Bow shape

[seadragon]

There are many discussion about HP require to move a vessel at a desire speed for a displacement hull. How does the bow shape effects the coefficient of water resistance. Is there an equation we can use to calculate the water resistance for a displacement hull. Any info are welcome

[Thunderhead19]

Prismatic coeficient?? or are you talking about bulbous bows? Hull speed fora displacement hull will give you a rough idea. There's a rather lengthy discussion on hull speed in another forum.

[seadragon]

Sorry Thunderhead19, I didn't make myself clear. I mean prismatic coefficient. If the bow is narrow, which have a fine entry into the water, which means less power to move the vessel. On the other hand we will more wet area, which means more water resistance. If I would like to design a boat, say 45 ft LWL, 10 ft beam LW beam, and the desire cruising speed of 20 knots. What would be the best angle of the bow in terms of fuel efficiency.

[Thunderhead19]

Originally Posted by seadragon

Sorry Thunderhead19, I didn't make myself clear. I mean prismatic coefficient. If the bow is narrow, which have a fine entry into the water, which means less power to move the vessel. On the other hand we will more wet area, which means more water resistance. If I would like to design a boat, say 45 ft LWL, 10 ft beam LW beam, and the desire cruising speed of 20 knots. What would be the best angle of the bow in terms of fuel efficiency.

That's one of those interesting questions that has a direct bearing on the seakeeping qualities of your boat. A very fine entry like a scull is obviously the most efficient, ie:making your boat side one continuous fairly tight curve from stem to stern. with little buoyancy in bow and stern you will have problems with the bow, stern plunging. This doesn't necessarily have anything to do with prismatic coeficint, and even diminishes how accurately this coefficint of form describes the fineness of the hull. The important thing is that the hull is long and narrow. the entry has to be fair and fine, but I'm not sure that you'll find an optimum angle