Displacement numbers

[TrentePieds]

Ah, the memories :-)

My planimeter is still in my desk drawer. The foam that lines it has long since perished as has my ability to apply Simpson's Rule just off the top of my head. My drafting instruments are also still there, but the "ducks" have long since flown off to where the foraging is better :-)

You caught me out in my misapplication of the term "prism". Your implied statement of what the "prism" is, is absolutely correct of course. I shoulda said "canoe body" although we can then argue about whether that term should include the keel or not if we are talking fin-keelers :-). Still, my opinion of what "displacement" means holds.

So does my opinion that for the skipper of an "off the shelf" boat, new or used, displacement need not be known accurately, as evidenced by the fact that we sail successfully in boats with all sorts of deviations from the design displacement.

As for "weights and balance", yes of course the schedule is long, not to mention tedious, since every component of the vessel with its weight and its distance from the desired Centre of Flotation, i.e. their moment arms, must be listed. If you can shift things around so not only will total weight of the vessel be consonant with her design displacement while also having the vessel float correctly fore'n'aft, you are damn good. And then the owner will throw it all off anyway according to what he takes aboard and where he places it.

All the best :-)

TP

[Bowdrie]

Originally Posted by TrentePieds

we can then argue about whether that term should include the keel or not if we are talking fin-keelers

Yeah, that same argument pops up when dealing with CLP/CLR.
Some designers include the whole rudder, some 1/2 the rudder, and some not at all.
They're seeking in an area that's quite "slippery" of exactness, trying to achieve the perfect "lead" of the CE forward of the CLR to get "just a touch" of weather helm, (when you hold ~2>3 degrees of rudder).
We do know that many of the older CCA designs did not have enough lead, big mains and small foretriangles.
I do think that the lower the DL ratio the more one should at least monitor the additional weight brought aboard if they seek to maintain performance capabilities.

[h20man]

Originally Posted by Adelie

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

I would say that displacement is a function of boat dimensions.


All of the data is important...



Both practical sailor and Boat US say that displacement is an important factor, not just boat length.



But you are correct in that FULL boat dimensions can give one displacement, as well as all the data on needs to undertake a good analysis.



I was just trying to show that displacement does have a quick value.

[Bowdrie]

Originally Posted by h20man

I would say that displacement is a function of boat dimensions.
But you are correct in that FULL boat dimensions can give one displacement,

Neither of those two statements are correct.
Two boats with identical length/beam/draft can have vastly different displacements.

[s/v Jedi]

When you carry fluids as ballast, you carry it against the hull sides, not on the centerline.

We have a total of 500 US gallons capacity at each side, 300 gallons water and 200 gallons diesel. This allows two tons of ballast on one side, which translates in about 4 degrees less heel.
Of course it requires pumping fluids from one side to the other.

[Pete7]

Originally Posted by MicHughV

What if car manufacturer's stated..."this car comes in at 3,500 lbs, without wheels, seats or engine...optional extra's which may affect trim and performance of car..

You must be talking about Volkswagen and their fictitious fuel consumption figures rigged by altering the fuel mapping prior to tests and then reverting it back afterwards to make a better driving experience Those law suits are still going on across Europe and not only VW.

Pete

[MicHughV]

I saw the word " planimeter" and thought...damn, I thought I owned the last one still in existence

But wanted to throw this out.
While looking for a new boat, I came across a Beneteau, which I really liked, but the boat had a pronounced list to the starboard side. I learned that the current owner had a generator installed under the starboard cockpit locker, which likely caused this list.
The list perturbed me, and I walked from the boat.

Shortly thereafter, came across a similar Beneteau, this too, had a generator under the starboard cockpit locker, but no list.

This particular generator was factory installed.

Some weeks after I had purchased the boat, I was rummaging around behind the nav station on the port side and came across several large chunks of lead glassed into the side of the hull. Thus, the mystery of the "no list" was solved, as this was obviously placed there to counter balance the generator installation in the boat.

The point of this tale is that several hundred pounds of lead was added to the displacement of the boat without me having any idea.

[Adelie]

Originally Posted by h20man

I would say that displacement is a function of boat dimensions.


All of the data is important...



Both practical sailor and Boat US say that displacement is an important factor, not just boat length.



But you are correct in that FULL boat dimensions can give one displacement, as well as all the data on needs to undertake a good analysis.



I was just trying to show that displacement does have a quick value.

The BoatUS link indicates “Boats with heavy displacements or superstructures that present a great deal of wind resistance need heavier gear.” But in the tables they provide all loads are predicated on vessel dimensions with no reference to displacement.

The PS article indicates displacement is important but does not quantify it in any way.

The ABYC charts originating decades ago only reference boat dimensions, not displacement. My research indicates they were originally based on USCG testing. Which apparently showed that displacement was a minor contributor to anchor loads.

So let’s go back and look at the physics. Let’s assume we have 2 identical boats with one boat loaded with enough cargo that it has 50% more displacement than the other.

