Displacement numbers

[Kd9truck]

Originally Posted by Bowdrie

Sure enough.
You'll need several sizes of Crescent wrenches, both metric and SAE in both left and right hand configuration.

I am not a boat junk hoarder , but I am proud of my LEFT HANDED tool stores, organized in a custom cabinet that I built it sits anchored to the in cabin mast support in the center of my boat over the front of my keel.

[Adelie]

Originally Posted by Mike OReilly

I have no idea where the sailboatdata comes up with their data. I know my boat is listed at 26,500#, but a recent crane operator called out 30,000 Pounds!

SBD uses the manufacturer’s published data. I used to interact with Randy Brown a lot about 8-10yr ago providing missing info or pointing out typos. I even got name checked in one entry.
He’s died since but the new guys seem to be following his protocol.

From the SBD list of definitions:
“DISPLACEMENT: If you weigh the boat on a scale, that is her actual displacement. It is the weight of sea water that she will displace when she is afloat. Many designers figure displacement when half loaded with stores, liquids and crew. Some report light displacement which is the weight of the hull and permanent equipment but with empty tanks. Light displacement does not include stores, removable equipment (ex. Sails) or crew.”

So so the meaning of “displacement” varies some boat to boat.

[TrentePieds]

Ah, but k9, you did say "for ballast" :-) Don't go misleading simple souls like me :-)

If you are thinking cargo or provisions, then by all means! The number that will then interest you is the one called "pounds per inch immersion". You boat will carry a LOT. The trick is to know when enuff is enuff.

You might already know that in the old days, the days of the tall ships pounding around Cape Horn to pick up guano to fertilize the fields of the German Plain, the emergency ration of water was half a pint per man per day. No fuel of course, ' cept coal for the galley which you couldn't use anyway when it blew. But then, it wasn't when it blew and you were making way you were worried about rations :-)

And in you boat, when it DOESN'T blow, aren't you better off just waiting it out rather than be burning fuel since even you boat cannot cross an ocean on its native engine regardless of how much extra fuel you carry.

If you don't know how to work out the Pounds per Inch Immersion, I'll be glad to show you :-).

TP

[sandy stone]

I would venture to say that a boat's listed displacement almost never comes from weighing an actual boat. Usually it would be the designer's estimate of the total weight of all the bits and pieces that go into its construction, along with an estimate of the hull weight based on the designer's layup schedule. I would further venture to say that these estimates almost always fall on the optimistic side, given the pressure from the advertising department for competitive SA/D, D/L, and ballast ratios, all of which are negatively impacted by higher displacement.

Going even farther out on a limb, I would be surprised if any two randomly chosen 40' boats of the same model, from the same manufacturer, weighed within a few hundred pounds of each other, or came even that close to the listed displacement. I consider listed displacement numbers to be no more than a rough guide for comparing somewhat similar boats.

[TrentePieds]

Well, yes, I agree, but why do we even care? Spurious accuracy is the bane of the sailorman, and an affliction that visits primarily them wot's homeported in NA. I don't give a hoot about what is the weight in pounds of TrentePieds. Nor what it is in kilogrammes. For me it's close enuff that she's a "five-tonner".

TP

[h20man]

Originally Posted by TrentePieds

Well, yes, I agree, but why do we even care? Spurious accuracy is the bane of the sailorman, and an affliction that visits primarily them wot's homeported in NA. I don't give a hoot about what is the weight in pounds of TrentePieds. Nor what it is in kilogrammes. For me it's close enuff that she's a "five-tonner".

TP

I care.. as I am asked at when I got a mooring for displacement. (WET, not dry, so full as she is now.).


Also for choosing anchors, or drogues.. Factor into all the stopping or non sailing part..

[Kd9truck]

[QUOTE=TrentePieds;3790898]Ah, but k9, you did say "for ballast" :-) Don't go misleading simple souls like me :-)

Apologies TP, I try to put very few things on my vessel that have one purpose. I also think of everything as ballast and where the tank is going it will be sitting on top of a portion of the keel.

If you are thinking cargo or provisions, then by all means! The number that will then interest you is the one called "pounds per inch immersion". You boat will carry a LOT. The trick is to know when enuff is snuff.

100%, besides my obsession of all the correct tools and extra parts for almost every job with safety equipment in this mix my vessel is light.

A note about extra fuel.
I start licking the mast when there is not enough wind to move my boat. I was a chef before I was a firefighter so I do not do slow it’s not in me, I fire up “the beast” when the wind drops below 4 knots on passage, so extra fuel will have a place inside on the centerline of my vessel when making passage.

[Kd9truck]

I don’t know about boat lifts but I worked briefly as a rigger and we worked with many different cranes. If a crane operator tells you the weight his Crane is lifting, it’s pretty spot on. As for published data I would think they are in the ballpark maybe not the same ballpark but ya gotta start somewhere.

