Trim tab usage

[mciaio]

On boats with hydraulic trim tabs, what is more fuel economical out of the following:

1. Running the boat at the slowest possible speed while keeping the boat on plane with the tabs all the way up?
2. Keeping the tabs partially down, allowing the boat to plane a slower speed?

 

[Tolyn]

Re: Trim tab usage

Greetings!

Pure speculation on my part but, lower RPMs = better fuel economy? Probably relatively insignificant savings given the low difference between the two speeds however.

Respectfully,
-T

 

[smokeonthewater]

Re: Trim tab usage

unless you are fighting a current the answer (for boats) is almost ALWAYS that the slower you move the better your economy.... that doesn't just mean planing.... you will do even better a displacement speeds although the speed change will often be more dramatic than the mpg change

 

[hungupthespikes]

Re: Trim tab usage

A little bit of trim goes a long way. Better speed at same RPM, much better handling, much better ride, better mpg and above all safer IMHO.

 

[sasto]

Re: Trim tab usage

Depends on your boat. Record the speed and RPM in both ranges and do the determination.

If you are refering to the Chaparral 2300. I would assume option 2.

 

[Home Cookin']

Re: Trim tab usage

there's a point with cars and boats that fuel economy is worse at low speeds because even though you burn less per minute, you take more minutes to get there--the "gallons per hour" function.

 

[bruceb58]

Re: Trim tab usage

unless you are fighting a current the answer (for boats) is almost ALWAYS that the slower you move the better your economy....

I used to think that as well until I bought a flow meter. All depends on the boat and engine and where the engine is most efficient. With my boat I am more efficient at 500 RPMs over what I thought was the most efficient which was just above planing speed.

My flow meter is connected to my GPS so I don't have to do any calculations. It coverts to MPG.

 

[steelespike]

Re: Trim tab usage

I used to think that as well until I bought a flow meter. All depends on the boat and engine and where the engine is most efficient. With my boat I am more efficient at 500 RPMs over what I thought was the most efficient which was just above planing speed.

My flow meter is connected to my GPS so I don't have to do any calculations. It coverts to MPG.

I agree comepletely depending on the boat and motor cruising speed could be up to 1,000 rpm higher than just on plane.

 

[smokeonthewater]

Re: Trim tab usage

yeah... I concede that there ARE bad speeds... falling off plane and barely planing being a couple... sometime try cruising a while at just over idle and see what your mpg's are obviously you'd need a hull that didn't tailwag at that speed to do any good

In my carver I got at least twice the mpg at displacement speed that I got at any planing speed....

Of course that isn't really all that relevant to the o/p's question. If only we all had flow meters

 

[bruceb58]

Re: Trim tab usage

... sometime try cruising a while at just over idle and see what your mpg's are

That's where mine is best too but obviously not a practical speed for a 24' boat that can go nearly 50 MPH which mine does.

 

[mciaio]

Re: Trim tab usage

Some Carvers were designed with the dual purpose Hull that would be efficient at both, displacement or planing.

 

[Mischief Managed]

Re: Trim tab usage

Some Carvers were designed with the dual purpose Hull that would be efficient at both, displacement or planing.

Actually, it's just the opposite. Dual purpose (semi-displacement) hulls allow planing, but are not very efficient at displacement speeds or on plane. They are for people who want the occasional turn of speed and can live with slow displacement speeds most of the time.

I have a fuel flow meter and it also measures speed. I get real time MPG readings and am happy to say that my boat is most efficient at idle (4 MPG) and at 28 to 36 MPH (2.5 to 2.6 MPG) with the tabs all the way up, which is well above my minimum planing speed. My MPG does not drop terribly from 36 MPH to WOT (which is about 50 MPH) either; at WOT, I'm still getting just over 2 MPG.

Minimum planing speed with tabs down (about 18 MPH) yields 1.8 to 2 MPG. Tabs up minimum planing speed (about 22 MPH) yields just over 2 MPG (the same as WOT). Plowing at just under planing speed gets me to just a little over 1 MPG.

When gas was $4 a gallon a couple of years ago, I saw lots more people plowing at just below planing speed, I bet they thought they were saving money; in fact, they were burning gas at more than double the rate they would have been if on-plane.

 

[steelespike]

Re: Trim tab usage

It's nice to see some actual mpg figures.I think its clear tabs likely can improve hole shot and comfort in certain swells but as I have suspected careful application is needed or you end up dragging them through the water wasting fuel.I can see that running with tabs at reduced planing speed could improve comfort under certain conditions.It seems tabs may not be a way to save fuel but to improve handling and comfort.

