Chrysler 20 HP and Boston Whaler 11'4' Speed

[rbruce63]

Boston Whaler has sent the following information about my recently restored boat:<br /><br />"This comes up as an 11' Sport model (console steering). The 11' Sport is 11'4" long, 5' beam, 6" draft, 250lbs estimated dry weight, 560lbs max weight capacity, 750lbs swamped capacity, 3 person<br />capacity, 20 hp maximum and needs a 15" (short shaft) engine. This boat is a 1986 model."<br /><br />Theoretically speaking my 1969 Chrysler 20 HP model 2003 S/N 5792 produces at 4,500 RPM 20 HP. Perhaps 3% less at the propeller shaft since.<br /><br />What is the maximum attainable speed at 4,500 RPM with this motor and propeller 8 1/2" diameter X 8 1/2" pitch?

 

[force01]

Re: Chrysler 20 HP and Boston Whaler 11'4' Speed

Your motor will produce peak power @ 5100 rpms at that rpm you will have 20 hp prop rated. with your trim angle properly set i would guess you will run 23-25 miles per hr. hang on for dear life at those speeds anything could happer lol. <br /><br /> If you need further assistance please email me @ forcepower01@yahoo.com. thanks,Bret

 

[rbruce63]

Re: Chrysler 20 HP and Boston Whaler 11'4' Speed

Force1:<br /><br />I arrived at a similar speed with the formulas found in Dave Gerr's "The Propeller Handbook" of around 28 knots per hour. <br /><br />At Río Indio in San Juan del Norte, Nicaragua, without the bimini top and my brother 6'2" and 185# at the bow of the Whaler, we clocked close to 20 MPH. <br /><br />I would guess that the next step is to find RPM at wide open throttle to see if the boat will handle an 8 1/2" x 12" pitch propeller and increase the speed. In this rally the motor overheated on the way back to Costa Rica but I think it was negligence on my part, and debris which was very common in these rivers.<br /><br />The ventilation plate is 1" below the hull end, I don't think I would like to have it jacked up more.<br /><br />Thanks for your input!