Thanks all

[TrentePieds]

Uhm... zere vee go mit ze accuracy again :-)

If a construction crane topples, you get dead people. Therefore cranes are fitted with strain gauges that let you know when you are about to go for a burton and what to do about - ahem - "ballast" :-). That is also the reason crane operators can be pretty cantankerous, not to say inflexible, people.

If you overload a Travelift, nothing really happens unless the staps give out, so there is no need for accuracy. If you really, really what to know the weight of your boat with accuracy - tho' heaven knows why you would - you can of, course, jack it under the keel and read the hydraulic pressure. I wouldn't recommend that you try to do that yourself :-)!

TP

[Adelie]

Originally Posted by h20man

I care.. as I am asked at when I got a mooring for displacement. (WET, not dry, so full as she is now.).


Also for choosing anchors, or drogues.. Factor into all the stopping or non sailing part..

Counter intuitively anchor loads are much more related to boat dimensions than displacement.

[wingssail]

Coming up with an actual displacement of your vessel can be challenging.

On sailboat data my vessel is listed as 17800 lbs. On various travel lifts it's been all over the place, from 14000 to 21000lbs. On my measurement certificates it is 20495 (ORR), 18823 (IRC), 18782 (IMS), 16508 (IOR)

When I bought the boat in San Francisco it had 1200 lbs of extra led in the bilge which I removed for sailing in Seattle. Now I am carrying very close 2200lbs of extra cruising gear weight (above what would have been on the boat when delivered new) not including fuel and water, so if we take the average empty weight of 18000lbs and add 2200lbs it comes to 20200lbs plus fuel and water.

So when you ask about a boat's displacement, it can be complicated.

BTW, I am in the process of removing weight. It seems that I now have some extra gear I no longer need.

[AiniA]

Originally Posted by Mike OReilly

I have no idea where the sailboatdata comes up with their data. I know my boat is listed at 26,500#, but a recent crane operator called out 30,000 Pounds!

Our Bristol 45.5 was nominally 36,000 and change pounds. When we got hauled in Oz it was 40,000. We certainly didn't scrimp on the 'essentials' onboard but the boat was not severely loaded compared to some.

[TrentePieds]

And "the beast" would be about a fifty-horse, nicht wahr? With a fuel burn at croosin revs of about 2 gallons/hour? Call it 8 litres for easy calculations. So that about 192 litres noon to noon. In which time you would run maybe 240NM - if the sea is flat.

A tank 30 cm x 30 cm x 180 cm (12" x 12" x 72"), such as you might find room for in your bilge, and which, when full, will weigh about the same as a well-grown man, holds 162 litres, i.e. just about enuff to take you just about 200NM. Better than nowt, of course, but IMO hardly worth the effort when a few days' wait will get you wind again. Massage the numbers to your heart's content but you will find that if you want to cross oceans by burning fuel, a Boeing 747 is the way to do it ;-0)!

But don't give my opinion too much credence. I'm strictly a coastwise man. I hate the ocean ;-)!

TP

[TrentePieds]

Well, coming back to "displacement": That figure is NOT the weight of the water displaced by the ACTUAL, real live boat.

It is the weight of water that WOULD BE displaced by a "prism" represented by the boat's hull as given by its "lines" if that prism were floating to the hull's design waterline. i.e. it is a CALCULATED figure. There are many good reasons for naval architects and even yacht designers to calculate that figure. To the practical sailorman it means very little. What matters is how much weight you stow and where in the boat you stow it, mainly because what you stow and how you stow affects stability and what in a loosey-goosey way is known as seaworthiness.

TP

[Bowdrie]

Today a designer uses a computer.
In the past they used a device called a "Planimeter" to measure the area of the underwater sections.
This, together with Simpsons Rules would give them a pretty close estimate of the actual volume of the underwater hull at the designed waterline.
They also performed a rigorous weight-and-balance study to determine the amount of ballast and its fore-and-aft location to maintain the proper CG/center of buoyancy.
I have a copy of the "Weight and Balance" study done by the designer of my boat.
It's several pages long and quite detailed.
When my boat was launched it floated within 1/4in. bow and stern of where it was supposed to.
However, I cheated, I actually weighed the boat and its ballast before it was launched and had done my own weight-and-balance study and was in contact with the designer because of changes made.
Make no mistake, the "displacement" of a vessel IS the actual weight of the water it displaces, and it IS the actual weight of the boat.
Archimedes law still prevails.
In many racing classes they will actually weigh a boat using slings attached to a dynamometer.
The volume of the underwater part of the hull vs a prism is used for determining the "prismatic coefficient", and that has a bearing on not only the most efficient speed of the vessel but also its "fineness/fullness" of hull shape, it's only related to displacement in a peripheral manner. (look at the cross section of a container ship).