Outboard - Small Power Cat

[fritzdfk]

I have almost finished construction on a 18' power cat. http://www.ikarus342000.com/ECOmotorboat.htm

I am thinking about an outboard. The designer mentions 15-20 hp and the boat should cruise around 12 knots. I was just at the Yamaha dealer and the guy there was trying to sell me on a high thrust 9.9 yamaha. He said for my application that motor would perform certainly just as well or better than the regular 15hp. He said the high thrust 9.9 is comparable to a 18 hp regular motor at the lower speeds and my hull type. 10-12 knots would be a fine cruising speed for me. There probably isn't any way to be sure other than trying them out.

 

[Steve W]

I love the high thrust yamahas, I have two of them, they are an amazing motor, but they are made to push displacement boats like keelboats at displacement speeds, they are all about thrust. Im no expert on motor selection but I would think that Bernd is right. I motored 1500 miles last summer with the 9.9 Yamaha on a 30ft Gemini which is about 7000lbs and it was unstoppable, but I don't think it is the right choice for your boat.

Steve.

 

[tsharac]

I have a T9.9 as well, I have a hard time believing it'd get 10-12 knots with 9.9 hp; but that hull weighs around 600 lbs per the website.

Using Crouch's formula; assume 1000 lbs of total weight, hull factor of 170 (worst case scenario) and 9.9 hp; that puts you at 16.9 MPH or 14.7 knots.

Play with the formula for ideas:

http://continuouswave.com/cgi-bin/crouchcalc.pl