On Sun, 30 Nov 2003 16:09:33 +0000, bowgus wrote:

Well that's the thing ... I did the calculation then, using formulae scrounged
from sources I no longer remember, so ... the answer I got then was that my
electric's 35 lbs thrust was about equivalent to a 1/4 hp outboard. But I see
from "user" that a 36 lbs thrust electric is equal to about a 2.5 hp (gasoline)
outboard ... which is A LOT MORE than I calculated. And the reason I originally
did the calculations was to decide the purchase of an electric say 60 lbs or a 4
cycle outboard say 3 hp.

"Harry Krause" wrote in message
...
bowgus wrote:

Question ... when I did my 36 lb Min Kota calculation afew years back,
the
answer I got was about 1/4 hp. Would you mind sharing your calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs Minn Kota
Endura motor, about the same thrust as a 2.0 or 2.5 hp gas engine... OK, it
doesn't plane. I'm not so sure it would plane with the maximum 3.5 hp
motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine. any suggestions
would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle,
you
don't have to mix oil in the gas can, and their exhausts smell
better. I
use a no-cycle outboard on my inflatable: an electric trolling motor.




Wouldn't there be a way to compare pounds of thrust? My motorguide trolling
motor produces-oh, I really don't remember-something like 65 pounds of thrust,
maybe. How many pounds of thrust might a 3 hp gas outboard produce at the
prop? A similar number? Larger?

The electric trolling motor is enough to push our rubber ducky around the
harbor for a couple of hours, but if it craps out, no worry. We have oars.

--
Email sent to is never read.

I too use a Minn Kota to push Near Cove around, and I too am confused about
power.

Theory is pretty easy: Power can be measured in HP or Watts (746 Watts = 1 HP).
And wattage can easily be calculated: Max current for a 40-lb Minn Kota
(according to their website) is about 40 Amps. 40A at 12V is 480W, or 0.6 Hp.
That's "going in" so allowing for inefficiencies, say 1/2 Hp. BUT, even the
30-lb'er I use pushes Near Cove like a 1 - 2 hp gas.

A couple of considerations: gas engines are a lot less efficient than electric
motors. Also, there's a tendancy to run gas engines at less than full throttle,
both for quietness and longevity, whereas there's no reason not to run an
electric "full throttle" except for how much it drains the battery.

Bottom line IMHO: an electric can push a dinghy just as well as a gas as long as
you're not interested in planing, and it does it quieter and less smelly, don't
need to store gas, etc.

Lloyd Sumpter
"Near Cove" Walker Bay 8 c/w Endura 30
"Tin Boat" Mirrocraft 12 c/w Maximizer 40

"Can push as well as gas" ... but for a much much much shorter length of
time ... here's one for you ... 1 hp, the amount of energy it takes to drag
a dead horse 100 yds :-)


"Lloyd Sumpter" wrote in message
...
On Sun, 30 Nov 2003 16:09:33 +0000, bowgus wrote:

Well that's the thing ... I did the calculation then, using formulae
scrounged
from sources I no longer remember, so ... the answer I got then was that
my
electric's 35 lbs thrust was about equivalent to a 1/4 hp outboard. But
I see
from "user" that a 36 lbs thrust electric is equal to about a 2.5 hp
(gasoline)
outboard ... which is A LOT MORE than I calculated. And the reason I
originally
did the calculations was to decide the purchase of an electric say 60
lbs or a 4
cycle outboard say 3 hp.

"Harry Krause" wrote in message
...
bowgus wrote:

Question ... when I did my 36 lb Min Kota calculation afew years
back,
the
answer I got was about 1/4 hp. Would you mind sharing your
calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs
Minn Kota
Endura motor, about the same thrust as a 2.0 or 2.5 hp gas engine...
OK, it
doesn't plane. I'm not so sure it would plane with the maximum 3.5
hp
motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine. any
suggestions
would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle,
you
don't have to mix oil in the gas can, and their exhausts smell
better. I
use a no-cycle outboard on my inflatable: an electric trolling
motor.




Wouldn't there be a way to compare pounds of thrust? My motorguide
trolling
motor produces-oh, I really don't remember-something like 65 pounds of
thrust,
maybe. How many pounds of thrust might a 3 hp gas outboard produce at
the
prop? A similar number? Larger?

The electric trolling motor is enough to push our rubber ducky around
the
harbor for a couple of hours, but if it craps out, no worry. We have
oars.

--
Email sent to is never read.

I too use a Minn Kota to push Near Cove around, and I too am confused
about
power.

Theory is pretty easy: Power can be measured in HP or Watts (746 Watts = 1
HP).
And wattage can easily be calculated: Max current for a 40-lb Minn Kota
(according to their website) is about 40 Amps. 40A at 12V is 480W, or 0.6
Hp.
That's "going in" so allowing for inefficiencies, say 1/2 Hp. BUT, even
the
30-lb'er I use pushes Near Cove like a 1 - 2 hp gas.

