Increase fuel economy on outboard powered boat

Jmac572

Jmac572

Active Member
Dec 20, 2016
212
Plymouth, MA
Boat Info
2004 Sea Ray 390 Sundancer "Freedom"
Engines
Twin Mercury 8.1
Hi everyone... So I am looking at a new outboard powered cruiser (twin 350 Verado's) and the fuel figures aren't too appealing. The review for the boat says it won't stay on plane under 4500 RPM (33 mph) so I am wondering if anyone has any experience on ways to either keep the boat on plane at lower speeds or increase fuel economy some other way.

33 miles per hour is a fast cruise speed so I'm not sure how I feel about that, something around 25-28 mph seems better to me, if I can keep my boat on plane at that speed I'm sure I will see increased fuel economy. The only way I can think to keep it on plane is to bury the trim tabs, but I'm not sure if that will lower the fuel economy with the extra drag. Below is the fuel chart, appreciate any feedback.


O3KCFVP62ZBUTDW54L7JHLBHLQ.jpg
 
Not much you will do to change that. I am an avid OB guy, but they have their limitations. Lower torque and torque at higher rpm requires less prop pitch then the same hull with big displacement IO engines, so spinning higher rpm is the result. Fuel economy is about the same, but higher cruise speeds developed to get the motor to its sweet spot.
 
paulswagelock said:
Not much you will do to change that. I am an avid OB guy, but they have their limitations. Lower torque and torque at higher rpm requires less prop pitch then the same hull with big displacement IO engines, so spinning higher rpm is the result. Fuel economy is about the same, but higher cruise speeds developed to get the motor to its sweet spot.
Click to expand...

I'm having a Four Winns H350 OB surveyed on Friday so the deal is almost finished. This was one of the few things I disliked about the boat. 400 Verado's would have been better but the boat is almost half the price of the Sea Ray 320 Sundancer OB, and its 3 feet longer with a wider beam, so ill take it haha
 
Try evaluating the cruise rpm as a percentage of WOT to determine your comfort level with running the engine like that.

For your 4500 rpm cruise, it would be about 70 percent of the 6250 WOT.

For my old 4.3L Vortecs, I cruise at 3300 rpm with a WOT of 4800. That is about 70 percent of WOT.

Outboards are just wound up tighter.

Mechanics, is my logic wrong?
 
Take a look at that chart 4000 and 4500 rpm. It’s only a 500 rpm difference, but it goes from 16 mph to 32 mph.

I think that very narrow RPMs band would make throttle / speed adjustments very hard in any kind of lumpy seas. Are you said, it’s a fast cruise. If you hit choppy seas and want to back off the throttle for a better ride you’re going to fall off plane. I don’t know if that’s a function of the outboards, the hull design, or both.

By comparison my 270 AJ felt best at 3800 (80% of WOT) and did 27-28 mph. In lumpy seas I could back it down to 22 mph for a easier ride and still stay on plane. I still needed to work the throttle but only started to fall off plane around 18 mph. I had enough speed / rpm buffer recover. From looking at the performance chart and your comments from the review I think that boat doesn’t have that buffer.
 
b_arrington said:
Take a look at that chart 4000 and 4500 rpm. It’s only a 500 rpm difference, but it goes from 16 mph to 32 mph.

I think that very narrow RPMs band would make throttle / speed adjustments very hard in any kind of lumpy seas. Are you said, it’s a fast cruise. If you hit choppy seas and want to back off the throttle for a better ride you’re going to fall off plane. I don’t know if that’s a function of the outboards, the hull design, or both.

By comparison my 270 AJ felt best at 3800 (80% of WOT) and did 27-28 mph. In lumpy seas I could back it down to 22 mph for a easier ride and still stay on plane. I still needed to work the throttle but only started to fall off plane around 18 mph. I had enough speed / rpm buffer recover. From looking at the performance chart and your comments from the review I think that boat doesn’t have that buffer.
Click to expand...

Their test day was in moderate chop I believe so I am wondering if that's why the drop off plane was such a quick hit between that narrow RPM range. I am going to have to see how it is in person, the boat comes stock with 4 blade props so that switch for added prop surface area is out the window. I am wondering how running with the tabs fully deployed would help keep the boat on plane.
 
Jmac572 said:
Their test day was in moderate chop I believe so I am wondering if that's why the drop off plane was such a quick hit between that narrow RPM range. I am going to have to see how it is in person, the boat comes stock with 4 blade props so that switch for added prop surface area is out the window. I am wondering how running with the tabs fully deployed would help keep the boat on plane.
Click to expand...
A test run it the best way to see if it’s ok for you. What kind of boat is it?

I’m very happy with a cruise speed of 26-28, by the way. My current boat will cruise at 29 in smoother seas and 27 in heavier stuff and still be comfortable so I’m quite pleased. Going 35 or 40 mph just doesn’t appeal to me.
 
If you’re using the chart as your only reference, The data on the chart might not tell the whole story.

Collecting data every 500 rpm Increase is good, but doing the same in reverse is more helpful.

From 4500 rpm, what happens as you decrease rpm in small, say 100 rpm increments.

Another set of data points while decreasing speed is needed.
 
b_arrington said:
A test run it the best way to see if it’s ok for you. What kind of boat is it?

I’m very happy with a cruise speed of 26-28, by the way. My current boat will cruise at 29 in smoother seas and 27 in heavier stuff and still be comfortable so I’m quite pleased. Going 35 or 40 mph just doesn’t appeal to me.
Click to expand...

Its a 2018 Four Winns H350 Outboard... twin 350 Verado's. I love my 290SDX for the open outside layout but we need a cabin for overnighting and stand up shower that you can actually shower in. This boat provides the best of both worlds at a far more reasonable price than Sea Ray's new 320 Sundancer.
 
FWIW most builders equip their boats with inadequate trim tabs. Here on CSR you can find many threads about modifying trim tabs to improve lower planing speeds and improving fuel economy. My 390EC Sea Ray is on of those boats. Not sure if and how it could be applied to your Four Winns but it is something to consider.
 
Do you think the difference in planing speeds is related to moving the engine weight back to very end of the boat?

That is a big shift in weight distribution compared to an I/O of the same size. It is not like they can just add weight up front because that would make the entire boat very heavy.
 
DWABoat said:
Do you think the difference in planing speeds is related to moving the engine weight back to very end of the boat?

That is a big shift in weight distribution compared to an I/O of the same size. It is not like they can just add weight up front because that would make the entire boat very heavy.
Click to expand...

It's possible, although the outboard motors are about 300 pounds lighter than a 6.2 I/O package. Also the Duo props on the bravo 3/VP drives increase the prop surface area which I'm sure has a massive impact on the ability to slow planing speeds.
 
Last edited:
Jmac572 said:
It's possible, although the outboard motors are about 300 pounds lights than a 6.2 I/O package. Also the Duo props on the bravo 3/VP drives increase the prop surface area which I'm sure has a massive impact on the ability to slow planing speeds.
Click to expand...
I hadn’t thought of that. We run duo-props on Konrad 680 I/Os duo-props on a 45-ft boat with Cummins 6.7’s. The boat pretty much stays on plane from DIW to WOT. Almost no bow rise. With the forward weight, torque of the diesels plus to duoprops, it accelerates easily. Makes for a very smooth and comfortable ride.
 
You must log in or register to reply here.

Forum statistics

Threads
113,155
Messages
1,427,399
Members
61,061
Latest member
Rod01
Back
Top