Go fast or go slow? Which is the most economical?

[SADS 669]

Quick question? Which is more economical? (With a 250 hp 2 stroke outboard on a fairly heavy boat, 17 inch prop, photo below)

1. Going 20 miles at just above idle ( 5 or 6 knots) for 4 hours or so

2. Going 20 miles at 4,000 RPM in an hour or so.

Planning to trial this but thought a request for opinions would be a good idea

 

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[OWK]

The boat will travel with the highest fuel economy when you give it just enough throttle to keep it on plane.

 

[Boatlode]

I always drive my boat as fast as the conditions allow. With all the money I spend on toys, gas is my least concern. I want to get where I'm going to maximize my play time.

 

[releventchair]

Sea and hull conditions a factor too. I have a hard time going slow with a two stroke. Boat control is one reason. The motor wanting to breath another,or my wanting to go zoom more likely. My tiny ones compared to yours, seem to load up at sustained low speeds after a while. As mentioned being on plane is less resistance and should be best economy for cruising. A kicker motor in a four stroke would add weight but may reduce fuel use if just out putting or trolling. Linked to steering arm of main motor when in use would add familiarity to set up.

 

[SADS 669]

Thanks guys,

I am hoping to give an opinion based on trialling this soon, it was interesting that I did 10 knots once in a 54 Bertram for 5 hrs and used EXACTLY the same amount of diesel by completing the same trip ( weather the same) at 25 knots in two hours.

Just wondered if anyone has done this in a heavy outboard boat??

 

[BVI Hunter]

Go fast or go slow? Which is the most economical?

............ as I said to the wife the other night....

 

[Jolly Mon]

I am certainly no expert, but early 2000's 2 stroke Yamaha Vmax 250's ( I don't know if that's what you have...just guessing) are supposed to have fuel consumption figures something like:

RPM GPH
1000 2.5
2000 7.8
3000 10.5
4000 15.2
5000 20.5
5900 26.9

So, throwing out variables like weather and sea conditions it probably mostly depends on the boat's hull design and displacement. ( I am going to guess your boat is around 4000 pounds fully loaded ?? ).

If these fuel consumption figures are accurate and your boat can make 40 knots at WOT (5900 RPM)...20 knots at 4000 RPM and 5 knots at idle (1000 RPM)...theoretically, at least, you could make the trip with 10 gallons at idle...15.2 gallons at cruising speed and 13.5 gallons at WOT.

These are just theoretical figures based on guesses at your boat/motor combination performance.

For my part, I am going to take a wild guess and say you might actually save 3 to 5 gallons on the trip if you can actually make 5 knots in your boat at idle speed...

It is an interesting question. Cheapskates like me are interested in your results. I try to never be in a hurry and I have spent days camping in little cuddy cabins...with a Bimini like that, a little forethought and enough beer, you could stay on site fairly comfortably for 2 or 3 days at a time and possibly save $100.00 to $150.00 on fuel vs. travelling back and forth each day...

 

[SADS 669]

I am certainly no expert, but early 2000's 2 stroke Yamaha Vmax 250's ( I don't know if that's what you have...just guessing) are supposed to have fuel consumption figures something like: RPM GPH 1000 2.5 2000 7.8 3000 10.5 4000 15.2 5000 20.5 5900 26.9 So, throwing out variables like weather and sea conditions it probably mostly depends on the boat's hull design and displacement. ( I am going to guess your boat is around 4000 pounds fully loaded ?? ). If these fuel consumption figures are accurate and your boat can make 40 knots at WOT (5900 RPM)...20 knots at 4000 RPM and 5 knots at idle (1000 RPM)...theoretically, at least, you could make the trip with 10 gallons at idle...15.2 gallons at cruising speed and 13.5 gallons at WOT. These are just theoretical figures based on guesses at your boat/motor combination performance. For my part, I am going to take a wild guess and say you might actually save 3 to 5 gallons on the trip if you can actually make 5 knots in your boat at idle speed... It is an interesting question. Cheapskates like me are interested in your results. I try to never be in a hurry and I have spent days camping in little cuddy cabins...with a Bimini like that, a little forethought and enough beer, you could stay on site fairly comfortably for 2 or 3 days at a time and possibly save $100.00 to $150.00 on fuel vs. travelling back and forth each day...

