Freeship Draft Problem

Hi everyone. In my design i have specified the draft of my hull at 0.12m. When I look in the Design Hydrostatics report it says "Design Draft : 0.134 m". Does anyone know what causes this discrepancy? I need to iron it out so i can trust the other I've uploaded a screenshot of the body plan. I notice my ship hovers above the baseline a little.

 

With a bit more measuring I can see that the distance the hull hovers above the baseline is equivalent to the difference between my specified draft and the draft given in the Design Hydrostatics report. Looking at other models that come with Freeship I see that some models sit directly on the baseline and some don't.
My main worry is that this is affecting the displacement calculation. A couple of quick calculations of my own indicated that Freeship is calculating displacement based on the larger, erroneous, draft value.

 

Screens showing the disparity between my specified draft and the draft in Design Hydrostatics, and the bodyplan showing the specified waterline and the waterline marked on the hull by Freeship.

 

Look in the Hydrostatics tab for a check box for moving the hull to the baseline, Delftship Free is similar.

 

Thanks so much. Try as i might I can't figure out what the setting does. It seems to do noting, and even worse it does not save between sessions! Looking at the sample ships, some have it checked and some do not, and there doesn't seem to be a a strong relation between the setting and the position of the model. I suspect some other setting somewhere is preventing me from doing what I want.

 

Are cornerpoints on the centreline important here? A few off my centreline conrtol points are not "corner points".

 

Did you start the design in Freeship or is it read in / imported from another software?

 

Started in freeship - a modification of the standard "new' ship.

 

I get a small delta between design draft and Calculations>Hydrostatics before I save and doublecheck the Project settings in the file.
If I save the file and confirm the Project settings, the draft in the Hydrostatics matches the design draft.

 

By moving control points to the midship distance, design draft in settings and and design draft in the Design Hydrostatics report are now equal on 0.140m
Enable auto move hull to base line will not stay set.
The body plan shows Base -14 and DWL 126.
Maybe getting somewhere but a lot of this makes no sense!

 

There will be a difference between the nodes in the control net that determines the shape of the splines, and the final control curves that determine the panel edges. Note that the control nodes/points are not placed on the splines; this is a bit confusing as the splines don't follow "on top" of the control points.
If you switch off the control net and switch the control curves on, you will probably note a difference. Also check what it looks like with wire-frame contra shaded presentation (and also check side views).

 

Interesting. many, many thanks. It seems like there s an issue. trying to sort it out now, but I'm getting a lot of crashing trying to delete and modify points and edges in the bow area.

 

I think I have solved the mismatched draft problem. The keel is a flat panel and I have the lowest control point set at Z=0. This control point is about amidships making it the lowest point of the keel panel. Setting an adjacent control point to Z=0 made the curve between the two points into the lowest point, and the hull settled further down to the baseline. The discrepancy is now only 2mm.

 

Nope, the control point IS NOT A POINT ON THE PANEL, it is a point OUTSIDE the panel, controlling the shape of the spline that is the edge of the panel. If the lower side of your bottom panel is convex, then the control point is lower than the panel itself. So, if you want that panel to rest on the baseline, then at least one of the cl control points must be below the baseline.

You got to understand that the control points do not fall on the hull surface; they just control the shape of the splines that define the panel surface.

 

The model would not let me have a negative control point. When I did so the baseline just moved itself lower.