I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
..28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts

On Wed, 26 Jul 2006 08:43:23 -0700 Bob S
) wrote:
I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

See the extra power needed for that 0.5 mile faster?

I would be pleased to hear from any of you who are also interested in
electric propulsion.

The Queen Mary II has electric propulsion.;-) For now I think battery
propulsed boats will be a niche product, nice for fishing or cruising
protected areas or coming home but not for holidays.

Do the math and find out yourself. My boat has also electric propulsion.
When the day comes that fuelcell or other electric generation becomes
cheaper then a diesel generator set, I'm prepared. Untill then the
diesel keeps running.

--
Richard
e-mail: vervang/replace invalid door/with NL.net
http://web.inter.nl.net/users/schnecke/

Richard van den Berg wrote:
On Wed, 26 Jul 2006 08:43:23 -0700 Bob S
) wrote:
I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

See the extra power needed for that 0.5 mile faster?

I would be pleased to hear from any of you who are also interested in
electric propulsion.

The Queen Mary II has electric propulsion.;-) For now I think battery
propulsed boats will be a niche product, nice for fishing or cruising
protected areas or coming home but not for holidays.

Do the math and find out yourself. My boat has also electric propulsion.
When the day comes that fuelcell or other electric generation becomes
cheaper then a diesel generator set, I'm prepared. Untill then the
diesel keeps running.

Bear in mind that many of the boats on the water today ARE used for
fishing or utilitarian functions on protected waters. No one suggested
serious cruising. But my 16-footer does have a 30 mile range at 4 mph at
the 70% discharge level which is more than adequate for its crabbing and
homecoming (sail backup) functions. The math does work in many cases,
but, as you say, picking the right niche is important.

BS

On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:

I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts


Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.


Take it with a pinch


Brian Whatcott Altus OK

Brian Whatcott wrote:
On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:

I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts


Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.


Take it with a pinch


Brian Whatcott Altus OK

Basically I agree with your analysis, but you assume equal over all
efficiencies for both motors, which negates both Minnkota's published
specs and my observations. Modern small electric motors can (not saying
this is actually the case)have efficiencies well above 90%. One of the
problems is that neither Minnkota nor MotorGuide will state efficiencies
for their motors or props. But IF this is the case with the Minnkota 50
lb model, then an over all efficiency of 80% would not be impossible. In
any case, without being able to accurately measure the current or thrust
at speed, it is difficult to wring much more information from my
results. Also, there were probably a couple hundredths of a volt more
drop along the internal and external motor cable (I measured at the
input end of the factory cable). Note, though, that your theoretical
values of available hp (.3 and .43) are not in bad agreement with my
observed results of .28 and .53 hp.

I sure would like to see some more people contribute some actual numbers.

BS

With a GPS and a spring scale, you could get a pretty good approximation of
the thrust. Attach a drag; e.g. a triangle of plywood, to a line and
measure both the reduction of speed and the pounds of drag. Assume as a
first approximation that the thrust/drag curve is about linear for a small
increment of drag. (We know it's exponential, but you're probably not up
near hull speed where it goes vertical.) If there's any wind or current,
make runs in several directions.

Add drag in increments and you'll generate a series of slopes. Find an
exponential curve that fits and you should have a pretty good guess at the
thrust curve.

Roger (or maybe not)

http://home.insightbb.com/~derbyrm

"Bob S" wrote in message
...
Brian Whatcott wrote:
On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:

I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts


Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.


Take it with a pinch


Brian Whatcott Altus OK

Basically I agree with your analysis, but you assume equal over all
efficiencies for both motors, which negates both Minnkota's published
specs and my observations. Modern small electric motors can (not saying
this is actually the case)have efficiencies well above 90%. One of the
problems is that neither Minnkota nor MotorGuide will state efficiencies
for their motors or props. But IF this is the case with the Minnkota 50 lb
model, then an over all efficiency of 80% would not be impossible. In any
case, without being able to accurately measure the current or thrust at
speed, it is difficult to wring much more information from my results.
Also, there were probably a couple hundredths of a volt more drop along
the internal and external motor cable (I measured at the input end of the
factory cable). Note, though, that your theoretical values of available hp
(.3 and .43) are not in bad agreement with my observed results of .28 and
.53 hp.

