Does increasing Prop Pitch reduce fuel at hull speeds

[jmunro123]

It must be winter - I am going to hypothetical question mode.

I was wondering something. The Props on our boat are 22x22's. At about 2000 RPM the boat travels about 10MPH and burns about 10GPH total. At 3600 RPM it travels 20MPH or so and burns about 42 GPH.

I was wondering if I wanted to use the boat like a Trawler and travel at hull speed what props would make the most sense. I was told if I went to a 22x24 the speed would stay the same but the RPM would be closer to 1700. So how far could I increase the pitch assuming the boat was not driven over 10MPH?

So this is for a Trawler conversion, I understand if the boat had a 22x (something to large) and I tried to go fast it would heat up the engine. But a trawler speeds I doubt it would be a problem.

Thoughts - btw - when will summer be here again???

 

[Southpaw II]

As you might expect, there are several factors that go into what it takes the achieve maximum hull speed at minimal cost; ask any blowboater. For your boat, hull speed is probably about 10-12 kts. After that, you start pushing water as the boat, determined by hull design, starts to squat and climb on plane. Then the sucking sound you hear is fuel.

Calculate the max hull speed for you boat. If you are not there at just above idle, there is not uch you can do because as you increase speed the boat will start pushing water and the economy goes down the tube. In other words, pitch will give you more thrust, but at an expense.

Don

 

[Steve S]

Well on my 21' Monaco I have used both a 17 pitch and 19 pitch prop.

The 17 pitch gave me great torque to get up on plane and to pull skiers up etc, but the top end was lacking.

The 19 pitch took longer to get up on plane but cruised smoother at a lower RPM.

At 3200 RPM I got about 4-5 mph more out of the 19 pitch. OR running at the same speed (Using GPS) it took less RPM's to maintain that speed with the 19 vs the 17.

-Steve

 

[Gofirstclass]

If you're looking for maximum efficiency for your hull you will need to measure the length of the boat at the water line. This is your LWL (Length at Water Line). The formula for determining the correct hull speed for your boat is:

1.34 x sqrt of LWL

To help determine what your most efficient speed is, watch how your hull acts as it goes through the water at low speeds. At a "no wake" speed, there is very little bow wave generated (that's the water the boat is pushing ahead of it). In water that's deep enough to absorb the bow wave, and as boat speed increased, the bow waves gets bigger and moves along the bottom of the hull.

In the photo below of a Grand Banks trawler you can see the bow wave, then there's an area where the bow wave has been pushed down, then near the stern you can see where the bow wave has come back up and is supporting the stern of the boat. This speed, for this boat, is very efficient.

As this boat starts to increase its speed, it starts to outrun the part of the bow wave that supports the stern. In a boat that has a planing hull you'll see what this looks like. As the planing boat makes the transition from hull speed up to planing speed, you'll see the stern of the boat squat down into the water. This is the least efficient speed because the boat is pushing a LOT of water. Then, when the boat gets up on plane it flattens out, the bow settles down and the boat is riding on top of the water, not in it.

So for your 390, the trick is to find that speed where the hull is moving through the water in a flat attitude (now running bow high) and the bow wave is still supporting the stern.

The props that came on your boat should be the best ones for that job. Factories spend a lot of money to determine what props will work best for every boat/engine combination. It's up to us to learn how to use them.

 

[billnpat]

Factories spend a lot of money to determine what props will work best for every boat/engine combination. It's up to us to learn how to use them.

EXACTLY.......:thumbsup:
__________________

 

[HUMPH]

Then why did they put little 3 blade props on the 320DA? There are some on this site that have moved to 4 blade and improved efficiency. It seems as though Sea Ray wanted to keep the high end performance better than average cruise performance.

 

[Gofirstclass]

You'd have to ask Sea Ray that question, but I'd bet they have a good answer for it.

My 330 with 5.7's had 3 blade props. A friend's 330 with big blocks had 4 blade props. I never asked Sea Ray cause it just didn't matter to me. My cruise mpg was better than his but he cruised about 2mph faster. Go figure.

 

[Searaybio270]

So, would I be better off with a 4 blade prop on my 400EC? Not sure what size I have on there right now. However, the 3870 Cruiser next to me has 4 blade and they look bigger than mine.
:huh:

 

[Four Suns]

As this boat starts to increase its speed, it starts to outrun the part of the bow wave that supports the stern. In a boat that has a planing hull you'll see what this looks like. As the planing boat makes the transition from hull speed up to planing speed, you'll see the stern of the boat squat down into the water. This is the least efficient speed because the boat is pushing a LOT of water.

Good grief... not this discussion again... there is no "supporting the stern" stuff going on with a displacement hull/wave problem ... It's classical wave drag theory at it's finest... not how the boat is floating... and if you got this information from wikipedia, it's an example of how information on that site can be wrong.

 

[Southpaw II]

Good grief... not this discussion again... there is no "supporting the stern" stuff going on with a displacement hull/wave problem ... It's classical wave drag theory at it's finest... not how the boat is floating...

Gary, what took you so long to get involved here? I've been waiting....and what is a displacement hull, anyway?

Don

 

[Gofirstclass]

Good grief... not this discussion again... there is no "supporting the stern" stuff going on with a displacement hull/wave problem ... It's classical wave drag theory at it's finest... not how the boat is floating... and if you got this information from wikipedia, it's an example of how information on that site can be wrong.

So, Four Suns, what's your explanation for what happens in the fluid dynamics when the boat increases speed above its hull speed and the stern of the boat starts to drop down into the water? The explanation I have seen for many years suggests that the boat outruns the trailing wake as it goes above the hull speed.

I have studied this for many years and have seen the explanation I gave in many forms and in many places. One of those was Wikipedia.

Southpaw, from my understanding, a displacement hull is one that is designed to move through the water rather than get up on top of the water as a planing hull would. A displacement hull, moving at the speed it is designed for, is very efficient.

There was a ~35' boat in my marina that had a displacement hull. It was powered by a 90hp Perkins and moved quite nicely at its designed speed. Even with huge additions of horsepower this hull would never be able to get on plane. It just was not designed to do that.

 

[Southpaw II]

I understand, thanks. We 've had this discussion before, as you might have guessed. I was being a bit facetious.

Don

 

[dpmulvey]

It must be winter - I am going to hypothetical question mode.

But a trawler speeds I doubt it would be a problem.

Thoughts - btw - when will summer be here again???

More than you would think. By basically asking for more thrust at a lower RPM than designed, you are shifting the torque curve left. The engine was made to produce "X" thrust at "Y" RPM.

IMO, you would be better served with 4 blade props (greater low end thrust) that provided a max RPM that falls in the middle of your Merc (I am assuming here) WOT range. A good prop scan shop can do this for you on a machine.

Pluses to having the correct props is engine longevity (cheaper than fuel) and the ability to get the heck home in the event of a storm.

Another option is to convert to diesel. There is a company in FLA that drives 2 props from a single diesel. Very interesting development.

HTH.