60 ton boat, 120 hp engine, how does that work?

[Cadence]

Originally Posted by Cpt Pat

Off topic, but this is something that has always puzzled me. Why do we express vessel weight as: "displacement of water" when, if it's buoyant (and not subject to some kind of acceleration or deceleration) a boat displaces exactly as much water as it weighs?

If my boat weights 6,000 pounds it displaces 6,000 pounds of water (assuming it's not taking on water, weighing more as a consequence, and in the process of sinking). If it were floating in any fluid, it would displace exactly as much of that fluid as it's own weight. So wouldn't it be just as valid to state: my boat displaces 6,000 pounds of wet concrete?

These displacement numbers seem to be a way of relating cargo capacity: "if I filled my entire boat with water, it would weigh this much." We who have non-cargo-carrying vessels try very hard not to fill it to the brim with water.

It is only the weight of water if filled to the waterline as a void. Displaced water. It may have no ballast or tons makes no difference. Makes more sense than a cubic area and an arbitrary assigned weight.
It is apples and oranges.

[Simi 60]

Originally Posted by Cpt Pat

?

If my boat weights 6,000 pounds it displaces 6,000 pounds of water (assuming it's not in the process of sinking). If it were floating in any fluid, it would displace exactly as much of that fluid as it's own weight.

If it was floating in salt water it would be floating higher than in fresh water
If was floating in a sea of liquid mercury there wouldn't be much boat in the liquid at all

[Cadence]

Originally Posted by Simi 60

If it was floating in salt water it would be floating higher than in fresh water
If was floating in a sea of liquid mercury there wouldn't be much boat in the liquid at all

A good point! If used as a standard what should be used? Probably fresh since salinity varies. I'd bet when stated by a naval architect it is fresh?

[Cpt Pat]

Originally Posted by boatpoker

No. The weight of water and wet concrete are not the same per cubic ft.

Correct. That's why I chose concrete for the example. My 6,000 pound boat while floating will displace a smaller volume of concrete - but if you weighed that volume of displaced concrete, it would weigh exactly 6,000 pounds.

And in acknowledgement to any physics majors reading this, I'm using the term "weight" instead of "mass" as a colloquialism, assuming an unaccelerated 1 G field where the two terms are exactly the same. I'm hampered by common English - most other languages correctly use the term "mass."

Archimedes' principle, physical law of buoyancy, discovered by the ancient Greek mathematician and inventor Archimedes, states that any body completely or partially submerged in any fluid (gas or liquid) at rest is acted upon by an upward, or buoyant, force the magnitude of which is equal to the weight of the fluid. Wet concrete is a fluid. Feel free to substitute any other fluid that is as dense or denser than water (my boat will sink in liquid hydrogen). The outcome will be the same. My boat will displace an amount of that fluid exactly equal to its weight -- unless Archimedes was wrong.

The root of the disagreement is the confusion of terms used for volume (important for cargo) versus mass. A vessel's rated (documented, registered, etc.) displacement describes its volume -- not its mass ("weight"). It describes "how big the boat is" but not how much mass is being pushed through the water. When we are talking about power needed to propel a boat (such as in this thread) the volume of the boat isn't as important as how much water it displaces while floating -- which is the same as its weight. So in acknowledgement of old Mr. Archimedes, why not just say "weight" and avoid the confusion?

My boat is documented. It's documented displacement is 5 tons (10,000 pounds). That number is calculated by measuring its outer dimensions. But it only weighs 3 tons. And I'm sure (though I've never tired it) that I could not put another 2 tons of stuff (water, or whatever) into it without sinking it.

5 tons does sound bigger than 3 tons. So maybe that's why people like to state displacement instead of weight. "Mine's bigger than yours."

[Cpt Pat]

Originally Posted by Simi 60

If it was floating in salt water it would be floating higher than in fresh water
If was floating in a sea of liquid mercury there wouldn't be much boat in the liquid at all

Correct. Salt water is heavier than fresh water. But the weight of either of the displaced waters would be the same: the weight of your boat.

Unfortunately, someone long ago decided to express volume as an equivalent weight of water, and started this confusion of mass versus volume. Had they chosen cubic feet instead to describe volume, there would be no confusion.

A hollow sphere could weight almost nothing and yet have a high "displacement" (volume). Because of this confusion (which none of us here are to blame for), I suggest using the term "displacement" only when referring to volume (the boat's dimensions), not when referring to weight.

[NevisDog]

Originally Posted by Cpt Pat

... Because of this confusion.., I suggest using the term "displacement" only when referring to volume (the boat's dimensions), not when referring to weight.

Whoa! Since "displacement tonnage" means precisely the vessel's weight (mass of fluid displaced by the vessel afloat, whether it be in salt or fresh water, or wet concrete), adopting that term when referring to volume would surely add to the already confused confusion. "Registered tonnage" (gross or net) would be a better term for measured volume (registered cargo capacity).

[Cpt Pat]

Originally Posted by NevisDog

Whoa! Since "displacement tonnage" means precisely the vessel's weight (mass of fluid displaced by the vessel afloat, whether it be in salt or fresh water, or wet concrete), adopting that term when referring to volume would surely add to the already confused confusion. "Registered tonnage" (gross or net) would be a better term for measured volume (registered cargo capacity).

Agreed. But wouldn't it be far simpler to just use the term weight, or better yet, mass? I've found no fewer than 10 qualifiers for the word "displacement."

