Yanmar 3gm30 & prop selection

[Lee Sandow]

I'd like to hear any experience you all may have had repowering your
sailboats with the Yanmar 3gm30.

Specifically which prop you chose and why, or did anyone use their old shaft
and prop and was performance adversely effected and so on.

We're going from an Atomic 4 to the 3gm30 and wondering if we can use the
same prop.

Thanks.

[Bryon Kass]

From a prof point of view I have a few questions. Does the AT4 have a
reduction gear? If it does what is the rotation of the prop. The new
Yanmars can use either rotation in most instances. Check with the dealer
for specifics on the transmissions capability. With the standard
reduction
of 2.62:1 for Yanmar if your AT4 has a 2.5 or 3:1 the prop can be
tested.
If the AT4 was direct drive it cannot. With a 1:1 the AT4 would only
drive
an approx 10x8x3 . With a reduction it would be approx 16x10x3. This
prop
is about correct for the 3GM30 under most conditions. The shafting
should
be min 1" dia. SS is best but bronze should be OK if in good condition.
You will have to change the shaft coupling.
Bryon Kass
webmaster and
Custom Design
150 Mechanic St.
Foxboro, MA 02035
508-543-9068 or fax 508-543-5127, Foot yard 508-384-2415
in THE ENGINE ROOM http://home.ici.net/~cusdn

[macnaughton.com]

Whenever you change engines or transmission ratios you should have new prop
calculations done. Just go to your favorite naval architect. It is not very
expensive.

Tom MacNaughton
Naval Architect
http://www.macnaughtongroup.com

Lee Sandow wrote in message ...

[Dave]

I had a C&C27 with a yanmar 2GM and noted the differences
between my boat and identical boats with the atomic 4:

The atomic 4 was a 1:1 reduction (no reduction) and the yanmar
was a 2.5 reduction. Combine this with the fact that the gas engine
revs up to 5000 rpm, and the diesel is happier around 2500 rpm, and
what you get is the basic principle that the atomic four prop turns
about 4 times as fast, so is ussually quite small. This is great on
a race boat, lousy if you want power.

the atomic4 boats typically had a small martec prop, something like
11x8. My yanmar had a 13x13 prop... a real monster in comparison, since
it would turn much slower.

Dave

[Wayne Lilley]

In article <753ics$qaj$1...@knot.queensu.ca>, da...@biomed.queensu.ca (Dave) wrote:

> I had a C&C27 with a yanmar 2GM and noted the differences
> between my boat and identical boats with the atomic 4:
>
> The atomic 4 was a 1:1 reduction (no reduction) and the yanmar
> was a 2.5 reduction. Combine this with the fact that the gas engine
> revs up to 5000 rpm, and the diesel is happier around 2500 rpm, and
> what you get is the basic principle that the atomic four prop turns
> about 4 times as fast, so is ussually quite small. This is great on
> a race boat, lousy if you want power.
>
> the atomic4 boats typically had a small martec prop, something like
> 11x8. My yanmar had a 13x13 prop... a real monster in comparison, since
> it would turn much slower.
>
> Dave
>

Big though the 13x13 prop may have been on the C&C 27 with the Yanmar 2GM
(with a 2:62 to 1 reduction BTW) it was the wrong prop. The boat lacked
any punch at all into a headwind, responds poorly in reverse (and forward)
at almost all engine speeds. It is also too large, placing the tips too
close to the hull which also hurts performance under power.
Hoping for improved performance under power and sail, the prop is being
changed for a foolding Flexofold (similar to Goris) with larger blade
surface and less pitch. The hub of the folder moves the effective blade
area further aft without interfering with the rudder, thus increasing the
tip-to-hull distance.
Will report on effectiveness of the change when launched in April, but it
cannot be worse than the 13x13 which was godawful.