Thank you for reporting the issue! You are right when there is only one grouped point on the chart the anchors are applied twice- demo. Internal note: Adjust the method here: highcharts/ts/Extensions/DataGrouping/DataGroupingSeriesComposition.ts Line 195 in ba88c1c function anchorPoints( ...
If you are unsure about what to choose, it is best to go online and search for aboat anchor size chart. Manufacturers will have varying recommendations. In most cases, they will recommend the right size depending on the boat length, as well as wind and water conditions. As for the weight...