Fantastic reply, thanks so much. Your description of camping is spot on and your other comments about the boat are accurate too. I will let you know how I do, I am off out at the weekend.

BVI, you are a very naughty chap, OWK, relevant, and boatlode, thanks for the guidance

 

[BVI Hunter]

Stay Safe!!!

 

[bungyboy]

When I was driving a salvage/recovery and vessel assist boat, I was taught to punch it out of the hole, then back down to just ABOVE planing speed, that way it was using the least amount while not chancing lugging down going into a wave, etc. Keep her running in a straight line as any deviation wastes fuel. Turn and look behind you every few minutes to see if you are drifting with the variable conditions. Hull shape is a factor, I drove a v-hull, and a tunnel hull, both 23ft boats with Evinrude 250hp 16/16 prop with 2/1 gear reduction (at least that's how it was explained to me) With that setup, I pulled an 80ft two story paddlewheel boat off a sandbar. The fuel tank took up most of my work space lol.

 

[SADS 669]

BB,

Thanks so much for the useful reply, I'll let you know how I do?

 

[Jolly Mon]

I thought this performance chart was interesting.

The boat is a similar hull design to yours. She displaces around 4500 lbs with motor, crew, gear and fuel.

The Evinrude engine is smaller than yours, only 200 HP, but is a two stroke and can obviously generate a lot of torque at low RPM's. You won't get the same economy, but the engine is similar to yours and the relative results of speed to fuel consumption should be very similar.

Some things jump out immediately.

First is the incredible fuel economy of the rig if you had the patience to creep along at 3 MPH at 500 RPM. I think if you told most people it was possible to make a 16 mile trip in a 4500 pound boat with a 200 HP two stroke outboard---and do it on ONE gallon of gas---they would call you crazy.

But in real world conditions, even if you had the patience to creep that slowly, it might not be possible. Wind, current, chop, etc., would probably force you to increase RPM's just to keep a steady course.

If you look at the fuel consumption column, the overall efficiency is not linear.

Once the probably most realistic minimum speed is achieved---4.5 knots at 1000 RPM, there is a steady drop in performance until optimum planing speed is reached at around 3500 RPMs. At that point efficiency increases from 2.5 to 3.4 MPG---and falls with increasing RPM's after that.

But look at the drop in efficiency between 1000 and 1500 RPMs. It is huge. The boat in question has a displacement hull speed of 5.7 knots...after that, it becomes a planing hull.

I hope I am not sounding like a snotty yachtty, but if you have the time, figure out your boat's theoretical hull speed. It is very easy. Just take the square root of the boat's length at the water line in feet (LWL) and multiply that figure by 1.34. After that, increasing power from the engine turns the boat into a planing hull and radically increases its speed potential.

I guess the point I am trying to make in my own bombastic and long winded way, is that you might find the best realistic fuel economy for your boat at right around its maximum theoretical displacement hull speed...

Have a great weekend !!!

 

[Au_Dreamers]

Fuel consumption is probably nearly the same.

One burns the same calories if they walk a mile or if they run a mile...

The factor to the equation is duration of time.

Is your time worth saving 3-6 gallons of fuel ($15? - $30?)? *using $5/gal

What more could you accomplish with 3 hours of being at your destination?

Those can be exponential too as those are one way figures.

If you need to cruise there and back in daylight then you're talking about 6 extra hours at your destination. 6-12 gallons of fuel ($30 - $60)...

We have the same considerations when choosing which 1715 site to go to.
Travelling time is travelling time,
Digging time is treasure time!!

 

[SADS 669]

Thanks so much guys,

I may leave at 2am and come back the same way 3 or 4 days later and fuel here is over 6 bucks a gallon ha ha.

 

[Jolly Mon]

No need to reply, K, but another advantage of cruising at very low RPMs is that it opens up the possibility of using an inexpensive Side Imaging sonar on the way to your destination. These units have fallen so much in price in recent years that just a few weekends of fuel savings would pay for a brand new unit.

Shoot man, in your location there is just no telling what you might stumble across.

You might find yourself reclining on the beach in retired splendor with that friend of yours down in the Virgin Islands...