Model Trial Blog 4: Bow Sail

Tim Lovett April 05 | Home | Menu

Escaping the dreaded Beam Sea


The model is running to the wind, using a sail fin at the bow.

"But the Bible never mentions any sail on Noah's Ark" 

True, but the Bible deals with naval architecture for only a few verses - even the shape of the hull cannot be determined from the text, and many essentials are never mentioned - such as drinking water. So a sail-like feature is not disqualified, especially if it happens to be the most efficient way to pilot the vessel.



A bow mounted "sail" steers the bow as the model maintains orientation in the passing waves.

A polystyrene sheet acts as a rigid sail or fin mounted at the bow. This is very similar to a jib sail mounted at the foremost extremity, which can help to steer the bow like a weather vane (wind vane).

Controlled navigation in a following sea makes some use of the jib sail (flying jib not employed here), but sail area is generally concentrated towards the bow, although the amount of canvas is restricted due to the strong wind.


Bow "Sail" Tests

24 April 2005

The following tests were done in Botany Bay south of Sydney. A light wind of 5-10 knots was blowing steadily, although throughout the day it turned gradually from SE to due E, with the wave direction changing with it. Waves were generated over a fetch of some 10km (6 miles), producing waves up to 0.3m (1 ft).    

All tests followed the same format. The model Ark was pushed bow-first some 20m (60ft) from the shore, into a headwind. The hull always turned to broadside within 10-20 seconds, but the vessel only yawed the full 180 degrees (to a following sea) with the help of a bow mounted "sail".  

Test 1. Skeg and trim to stern.

Without any wind catching features, the hull performs the classic broadside roll in the passing waves.

With approx 1 degree slope (trim) towards the stern [1], and with the action of the skeg (fin at stern) the hull remains almost parallel to the wavefront.

Here, the center of wind area above the waterline is effectively amidships (neither fore or aft), so the wind does not turn the hull (negligible yawing moment). The skeg alone does not appear to be capable of steering the vessel to get into a following sea.

Test 2. Drogue and forecastle.

The forecastle and drogue work to yaw the model around, but wave action fights to keep the hull broadside. The waves win. 

Same as Test 1 but with addition of forecastle and drogue.

The axis of the vessel sits approx 10 to 20 degrees ahead of the wavefront (quartering sea). The combined effect of wind pushing the forecastle (raised area at the bow) and the additional drag of the drogue had some effect, but not enough.

The drogue was made from the top half of a plastic (PET) drink bottle and tied to the stern eyebolt with a 1.5m (5 ft) string. The drogue is approx 100mm (4 in) in diameter.

Drogue cone attached to stern

Test 3. Large Bow "Sail"

A generous "sail" does the job easily.

The model ark negotiates a following sea with no hint of broaching or surfing.

The rigid "sail" is made from polystyrene foam measuring 270 x 410mm (10.5" x 16"), and 15mm (5/8") thick. 

The hull rapidly recovered from off-axis waves and maintained a strict heading with the wind, within approx 10 degrees. It appears the "sail" is more than adequate to steer the vessel in a steady wind.

Test 4.  Large "Sail" with Drogue

The "sail" steers the vessel even in breaking waves.

The model moved more slowly through the water with the added drag of the drogue. Since the large sail was already doing the job no real advantage is evident. The hull did seem to handle a little more roughly (some slight yawing oscillations), but no measurements were taken to confirm this.

Test 5.  Stern "Sail"

A stern-mounted sail battles the skeg and trim of the hull.

To test the yawing effect of the skeg, the sail was re-mounted at the stern. In this case the model remained in a virtual beam sea throughout the test, proving that the skeg and stern trim contribute to broadside avoidance. Logically, this test proved that the center of transverse area of the wind was similarly located longitudinally to the transverse center of water pressure. In other words, the wind was pushing at about the same spot as the water was resisting. The resulting lack of yawing moment meant the hull was responding to waves alone (as if there was no wind), a situation that appears to favor a beam sea.

Test 6.  Medium "Sail"

A reduced bow mounted "sail" proves adequate.

This time the bow sail was reduced to 270 x 250mm (10.5" x 10"). Nothing scientific about the measurement, I just chopped it down to approx 60% so I would have 40% left for the last test. 

The model successfully maintained a following sea.

Test 7.  Small "Sail"

A reduced bow mounted "sail" proves adequate (just).

The final arrangement kept the hull in a following sea but with increased yawing motion with passing waves, reaching angles of around 20 degrees from the wind direction. This "sail" is 270 x 160mm (10.5" x 6 1/4"). This test included an increased bow buoyancy (removal of the wet sand in the bow section container). The container has diam 125 x 160mm tall, giving volume of 1.9 litres. The wet sand would weigh approx 4kg (8.8lbs). In terms of the real thing, we just took 4kg * 76^3 = 1756 tonnes out of the bow of Noah's Ark. (Mass scales to the cube of linear scale).  


Discussion

The bow mounted rigid "sail" or "fin" demonstrated significant steering effect. A traditional fabric sail would not be needed because no adjustments would need to be made in a steady, unidirectional wind where a following sea is the only goal. The vessel is effectively running to the wind at all times, with the assumption that the global scale wind and lack of localized storms meant that waves were nearly always running with the wind.

Such a "sail" could be a rigid structure, such as wood. This could ensure there is zero maintenance during the voyage.

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 escape contact with waves, and as large as possible without compromising stability.


References

1. Trim to stern. Since the hull is designed for this slope towards the stern, the correct term would be drag to stern. i.e. Only the bottom is on a slope, the other decks are level. Of course, a simple way to achieve this is to let Noah use up food from the bow so that by the time the drying wind arrives the Ark is lower at the stern. However, the drying wind occurred after the water level had climaxed, which is only a few months into the year-long confinement. So there is limited time to use up food stores during the voyage, most of the time was spend sitting on the ground waiting for the mud to dry out.  Return to text


www.worldwideflood.com