the hypotheses proposed in this thread, I attempted to demonstrate 'load' in my car by driving up a hill at a constant speed in a gear that is optimized (by my automatic tiptronic transmission) for that speed and load. While establishing the control run, I noted the instantaneous MPG that my dash indicated.

 

Upon shifting to one gear down, which increased my rpm by ~700, and maintaining the same speed on the same incline, I noticed that my instantaneous MPG decreased slightly. This backs the theory that the increased rpm, although while reducing load per rev, actually increases fuel consumption due to the higher revs needed to keep up speed.

 

It seems like opinions run both sides and there is no real hard data available to back any one hypothesis.

 

To H's point, the 226 I demo'd with the upgraded 1100 in the rear ran at 3700rpm at 11.4 on the stock prop. I can't even imagine what it would run at with a high altitude prop- 4000rpm? That would worry me tbh.

Edited September 25, 2016 by Cwazy1

[h20king 794]

Yep close to 4000 rpms with all ballast full using quick surf. we list our boat so less weight thats why are rpms are only 3600 -3700 rmps at 11.8 mph. More fuel consumption is what I expected but am averaging 3.8 gph. There are others here that are getting similar results.