Qsl 9 prop sizing

[Thomas smith]

I am repowering to a Cummins qsl 9 405 hp engine . I have a 43 ft displacement hull boat that is a dragger . Max speed is around 9 to 10 knts . The gear ratio is 2:1 and maximum propeller diameter is 28” . Any suggestions on pitch size for this motor . One prop calculator suggest a 30×21 .

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[Thomas smith]

A 30×21 was his recommendation but because I can t swing a 30” and can only swing a 28 I was thinking either a 28×26 or 28×25 . I saw where you had installed several of the qsl 9 so I thought you might know somewhere in the ballpark what size t might turn wheel wise .

[Tony Athens]

When you are trying to make the ‘best” out of a less than best situation, this is something your “prop” guy needs to help you with.. You want the most “Bollard Pull” you can get “dead headed” with 400hp/2100 rpm AND 28″ of diameter max……….. At best, It will only be a compromise as you have a 1000 RPM 28″ prop.. Not good for your application…. Blade area would be #1 on my list.. Let’s see what he comes back with..

[Thomas smith]

Tony if the qsl9 is rated for 2100 rpm I want the maximum pitch at 28” of diameter I can get and it still turn the 2100 rpm tied down . Thanks

[Thomas smith]

Thanks again Tony . I m familiar with props diameter and how it pulls . From watching your videos and looking at your website I knew you were familiar with the qsl9 and had put them in boats before . So my question basically is what size prop will the qsl9 swing on a 2:1 if 28 “ of diameter is the most I can swing . Is the 28×21 the most pitch it will turn and still be free or can it turn more pitch ? Thanks

[Tony Athens]

Power or HP is not your issue–It’s the fact that a deeper ratio gear and a bigger diameter prop would be way more suited to your application.. A 3:1 gear and a 38″ x 30 prop would be way more efficient for dragging — At the same power input ( let’s say at 10 gallons per hour/200HP, you’d have approx 50% more pulling power).

[Thomas smith]

Thanks Tony . I m guessing your prop size is based on personal experience with the qsl9 ? I know of a couple other boat owners that have the qsl9 405 hp that are swinging 28×32 props on planning hills ( lobster fisherman ) . That s not a equal comparison to a displacement hull dragger . I m just trying to make sure the qsl 9 is going to have sufficient power for my application.thanks

[Tony Athens]

With only a 28″ max diameter , then you need to go with a 28 x 21 4-Blade high DAR –Acme would be my choice if they offer something a flat pitch.. Being a dragger, at times you’ll be at well above a 75% slip and IMO, you really need as high strength prop.

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