I suspect a water jet is not as efficient as a propellor.
Since you are evidently planning on 12v on your cat, why don't you get a
small 12v trolling motor ? That would take dumkings better than a gas motor,
although you would need to rinse it off.

"Shaun Van Poecke" wrote in message
...
ok, Ive heard this idea brought up before and seen it poo poo'd, but can
it work to some extent?

Ive got a 14' cat that i store next to the beach in sydney harbour and i
like to get out for a sail when i can. Unlike most cats this size, it has
heaps of flotation - 3-4 adults and still not sinking anywhere near as
much as a hobie 14 with 2 POB. It has an outboard bracket on the back
which was on it from the factory, but i've destroyed 2 admittedly cheap
and nasty second hand outboards on it so far. I have a new honda 2hp 4
stroke on my dinghy that i dont want to put on the cat because it would
probably suffer a similar fate.

What happens is that because your moving so fast, the outboard gets
completely drowned, as do the people sailing for that matter ;-) an
outboard might work on a lake or something, but sydney harbour gets
choppy. The thing is, this boat was designed for island hopping the
whitsundays and the manufacturer claims enough flotation for 4 people,
plus camping gear, a stove, and food and water for a week! I cant see how
they get the outboard to survive...

i do ok if i get the outboard started and there are no waves and i just
motor round on it. a 2 or 3 is plenty to move along. its rated for a 5
at maximum. the trouble is, as soon as you sail if there's a bit of chop,
that motor is getting a drink - then you'll never get it started. I want
a bit of propulsion for getting into some of the finnicky bay areas rather
than spending a couple of hours tacking to death, 5 minutes of propulsion
would get me there. propulsion would also be nice for getting back in if
the wind dies, and finally as a backup just in case something breaks. i
like to go out when the wind is big, and ive already had a couple of
incidents with snapping a gooseneck fitting, and destoying a couple of jib
cleats.

so here's what im thinking; a 1250GPH bilge pump in each hull, intake
hoses going into the water, outlets hooked up to hoses facing rear. these
draw 5A each and are cheap to buy. A 40-50AH sealed gel cell would give
me a lot more power than i need. I cant see myself using ot for longer
than about 30 minutes to an hour at a time. Is this going to move me at
all? at least more than trying to paddle the boat by myself (which gets
me almost nowhere!) would i get any benefit from using smaller diameter
hoses to create a stronget 'jet', or am i just unnecessarily loading the
pump for no real gain? am i miles off in the GPH stakes.... do i need a
lot more pumping to get any effect?

im not looking for a speed boat here, just *movement* even 1 knot would
be something. benefits to this system are that its fully submersible,
completely unobtrusive, almost silent, would cost me about $100, starts
instantly when i need it.

thanks,
Shaun

"Garland Gray II" wrote in message
...
I suspect a water jet is not as efficient as a propellor.
Since you are evidently planning on 12v on your cat, why don't you get a
small 12v trolling motor ? That would take dumkings better than a gas
motor, although you would need to rinse it off.

the voltage issue is pretty redundant for me as the propulsion will be the
only thing powered, and i will be recharging at home, so it could be 12, 24
or 48. the trolling motor was something i originally considered, but i
heard a lot of bad reports about them, and they seem expensive for what they
are; the salt water ones even more so. There are no fresh water supplies
when my boat is, and its stored at the beach, so washing down is pretty
unlikely.

The only jet boats ive been in are the ones in new zealand, aluminum hulled
driven by either a single or a twin v8 engine and they sure do seem to suck
the gas down, but they have performance to match. I thought at the time
that they were very fuel inneficient, but compared to a similar powered
sterndrive, maybe not.

the 12v trolling motors have come down a lot in price, but they still seem
expensive to me (in australia at least) for what they are.

