Bow Fin

Tim Lovett  April 05 | Home | Menu

Bow Fin Design Discussion

Preliminary sea trials indicate the superiority of a bow mounted sail for directional stability. How could this be interpreted in the context of Noah's Ark or ancient ship-building?


See Model Trial Blog 4: Bow Sail

The bow mounted rigid "sail" or "fin" demonstrated significant steering effect. The optimum design would have the feature as far forward as possible (maximizing the yawing moment arm), relatively high (increasing the wind velocity and away from wave induced air turbulence), high enough to avoid contact with waves, and as large as possible without compromising stability (wind heeling moment).

By maximizing the distance between the transverse area center of wind and water forces, the size of the features can be kept to a minimum while still achieving adequate yaw moment. With waves hitting the stern, the area would need to be shaped like the typical bow - some flare to deflect waves, relatively fine entry to cut through waves rather than ride over them. If the stern lifts too much there is danger of broaching.

This tends towards an overhanging, vane shaped feature at the bow, and a submerged  protrusion at the stern (like a skeg or fixed rudder).

Look familiar? It is reminiscent of the Mediterranean navy ships [1], such as this Greek Trireme.

Interestingly, one striking feature of many ancient ships is the extreme upsweep of stem at bow and stern, well beyond what would be needed for simply deflecting waves. One suggestion is that it is a tradition passed down from Egyptian reed boats with their anti-hogging tension rope pulling bow and stern together. For the big wooden ships like the Greek Trireme, the passive storm seakeeping afforded by the bow-first behavior is a better explanation.  It is highly unlikely that the engineering know-how behind these ships would have wasted so much effort (and weight) on a stern ornament. 

Such widespread emphasis on the upswept stem could imply;

Direction Keeping Features

The animation above shows five ways to maximize the wind induced yawing moment (turning effect) in order to maintain a following sea.

Referring to the six motions as shown below;

1. Bow Cutup. The cutup would probably be less pronounced than shown in the animation. Ample bow buoyancy helps to prevent a nose-down, which can lead to broaching. The bow is supposed to have reduce water resistance when moving sideways (sway) so that it steers relatively easily.

2. Bow Sail. Wind forces maintain the bow ahead of the stern, rather like a weather vane (wind vane), or the tail fin of an aircraft. 

3. Stern Fin. This is a substitute for (or supplement to) the skeg. However, the advantage is that less cut-up is needed, and a finer entry is possible to help slice through the passing waves. The stern should have limited buoyancy relative to the bow to reduce the likelihood of broaching.

4.  Stern Flare. The role of the flared forecastle is to keep water off the deck (roof)

5. Trim to Stern. This tends to make the bow swing around more easily while the stern digs into the water.       


References

1. http://www.science.sakhalin.ru/Ship/Vlad_E1.html#P5 Vasily N. Khramushin, Saint-Petersburg - Yuzhno-Sakhalinsk  Return to text


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