Welcome, Guest
Username: Password: Remember me
  • Page:
  • 1
  • 2

TOPIC: Apparent Discrepancy in Displacement (Free, Imperial)

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5056

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
I've been working on a boat for some time in some FreeShip versions/derivitaves. I started re-creating it in DelftShip and noticed that the displacement was very much less despite the hull being very similar. I ran the report (see parameters screenshot) to show every inch right up to the first leak point. That ensured I wasn't misreading the units. The thing is, because I ran the range as 1 inch increments (.083 foot), I'd expect the TpCm (ton/inch) column to come very close to simply accumulating in the Displ FW column, and it doesn't. I end up with a 38' (11.5m) catamaran with little more than a ton and a half of displacement and nearly 2' of draft.

Am I misreading something or is there a problem (possibly with imperial units)?

Thanks!
Attachments:
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5057

  • Maryak
  • Maryak's Avatar
  • OFFLINE
  • Expert Boarder
  • I was tired, today I am retired
  • Posts: 156
  • Thank you received: 18
  • Karma: 4
Where is your model relative to the baseline?
Maybe the problem if it's not line up properly.

Regards
Maryak
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5058

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
The lowest point of the hull is at 0.
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5059

  • Icare
  • Icare's Avatar
  • OFFLINE
  • Expert Boarder
  • Posts: 120
  • Thank you received: 17
  • Karma: 5
It's the first time I hear from such a problem... :dry:

Is the water line correct?
Just a glance at the water line level may tell you there's a problem with it or not. Check for le color change location.
If the water line is correct, then...

What's the model's volume?
Since its displacement is 1.5 tons, does the volume fits such a datum?
In the metric system, a displacement of 1.5 tons means a volume of nearly 1.5 cube meter. Check it broadly in the imperial system.
If you find an inconsistancy, then...

Did't you change the water density datum?
Broadly, in the metric system, fresh water has a density of 1 Kilogram/liter, ocean water 1.015 and the mediterrannean 1.033 (as far as I remember :whistle: ). Check for your water density, convert it in metric system and make sure you get a consistent number.

I grew up with the metric system, and trust me: Imperial system %ยงยค# ! :sick:
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5061

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
Look at the attachments. The far right column is displacement for the next inch in tons. The left column is displacement in tons. Look at the parameters. You'll see that the report is inch by inch, yet the displacements don't accumulate accordingly. That's my question: why don't they?
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 2 weeks ago #5062

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
1. The water line is correct.
2. The report doesn't show volume, but each of the two hulls is 40' long and the beam is over 4'. At 1.75' draft, I've got a lot more than the needed 50 cubic feet of volume.
3. Why would I do that? Broadly, in the English system, "a pint's a pound the world around" in distilled fresh water (a gallon's 8# and a cubic foot is 8 gallons) and seawater is about 65# per cubic foot.

I grew up with the Imperial system, and I have to order parts and materials in it. DelftShip does everything in decimal feet, so what's the issue?

As per my original post, it's an apparent discrepancy ON THE REPORT that I'm asking about, not the volume of my model.

Look at the attachments. The far right column is displacement for the next inch in tons. The left column is displacement in tons. Look at the parameters. You'll see that the report is inch by inch, yet the displacements don't accumulate accordingly. That's my question: why don't they?
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 1 week ago #5064

  • Maryak
  • Maryak's Avatar
  • OFFLINE
  • Expert Boarder
  • I was tired, today I am retired
  • Posts: 156
  • Thank you received: 18
  • Karma: 4
What does the Design Hydrostatics report show?
Regards
Maryak
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 1 week ago #5065

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
It's very similar to the left column, but that still doesn't answer my question: why do the leftmost (Displ FW) and rightmost (TpCm) columns on the hydrostatics report not seem to be consistent with each other? With the draft increments set to one inch (.083 ft), I would expect that displacement would fairly closely be an accumulation of the TpCm weight to displace in tons per inch. It's nowhere close.
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 1 week ago #5066

  • SV Heyoka
  • SV Heyoka's Avatar
  • OFFLINE
  • Fresh Boarder
  • Posts: 11
  • Karma: 0
The really amazing thing is this: when I run the design hydrostatics in Imperial units, I get a CP of .0947 (I can't imagine how I could make such a thing), but on the same hull, run in metric, the CP is .3731. Now we all know that CP is dimensionsless, so the units of measure shouldn't matter.

Here's the model (slightly modified since initial numbers were given, but still showing discrepancies). Try it yourself.
Attachments:
The administrator has disabled public write access.

Apparent Discrepancy in Displacement (Free, Imperial) 8 months 1 week ago #5067

  • Maryak
  • Maryak's Avatar
  • OFFLINE
  • Expert Boarder
  • I was tired, today I am retired
  • Posts: 156
  • Thank you received: 18
  • Karma: 4
The design appears to be in unstable equilibrium which may account for all the weird numbers in the hydrostatics report.

From your fbm
G= 1.877m
M= 0.718m
B= 0.252m

I don't know why there is no baseline or perpendiculars in your fbm so z=0 relative to?

HTH
Regards
Maryak
The administrator has disabled public write access.
  • Page:
  • 1
  • 2
Time to create page: 0.119 seconds
Powered by Kunena Forum