Roger Long wrote:
And:

If your boat is marginally powered, you could reduce the pitch so that
the engine will reach maximum RPM slightly before hull speed. This
will optimize it for lower speed you make in a headwind but you may
have to be careful about over speed depending on your governor set up.

You will only gain from this if blade area is sufficient. In most
sailboats, it is not. This also means that the engine will be running
at a slightly higher RPM for all boat speeds which means more noise
and wear.

If you have the engine power you need in headwinds, and the prop area
for maximum efficiency free running, optimizing the prop as I
described will still leave you with the bucking power you need in all
but the worst conditions. Very few people would want to lug the prop
necessary to deal with those forces around all the time unless it is
feathering. Very few sailboats will have been built with sufficient
prop clearance to install such a prop even if it is feathering. You
would have to plan for this at the design stage.

G'day, folks
I've just bought an H28 with an MD2B auxiliary. I haven't had a chance
to experiment yet but I'm looking for a starting point. I know there's
a bunch of people here with MD2B's in similar size boats, what do you
cruise at and what sort of consumption do you get at cruise?
Preliminary indication seems to be that 1600RPM in smooth water is
getting close to hull speed, so there seems to be a heap in reserve.
Interested in comments.
Thanks
JM

My 13 hp 2gm is probably good enough for my 28' S2. My question was
purely hypothetical. However, i used to have a 6.5 hp 1gm that
probably did have too little power.

In article ,
Moores family wrote:

I've just bought an H28 with an MD2B auxiliary. I haven't had a
chance to experiment yet but I'm looking for a starting point. I
know there's a bunch of people here with MD2B's in similar size
boats, what do you cruise at and what sort of consumption do you get
at cruise? Preliminary indication seems to be that 1600RPM in smooth
water is getting close to hull speed, so there seems to be a heap in
reserve. Interested in comments.

You may or may not have reserve. If you firewall the throttle and can
only pull 17-1800, you're probably overpropped and have little actual
reserve.

We don't have the MD2B, but the Yanmar 2GM20. We consume anywhere from
0.25 gph at about 5 knots and 2200 RPM to a bit over 1 GPH at 3200 and
hull speed of 6.6 knots. We can go faster (up to 7.2 at 3500), but not
long enough to get a good reading of the consumption. We tend to cruise
at about 1/3rd GPH at 5.5 +/-.


--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Jere Lull wrote:
In article ,
Moores family wrote:


I've just bought an H28 with an MD2B auxiliary. I haven't had a
chance to experiment yet but I'm looking for a starting point. I
know there's a bunch of people here with MD2B's in similar size
boats, what do you cruise at and what sort of consumption do you get
at cruise? Preliminary indication seems to be that 1600RPM in smooth
water is getting close to hull speed, so there seems to be a heap in
reserve. Interested in comments.


You may or may not have reserve. If you firewall the throttle and can
only pull 17-1800, you're probably overpropped and have little actual
reserve.

We don't have the MD2B, but the Yanmar 2GM20. We consume anywhere from
0.25 gph at about 5 knots and 2200 RPM to a bit over 1 GPH at 3200 and
hull speed of 6.6 knots. We can go faster (up to 7.2 at 3500), but not
long enough to get a good reading of the consumption. We tend to cruise
at about 1/3rd GPH at 5.5 +/-.


Thanks, Jere
I'm using less than 1/2 throttle to get to 'round 1500, so indications
are she's probably not overpropped. Is the Yanmar rated similar to the
MD2B- 25HP at 2500 RPM?
I'll continue happily experimenting...
JM

Moores family wrote:
Jere Lull wrote:

In article ,
Moores family wrote:


I've just bought an H28 with an MD2B auxiliary. I haven't had a
chance to experiment yet but I'm looking for a starting point. I
know there's a bunch of people here with MD2B's in similar size
boats, what do you cruise at and what sort of consumption do you get
at cruise? Preliminary indication seems to be that 1600RPM in smooth
water is getting close to hull speed, so there seems to be a heap in
reserve. Interested in comments.



You may or may not have reserve. If you firewall the throttle and can
only pull 17-1800, you're probably overpropped and have little actual
reserve.

