weight and fuel burn

[skipperdude]

Does it really matter how much weight you carry in a full*displacement hull. When it comes to the fuel consumption.
SD

[Marin]

Well, heavy is heavy. The heavier the boat the more of it will be in the water and the higher the resistance to move it will be. That said, with a typical trawler like a GB, CHB, etc. there probably isn't much difference in fuel consumption between the boat at its empty weight and it's maximum allowable weight. If the weight affects the trim of the hull in the water this could cause excess drag (or not).

[RickB]

skipperdude wrote:does it really matter how much weight you carry

******************* *f=ma

[Bendit]

I've often thought about this when heading north to places where fuel is generally more expensive than at home.

Although we are a slow planing hull, we generally travel at max displacement speed (9.2kn). (I know we should go slower for better efficiency, but this speed is much more efficient than planing - half the fuel burn for 2.5 kn slower - and keeps a reasonable load on the engine).

Of course it must take a bit more HP to push a heavier hull through the water, but I think this may be offset somewhat by more momentum in a seaway and a tiny little bit of added waterline.....I haven't been able to quantify any appreciable difference in fuel burn and therefore fill the tanks before setting off.

A clean hull and prop is a much bigger factor inmo. The effect of a dirty hull is immediately apparent.

[RickB]

Bendit wrote:

max displacement speed (9.2kn) ... half the fuel burn for 2.5 kn slower

Those are interesting numbers. Another way to look at it: if you paid $3 per gallon for fuel then departed on a 24 hr, 220.8 mile trip at 9.2 knots, you would get there 8.9 hours earlier than if you slowed down 2.5 knots but you would pay $40.45 an hour for each one of them.

It's got to be some pretty awful weather before I am willing to pay $40 an hour to get off a boat.

[Bendit]

Perhaps I was misleading, our fast cruise is 11.7 kn. This uses about twice the fuel as 9.2kn.

The actual burn at 11.7 kn is 45l (11.6 gal) per hour. At 9.2kn it's 20l (5.3 gal) per hour.

A 220 m trip at 9.2kn takes 24 hrs and at $3/gal costs $380
A 220 m trip at 11.7kn takes 19 hrs and costs $654 takes
Our slowest cruise, while maintaining engine oil temp, is 8kn. Fuel burn is 11l (2.9 gal) per hour.
A 220 m trip at 8kn takes 27.5 hrs and costs $239.

9.2 kn is a good compromise for us with limited time available. If we were retired, we might go slower..

Diesel here in NZ is currently NZD1.20 per litre or USD3.15 per US gal.

[Forkliftt]

Rick,
f=ma
I looked it up and a seems to stand for acceleration. If that were the case- once you reached your cruising speed would a different law take effect?

[RickB]

Forkliftt wrote:

Rick,
f=ma
I looked it up and a seems to stand for acceleration. If that were the case- once you reached your cruising speed would a different law take effect?

You are correct, mostly, it was an off the cuff catchall response to the question of power to move weight.

It requires a certain amount of power to move a given weight a given distance in a given time. Add weight or reduce time and it takes more power. Reduce power and it takes longer to move the same or more weight.

You could almost say that a boat is always accelerating, at least it is accelerating to get back to its set point speed because everything wants to slow a boat. Each time the bow goes into a sea, the boat slows, each time the rudder moves, the boat slows, each time a sea hits the quarter and the boat wallows, it slows. Unless you are going downriver with a tailwind you just can't win. Ever notice that you set the throttle at your desired cruise rpm then eventually the boat reaches cruising speed and you stay at the same power setting. The same boat will take longer to get to speed and more power to stay there when it's heavy than when it's light.

That doesn't apply to a planing hull of course, they're a whole different world.

*

[Forkliftt]

Got it. Love the Avatar

[FF]

Most boats that can carry enough fuel to matter wont notice the difference if any caused by the weight.

Weight translated via "inches of immersion" to extra wetted surface to be dragged they the water.

For most boats an extra ton of fuel will only lead to a 1 or 2% increase in wetted surface.

Hardly noticible as few boats of our size have fuel management down to 1 or 2%.

Want to save money , slow to between .9 to 1.15 of "hull speed.

Usually 1/4 the fuel burn , for most its about 2K slower.

[RickB]

Forkliftt wrote:

Got it. Love the Avatar

Thanks, when I get the waterproof cuffs figured out I will have really cheap propulsion. Not much of a view though.

*

[Keith]

One of the rags out there did a story on this subject. Did some runs with the boat loaded up and weighed in the slings, then removed a lot of weight, weighed it again and did some more runs to get fuel burn. I think it was in an old Passagemaker, but it could have been Power and Motoryacht or something else.

The short version was that even with a trawler, the difference was surprising! They had assumed it didn't make much difference to a big, slow trawler but it did. I wish I could remember real numbers but my brain doesn't work that way. It's worth it to try to keep your boat as light as possible.

[Marin]

Keith wrote:The short version was that even with a trawler, the difference was surprising! They had assumed it didn't make much difference to a big, slow trawler but it did.

That's interesting, as from what I've heard said I assumed the same thing.* But if you think about it, we do put a lot of weight on our boats what with cans of food, equipment, dinghy motors, tools, spare parts, books, clothes, cooking stuff, etc., etc. etc.* I bet if all this stuff was stacked on a scale it would amount to way more weight than we thought.

And the greater the wetted surface, the more drag there will be.

Maybe this is why charter boats always seem to go so fast--- other than essentials they are "stripped out" as far as on-board "stuff" is concerned.* Or maybe it's just because since they don't own the boat, the charter folks figure they'll just floor it and go-- the consequences are not their problem

*

[FF]

Or maybe it's just because since they don't own the boat, the charter folks figure they'll just floor it and go-- the consequences are not their problem smile



Charter boats have someone e;lse , the customer , paying for the fuel and rebuilds.

A DD 6-71 will go 10,000 hours with ease , many have been nursed for 2x or 3x that life.

The Turboed version use on fish killers seldom sees 1000 hours with out a removed from the boat rebuild , not just a quickie inframe.