Is this the right prop for this low speed inflatable?

[bill fish]

Hi all....

Here is the info...

Inflatable Avon Redcrest (pointed uplifted bow and rounded stern) - 9 ft long, 4 foot wide, TOTAL all up weight = 300 lbs

4 Hp at 4500 RPM evinrude, 2.6 gear ratio, therefore 1700 prop rpm at target hp (engine says top rpm range 4000-5000 rpm).

Prop specs 6 1/4 by 6 (which is diameter and which is pitch BTW?).

Riding the boat in extremely calm waters and taking care to make the bottom of the inflatable as flat as possible as well as getting the center of gravity in the best location to maximise speed...

I get absolute top speed = 5 mph, most likely speed 4mph, and absolute lowest speed 3 mph (errors due to eyeballing charts and approximate travel times)...

5 mph = 440 feet per minute, 4 mph =350 fpm, 3 mph = 260 fpm.

Taking 1700 rmp for max hp output, 1700 times 6 inches of pitch (propellor not slipping any?) = 850 feet per minute.

So, if 5 mph, 850/440 = 1.9X "slip factor"

if 4mph, 850/350 = 2.4X (most likely value)

if 3 mph 850/260 = 3.3X

So, my question is...is my prop way over/under? pitched for such a draggy inflatable boat?

And if so, what pitch change do think I should go to to increase speed/hp output without overeving the engine (assuming I am careful to not ventilate or cavitate the prop due to improper engine trim)

thanks


take care

Blll