Rick, "So slow boats soft chines and attached flow. Fast boats hard chine and detached flow.
There is a good deal of custom and practice in boat design and building. The general belief is that sailing boats need soft chines and power boats intended to plane need hard chines. These days there are quite a few sailing hulls intended to plane or at least semi-plane. With fast sailing boats I see a gradual progression toward harder chines. My own data shows the hard chine to be beneficial to performance in the slender hulls I design and build.
If you have good researched data to the contrary then happy to review it and identify where it is wrong or why it does not apply."
Let's use Savitsky's research that you referenced and your 8 meter race boat that you have optimized for a SLR of 1.3 (3.3meters/s (6.4 knots) / √ length (26 feet)) and compare its hull features to Savitsky's descriptions.
Savitsky in figure 4 shows that the difference in resistance between a displacement hull and a semi-displacement hull at a SLR of 1.3 is ZERO. The cost in increasing the speed to an SLR of 1.8 (6.5 knots to 9 knots) for a semi- displacement hull is double the resistance and off the chart (literally) for a displacement hull.
The target speed for your race boat seems to be as fast and efficient as possible : ) An SLR of 1 is the most efficient point (5.5 knots).
Your design seems to follow the Semi-Displacement hull the closest. The exceptions are "straight buttock lines in the
afterbody with a slight steady rise aft." Your design is concave there which increases squatting and is similar to a displacement hull shape.
"Round bilges along the entire hull -although some designers
may prefer a combination of sharp chine aft with round chine
forward or vice-versa." Your design has sharp chines fore and aft which increases separation and drag. I happen to think that his description here is very poor though.
To summarize let me again quote Savitsky "In summary, the semi-displacement hull form is recommended for
operation in the region of SLR between approximately 1.3 and 3.0. For higher speed the
hard chine planing hull form is required.
Obviously your boat is not designed to exceed a SLR of 3 (16.5 knots) so a hard chine is detrimental to its performance at speeds less than 16.5 knots. Your boats performance would be improved by using a standard semi-displacement hull.
Talador
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Posted by: taladorwood@yahoo.com.au
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