As a very rough rule of thumb, you can assume that increasing speed
one knot requires doubling the horsepower, so between 26, 29, and
31hp, you won't see much difference.
Disclaimers: This is a rough and ready rule of thumb. It works only
at speeds between S/L (speed in knots divided by the square root of
the waterline length in feet) of 1 and S/L of 1.5. It works only for
full displacement vessels, those whose hull form gives them no "lift"
at higher speeds. And, it works only for vessels in the small to
midsize area -- LWL 25 to 49' or so.
Your 5.5 knots on a 30.25' LWL (Catalina 36, I think) is exactly an
S/L of 1, so you might do a little better than my rule, but if you
figure adding half a knot for the 29 or 31hp engines, you're close.
All of which shows that adding power to increase top speed is
difficult. More power will help in maneuvering, acceleration, and
punching into wind or seas, but don't look for much increase in flat
water top speed.
I don't know any of these engines at all well, but on general
experience and predjudice I would take a Westerbeke over a Volvo any
day. And have you looked at Yanmar?
Jim Woodward
www.mvFintry.com
"Lloyd Sumpter" wrote in message ...
On Thu, 25 Sep 2003 15:17:23 +0000, Lloyd Sumpter wrote:
Hi,
As soon as I sell the appt, I'm repowering Far Cove (36ft sailboat).
The existing engine is old, tired and underpowered. 21hp is NOT ENOUGH
for a 36-ft boat!
However, I don't have a lot of space, so I need something that's
about
the same size, but hopefully more power, and also hopefully as quiet,
smooth, and reliable as the ol' M25 has been!
Options I'm looking at:
1. Universal M25XPB: 26hp, pretty much exact, drop-in replacement.
Basically the same engine, bored/stroked a bit to get an extra 5 hp.
2. Westerbeke 35D: 31hp, 3" longer. Have to build a new engine cover,
but I THINK I could shoehorn this in. I think this is the engine the new
C36's have.
3. Volvo MD2030: 29hp, slightly larger. Probably an easy fit, size-wise,
but I would expect mods to engine mounts, exhaust, etc.
Comments? In particular, would the Volvo be quieter/smoother than the
Universal or Westerbeke? Any other options/mfrs I should be looking at?
Lloyd Sumpter
"Far Cove" Catalina 36
BTW: I see lots of equations, etc. for estimating top/cruise speed of
various boats with various engines. Anyone have anything I could use to
estimate theoretical cruise/top speed with these various engines?
Before I had a GPS, the M25 (21hp) could (I think!) push me at 6.5knots
WOT (I'm guessing my knotmeter might have been a bit optimistic). Now I'm
lucky to get 5.5, even with a clean hull. Any guesses what 26, 29, or 31hp
would get me?
Lloyd Sumpter
"Far Cove" Catalina 36