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A Better
Sportmaster?
Modifying a Mercury
Sportmaster
Article by Greg Terzian and Helmut Freitag
Our test
was performed on a 1998 STV Euro with a ski bottom design. This boat was
powered by a modified Mercury 260 EFI bolted to a four inch hydraulic
setback unit. We used a Mercury 14.5 x 32P cleaver propeller for all
testing.
First, we
started with the stock Sportmaster and made several runs in the calm bay
water. The result was a best of 121 mph. The boat felt very stable, as it
always did, with minimal driver correction to keep on track. There were no
signs of blowout or any other anomaly associated with extreme speeds on the
water.
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Left:
Changing
to the modified unit.
Right: Running with the modified
Sportmaster. |
The boat
went back to the trailer and the lower unit was changed to the fully
modified Sportmaster. The modified Sportmaster has a number of changes to
the original design. These changes are as follows:
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Skeg cut and swept back
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Reduced the skeg height by
3/4 of an inch
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Torque tab reduced in height,
and the flat side has been flared in
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Skeg thickness has been
reduced
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Skeg leading edges have been
chamfered, rolled, and sharpened
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The blowout lip has been
completely grinded and polished off
The STV
behaved well at first, however it exhibited some added stern instability
when accelerating hard. This was most likely the result of the shorter skeg.
Remember, a tunnel hull generates more lift at the stern than a V-bottom,
and therefore would depend on the lower unit more for stability. This was
to be expected, and the problem became worse, even as the boat settled down
at speed. The boat also generated more bow lift – a lot more than with the
stock Sportmaster. These situations were manageable up to about 115 mph,
when from there it became a battle to tame the boat. We witnessed a pretty
hard blowout, and the boat had a very hard time tracking straight. After
several runs, it was surmised that running with negative trim was the only
solution at speeds above 115 mph. This helped the STV achieve a top
GPS-recorded speed of 122.4 mph. Indeed we gained 1.4 mph from cutting up
the Sportmaster, but was the gain worth the increased instability and risk
of blowout? I would say no.
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Running the
STV at top speed with the stock Sportmaster |
Safety is always the first concern when boating – even when conducting
experiments such as this. I would conclude that, although the results might
be different on a V-bottom hull, modifying the Sportmaster for increased
speeds carries with it serious compromises. The 1.4 mph gained is not worth
the safety risk, nor was it worth the detriment to the boat’s handling. We
took a very well behaved STV and turned it into a white-knuckled battle for
top end speed. I would choose to sacrifice the 1.4 mph for the
stability. Remember, a stable boat always has more performance
potential than an unstable one.
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Top Recorded Speeds |
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Stock Sportmaster |
Modified Sportmaster |
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121.0 mph |
122.4 mph |
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Article Information |
| We test a
modified Mercury Sportmaster lower unit to see what, if any
performance will result. |
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Created: |
| October 15,
2002 |
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By: |
| Greg Terzian |
| Helmut Freitag |
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Category: |
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Product Evaluations |
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