Use caution in the use of that manual. Notice they make several mentions of positioning the floats relative to the
empty weight of the airplane and floats. How many airplanes do you know of which are flown
empty? The positioning of the floats should be based upon the
loaded weight and CG. Whether you choose maximum gross weight or your average loaded weights is up to you.
Fish mouth angle relative to horsepower. First, less angle (less aerodynamic drag) is best for speed. More angle reduces take off speed.
Second, water drag is highest while accelerating the plane from at rest to the planning position (on the step). The length of time required to "get on the step" is the primary take off distance limitation. This is why they mention more angle for less horsepower. With the engine and wing pointed more up, it helps in lifting the floats up on the step using up thrust and wing lift. With higher horsepower, the engine will pull the floats up on the step through acceleration and water pressure against the bottom of the floats. The best angle is a compromise between the two.
Third, once on the step the relationship of the airplane to the water surface and floats (combined) must be considered. IF (for this example) you use 7 degrees, the chord line angle to the relative wind during the on step take off run phase will be 15 degrees. Optimum angle between float keel and the water surface while on step is 8 degrees. 15 degrees is on the high drag side of the best L/D. This means that your acceleration while on the step will be held back by aerodynamic drag. The angle above which the drag exceeds the optimum L/D is just under 13 degrees. This suggests the maximum float angle ought to be no more than 5 degrees.
This is the Cub's airfoil performance chart.
Fourth, the shape of the bottom of the floats also enter into consideration. Some shapes have better "climbing on the step" characteristics than others. This is only determined by actual testing.
All of that being said, be prepared to change the angle once you get into the flight test phase. Particularly if your initial settings are near an extreme. Making provisions to change the length of the strut and/or the attach fittings is a wise consideration. Remember, changing the strut lengths effects the cross wire lengths. And the water rudder steering cables.
One of our members had a friend contact me. That friend had just purchased a set of Clamar amphibs which were installed by the Clamar folks in Maine. That installation was dangerous in that it had bad porposing characteristics and the bows buried under water while taxing. This was on a Cub. Cubs have been going on floats for 80+ years. This is not rocket science. Take what is in that manual with a grain of salt.
I noticed Olibuilt thanked the post. He has a set of amphibs on order. In his case, he has slats on his wing. Slats require high angles of attack to be effective. To utilize the slats on floats he will need a large fish mouth angle. However, once he is in the air his cruise speed will be very slow. He needs to think of this while he's making his installation. He could have a situation where he drags the aft end of the floats while taking off.
The optimum planning angle of 8 degrees between the float keels and the water is important. Any more or less angle increases drag. This 8 degrees is what is commonly call the "sweet spot". If the fish mouth angle is too small, when you rotate to take off, the water drag will increase. Sometimes this can prevent continuing the take off, particularly with low horsepower. If the fish mouth angle is too great, the aerodynamic drag may prevent take off.
It's a juggling act.