Thanks Greg V, I agree with you and Cougar. The rear parts of the Kiwi hula was too wide, 500 mm. It should have been the trailing edge of a fin. Avoiding the slipstream effect. I tested the hula concept on my 2.4mr and it extended the waterline effetively. You can see the result on the web site:
Under the title Trimtips from U391.
Regards,
Booster