Sea-Kindliness

Of all power vessels the type that commands the greatest admiration and respect is the North Sea Trawler; the vessels of a type that have developed along the coasts of the British Isles; England, Scotland and Ireland, the Scandinavian Countries and those of the Atlantic Coast of Spain and Portugal. These are the original “long range motor boat”. Not only do these vessels work the North Sea but also the North Atlantic and higher latitudes of the Icelandic waters, summer and winter.

That these vessels are considered so highly is only appropriate. Of all vessel types the commercial fishing vessel is unique. It is the only type that returns to port heavier than when it left. It is required to carry a pay load. The design must have the ability to cater to this requirement, at the same time meeting the demanding working conditions that can be expected to be found in the area of operations.

Big Wave

The vessel operator; the fisherman, has far more stresses to deal with than any other mariner, not only are vessel safety and weather conditions major concerns, but also the running of deck equipment, adequacy of fishing gear, care of the catch, careful loading of the vessel and crew working conditions and safety. Then back in port there are the fish processor and bureaucratic requirements; quota management and fish returns to deal with and finally the Bank to satisfy!

Sweeping Wave

By absolute necessity the North Sea trawler is a sea-kindly vessel. This explains its record for comfort and safety; a safety record built up over a long history.

When we use the term sea-kindliness we are referring to comfort and safety. Today an ocean passage in a motor yacht does not need to be an exercise in human endurance. Cruising yachtsmen now are far more conscious of noise, vibration, sea motions and the discomfort they invoke, particularly sea motion.

Rolling, Pitching, Heaving, Slamming, Yawing, Surge and Sway are the features inherent in any vessel that contribute to a lack of comfort. Since there is no word that can be made up from RPHSYSS, many designers simply refer to this as vessel “rhythm”.

Decreasing the rhythm in any design increases your comfort and to an extent lessens your proneness to being seasick. It also markedly increases your safety.

Although none of these features can be completely eliminated, every effort should be made in hull design to minimize them; to lessen the vessels rhythm.

Any vessel in a sea way is subjected to that mix of sea motions in descending order of importance as far as comfort is concerned:

Rolling is the most objectionable of all motions but fortunately is also one effect that can be cured mechanically by the installation of active fin stabilizers or passively by anti roll paravanes. The makers claim, and it is substantiated, that active fin stabilizers can reduce roll amplitudes by up to 90%; so even hull forms that would normally roll in “wet grass” are saved.

Excessive pitching is also highly objectionable. Experiments have been made with anti pitching fins in the past. Most notably in Royal Navy Anti-submarine Corvettes in 1970. While pitching reductions of up to 30% were obtained the forces on the fins were so high (five times that due to rolling) they were soon damaged and further trials abandoned.

The aim then must be to dampen this motion by good hull design. Research and model testing have focused on fore ship design where it has been found that ships with deep Vee shaped underwater sections in the bow, as apposed to shallow U shaped sections, lead to amplitude reductions of 25% and acceleration reductions of 30%. The model ships in the tests had a Draft/Length ratio of 0.175. The most severe pitching was found to occur in wave lengths of 1.25 times the ship length. Research has concluded that deep draft ships with Vee sections forward were better able to damp motions due to pitching.

Heave co-exists with pitching and is the bodily vertical rise and fall of the hull in a seaway.

The research and model testing for pitching has also shown that the deep Vee shaped bow sections reduce heave by up to 50% in the case of the critical 1.25 L wave length over shallow bow sections.

The position of the ships longitudinal centre of gravity also has an influence on the heave motion. In fact we have found that the position of the LCG is critical in this respect.

Slamming like heave co-exists with pitching and is caused by the emergence of the lower bow in head seas. In shallow draft craft this leads to very high bottom pressures on the hull followed by structural vibrations extending the whole length of the vessel. Bulbous bows of the simple cylindrical type fitted close to the waterline are particularly susceptible to slamming.

Yaw is an irregular and persistent change of heading caused by quartering seas acting on the bow or stern and wind gusts acting on the superstructure.

Not only is this a disconcerting motion but also hard on the autopilot or helmsman.

This motion is dampened by heavier deep draft hull forms that possess inherent directional stability and low A/B ratios.

Surge is the periodic speed reduction on meeting and passing through waves. This action is very pronounced in light displacement vessels and is akin to the random acceleration and deceleration of a car by applying the brakes.

Hull forms possessing high displacement/length ratios and fine angles of waterline entry coupled with large diameter propellers are far less prone to this action.

Sway is an irregular athwartship or sideways motion caused by waves and wind gusts on the beam. This also means that the vessel is likely to be sliding to leeward, taking a longer path to its destination.

The only way to mitigate this action again is large immersed underwater lateral plane area and low A/B ratio.

Over the past 70 years T.C. Watson & Sons have designed and/or built nearly every type of power boat hull form. Some of these vessels have now covered well over a million miles, often working in some of the most treacherous sea conditions. Our conclusions regarding sea-kindliness are drawn from this experience, observation and reference to a large body of published research material specifically investigating and experimenting with sea motions. In the experience we have gained the main stand out characteristics required in an ocean capable vessel are the following:

• Deep vee shaped sections in the vessel underwater fore section, in other words a “fine and deep entry”. We always maintain deep Vee shaped sections in the fore ship for the first 25% of the waterline length in our designs.

• A sensible amount of draft allowing a reasonable lateral plane

• Low A/B ratio

• And the ever critical positioning of the LCG; Longitudinal Centre of Gravity.

Watson 48

Our designs are based on the proven requirements for good seakeepng ability, differing from their commercial cousins in that the hull form is more fined for greater propulsive economy.

Watson 48 bow view

It is clear that correct hull proportions are essential to produce a sea kindly vessel capable of comfortable ocean voyages.

So often we see otherwise excellent designs spoilt by compromising these essential hull form elements. Chief among these is the obsession with the so called “Bahamian Draft”. The designers of these vessels have our sympathy as it has severely restricted their ability to incorporate sea kindly shapes into the design. This is often compounded by excessive top hamper resulting in very high A/B ratios.

Footnote: the photos shown "at sea" are from the web site www.trawlerpictures.net each copyright owner of these images was asked for permision to allow their images to appear here, each gave their permission un-reservedly; we thank them and recommend this web site to anyone interested in the design, construction, operation and history of the North Sea trawler fleet; much of which has now been destroyed, not by accident, but by Government mandate.

 

Last Updated (Wednesday, 08 September 2010 23:02)