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Old 02-12-2013, 11:37 AM   #112
Underway
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Join Date: Aug 2012
Posts: 741
Quote:
Originally Posted by sunchaser View Post
Skidgear

The way the pros (commercial Post type charter fishermen, offshore racers etc) do this is to first understand that each boat is different. Then they do as you are, dial in on paper their best prop setup and have a prop on either side of the diameter and pitch equation to test as well to see if any benefits arise by going bigger or smaller. I had this exact test done years ago on a high speed boat. Many choose to go for quiet and smoot props which are not necessarily the most fuel efficient. I've a friend who tinkered with this for years after his boat was initially - "perfectly" propped. Your quest can be frustrating and expensive.

Accurate fuel flow and speed measurement is a must. But, and a very big but, at what boat load conditions do you plan ot run the majoriity of the time because the prop selection goes to hell as you add or subtract tons of water, gear and fuel or hit a small stick that bends a prop.

What is your boat, desired perfect prop speed range and engine(s) combination?
For discussion purposes say the boat is 30,000 pounds, semi-planing, twin turbo 250's, with 2:1 transmissions. The target "efficiency" speed for discussion purposes is roughly 8.5 knots. Prop speed is currently 750 rpm at that hull speed. It should change incrementally with larger, more efficient props.

But keep in mind that I'm not thinking in terms of optimizing the "perfect/correct" prop as seems to be the case in your charter boat example. The cruise prop scenario at its most basic, installs a bigger "wrong" prop while keeping the rest of the drive train unchanged...same engines...same gear ratio, etc. So, start with a perfect prop and increase the diameter an inch at a time. Then plot the fuel specifics across the speed range for each new prop size. I understand that efficiency gains will be small (and boat specific)...but that's not the point of the exercise. Documenting an examplle of what we're really talking about when it comes to cruise props is what I'm trying to quantify.....curiousity if nothing else.

Regards
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