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Old Aug 14, 2014, 02:59 AM
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Joined May 2013
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A plea for enlightenment on account of I'm not very clever and easily confused, nor can I find any explanation in the manual or elsewhere on line.

I can't fathom how "draft" works in Freeship. As far as I can make out, one enters what one confidently expects will be the final draft of the finished design and from then on this entry apparently governs all calculation of the hydrostatics. However, if in the careless rapture of artistic expression one should happen to change the depth of the keelson, what happens then? I can find no way of determining what the new draft might actually have become, and despite this change the calculations are still apparently based on the notional depth first entered.

Stranger still, the design draft one might specify is not necessarily the DWL that the program will offer on the actual drawing. If one starts a new file with say a draft of 590mm, the DWL will appear on screen as 603mm first off before anything has been touched.

Furthermore, changes to the bottom sections themselves can evidently alter the height of the waterline -- one can see it physically realign on screen, presumably because of the change in buoyancy thus caused-- but nowhere can I discover what this actually might measure or how to interpret it.

Finally, while on the subject, given that the horizontal scale is rendered in straightforward 500mm increments, why does the program use what seems to be a thoroughly arbitrary, not to say weird, vertical scale, with intervals that make no sort of sense to a simple country boy without a mathematical bone in his body.

If someone can elucidate all or any of this, I will be greatly in your debt.
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Old Aug 14, 2014, 05:23 AM
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Germany, Munich
Joined Dec 2006
56 Posts
Important: Work with scaling!
100:1
Then it will be more accurate.
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Old Aug 14, 2014, 10:20 AM
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United States, WA, Port Townsend
Joined Feb 2006
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Do you have to input the weight of the vessel and it's center of gravity at any point? If not, how can the program determine the floating waterline. Likely the draft you initially enter is being used to solve for the weight and longitudinal center. Once the initial wt and center is know, then any subsequent changes to the buoyancy of the hull can be accounted for by solving for a new draft/trim.
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Old Aug 14, 2014, 07:34 PM
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Australia, QLD, Peachester
Joined Dec 2013
95 Posts
The design is for the outside skin and the displacement can be set at 4kg. The software then calculates all the hydrostatics on that number.
The default water density is 1.025 (salty). I opted for fresh water - 1kg/m3 to err on the more bouyant side, even though I usually sail in brackish.

I set the draught at 56mm and also wondered if that is what it would end up as even though I fiddled with the variables that might affect it.
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Old Aug 14, 2014, 08:24 PM
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United States, MI, Bloomfield Hills
Joined Feb 2010
556 Posts
Quote:
Originally Posted by lhurt View Post
Do you have to input the weight of the vessel and it's center of gravity at any point? If not, how can the program determine the floating waterline. Likely the draft you initially enter is being used to solve for the weight and longitudinal center. Once the initial wt and center is know, then any subsequent changes to the buoyancy of the hull can be accounted for by solving for a new draft/trim.
A few guys were discussing design numbers back on page 29, well worth taking a look.
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Old Aug 15, 2014, 12:04 PM
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United States, CA, Danville
Joined Jul 2013
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Quote:
Originally Posted by Emintaka View Post
A plea for enlightenment on account of I'm not very clever and easily confused, nor can I find any explanation in the manual or elsewhere on line.

I can't fathom how "draft" works in Freeship. As far as I can make out, one enters what one confidently expects will be the final draft of the finished design and from then on this entry apparently governs all calculation of the hydrostatics. However, if in the careless rapture of artistic expression one should happen to change the depth of the keelson, what happens then? I can find no way of determining what the new draft might actually have become, and despite this change the calculations are still apparently based on the notional depth first entered.

Stranger still, the design draft one might specify is not necessarily the DWL that the program will offer on the actual drawing. If one starts a new file with say a draft of 590mm, the DWL will appear on screen as 603mm first off before anything has been touched.

Furthermore, changes to the bottom sections themselves can evidently alter the height of the waterline -- one can see it physically realign on screen, presumably because of the change in buoyancy thus caused-- but nowhere can I discover what this actually might measure or how to interpret it.

Finally, while on the subject, given that the horizontal scale is rendered in straightforward 500mm increments, why does the program use what seems to be a thoroughly arbitrary, not to say weird, vertical scale, with intervals that make no sort of sense to a simple country boy without a mathematical bone in his body.

If someone can elucidate all or any of this, I will be greatly in your debt.
I use Freeship. I work on 10 meter scale for IOM other wise there is not enough decimal places in the Hydrostatics. I set the draft to what I want. scaled up of course. then as you play with the deepest part of the rocker the "base" value moves and with it the draft line. consequently your displacement will change. So I change shape- check displ, change shape - check displ...

as for ref grid you see that's based on your "stations" and "Water Lines" that you can change in "calculations/Intersections" very useful you can add specific stations at you preferred locations. comes in handy to increase the water lines view and after you have finalized your design you can change the number vertical stations and position to match foam thickness if that's how you are making your hull...
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Old Aug 15, 2014, 05:52 PM
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Quote:
Originally Posted by Whitaker View Post
I use Freeship. I work on 10 meter scale for IOM other wise there is not enough decimal places in the Hydrostatics. I set the draft to what I want. scaled up of course. then as you play with the deepest part of the rocker the "base" value moves and with it the draft line. consequently your displacement will change. So I change shape- check displ, change shape - check displ...

as for ref grid you see that's based on your "stations" and "Water Lines" that you can change in "calculations/Intersections" very useful you can add specific stations at you preferred locations. comes in handy to increase the water lines view and after you have finalized your design you can change the number vertical stations and position to match foam thickness if that's how you are making your hull...
Thank you. Most useful. After admitting defeat and stating the problem here, had more or less worked this out for myself but good to have what I laughingly call my thinking confirmed.

FYI a friend of mine is putting the finishing touches to a spreadsheet application for comparing the drag curves of different boats, or variations of the same boat. Have been testing it for him and it seems a brilliant tool. He will be posting it on his website within days and I'll flag the link.
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Old Yesterday, 09:14 AM
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Joined Aug 2013
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Finally there!!

Well the time has come the new boat has finally hit the water!! I have only sailed her a handful of times but I am pleasantly surprised by her performance. For all my guesstimations and 'by eye' shaping everything appears to be in vaguely the right place and she is not to far off the pace at a club level.

If i am brutally honest I think her Achilles heal will be downhill in gusty conditions. She has a fairly fine entry and feels a little tender in the puffs. That said she has no lack of pace and has knicked a few races here and there provided I can point her in the right direction.

I don't know whether anybody else does this but after the first sail I was already planning the changes I will make on the mk 11 but I guess thats what keeps all interested!!
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Old Yesterday, 03:21 PM
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United Kingdom, London
Joined Jun 2005
832 Posts
the attention to detail in the second picture of the mast ram is staggering very well done I can see you have a slot on the mast holder to stop the side to side slop!
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Old Today, 03:38 AM
NORTHLAND NEW ZEALAND
D design's Avatar
New Zealand, Northland, Totara North
Joined Feb 2012
568 Posts
Displacement : 0.004 tonnes

This is what I get when looking at the displacement in freeship. Normally we don't measure IOMs in tons lol. Is there a way to change it to kg?
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