 

[smokeonthewater]

Re: Trim tab usage

this is quite interesting.... my experience has been different than some here, but I have not had the ability to accurately measure.... I had also taken some economy charts from manufacturers as gospel. ..... I definitely see a need for my own fuel flow meter now. I would be very surprised if I could get that close to my best mpg at wot but would be thrilled to be wrong there.... I HATE going slow unless someone else is driving

 

[mciaio]

Re: Trim tab usage

Actually, it's just the opposite. Dual purpose (semi-displacement) hulls allow planing, but are not very efficient at displacement speeds or on plane. They are for people who want the occasional turn of speed and can live with slow displacement speeds most of the time.

I have a fuel flow meter and it also measures speed. I get real time MPG readings and am happy to say that my boat is most efficient at idle (4 MPG) and at 28 to 36 MPH (2.5 to 2.6 MPG) with the tabs all the way up, which is well above my minimum planing speed. My MPG does not drop terribly from 36 MPH to WOT (which is about 50 MPH) either; at WOT, I'm still getting just over 2 MPG.

Minimum planing speed with tabs down (about 18 MPH) yields 1.8 to 2 MPG. Tabs up minimum planing speed (about 22 MPH) yields just over 2 MPG (the same as WOT). Plowing at just under planing speed gets me to just a little over 1 MPG.

When gas was $4 a gallon a couple of years ago, I saw lots more people plowing at just below planing speed, I bet they thought they were saving money; in fact, they were burning gas at more than double the rate they would have been if on-plane.

This is excactly what I was looking for. THANK YOU!

You are not a Harry Potter fan, are you? LOL!

 

[dingbat]

Re: Trim tab usage

How a boat performs is a function of hull design, power, etc.

My hull was designed for the sole purpose of running in the slop. When I first bought it was quite surprised at how poorly the boat performed in rough water. It took me a while to figure out that I was running the boat to slow for the conditions. The hull needed speed to get it up and out of the water to put the bow into position to do its job. Who would have thought you needed to increase your speed as it go rougher?

To answer the question, none of the above. Anytime I have tab in the water my fuel economy suffers. The only reason I have tabs is for rough water performance or when I'm loaded down with fuel or people and need to balance the load. Otherwise my tabs are up in fuel saving mode.

 

[QC]

Re: Trim tab usage

What a cool thread with very good info (data) . . .

Question for the OP. The ultimate answer for fuel efficiency above planing speed as noted by others here, which I agree with based on all data I have seen and collected, does not fit either one of your scenarios in your poll. So where did you end up? Deciding to cruise faster?

 

[mciaio]

Re: Trim tab usage

I decided that it is different for every boat and every condition. To get the best mileage on your boat, you need a fuel flow gauge. I decided I am getting a fuel flow gauge! LOL!

 

[Mischief Managed]

Re: Trim tab usage

This is excactly what I was looking for. THANK YOU!

You are not a Harry Potter fan, are you? LOL!

My wife and kids are HP fans; they named the boat, and the boat's name is my screen name. I enjoy the HP movies and thought the name was kinda cool for a boat. It starts a lot of conversations.

 

[QC]

Re: Trim tab usage

I want a fuel flow meter as well, but the fact is it will take a long time to pay it back with savings if that's your plan. I have studied data for years and almost every single I/O combo out there, especially 18 - 25 ft, has it's best planing speed fuel efficiency at 3000 - 3500 RPM, and between 25 and 35 MPH. Be really careful to draw a conclusion that these engines are most fuel efficient at that RPM, they aren't!!! The combination of these engines, drives, hulls, gearing, propellers, speed and hydrodynamics makes them most fuel efficient at that RPM, and as some have noted, even more efficient at idle where gasoline engines (Otto cycle) are actually at their worst efficiency as an engine alone.

Maybe something to help understand this is that in a wheeled vehicle, most gasoline engines are spec'd to cruise between 1500 and 2000 RPM. Just so happens with their combination of load, speed, aerodynamics, tire deflection, gearing, operating flexibility etc. etc, etc. that that's where it falls. Annnnnd, if you were setting up a fixed speed and load application for an Otto cycle engine (4cycle, spark ignition and a throttle), and you didn't care about engine life, or potential damage etc. it would probably be most fuel efficient at peak torque RPM and wide open throttle.