Boat Maintenance: 4.3 ltr 190 vs. 220
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Name:
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CAT BOAT
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Subject:
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4.3 ltr 190 vs. 220
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Date:
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8/20/2013 2:44:47 PM (updated 8/20/2013 3:16:34 PM)
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I have a strong opinion on that subject. Make your comparison 4.3 vs. 5.7. If you go with the larger engine, and operate it the way you say you plan to, you will use LESS fuel. You will also enjoy better performance with the larger V-8 when you need it. The larger engine will have a more aggressive propeller pitch that is measured in inches. Example, a 4.3 ltr engine will probably be equipped with a 19"pitch. A 5.7 ltr would most likely have a 23" pitch propeller. That means (provided gear ratios are the same) the 4.3ltr boat would travel 19" through the water every prop turn, and the 5.7 23 inches per turn. Now, consider this.... The 4.3 ltr is working at a higher RPM range, burning fuel while the 5.7 is operating at a much lower RPM range working less while maintaining the same speed. Here is a speed chart for comparison.
Prop pitch X RPM (_____) divided by Gear ratio X 1056 = SPEED
You must decuct a percentage for Prop slip. 8-10% for speed hulls/12-15% for heavier boats, you get the picture.
Example.
23" X 5,000 Engine RPM = 115,000
1.50 gear ratio X 1052 = 1578
115,000 divided by 1578 = 72.88 MPH. Use a slip factor of say 15% for the boat you speak of and you get 62 MPH.
Do the same calculations for the 4.3 ltr with a 19" prop and you will get the picture. However, if you run both set-ups wide ass open all day you will use MORE fuel in the 5.7 BUT you will travel a whole lot farther. I have people ask me all the time "How much fuel does your boat use"? (speaking of the Catamaran with twin engines) And the answer is "Probably less than yours". The gear ratio on performance boats are normally 1.115 to 1 and prop pitch 32-36" range.
36" X 2200 Engine RPM + 79,200.
1.15 X 1056 = 1214.40
79,200divided by 1214.40 = 65.22 Take 9% for slippage and you get 59.35 MPH. ; )
I hope this helps. Sorry for the long answer.
Respectfully,
Steve
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