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

Lloyd:

There is a quick way to guess at the speed increase for a modest repower,
given the following assumptions.

Assuming,

a. the existing engine is producing full power and it has a properly matched
gear and propeller,

b. your new engine will also have the proper gear and propeller,

c. the vessel is of a displacement type (i.e., not of a planing craft form,
as a significant immersed transom changes speed-power relationships),

d. your operating speed is below a speed-length ratio of about 1.2 (this
keeps things well-behaved),

e. the loading of the boat (draft, displacement and waterline length) will
not change, and

f. your speed data is reliable,

then, you can scale the expected speed increase by the cube-root of the
power increase. For example, if you were going 6.5 kts with 21 hp, and your
new power is 31 hp then your speed increase is the cube-root of 31/21, which
equals 1.14 (or a 14% increase in speed). So, your expected speed (with all
of the above assumptions) is 7.4 kts. In our experience, this offers a
pretty reasonable answer (if not a wee bit conservative).

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366





"Lloyd Sumpter" wrote in message
...
On Thu, 25 Sep 2003 15:17:23 +0000, Lloyd Sumpter wrote:


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

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

Thanks for the reply, Jim!

On Fri, 26 Sep 2003 06:13:13 +0000, Jim Woodward wrote:

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.


I'm beginning to think this way. I should see a marked increase from the
18 (maybe) I'm getting now, though, and probably over the 21 I'd get with
a rebuild.


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.


Yes, that's definitely another reason to up my power: any serious
headwind/waves and I'm down to 3 knots.

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?


Interesting...My "take" on the various reputations was that Universal (now
Westerbeke) is pretty low on the food chain. I HATED my Volvo furnace, but
Volvo cars and trucks seem pretty well-built and reliable, and Volvo-Penta
at least is easier to get parts than Universal/Westerbeke. I may have to
re-think this...

Trouble is, I'm not sure I can fit the bigger Westerbeke in, and if I do,
it might intrude in the galley too much (I'd have to build a new engine
cover). And I'd SURE like 30hp!

Lloyd

Lloyd, I've had two different Volvo diesels on past sailboats and I
was not terribly impressed with either one. Parts and repairs are
expensive and reliability/durability was nothing to write home about.
Frankly, the most dependable and long lived sailboat aux I ever owned
was the Atomic-4 gas engine on my old Cal-34. Very reliable, very
easy to repair when it wasn't, and lots of power at 30hp. Motor
sailing at slow cruise it would run almost two days on 30 gallons of
gas.

That said, the A-4s are of course no longer available new, and many
folks are allergic to gas engines.

The small Yanmar diesels are popular here on the east coast.

============================================

On Thu, 25 Sep 2003 15:17:23 -0700, "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

Don:

Interesting method -- I haven't seen it in any of my reading. (I
should say I've been to your web site, so I assume that you know what
you're talking about.)

Perhaps you can help with the following facts, then. (For background,
see www.mvFintry.com). The Royal Navy made it very easy to measure
fuel consumption on the Fleet Tenders (and, I would guess, on its
other vessels as well) with a special three tank and sight gauge
permanent setup. As a result we have good fuel consumption figures at
various speeds. Assuming that fuel consumption is a good proxy for
horsepower (ignoring the typical small increase in specific fuel
consumption as engine speed decreases), we have pretty good numbers
for the horsepower required at various speeds.

Base point is 325hp at 10.5 knots, S/L 1.21. From there down to seven
knots, your cube formula gives a higher horsepower than the fuel
numbers, topping at 144% high at nine knots. At seven knots (S/L
0.81) the two curves cross, and below that the cube rule falls off
much faster than the fuel consumption would indicate.

Am I correct to guess that the cube formula applies in the "sort of
well behaved" area where wave making dominates, but not overwhelms,
from around S/L 0.70 to 1.20? Above that, it fails low and below
that, friction is more important and not very speed dependent.

Is there an explanation for the 44% error? Given that cube formulas
are sensitive to starting point, it could well be that the full speed
horsepower is less than 325, which would change the SFC number from
the 18.55hp-hr/USG used.

Am I missing something?

Jim Woodward
www.mvFintry.com




"D MacPherson" wrote in message ...
Lloyd:

There is a quick way to guess at the speed increase for a modest repower,
given the following assumptions.

