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Old 11-06-2013, 11:39 PM   #47
Pete37
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City: Grasonville, MD
Join Date: Oct 2013
Posts: 52
Hi Tim,
You don’t have any specs listed on your boat so I had to look her up on Yachtworld. She’s a rare bird; only one listed. She’s 35.5’ long and assuming a normal hull form her waterline length (L) would be about 92% of that or 32.7’. That would make her speed-length ratio (SLR) at 7.4 knots 7.4/sqrt(32.7) =7.4/5.7=1.30. At 6 knots it would be 6/5.7 = 1.05. I’m going to assume that she’s a light trawler and therefore use the del=150 curve.
Run a line vertically up from SLR=1.05 to where it intersects the del=1.5 curve. You’ll find that the specific resistance is about 16 lbs/ton. Run a second line up from SLR=1.3 to where it intersects the del=150 curve and you will find the specific resistance is about 57 lbs/ton. So by shutting down one engine the specific resistance drops from 57 lbs/ton to 16 lbs/ton. You have cut your resistance to only 16/57 = 28% of the two engine resistance. Your change in speed should be (1.3-1.05) x 5.7 = 1.43 knots. That’s pretty close to the 1.4 knots you estimated.
The reason the curve is so steep between SLR=1.2 and SLR=1.6 is because this is where your boat is building up its wave train. Wave making resistance is the largest source of resistance.
You don’t give any hull weight or current twin engine fuel consumption at 7.4 knots so I can’t estimate the fuel burn rate but the single engine fuel burn should only be about ¼ of the two engine rate.
Everything sounds wonderful except that you are operating your engines at idle speeds and I doubt the engines will get up to their normal operating temperatures. Do that regularly and you’re going to damage your engines. The exhaust system parts will be the first to go. Look for transom soot. Later, if you don’t get your engines up to proper operating temperature you will have piston, cylinder and block damage. Check your temp gauges and make sure you are getting up to around 180F. Your engine manual will give you the exact temps the manufacturer wants you to maintain.
Keep in mind that the graph is most accurate for trawler speeds up to SLR=1.6 ( 9 knots for your boat). At planing speeds (SLR>1.6) the curve of specific resistance is nearly flat so there would be little difference in fuel economy between single and twin engine performance (provided you remain above SLR=1.6 for both modes of operation).
Parasitic drag is the drag caused by dragging the propellers, shafts, struts and rudders through the water. Parasitic drag is usually small potatoes compared to wave making and surface friction drag. But for high speed planing hulls it can become a significant factor.
Pete37
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