Marga — trim, displacement and keel investigation

Scope

Hydrodynamic and stability performance investigation only, focused on mass and mass distribution. Class rule compliance, measurement and sail area are not yet included.

Method and assumptions

RANS CFD and a simplified VPP study on the supplied scan/CAD geometry, and a supplied rudimentary weight estimate.

01Scope

Study scope

Focused solely on performance for mass, trim and keel changes, independent of class rule considerations.

This document sets out the performance case for changing Marga's mass, trim and keel shape, based on a CFD and VPP study against the current baseline. It does not assess class rule compliance, measurement or sail area impact — those implications, and what is practically achievable, are the next step for Andreas and his team.

02Baseline

Assumed baseline

9,440kg

Supplied displacement, weight estimate

−7,353mm

Supplied LCG, weight estimate

−508mm

Supplied VCG, weight estimate

10,277kg

Calculated CFD baseline, “sailing trim”, LCG −7.421m

Based on an ORC light-ship-to-sailing-trim estimate; baseline sailing condition assumed at 10,277kg.

Routes to the proposed configuration under consideration: keel lead/support cut position — maximising VCG down whilst accepting LCG aft; a small, subtle increase in section volume in the lower keel sections; an electric saildrive aft of the rudder, with batteries and any other ‘free’ equipment located as far forward as practical; trim correction weights in the bow.

03Geometry

Scanned hull data

Mismatch between the supplied scan data/hull surface and the original lines plan.

$CAD keel-volume solid overlaid on the 1919 Salander archive lines drawing$

Figure 1 · Overlay of CAD solid on archive lines drawing (Design No. 166, Otto Hillerström, Helsingborg, 1919–20; CAD keel-volume extraction 616.6 million mm³, shaded). Source: Sjöhistoriska Museet; N.RD overlay.

CAD solid vs. archive lines: forward hull volume, rocker line forward of keel, and keel placement and design all differ.

Is the keel shown representative of the scanned data, or is it a new design?

The rudder

Intent was hard to infer from the supplied data; the CFD study used a representative rudder shape. Expect a rudder closer to the original archive drawings, particularly in vertical area distribution — not top-loaded, with a clear hull/rudder junction at the head.

04Method

Test matrix

Condition	Boat speed	Heel
Upwind, 12kt TWS	7.56kt	25°
Downwind, 8kt TWS	6.50kt	0°
Downwind, 12kt TWS	7.82kt	0°
Downwind, 16kt TWS	8.71kt	0°

RANS CFD, yaw sweep at each condition. Variable sweeps: displacement −500kg, +1,000kg and +1,500kg; LCG offsets to ±400mm. Baseline 10,277kg, LCG −7.421m. Separate VPP check: VCG variation impact.

05Results

CFD sweeps

+40s/NM

Upwind, 12kt TWS — gain

−65s/NM

Downwind (light), 8kt TWS — loss

≈0

Downwind (mid/heavy), 12/16kt TWS — negligible, slight gain

$RANS CFD force and time-delta sweep across displacement and LCG offsets, all four conditions$

Figure 2 · RANS CFD sweep, all four conditions. Force and elapsed-time delta vs. LCG offset, by displacement offset. Proposed trim = +1,000kg, LCG 200mm aft. Source: CFD performance study; underlying data available on request.

Upwind: gains are clear in all conditions with +1,000kg displacement (11,277kg sailing trim).

Downwind: worst at 8kt TWS, negligible from 12kt upward.

Assumes racing is level within class (no rating correction) and no sail area impact.

06Results

VCG variations

Moving 100kg down 1m: ≈3 s/NM. This is an unrated gain — pushing VCG lower, primarily through keel lead-cut optimisation and keel shaping (with LCG management to enable it), provides a significant free performance gain.

07Proposal

Recommended updates for consideration

Mass and trim

10,440kg (+1,000kg), LCG −7,553mm (200mm aft).

Keel lead cut and shaping

Optimise the lead-cut shape to ensure LCG in the keel sits as low as possible (LCG aft, plus the following points, will aid this).

Weight forward

Optimise equipment placement; electric drive places batteries etc. forward, buying LCG aft-and-down budget for the keel.

Bow corrector option

Small trim weight at the bow, taken from the keel, so keel lead concentrates for depth — optimised for minimum VCG.

08Andreas and team

Next steps

Geometry

Confirm whether the scanned keel (Section 03) matches Marga's original, or is a new design — and if new, what drove it.

Rudder

Provide the intended rudder design.

Rule and measurement

Assess the Section 07 proposal against class rule and measurement procedure.

Sail area

Calculate resulting sail area for the proposed configuration, to reassess Sections 05–06 on a confirmed basis.

09Conclusions

Conclusions

If racing is level and compliant, with no rating correction or sail area change, the proposed displacement and LCG changes give a clear net gain from 12kt TWS upward, with a cross-over at approximately 9kt TWS around a windward–leeward course.

Once the impact of rule and sail area limitations is understood, final gains can be confirmed.

VCG — via LCG, lead-cut position and keel shaping — is a free gain of c. 3 s/NM per 100kg moved down 1m, in almost all conditions.

0AAppendix

Supporting CFD imagery

For information — not referenced directly in the findings above.

Upwind, 12kt true wind speed

Baseline vs. +1,000kg / LCG 200mm aft · wave elevation and pressure

CFD wave elevation imagery, upwind 12kt TWS, baseline vs proposed trim

Figure A1 · Upwind, 12kt TWS, 2° yaw. Baseline (10,277kg, LCG −7.421m) vs. proposed trim (11,277kg, LCG −7.621m).

Downwind, 8kt true wind speed

Baseline vs. +1,000kg / LCG 200mm aft · wave elevation and pressure

CFD wave elevation imagery, downwind 8kt TWS, baseline vs proposed trim

Figure A2 · Downwind, 8kt TWS. Baseline (10,277kg, LCG −7.421m) vs. proposed trim (11,277kg, LCG −7.621m) — the condition carrying the downwind cost in Section 05.