Assuming,

a. the existing engine is producing full power and it has a properly matched
gear and propeller,

b. your new engine will also have the proper gear and propeller,

c. the vessel is of a displacement type (i.e., not of a planing craft form,
as a significant immersed transom changes speed-power relationships),

d. your operating speed is below a speed-length ratio of about 1.2 (this
keeps things well-behaved),

e. the loading of the boat (draft, displacement and waterline length) will
not change, and

f. your speed data is reliable,

then, you can scale the expected speed increase by the cube-root of the
power increase. For example, if you were going 6.5 kts with 21 hp, and your
new power is 31 hp then your speed increase is the cube-root of 31/21, which
equals 1.14 (or a 14% increase in speed). So, your expected speed (with all
of the above assumptions) is 7.4 kts. In our experience, this offers a
pretty reasonable answer (if not a wee bit conservative).

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366

Jim:

It's hard to back-engineer what may be going on - I'd need to see your
numbers to know how you're correlating fuel rate to power. Are you deriving
the 325 hp from the fuel rate?

There are other things thrown into the mix. This approach assumes that
you'll have a "best" propeller at both speeds (to insure comparable
efficiencies). Its not uncommon to have measurably different propeller
efficiencies at a 30%+ spread of speeds (7 to 10.5 kts). Drag humps and
hollows might contribute a bit, but the 10%+ SFC differences found on
typical engines at low to high limits will probably have a greater impact.
Add all of these, and can you get 44% difference? It would take a
fully-baked sea trial analysis to know for sure.

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
email:
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366




"Jim Woodward" wrote in message
om...
Don:

Interesting method -- I haven't seen it in any of my reading. (I
should say I've been to your web site, so I assume that you know what
you're talking about.)

Perhaps you can help with the following facts, then. (For background,
see www.mvFintry.com). The Royal Navy made it very easy to measure
fuel consumption on the Fleet Tenders (and, I would guess, on its
other vessels as well) with a special three tank and sight gauge
permanent setup. As a result we have good fuel consumption figures at
various speeds. Assuming that fuel consumption is a good proxy for
horsepower (ignoring the typical small increase in specific fuel
consumption as engine speed decreases), we have pretty good numbers
for the horsepower required at various speeds.

Base point is 325hp at 10.5 knots, S/L 1.21. From there down to seven
knots, your cube formula gives a higher horsepower than the fuel
numbers, topping at 144% high at nine knots. At seven knots (S/L
0.81) the two curves cross, and below that the cube rule falls off
much faster than the fuel consumption would indicate.

Am I correct to guess that the cube formula applies in the "sort of
well behaved" area where wave making dominates, but not overwhelms,
from around S/L 0.70 to 1.20? Above that, it fails low and below
that, friction is more important and not very speed dependent.

Is there an explanation for the 44% error? Given that cube formulas
are sensitive to starting point, it could well be that the full speed
horsepower is less than 325, which would change the SFC number from
the 18.55hp-hr/USG used.

Am I missing something?

Jim Woodward
www.mvFintry.com




"D MacPherson" wrote in
message ...
Lloyd:

There is a quick way to guess at the speed increase for a modest
repower,
given the following assumptions.

Assuming,

a. the existing engine is producing full power and it has a properly
matched
gear and propeller,

b. your new engine will also have the proper gear and propeller,

c. the vessel is of a displacement type (i.e., not of a planing craft
form,
as a significant immersed transom changes speed-power relationships),

d. your operating speed is below a speed-length ratio of about 1.2 (this
keeps things well-behaved),

e. the loading of the boat (draft, displacement and waterline length)
will
not change, and

f. your speed data is reliable,

then, you can scale the expected speed increase by the cube-root of the
power increase. For example, if you were going 6.5 kts with 21 hp, and
your
new power is 31 hp then your speed increase is the cube-root of 31/21,
which
equals 1.14 (or a 14% increase in speed). So, your expected speed (with
all
of the above assumptions) is 7.4 kts. In our experience, this offers a
pretty reasonable answer (if not a wee bit conservative).

Regards,

Don

Donald M. MacPherson
VP Technical Director
HydroComp, Inc.
http://www.hydrocompinc.com
tel (603)868-3344
fax (603)868-3366