Shaun

Shaun Van Poecke wrote:
"Garland Gray II" wrote in message
...
I suspect a water jet is not as efficient as a propellor.
Since you are evidently planning on 12v on your cat, why don't you get a
small 12v trolling motor ? That would take dumkings better than a gas
motor, although you would need to rinse it off.

the voltage issue is pretty redundant for me as the propulsion will be the
only thing powered, and i will be recharging at home, so it could be 12, 24
or 48. the trolling motor was something i originally considered, but i
heard a lot of bad reports about them, and they seem expensive for what they
are; the salt water ones even more so. There are no fresh water supplies
when my boat is, and its stored at the beach, so washing down is pretty
unlikely.

The only jet boats ive been in are the ones in new zealand, aluminum hulled
driven by either a single or a twin v8 engine and they sure do seem to suck
the gas down, but they have performance to match. I thought at the time
that they were very fuel inneficient, but compared to a similar powered
sterndrive, maybe not.

the 12v trolling motors have come down a lot in price, but they still seem
expensive to me (in australia at least) for what they are.

Shaun


The Minnkota series electric trolling motors can be used successfully in
salt water. I do it with both a 30 lb and 50 lb model. Paint the lower
unit with resin to prevent the paint flaking off. The only thing in the
upper unit is a switch. If you are worried about the switch getting
soaked, simply remove it and connect directly to the battery, or else
move the switch elsewhere.

BS

Shaun Van Poecke wrote:
"Garland Gray II" wrote in message
...
I suspect a water jet is not as efficient as a propellor.
Since you are evidently planning on 12v on your cat, why don't you get a
small 12v trolling motor ? That would take dumkings better than a gas
motor, although you would need to rinse it off.

the voltage issue is pretty redundant for me as the propulsion will be the
only thing powered, and i will be recharging at home, so it could be 12, 24
or 48. the trolling motor was something i originally considered, but i
heard a lot of bad reports about them, and they seem expensive for what they
are; the salt water ones even more so. There are no fresh water supplies
when my boat is, and its stored at the beach, so washing down is pretty
unlikely.

The only jet boats ive been in are the ones in new zealand, aluminum hulled
driven by either a single or a twin v8 engine and they sure do seem to suck
the gas down, but they have performance to match. I thought at the time
that they were very fuel inneficient, but compared to a similar powered
sterndrive, maybe not.

the 12v trolling motors have come down a lot in price, but they still seem
expensive to me (in australia at least) for what they are.

Shaun


Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS

i havent priced a 30lb, but off the top of my head, a no name made in china
13lb trolling motor is about AU$200 while a 50lb is about AU$500. add in
battery, wiring etc and i could buy a brand new petrol outboard for those
prices. I think the electric outboards are still really expensive in
australia for what they are. they will probably come down in price in a
year or two but for now i'd feel a bit cheated paying for one.

Shaun


Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS

Shaun Van Poecke wrote:
i havent priced a 30lb, but off the top of my head, a no name made in china
13lb trolling motor is about AU$200 while a 50lb is about AU$500. add in
battery, wiring etc and i could buy a brand new petrol outboard for those
prices. I think the electric outboards are still really expensive in
australia for what they are. they will probably come down in price in a
year or two but for now i'd feel a bit cheated paying for one.

Shaun

Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS


That does seem rather high. My 30 lb was $99 and the 50 Lb $130. But
they work really well, appear to be well made, and are very reliable.

BS

Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

I think this general theme (Using inboard pumps for propulsion) is
worth pursuing. One interesting aspect is being able to use the same
pump(s) for forward propulsion and bow-thruster/steering.

Can we start with a decent idea of the efficiency of the trolling
motors?

How much current do some typical units draw (all 12Volts?) ??

Are they rated only in static thrust? Or also Horsepower? Horsepower
can be converted to Force VS Distance VS Time. (1.0 Horsepower == 550
Foot-Pounds per second, right??) HighSchool Physics was, um, 50 years
ago :-) Yes, I just see "1 horsepower [electric] = 550.221 382 975
foot pound-force/second" at
http://www.onlineconversion.com/power.htm

So, IF you knew the relationship of Drag (In Pounds) VS Speed for YOUR
boat, you could create a graph of Horsepower VS Speed. (This would
be for "Perfect Horsepower" which certainly will not happen with real-
word trolling motors and propellers, OR real-world pumps and
hoses.. )

But you'd have SOME idea...

