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Trading Ahead - Military bridging pontoon
A military bridging pontoon has a swim forward and a transom aft.
In a wind the water resisting leeway is centred around a point which is the after side of the centre of her sail area. So she turns and brings her head downwind of her stern.
The turning stops when low-pressure wind eddies around aft bring about a stabilising balance.
The pontoon now has a wind which is abaft abeam. Another way of describing it would be a “slightly-fair-wind“. So she gathers headway in addition to leeway.
Trading Ahead - Swim-ended Barge with Budget
The swim-ended barge fitted with a budget and in level trim trades ahead rather more than the bridging pontoon. With a proud head she will have a sail area centred around a point further forward resulting in wind pressure pushing her head even more downwind.
Her fair wind will be that extra bit fairer but her manoeuvrability will suffer.
When sailing vessels are making progress upwind there comes a time when they need to ‘Go About’. That is they need to turn into wind a matter of ninety degrees or more to bring the wind from one bow and onto the other.
Under oars we do not say “I had to go about”, instead we say “I had to wind (rhymes with sinned) her”. The technique if winding will be dealt with later.