When a wind gust hits them they are presenting the same windage so they both absorb the same amount of energy and start moving backwards with the same amount of kinetic energy the anchor rode needs to absorb. The lighter vessel is moving faster and the heavier one more slowly but the have the same kinetic energy.

When a smaller wave hits the boats same thing happens.

When a larger wave where the sit on a wave with a significant slope from horizontal (>30*) hits the boat the physics and kinematics gets way more complex I can see that displacement might become more important because the boats are accelerating in part due to increased immersion into the water but my sense is that the heavier boat is still accelerated less and spends less time on a wave and winds up receiving similar amounts of energy from wind and waves compared to the lighter boat.

I believe that this is what was shown in the USCG testing decades ago which was then reflected in the ABYC table.

A better discussion of this is in ERHinz’s book on anchoring if I recall correctly.

[TrentePieds]

Well, if you are a fanatic about list, then if you install anything off the centreline, say to starboard, you need to note the weight of that article and its distance (moment arm) from the C/L. Weight x MA then gives Heeling Moment. On the other side of the C/L, in this case the port side, you need to install something of some weight and with some moment arm so that W x MA on the port side equals W x MA on the stbd side.

For a designer, this matters, but for us practical sailormen, it doesn't generally.

For anything less than the weight of a man, why bother? We are back in the realm of spurious accuracy. Nice hobby, but trivial in practical terms.

TP

[Kd9truck]

Originally Posted by 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

I am almost entirely coastal as well, as my vessels disposition is a coastal cruiser, I could have afforded a mid 2000s “blue water” vessel but sailing is not my only interest nor is it my family’s.

I am not a purist in the sailing sense, I power sail under 4 knots near shore or offshore, or if I have to make a tide or to get out of weather, or because I’ve had enough. Or because I’d rather run the motor than the generator.

I have the room to store 2x my vessel’s fuel capacity over the centerline (just like the water storage, one of the reasons why I bought a boat the size I did …..is because I wanted the internal storage space. For me extra fuel is an insurance policy like two fuel separators, no unneeded stops, no un expected time delays, no navigating questionable inlets with a 7.5 ft draft, no close call bridges (air draft is 63 ft), and the big one no extra time bobbing around with floppy sails, and me licking the mast.

Respectfully I am not sitting in front of a calculator crunching numbers, that’s just not me, my question on the wet, dry, or 1/2 wet “published displacement” was more out of curiosity, I just naturally ask my self questions, it’s a part of who I am, I contemplate a great many things in structural design it was part of some of my occupations. The balancing of loads inside the hull is much more important than the actual “ published displacement “ as many have mentioned imho.

If my curiosity offended anyone please find a safe space it will happen again I innocently am oblivious. Cheers

[TrentePieds]

k9 said: "my question on the wet, dry, or 1/2 wet “published displacement” was more out of curiosity, I just naturally ask my self questions, it’s a part of who I am."

Good for you. We share the same songbook. To be a purist about anything can get on other people's nerves :-).

The displacement discussion took off cos Bowdrie and I apparently share the same affection for the design aspect of yachting and got to splitting hairs. Now see what you've done :-)! But hopefully it may have set other people - the ones we never hear from - thinking about these things and perhaps learning about them.

May you and your family have many wonderful years ahead sailing a boat that has more tankage than you will ever use :-)!!

All the best

TP



The essence of what you've said is that you have a boat that pleases your family and you sail it to please your family. That IMO is what it's all about.

[Bowdrie]

Originally Posted by TrentePieds

Bowdrie and I apparently share the same affection for the design aspect of yachting

"Bowdrie and I apparently share the same affliction for the design aspect of yachting".
Fixed it for you.

[Kd9truck]

Originally Posted by TrentePieds

k9 said: "my question on the wet, dry, or 1/2 wet “published displacement” was more out of curiosity, I just naturally ask my self questions, it’s a part of who I am."

Good for you. We share the same songbook. To be a purist about anything can get on other people's nerves :-).

The displacement discussion took off cos Bowdrie and I apparently share the same affection for the design aspect of yachting and got to splitting hairs. Now see what you've done :-)! But hopefully it may have set other people - the ones we never hear from - thinking about these things and perhaps learning about them.

May you and your family have many wonderful years ahead sailing a boat that has more tankage than you will ever use :-)!!

All the best

TP



The essence of what you've said is that you have a boat that pleases your family and you sail it to please your family. That IMO is what it's all about.

Yes! Also a tad of survivalist, and yes thread drift is not always a bad thing, and yes I read posts that I have “no skin in the game” just to see if there’s a knowledge nugget I can adulterate for my own savant purposes 😁☺️.
Cheers TP
Cheers all

[MWGDVC]

Originally Posted by MicHughV

What if car manufacturer's stated..."this car comes in at 3,500 lbs, without wheels, seats or engine...optional extra's which may affect trim and performance of car..

Sadly, car buyers today concern themselves more with on board wi-fi, self driving/parking options, and number of airbags. Anything to remove them from the driving experience.