If you had a 25 pound fish scale and 100 feet of line, and someone to
paddle the boat OUT so you could pull it IN, you might start to get
some numbers...

Other Related Idea: I have thought about running a medium-large (??)
pump from my inboard boat engine to bow ports for "Bow Thruster".
Anyone seen something like this?? A pump could be engaged with an Air
Conditioner Clutch....

So let's keep thinking about this????

wrote:
Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

I think this general theme (Using inboard pumps for propulsion) is
worth pursuing. One interesting aspect is being able to use the same
pump(s) for forward propulsion and bow-thruster/steering.

Can we start with a decent idea of the efficiency of the trolling
motors?

How much current do some typical units draw (all 12Volts?) ??

Are they rated only in static thrust? Or also Horsepower? Horsepower
can be converted to Force VS Distance VS Time. (1.0 Horsepower == 550
Foot-Pounds per second, right??) HighSchool Physics was, um, 50 years
ago :-) Yes, I just see "1 horsepower [electric] = 550.221 382 975
foot pound-force/second" at
http://www.onlineconversion.com/power.htm

So, IF you knew the relationship of Drag (In Pounds) VS Speed for YOUR
boat, you could create a graph of Horsepower VS Speed. (This would
be for "Perfect Horsepower" which certainly will not happen with real-
word trolling motors and propellers, OR real-world pumps and
hoses.. )

But you'd have SOME idea...

If you had a 25 pound fish scale and 100 feet of line, and someone to
paddle the boat OUT so you could pull it IN, you might start to get
some numbers...

Other Related Idea: I have thought about running a medium-large (??)
pump from my inboard boat engine to bow ports for "Bow Thruster".
Anyone seen something like this?? A pump could be engaged with an Air
Conditioner Clutch....

So let's keep thinking about this????


Generally speaking a figure of about 50% overall efficiency (prop +
motor) seems to be appropriate for my 30 and 50 lb motors if you believe
the factory-stated thrust and current, based on the speeds I've
measured. Current draw is 30 and 42 amps respectively. There are a few
(very expensive) motors that do better. Motors are available the run on
12, 24, 36 and 48 volts. Generally trolling motors are rated in static
thrust while "electric outboards" are quoted in hp. It is difficult to
directly compare electric hp with gas because the gas motor hp differs
tremendously with rpm. That having been said, volts x amps is still true
input power, and 550 ft-lb/sec is hp regardless of how it is generated.

BS

On Sun, 11 Feb 2007 07:33:28 -0800, R Swarts
wrote:


Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS

It's always the maker's claimed thrust that raises questions in my
mind. They take the thrust reading stationary, as far as I know.

Anyway: if you maintained 30 lb thrust at 3.4 MPH
that would need 203 watts or more
i.e. 17 amps at 12 volts
to provide about 0.27 HP.

Brian Whatcott Altus OK

Brian Whatcott wrote:
On Sun, 11 Feb 2007 07:33:28 -0800, R Swarts
wrote:


Should also have mentioned that my 30 lb Endura pushes a 14' Hobie at
about 3.5 mph in still water.

BS

It's always the maker's claimed thrust that raises questions in my
mind. They take the thrust reading stationary, as far as I know.

Anyway: if you maintained 30 lb thrust at 3.4 MPH
that would need 203 watts or more
i.e. 17 amps at 12 volts
to provide about 0.27 HP.

Brian Whatcott Altus OK

Yup, that's right. And the motor actually consumes about 30 amps or 360
watts. So if we believe the manufacturers specs in this case it gives an
efficiency of about 56%. The 50% number appears to be about right for
most trolling motors I've run across. Of course the motor is more
efficient, but that number includes the prop efficiency.

BS