A couple of considerations: gas engines are a lot less efficient than
electric
motors. Also, there's a tendancy to run gas engines at less than full
throttle,
both for quietness and longevity, whereas there's no reason not to run an
electric "full throttle" except for how much it drains the battery.

Bottom line IMHO: an electric can push a dinghy just as well as a gas as
long as
you're not interested in planing, and it does it quieter and less smelly,
don't
need to store gas, etc.

Lloyd Sumpter
"Near Cove" Walker Bay 8 c/w Endura 30
"Tin Boat" Mirrocraft 12 c/w Maximizer 40

You can not calculate HP from thrust alone. Horsepower is proportional to
force (thrust) times speed. One HP = 550 ft-lb/sec. For example, if an
electric motor can push a boat with a thrust of 35 lb. at a speed of 5 mph
(7.4 fps) the horse power delivered to the boat is:

35 x 7.4/550 = 0.47 HP

Actual motor power would be higher since there are losses from friction,
prop slip, etc.

Unfortunately, I suspect the manufacturers rate the thrust at zero speed
(highest thrust value) so you can't calculate motor HP.

"bowgus" wrote in message
le.rogers.com...
Question ... when I did my 36 lb Min Kota calculation afew years back,
the
answer I got was about 1/4 hp. Would you mind sharing your calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs Minn
Kota Endura motor, about the same thrust as a 2.0 or 2.5 hp gas
engine... OK, it doesn't plane. I'm not so sure it would plane with the
maximum 3.5 hp motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle, you
don't have to mix oil in the gas can, and their exhausts smell better.
I
use a no-cycle outboard on my inflatable: an electric trolling motor.

I like Mr. Ohms method (used by Loyd) ... convert watts to hp, e.g. ... the
35 lbs thrust Minn Kota is rated 34 Amps max so assuming 12V across the
motor = 408 watts (P=IxE). And converting that to hp by x 0.00134102209 :-)
= 0.55 hp. The calculation for my 55 lb Minn Kota is: 45 Amps x 12V = 540 =
0.72 hp. So good old ohms law calculations say 0.72 hp is the absolute max
(as Loyd pointed out, there's gotta be some mechanical loss in there
somewhere) ... let's call that good old "K".

So hp = f (((I*E)*1.34E-3) - K) :-)

Thanks Loyd, that works for me.


"Bill Sc" wrote in message
om...
You can not calculate HP from thrust alone. Horsepower is proportional to
force (thrust) times speed. One HP = 550 ft-lb/sec. For example, if an
electric motor can push a boat with a thrust of 35 lb. at a speed of 5 mph
(7.4 fps) the horse power delivered to the boat is:

35 x 7.4/550 = 0.47 HP

Actual motor power would be higher since there are losses from friction,
prop slip, etc.

Unfortunately, I suspect the manufacturers rate the thrust at zero speed
(highest thrust value) so you can't calculate motor HP.

"bowgus" wrote in message
le.rogers.com...
Question ... when I did my 36 lb Min Kota calculation afew years back,
the
answer I got was about 1/4 hp. Would you mind sharing your calculation
(I've
since upgraded to a 55 lb thrust)? Thanks


"user" wrote in message
...
I have a 9'2" Sevylor XR-86 inflatable. It is powered by a 36 lbs Minn
Kota Endura motor, about the same thrust as a 2.0 or 2.5 hp gas
engine... OK, it doesn't plane. I'm not so sure it would plane with
the
maximum 3.5 hp motor. I'm not so sure I'd want to do that anyhow.

Harry Krause wrote:
jake waldman wrote:


i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.
thanks
jake


It's your choice. The small two cycle and four cycle outboards are
each
reliable, and don't burn a whole lot of fuel. With the four cycle,
you
don't have to mix oil in the gas can, and their exhausts smell
better.
I
use a no-cycle outboard on my inflatable: an electric trolling
motor.

http://www.shipstore.com/ss/HTML/MIN/MIN1352363.html
I took their word for it...
May be right, may be wrong. Bought the inflatable and motor from these
people. Have been to their marina for fuel and lunch. They are good
competent people... but may very well be way off.

bowgus wrote:
Question ... when I did my 36 lb Min Kota calculation afew years back, the
answer I got was about 1/4 hp. Would you mind sharing your calculation (I've
since upgraded to a 55 lb thrust)? Thanks

"jake waldman" wrote in message
...
i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.


To provide any decent suggestions we would need to know more about how you
intend to use it and stow it.

If you have visions of zipping around then your 3Hp is unrealistic.
On the other hand, if you need to be able to deflate the boat and stow it in
a small locker then you are probably right on.

