Propulsion system for inflatable

[albatross]

Dear all,
Hi and would appreciate if you can help me.
I come from Hong Kong, working on a project that developing a electrical propulsive system for inflatable. I am not a Naval engineer and know nearly nothing about boat.

My story starting from April this year, I began the making of the prototype and eventually at early July, it was finished and tested. It is a 2.4m inflatable for 1 passenger and the max. loading is about 140Kg. Our target is it can travel at 8Km/hr with full speed but all I got is just 7.5Km/hr. Right now, I am trying to increase the speed, hopefully can achieve 10. Reducing the water drag is not easy as the hull is inflatable which has so much constraint on the structural design. Therefore, using a best matched propeller with optimal powering is the only way out.

Kindly see the attached photo. It is the propulsive part of the prototype. The propeller is from a 15Hp outboard. I don't know the type of this propeller and even the pitch etc. I cut out the gear part of a 5 Hp outboard which has about 2:1 gear ratio and drive it with a 800W DC motor. The RPM under full speed that I estimated out is about 1000~1200. You can see there is a nozzle added. The purpose of this is not for increasing the performance under heavy load but, for safety reason, to protect the user being hurt by the propeller.

Here comes the questions.
1. I know I should use some sort of nozzle propeller like the Kaplan type. Unfortunetly, I don't have any 3D profile which I can CNC one for testing. Is there any web-site I can get these 3D dwg?
2. our consultant said I can still use that 15Hp propeller with the nozzle, just the performance in low speed will be affected as it increase the drag under low speed, is that true?
3. any suggestion for the diameter & pitch if I directly drive the Kaplan propeller with 800W output power, 2400 RPM? Should I provide more info for your suggestion?



Thanks in advance.