Masthead29_Mar2014 - page 21

Know It All Contest Solution
to theDecember 2013Question
What are thedesign loads for awakeboard-boat towing tower?
TheKnow It All questions and correct answers are important design tips for students aswell
as othermarine professionals.We suggest that you file them away for future reference.
As the leaddesigner for ToughRider Boats, you’vebeenasked todrawup the towing tower for their newwakeboardboat,
theDXWakeRocket 220. She is a22-footer, witha19-footwaterlineand8-foot 6-inchbeam. TheDXWakeRocket 220
displaces4,600poundsat average load. Shewill bepoweredwitha standard330-HPgas engine, withahigh-power option
of 450horsepower. Configuration is single-engine inboardV-drive.
The450-HPengine runsat 5,500RPMmax, 3,500 to4,200RPM cruise. Theboatwill be fittedwitha1:75:1gear driving
a4-blade, 16-inch-diameter, 20-inch-pitchpropeller, withheavy cup. Top speed isabout 50knots, but normal wakeboard-
ingoperation is between18and24knots.
What design load should youuse for the towropeattached to the tower, andwhy?
TheAnswer is:
Maximum speedpropeller thrust is:
T= Thrust inpounds
SHP=Shaft horsepower at thepropeller
e=Propeller efficiency inopenwater
Va=Speedofwater at thepropeller, inknots.
For high speedplaninghulls Va canbe takenas98%of top speed inknots (V).
Va=50knots x0.98=49knots
Shaft horsepower (SHP) is the full brakehorsepower of theengine (BHP)minus looses for friction in thebearings andgear-
ing. Formost averageboatsSHP is96%of BHP, but V-driveshaveabitmore lossesdue to their extragearing, sowe’ll use
450BHP x0.95=427.5SHP
Propeller efficiency inplaningpowerboats varies fromabout 60% to78%. Since this isa fast planinghull andwedon’t
want tounderestimate thrust, we’ll usea reasonablebut highefficiency of 75%.
We cannow find thrust at speed:
Wealsoneed to check the static thrust or bollardpull. Todetermine the load should the linebe caught on somethingwhen
accelerating from still or nearly still. Anapproximate formula for static thrust is:
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