For the ultimate in stowability you will want the "dingy" style boat, which
has tube on all four sides and no hard transom. While these boats offer a
lot of floatation in a small space, they are hard to row and will never
plane. They often come with a motor mount, either built in or as an
attachment, and are generally suitable for only the smallest of motors. The
floor is generally just fabric, often with no floorboards at all, which mean
you have to be careful when setting heavy/sharp objects (such as anchors) in
the boat.

A "sportboat" style inflatable has a hard transom on the back which will
accept a higher horsepower motor. They often have some sort of floorboards,
either inflatable, wood or plastic. The boats will generally provide some
sort of keel which improves the rowing tremendously. The hard transom keeps
them from folding up into a small space, plus you have the floorboards to
deal with. Assembling the boat, especially dealing with the floorboards, is
often a hassle.

For either style, the size and material of the tubes can make a big
difference. Bigger tubes will provide a dryer ride, especially for the
sportboats. It will also make the inflatable harder to get back into from
the water. A heavier material will better resist punctures and tears but
will make the boat heavier and take more room to stow.

As for the motor:

Many of the small gas motors have a built in gas tank, which can be both a
blessing and a curse. It is nice on a small dinghy because then the gas
tank isn't taking up room in the boat. On the other hand, the tank doesn't
hold much gas, so you may have to take an extra can for a long outing. When
you are done using the boat and need to pack it away, the external tank can
generally just have the vent closed and put away. The built in tank will be
prone to leaking unless you can stow the motor standing upright, or have a
way to empty the tank.

As others have said, the electric motor might be a good option. The
advantage is that they are quiet and generally trouble free. The down side
is that the battery is heavy, and when it goes dead it will take a while to
charge it back up. If you are a sailing cruiser and have limited battery
charging capability then electric probably isn't the best approach. If you
are going to use it for an hour or two a day and can recharge at night then
it will do nicely. I highly recommend getting a gel or AGM battery rather
than a lead-acid.

Give us some more details and we can tell you more.

Rod McInnis

In article ,
"Rod McInnis" wrote:

"jake waldman" wrote in message
...
i'd like to get a 8'-9' inflatable with a 3hp engine.
any suggestions would help.
do i go 2 stroke or 4.
it will be used for light duty.


To provide any decent suggestions we would need to know more about how you
intend to use it and stow it.

If you have visions of zipping around then your 3Hp is unrealistic.
On the other hand, if you need to be able to deflate the boat and stow it in
a small locker then you are probably right on.

For the ultimate in stowability you will want the "dingy" style boat, which
has tube on all four sides and no hard transom. While these boats offer a
lot of floatation in a small space, they are hard to row and will never
plane. They often come with a motor mount, either built in or as an
attachment, and are generally suitable for only the smallest of motors. The
floor is generally just fabric, often with no floorboards at all, which mean
you have to be careful when setting heavy/sharp objects (such as anchors) in
the boat.

A "sportboat" style inflatable has a hard transom on the back which will
accept a higher horsepower motor. They often have some sort of floorboards,
either inflatable, wood or plastic. The boats will generally provide some
sort of keel which improves the rowing tremendously. The hard transom keeps
them from folding up into a small space, plus you have the floorboards to
deal with. Assembling the boat, especially dealing with the floorboards, is
often a hassle.

For either style, the size and material of the tubes can make a big
difference. Bigger tubes will provide a dryer ride, especially for the
sportboats. It will also make the inflatable harder to get back into from
the water. A heavier material will better resist punctures and tears but
will make the boat heavier and take more room to stow.

As for the motor:

Many of the small gas motors have a built in gas tank, which can be both a
blessing and a curse. It is nice on a small dinghy because then the gas
tank isn't taking up room in the boat. On the other hand, the tank doesn't
hold much gas, so you may have to take an extra can for a long outing. When
you are done using the boat and need to pack it away, the external tank can
generally just have the vent closed and put away. The built in tank will be
prone to leaking unless you can stow the motor standing upright, or have a
way to empty the tank.

As others have said, the electric motor might be a good option. The
advantage is that they are quiet and generally trouble free. The down side
is that the battery is heavy, and when it goes dead it will take a while to
charge it back up. If you are a sailing cruiser and have limited battery
charging capability then electric probably isn't the best approach. If you
are going to use it for an hour or two a day and can recharge at night then
it will do nicely. I highly recommend getting a gel or AGM battery rather
than a lead-acid.

Give us some more details and we can tell you more.

Rod McInnis




thank you all for your input.
i'm looking at the small west marine ru260(?).
i intend to just use it to putt around the achorage, speed is not an
issue.
the engine would be in the 3hp class. if it has an internal gas tank can
it be stored lying down? can a 4 cycle be stored lying down?
what about the differance between pvc and hypalond(?)

thank you all again.
jake