When someone says just "displacement," I have no idea which of those qualifiers they mean. Saying: "my boat has a mass of 3,000 pounds" seems much clearer to me. But maybe the word mass just sounds lubberly to some.

I am guilty of other non-salty verbage, like calling the trailing edge of my mainsail the "lee edge" instead of the "leech," since I don't normally speak English with a cockney accent.

[MartinR]

Originally Posted by Simi 60

Our sister(from a different mother) ship when converted at the same time was re engined with a 6lxb Gardner @ 152hp.
I imagine she runs near full noise vs our fast idle.

The sound of a Gardner, though.

[Warby12]

Originally Posted by NevisDog

From Performance Product Technology website:

Torque = HP x 5252 / RPM [units not stated]

It is interesting to note that the formula also verifies the typical torque bell curve when the torque trails off as rpm increases at the top end. ... It’s not by chance that most marine engine manufacturers continue using the larger displacement – lower rpm, cast iron marine engines because of this fact...

BOTTOM LINE
When looking to re-power spend as much if not more time focusing on the torque rather than horsepower.

I knew there was a reason for those slow-running diesels being so much more powerful...

Sooooo, does using a higher geared gearbox, increase the torque, at the shaft, of an engine?

[valhalla360]

Originally Posted by Cpt Pat

A hollow sphere could weight almost nothing and yet have a high "displacement" (volume). Because of this confusion (which none of us here are to blame for), I suggest using the term "displacement" only when referring to volume (the boat's dimensions), not when referring to weight.

If you've ever seen a ball floating in the water, you would know this is wrong as it is almost entirely out of the water with only a tiny amount of water displaced.

I believe it goes back to Archimedes and his theory of displacement as a method of measuring both volume and weight of oddly shaped objects.

There is no confusion in the industry but you deciding to redefine the term likely will add confusion.

[valhalla360]

Originally Posted by Warby12

I met this guy once and he had a 60 ton steel trawler, similar in style to a fishing trawler but built for him as a pleasure boat. I've been thinking about it ever since.

The boat was absolutely beautful and perfect in every way. He knew an enormous amount about boats and had been a pro fisherman for years.

He said it had a 120hp engine and a reduction gear number I should have paid more attention to but didn't. It cruised at six nots. It also had a very big prop.

How does that work? I would have thought 120hp engine would be too small for a 60ton boat but clearly not!

Is it all in the gearing? How would a boat like that behave?

Thanks for any explanation!

The reduction gear and big prop make it kind of like a tractor. They don't go very fast but they will keep going even without a lot of HP.

Torque is what determines if the engine can move the boat. A reduction gear increases torque, so you don't even need a high torque engine.

HP determines how fast the boat will go (assuming you have enough torque to keep spinning the prop and don't max out of RPM first)

The prop dimensions do factor into this also. In particular the pitch can function as additional gearing.

As long as you don't exceed displacement hull speed, it really doesn't take a lot of power to keep a boat moving (acceleration may be pretty marginal with a heavy low HP boat)

[denverd0n]

Originally Posted by boatpoker

To the OP ... I suspect that many of these posts are quoting their documented tonnage rather than displacement (weight) tonnage.

This was my first thought. A boat that is documented with the CG as "60 tons" is a completely different animal than a boat that displaces 60 tons of water (or wet concrete, if you must ).


I also note that the OP mentioned the boat cruising at 6 knots. Hull speed of 6 knots would mean a length-waterline of barely over 20 ft. That is EXTREMELY small for a 60 ton boat. Indeed, I would expect a typical 60 ton boat to have an LWL more than twice that. As such, I'm guessing the boat mentioned originally was not cruising anywhere close to hull speed, and could very possibly need more horsepower to get it to hull speed.

[Training Wheels]

Speaking of tonnage, here’s one for you. Our 44’ trimaran weighs 10,000 pounds. Our gross tonnage on our documentation is 68 TONS.

[Cadence]

Originally Posted by Cpt Pat

Off topic, but this is something that has always puzzled me. Why do we express vessel weight as "displacement of water" when, if it's buoyant (and not subject to some kind of acceleration or deceleration) a boat displaces exactly as much water as it weighs?

If my boat weights 6,000 pounds it displaces 6,000 pounds of water (assuming it's not in the process of sinking). If it were floating in any fluid, it would displace exactly as much of that fluid as it's own weight. So wouldn't it be just as valid to state: my boat displaces 6,000 pounds of wet concrete? Why bother mentioning the fluid? Why not just say how much it weighs and assume we aren't talking about a submarine?

These displacement numbers seem to be a way of relating cargo capacity (volume): "if I filled my entire boat with water, it would weigh this much." We who have non-cargo-carrying vessels try very hard not to fill them to the brim with water. Are we just trying to sound salty - when it isn't necessary?

Your right on the 6000# analogy. The difference maybe the design waterline weight.

[MartinR]

Originally Posted by Simi 60

If it was floating in salt water it would be floating higher than in fresh water
If was floating in a sea of liquid mercury there wouldn't be much boat in the liquid at all

Independent of the liquid, the boat will always displace the same amount of pounds. Saltwater, fresh water makes no difference. but the draft will change a little. A lot in mercury. Same displacement, though.

Displacement is the boats weight.
Gross register tons is basically the boats internal volume. 1GRT = 100 cubic feet