The manufacturer of the engine does not determine horsepower, and that is
really what you are talking about, assuming clean bottom and the right prop.
I am not sure about the range of engines available now from Westerbeke, but
Yanmar makes a range of engine sizes from too small, to much too large for
your application.
Don't forget that the theoretical hull speed for your water line length is
6.82 knots and that to achieve that speed, or go a little faster, requires
an enormous amount of power in getting that last knot. The somewhat larger
boats with a longer water line will require less power to go 7+ knots than
yours will. The power required to drive a displacement boat, or a car for
that matter, increases by approximately the square of the speed until you
get to within a knot or so of the hull speed, and then the ratio starts
going up enormously.
I towed a 30 foot boat with a disabled A4 in it for the last 20 miles coming
back from Ashtabula on the July 4 weekend. My boat has a Yanmar YSB12 which
is rated at 10 hp continuous output. We were able to maintain 4.3 knots with
the engine running at about 80%. I don't have a tach so I don't know the
exact percentage, but it was definitely not flat out, and I had been getting
5.6 knots on the same setting before starting the tow. I was surprised that
we were able to make such speed as we did, considering that the displacement
more that doubled with the tow, but the water was almost flat. Hull speed on
my boat is 6.34 knots.
Steve Thomas
C&C27 MKIII
Port Stanley, ON
----- Original Message -----
From: "Brian Morrison via CnC-List" <cnc-list@cnc-list.com>
To: <cnc-list@cnc-list.com>
Cc: "Brian Morrison" <brianm...@hotmail.com>
Sent: Tuesday, July 14, 2015 00:47
Subject: Stus-List Best Engine?
Hello,
I often sail with a group of sailors that have more modern and somewhat
larger boats than my 1979 C&C34. When there is no wind and we are forced
to motor I have difficulty keeping up. With a clean bottom my cruising
speed is about 5-5.5 knots. I have an Atomic 4 engine. I'm wondering would
a Yanmar or a Westerbeke give me 7+ knots?
Brian C. Morrison
S/V
Rekofa
1979
C&C34
Chesapeake Bay
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