We don't have the MD2B, but the Yanmar 2GM20. We consume anywhere from
0.25 gph at about 5 knots and 2200 RPM to a bit over 1 GPH at 3200 and
hull speed of 6.6 knots. We can go faster (up to 7.2 at 3500), but not
long enough to get a good reading of the consumption. We tend to
cruise at about 1/3rd GPH at 5.5 +/-.


Thanks, Jere
I'm using less than 1/2 throttle to get to 'round 1500, so indications
are she's probably not overpropped. Is the Yanmar rated similar to the
MD2B- 25HP at 2500 RPM?
I'll continue happily experimenting...
JM

No - IIRC, the MD2B is rated at 25 HP at 2500 RPM. The Yanmar 2GM20
is about 18 HP at 3600 RPM. Very different specs. My Yanmars will be
most efficient at around 2800, and any slower than that and the
exhaust header will tend to carbon up.

You made a comment that your boat displaces 6.5 tons, but when I
looked up the H28 specs, it shows 3300 Kg, or 7200 pounds. At the
lighter weight, 25 hp may be somewhat overpowered. Not necessarily a
bad thing, though it does mean that the engine will not be fully loaded.

Pushing a boat near hull speed requires roughly 1 HP per 500 pounds of
displacement, so if you can get to hull speed at only 1600 RPM the
weight is probably towards the lower number.

The "rule of thumb" for fuel economy of a diesel 0.055 Gallons per HP
per hour. For gas engines, use 0.1. Assuming 16 HP at 1600 RPM, that
works out to 0.88 Gal per hour. It may actually be worse because the
engine is not running in the most efficient range.

While this seems like a lot for a small boat, remember you can double
your fuel economy by backing off from hull speed to about 80%.

At one time, OMC did have a gas saildrive.
Then Volvo started to have diesel saildrives.
I was not too amenable to the saildrive concept and I am still a little
apprehensive about the idea.
However, today the concept has been used on the Queen Mary ll using electric
motors.
The Finnish and Sweetish sailboat manufacturers are powering their boats
with diesel saildrives. I wonder if this is the drive of the future or will
the jet drive take over?

"Jeff" wrote in message
...
Moores family wrote:
Jere Lull wrote:

In article ,
Moores family wrote:


I've just bought an H28 with an MD2B auxiliary. I haven't had a
chance to experiment yet but I'm looking for a starting point. I
know there's a bunch of people here with MD2B's in similar size
boats, what do you cruise at and what sort of consumption do you get
at cruise? Preliminary indication seems to be that 1600RPM in smooth
water is getting close to hull speed, so there seems to be a heap in
reserve. Interested in comments.



You may or may not have reserve. If you firewall the throttle and can
only pull 17-1800, you're probably overpropped and have little actual
reserve.

We don't have the MD2B, but the Yanmar 2GM20. We consume anywhere from
0.25 gph at about 5 knots and 2200 RPM to a bit over 1 GPH at 3200 and
hull speed of 6.6 knots. We can go faster (up to 7.2 at 3500), but not
long enough to get a good reading of the consumption. We tend to
cruise at about 1/3rd GPH at 5.5 +/-.


Thanks, Jere
I'm using less than 1/2 throttle to get to 'round 1500, so indications
are she's probably not overpropped. Is the Yanmar rated similar to the
MD2B- 25HP at 2500 RPM?
I'll continue happily experimenting...
JM

No - IIRC, the MD2B is rated at 25 HP at 2500 RPM. The Yanmar 2GM20
is about 18 HP at 3600 RPM. Very different specs. My Yanmars will be
most efficient at around 2800, and any slower than that and the
exhaust header will tend to carbon up.

You made a comment that your boat displaces 6.5 tons, but when I
looked up the H28 specs, it shows 3300 Kg, or 7200 pounds. At the
lighter weight, 25 hp may be somewhat overpowered. Not necessarily a
bad thing, though it does mean that the engine will not be fully loaded.

Pushing a boat near hull speed requires roughly 1 HP per 500 pounds of
displacement, so if you can get to hull speed at only 1600 RPM the
weight is probably towards the lower number.

The "rule of thumb" for fuel economy of a diesel 0.055 Gallons per HP
per hour. For gas engines, use 0.1. Assuming 16 HP at 1600 RPM, that
works out to 0.88 Gal per hour. It may actually be worse because the
engine is not running in the most efficient range.

While this seems like a lot for a small boat, remember you can double
your fuel economy by backing off from hull speed to about 80%.