MD6A optimum revs and Combi

M

marcin_palacz

.
Jan 17, 2005
99
Hi,

I have just installed a tachometer (Tiny-Tach),
sailed a short test,
and finally I know how fast my MD6A and Combi rotate!

To my surprise, it turns out that I never run it faster than about 2300
rpm. On flat water, the boat makes about 5.7 knots at 2250 rpm.
I actually feel much more comfortable running it 200 rpm lower,
at about 2050 rpm, which gives about 4.9 - 5.0 knots.
I think this is my usual engine cruising speed.

Do you think it is normal ?
I seem to remember that MD6A was designed to operate at ~2800 rpm.
I wonder if I should change the revs - Combi propeller pitch
synchronisation. I mean reducing the pitch
at a given throttle setting, i.e. reducing the engine load
(by turning the yellow cam at the combi).Another question:
since ever, at about 1600 rpm (now I know it is 1600 rpm :) ),
I get strange vibrations. Seems that this is the alternator
which periodically vibrates (a series of noisy, visible vibration
once per 2 seconds, or so).
What could be the reason of such vibrations and how
can I get rid of it ?Cheers,

Marcin
V1958 "Lotta"
 
M

marcin_palacz

.
Jan 17, 2005
99
Hi,

I have just installed a tachometer (Tiny-Tach),
sailed a short test,
and finally I know how fast my MD6A and Combi rotate!

To my surprise, it turns out that I never run it faster than about 2300
rpm. On flat water, the boat makes about 5.7 knots at 2250 rpm.
I actually feel much more comfortable running it 200 rpm lower,
at about 2050 rpm, which gives about 4.9 - 5.0 knots.
I think this is my usual engine cruising speed.

Do you think it is normal ?
I seem to remember that MD6A was designed to operate at ~2800 rpm.
I wonder if I should change the revs - Combi propeller pitch
synchronisation. I mean reducing the pitch
at a given throttle setting, i.e. reducing the engine load
(by turning the yellow cam at the combi).Another question:
since ever, at about 1600 rpm (now I know it is 1600 rpm :) ),
I get strange vibrations. Seems that this is the alternator
which periodically vibrates (a series of noisy, visible vibration
once per 2 seconds, or so).
What could be the reason of such vibrations and how
can I get rid of it ?Cheers,

Marcin
V1958 "Lotta"
 
M

mnhdl

.
Oct 31, 2019
83
Hi Marcin,

My question is about thé installation of the tiny-tach. Was it a simple job to do? Do you have any picture?

Thank you?

Marie

Envoyé de mon iPad

Le 2012-07-25 à 09:47, "Marcin Palacz" <palacz@...> a écrit :
 
N

n4lbl

.
Oct 7, 2008
307
If the vibration is associated with the alternator my first thought would
be to adjust the belt tension. If the vibration changes or goes away then
you know that you are at the right place. That said, I'd guess that the
vibration is from somewhere else since one half hertz (one vibration every
two seconds) seems an exceedingly low frequency for something turning so
fast.

Alan
 
N

n4lbl

.
Oct 7, 2008
307
Seeing the vibration in the alternator seems compelling. With a resonance
issue it can be difficult to separate causes and effects. If changing the
belt tension doesn't clarify the issue then I'd continue with more
guesses/questions: - Does it happen in neutral or in gear?
- If in gear, does forward or reverse matter?
- Do you think you can safely run the engine with the cockpit sole
removed? Perhaps there is something to be observed there. I'm grasping
here.
 
N

n4lbl

.
Oct 7, 2008
307
Forgot. I also didn't realize that the Combi doesn't have a neutral and
will always spin with the engine on. I hope changing the belt tension will
make the solution clear.
 
D

dougpol1

.
Feb 13, 2010
528
You are up against a tough thing. For example a wheel on a car is out
of a balance. The wheel is out of balance at all speeds and it is
hopping up and down but you only feel it at certain speeds.. It may be
in resonance with the car body for example 40 mph again at 60 and again
at 80. Those are the only speeds where the car shakes. Your boat may
be in resonance at some particular engine Rpm. Most machines have this
if there is any moving parts in them that are out of balance. My
solution would be not to run at the resonant speed just go a little
faster or slower.If You can't find anything that is shaking you can take
the engine out an have it balanced. I have rebuilt some engines for
performance and you must have them balanced for high
speed. Doug
 
M

mnhdl

.
Oct 31, 2019
83
Thank you Marcin, any picture would be helpfull for me ;-)

Envoyé de mon iPad

Le 2012-07-25 à 17:04, "Marcin Palacz" <palacz@...> a écrit :
 
J

johnadenney

.
Oct 30, 2011
221
Marcin, sounds just like mine. I have (yesterday) adjusted the ratio
between prop pitch and throttle and I am now getting the sort of
speeds you mention. Before it was "over geared" ie when full throttle
was selected the engine slowed down and I was pumping out lots of
black smoke. It now runs much smoother, quieter and seems to use less
fuel although I go about 1kt slower. It still puffed out a cloud of
soot yesterday when -, after idleing for a bit, I selected full
throttle. Maybe it is a bit coked up due to being poorly set up. I
adjusted the pitch by lengthening the control rod - I did not touch
the throttle cam. Is this correct?

I still have the leak from the back of the combi but I am trying to
fix that today - just a temporary fix with gasket goo - or similar.
Hopefully it will slow the leak down a bit.
 
M

mnhdl

.
Oct 31, 2019
83
Thank you Marcin!

Envoyé de mon iPad

Le 2012-07-30 à 05:12, "Marcin Palacz" <palacz@...> a écrit :
 
M

mnhdl

.
Oct 31, 2019
83
Hi!

i suppose that you bought the 6mm transducer?

Thank you!

Marie

Envoyé de mon iPad

Le 2012-07-30 à 05:12, "Marcin Palacz" <palacz@...> a écrit :
 
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