wrote in message
...
On Fri, 04 Jan 2008 20:14:09 -0800, Gordon wrote:

Any recommendations?
Gordon

Think about it this way. If the generator is 100% efficient, then to
generate 746 watts, one horsepower must be provided. Imagine a dinghy
with a one hp engine pulling your boat backwards and you get a sense
of practicality. In practice, two or three hp is probably closer to
what the effect might be.

So there is plainly an upper limit to the amount of power a towed
generator can realistically provide to a small sailboat.

Small sailboat? So, probably a 12 volt battery..
746 watts is 62 amps at 12 volts-far too much for your battery.
Certainly there is an upper limit to the power you can get, but there should
be no problem in getting quite enough to power up anything you have aboard a
small sailboat.

Gordon wrote in news:13nu10ihddb3o01
@corp.supernews.com:

Any recommendations?
Gordon


http://www.brownmarine.com/tg09.htm

Larry
--
Everybody sitting in the dark at the anchorage will be your friend!

"Edgar" wrote in
:

Small sailboat? So, probably a 12 volt battery..
746 watts is 62 amps at 12 volts-far too much for your battery.
Certainly there is an upper limit to the power you can get, but there
should be no problem in getting quite enough to power up anything you
have aboard a small sailboat.





Lionheart, my friend Geoffrey's Amel Sharki 41 has a Perkins 4-108 with
freewheeling hydraulic transmission and a shaft alternator that produces
about 25A of 12V power at 7 knots, about as fast as she'll go. It still
produces 15A at 6 knots, but drops off rapidly below 5 knots as the load
stalls it. There's a large wheel on the shaft about 6" behind the
transmission turning about a 3" wheel on the specially-designed-to-turn-
slowly alternator, which I think Motorola made. It's not self-exciting
and requires 12V, switched by a key in the control panel where the
ammeter shows you how we're doing. This key is on a welded ring with the
engine key and locks itself into the panel until you turn it off. This
prevents even the stupidest amoung us from running the shaft alternator
when the engine is running, which the instructions say will destroy it.

The key to all this is that transmission that allows the shaft to
freewheel without destroying the transmission.

Larry
--
Next time some broker tells you what a great investment he's selling,
ask him about Rhodium, a shiny metal used in Catalytic Converters.
Jan 1st 2004 2005 2006 2007 2008
Rhodium $452 $1341 $3006 $5339 $6775 PER OUNCE!
How much longer can we pay for new cars at this rate?
Feb '97 it was $182/oz

Gordon,
I have built into my 60' sloop a prop shaft with pressure fed oil bearings
and it drives a Sperry Marine rig that was designed to power winches on
lobster boats. The Sperry Vickers variable volume hydraulic pump drives a
fixed displacement hydraulic motor on the end of an Onen 6 KW 2 pole
alternator. The pump will handle input speeds of 1100 to 4400 rpm. The unit
adjusts the pump volume automatically while monitoring alternator output
frequency and will hold the frequency within 3 Hz. I use a toothed belt to
drive a gearbox and from the gearbox I drive the Vickers pump. At the most
inefficient speed the unit draws 22 HP at 6 KW. The plan is to use the
trailed prop to drive this rig. The 3 bladed prop I am using is 70 CM in
diameter with a 26" pitch and a 70% BAR. It has been sized to drive the
boat. I cannot tell you if it works, I have not tried it yet. When designing
this, I could find no data on the efficiency of props in drag mode. To my
knowledge, no study has even been performed and I asked everywhere I could
think of. I expect when in use the boat will slow somewhat, but how much
only time will tell. The unit will antomatically drop the load, if speed
drops below 1100 rpm. I am sure I will have to do some tuning, but I believe
it will work or I would not have spent all the time and money to build it.
Steve


"Gordon" wrote in message
...
Any recommendations?
Gordon

Edgar wrote:

wrote in message
...

On Fri, 04 Jan 2008 20:14:09 -0800, Gordon wrote:


Any recommendations?
Gordon

Think about it this way. If the generator is 100% efficient, then to
generate 746 watts, one horsepower must be provided. Imagine a dinghy
with a one hp engine pulling your boat backwards and you get a sense
of practicality. In practice, two or three hp is probably closer to
what the effect might be.

So there is plainly an upper limit to the amount of power a towed
generator can realistically provide to a small sailboat.


Small sailboat? So, probably a 12 volt battery..
746 watts is 62 amps at 12 volts-far too much for your battery.
Certainly there is an upper limit to the power you can get, but there should
be no problem in getting quite enough to power up anything you have aboard a
small sailboat.




Maybe once upon a time?

But these days....

...Small sailboat? So, probably a 12 volt battery..
746 watts is 62 amps at 12 volts-far too much for your battery...

Maybe once upon a time?

But these days....

The folks I've talked to with towed generators love them and claim
they get about an amp per knot and it takes about half a knot off
their boat speed... I suspect they are being generous. Nevertheless,
if you need 60 amps all the time you really need a genset. Even with
a 12 volt fridge and autopilot 5-6 amps should pretty much keep up
with demand.

-- Tom.