F18s My First Exposure
Posted: December 20th, 2013, 3:00 pm
At the 2013 Steeplechase I saw F18s up close for the first time. Now I know what the "box rule" means. It means that the hull cross section should be a box or very close to a box, maybe a slight curve from chine to chine. For years the hull botttoms on any and all performance catamarans was a rounded or semicircular or elliptical hull shape. These F18s have made a major departure from that to these box hull shapes with rounded corners. Well, what's going on here? What's the logic?
Let's do a little study and compare a box type hull cross section to a rounded hull cross section of the same displacement per foot of hull length. Let's go to a hull section of max size at or about the main beam. These F18s that I saw were very wide hulls. In our simple example lets assume that with one hull carrying the total boat weight, hull flying, our box rule hull shape underwater is 2ft wide by 1ft deep at this max section. The underwater area is 2ft sq and the wet perimeter is 4ft. The underwater hull cross sectional area is proportional to displacement. Simply multiply this area by 1ft of hull length and we have 2 cu ft of hull volume which will support 2 X 64 lbs per cu ft or 128 pounds. The wetted area of this hull is simply 4ft X 1ft = 4 sq ft of wetted area or drag making area.
Now let's go to the semicircular hull shape. Again the hull is 2ft wide with a 1ft radius. The underwater area is 1/2 X Pi X 1**2
and this equals 1.57 sq ft. Well the box hull had 2 sq ft underwater so we have to add a little rectangular area on top of the semicircular area to get it up to 2 sq ft underwater. So we have to add 0.43 sq ft of area on top of a 2 ft wide base. The height of this added rectangular area is 0.215 ft or 2.58 ins. Now both hull shapes have the same underwater area and therefore the same displacement per 1 ft of hull length. Now the perimeter of the semicircular hull shape is 1/2 X Pi X D = 3.14ft plus 2 X 0.215 ft = 3.57ft. The wetted area of this hull shape per foot of hull length is 3.57 sq ft. Well how about this, the rounded hull sahpe of the same displacement has 10.75% less wetted area or drag making area than the boxey shape. Now we understand why well rounded hull bottom shapes on catamarans have been popular on performance boats for many years.
I talked to some F18 owners and asked about the boxey hull bottom shape and I was told that the design is a semi planning hull shape. I have first hand experience with planning hull shapes on hydroplanes and one of the most important points to the hull shape is "hard chines". Hard chines capture the vertical component of the energy in the spray and make it lift the hull. Round chines let that energy escape. The logic being that it costs more drag to capture that energy than you get back. Therefore round hull bottoms and no hard chines go together. But flatish hull bottoms and hard chines might go together OK, especially if the boat is fast enough to plane or semi plane.
Another thing on hull shapes that was pointed out to me on one of the F18s was a slight kick down in the keel line/flat bottom just in front of the transom. This is a power boat thing. Power boats do this to stop them from porpoiseing while planning. If a power boat hull naturally has a tendenct to porporise, this can be stopped by making a slight hollow/concave shape in the hull just in front of the transom. It can also be stopped by putting downward trim on trim tabs at the transom. Either one of these correction methods comes with a slight increase in drag and reduces the top speed of the boat.
So, this is my first technical experience with F18s and I'm not too well impressed.
Let's do a little study and compare a box type hull cross section to a rounded hull cross section of the same displacement per foot of hull length. Let's go to a hull section of max size at or about the main beam. These F18s that I saw were very wide hulls. In our simple example lets assume that with one hull carrying the total boat weight, hull flying, our box rule hull shape underwater is 2ft wide by 1ft deep at this max section. The underwater area is 2ft sq and the wet perimeter is 4ft. The underwater hull cross sectional area is proportional to displacement. Simply multiply this area by 1ft of hull length and we have 2 cu ft of hull volume which will support 2 X 64 lbs per cu ft or 128 pounds. The wetted area of this hull is simply 4ft X 1ft = 4 sq ft of wetted area or drag making area.
Now let's go to the semicircular hull shape. Again the hull is 2ft wide with a 1ft radius. The underwater area is 1/2 X Pi X 1**2
and this equals 1.57 sq ft. Well the box hull had 2 sq ft underwater so we have to add a little rectangular area on top of the semicircular area to get it up to 2 sq ft underwater. So we have to add 0.43 sq ft of area on top of a 2 ft wide base. The height of this added rectangular area is 0.215 ft or 2.58 ins. Now both hull shapes have the same underwater area and therefore the same displacement per 1 ft of hull length. Now the perimeter of the semicircular hull shape is 1/2 X Pi X D = 3.14ft plus 2 X 0.215 ft = 3.57ft. The wetted area of this hull shape per foot of hull length is 3.57 sq ft. Well how about this, the rounded hull sahpe of the same displacement has 10.75% less wetted area or drag making area than the boxey shape. Now we understand why well rounded hull bottom shapes on catamarans have been popular on performance boats for many years.
I talked to some F18 owners and asked about the boxey hull bottom shape and I was told that the design is a semi planning hull shape. I have first hand experience with planning hull shapes on hydroplanes and one of the most important points to the hull shape is "hard chines". Hard chines capture the vertical component of the energy in the spray and make it lift the hull. Round chines let that energy escape. The logic being that it costs more drag to capture that energy than you get back. Therefore round hull bottoms and no hard chines go together. But flatish hull bottoms and hard chines might go together OK, especially if the boat is fast enough to plane or semi plane.
Another thing on hull shapes that was pointed out to me on one of the F18s was a slight kick down in the keel line/flat bottom just in front of the transom. This is a power boat thing. Power boats do this to stop them from porpoiseing while planning. If a power boat hull naturally has a tendenct to porporise, this can be stopped by making a slight hollow/concave shape in the hull just in front of the transom. It can also be stopped by putting downward trim on trim tabs at the transom. Either one of these correction methods comes with a slight increase in drag and reduces the top speed of the boat.
So, this is my first technical experience with F18s and I'm not too well impressed.