Masthead29_Mar2014 G.pub - page 21

TheMasthead
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.
TheQuestionWas:
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:
Where:
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
95%
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:
326xSHPxe
T=
Va
326x427.5SHPx0.75e
T=
=2,133lb.
49knotVa
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