Re: What makes for a ‘racing’ lower unit?

[bh]

quote benchrascal:

Some bathtub racers in Nanaimo BC experimented with racing units on their 6hp motors .Not sure now but at the time that was the top hp allowance .I could not believe how small a profile an lu could be and what high thrust you could get out of just six horsepower .Some of those bathtubs really fly .The one thing I saw that was common (irregardless of make ) was that they were all direct drive to reduce size and weight.

The motor of choice during those times, ie ’50s >’70s, (+?) was the 7.2", Mercury, KF5, Mark 5 & Mark 6.
This was also the motor used in APBA JU & later JSR racing, ie; ‘Junior Stock Runabout’.
A recent post on Hydroracer plots the rise in speed of these rigs, from 29mph in ’59, and 30mph in ’63,
to 33mph in ’73, and 36mph in ’75. At some point in the ’80s the carb restricted OMC motor became the J engine,
and speeds went up. The ‘bathtubs’ that I saw running in the ’70s were nothing more than JUs with plastic
tubs mounted on them. and their straight away speeds were comparable to JU/JSRs.
One design change was made in APBA racing, in that period, the introduction of the ‘J60’ by Mercury,
a stock Mark 6 with a very small special Quicksilver racing l/u. It would appear that,
all other things being equal,[??] the switch from the stock fishing unit to a ‘racing l/u’
at most resulted in an increase of ~20% in straight away speeds.
Not as great as one might expect, as the fishing l/u’s were direct drive, and a 16:21 ratio in some cases,
whereas the Quicky would have been a 1:1 ratio, making for a small case, but not necessarily higher speeds .
Another good comparison is in the large l/u of the Johnson racing PR40, and the small streamlined unit
of their later PR50. Speeds in that period doubled, from the 20s to the 40 mph range,
but for many reasons.
And one could compare the fishing 30hp Johnson RD vs the M30H with quicky,
36"Run vs CU, ~40mph vs 60mph, ie; roughly 50% +
Food for thought………. ❓