Masthead29_Mar2014 - page 28

Ts=62.72 x (SHP xD/12)
Ts=Static thrust or bollardpull, inpounds
SHP=Shaft horsepower at thepropeller
D=Propeller diameter, in inches
Ts=62.72 x (427.5SHP x (16 in. Dia./12))
=4,404 lb. static thrust
As is usual, static thrust is greater than free-running thrust at speedand thatmight beour design load, butwealsoneed to
check thebreaking strengthof the line that’s attached to the tower.
Wakeboard towropes areusually low-stretchDyneemaor spectra, withbreaking strengths around4,000 to4,100pounds.
Of course, youhave toassume that theboat’s users are likely toplaywithother types of sport towing—water skiingand
tubing for instance, and that these linesmight beattached to the tower.
Waterski towropes usuallyhavebreaking strengths around800pounds, but tubing towropes vary in recommendbreaking
strength from1,500pounds for a single rider of about 170pounds to six ridersat 6,000pounds breaking strength.
Now thequestion is should youdesign to the4,404 lb. static-thrust loador thebreaking strengthof the6,000 lb. 6-person
tube line? In reality theboat simply can’t producemore thrust than4,404 lb. In fact, even that’s agenerous number, be-
cause it’sbollardpull for tug-like vesselswith large-diameter, slow-turningprops.
Theactualmaximum loadon the towropes duringwakeboardorwaterskiing isabout 1.5 to2 times theweight of the
skier. For a6-person tube, themaximum loadwouldbeabout 1.5 times theweight of all 6people, say, 185pounds times
6or 1,110 lb. times1.5, or 1,665 lb.
Lines, shouldbeworkingat nomore thanonequarter toone third their breaking strength, which is consistentwith the
1,665 lb. for the6,000 lb. breaking-strength tubing towrope.
Wenowhave several numberswe canexamine to select for our design load:
4,100 lb. breaking strengthof thewakeboard towrope
4,404 lb. static thrust
1,665 lb.maximum load tubing six people
The static thrust is the largest of thesenumbers, so the4,404 lb. static thrust is thegoverning load.Weknow this is the
ultimatepossible loadand is unlikely toexperiencedexcept in themost unusual conditions, soa safety factor of 2over
ultimateor 1.5over yield isapplicable. Bothof thesewill result indesigning to strengths over thebreaking strengthof the
6,000-lb., 6-person tube towrope.
March 2014 Page28
Know It All Contest Solution
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