I sure would like to see some more people contribute some actual numbers.

BS

derbyrm wrote:
With a GPS and a spring scale, you could get a pretty good approximation of
the thrust. Attach a drag; e.g. a triangle of plywood, to a line and
measure both the reduction of speed and the pounds of drag. Assume as a
first approximation that the thrust/drag curve is about linear for a small
increment of drag. (We know it's exponential, but you're probably not up
near hull speed where it goes vertical.) If there's any wind or current,
make runs in several directions.

Add drag in increments and you'll generate a series of slopes. Find an
exponential curve that fits and you should have a pretty good guess at the
thrust curve.

Roger (or maybe not)

http://home.insightbb.com/~derbyrm

"Bob S" wrote in message
...
Brian Whatcott wrote:
On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:

I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts

Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.


Take it with a pinch


Brian Whatcott Altus OK

Basically I agree with your analysis, but you assume equal over all
efficiencies for both motors, which negates both Minnkota's published
specs and my observations. Modern small electric motors can (not saying
this is actually the case)have efficiencies well above 90%. One of the
problems is that neither Minnkota nor MotorGuide will state efficiencies
for their motors or props. But IF this is the case with the Minnkota 50 lb
model, then an over all efficiency of 80% would not be impossible. In any
case, without being able to accurately measure the current or thrust at
speed, it is difficult to wring much more information from my results.
Also, there were probably a couple hundredths of a volt more drop along
the internal and external motor cable (I measured at the input end of the
factory cable). Note, though, that your theoretical values of available hp
(.3 and .43) are not in bad agreement with my observed results of .28 and
.53 hp.

I sure would like to see some more people contribute some actual numbers.

BS


I am not sure this will work satisfactorily. The drag total drag force
is comprised of that from the plate and that from the boat itself. I
could measure the force on the plate as you suggest with a spring scale,
but unless it is significantly greater than the drag on the boat, I
can't solve for the thrust. And while it may be much greater than the
drag on the boat as its size increases and the speed decreases, that
will be at low speed, so I still don't have thrust at full speed.

BS

Nice post! Just as practical as it could be.

Brian Whatcott Altus OK

On Fri, 28 Jul 2006 12:46:44 -0400, "derbyrm"
wrote:

With a GPS and a spring scale, you could get a pretty good approximation of
the thrust. Attach a drag; e.g. a triangle of plywood, to a line and
measure both the reduction of speed and the pounds of drag. Assume as a
first approximation that the thrust/drag curve is about linear for a small
increment of drag. (We know it's exponential, but you're probably not up
near hull speed where it goes vertical.) If there's any wind or current,
make runs in several directions.

Add drag in increments and you'll generate a series of slopes. Find an
exponential curve that fits and you should have a pretty good guess at the
thrust curve.

Roger (or maybe not)

http://home.insightbb.com/~derbyrm

"Bob S" wrote in message
...
Brian Whatcott wrote:
On Wed, 26 Jul 2006 08:43:23 -0700, Bob S
wrote:

I have noticed that real numbers concerning electric outboards seem to
be few and far between. Therefore, I thought some of you might be
interested in some results I obtained the other day with mine.

I used two Excide 6 volt golf cart batteries in series. Open circuit
voltage at the time of the tests was 12.53 volts.

The motors were both Minnkota Enduras, one 30 lb, the other 50 lb. I
have no way of directly measuring thrust, nor did I have a calibrated
current shunt, so I assumed the factory-published values of 30 lb at 30
amps and 50 lb at 42 amps.

Voltages at the input to the motor leads were 12.14 and 11.96 with the
motors set to max. This indicates and combined internal battery and
external wiring resistance of about .013 ohm. The nominal input powers
are therefore .49 hp and .67 hp.

Again, assuming factory stated thrust is accurate, the output powers are
.28 hp and .53 hp for overall efficiencies of 57% for the 30 pounder and
80% for the 50. Incidentally, when I questioned Minnkota by phone they
would not state efficiencies but did say the 50 lb unit is their most
efficient.

These motors push a 16 ft flat-bottomed sailing skiff at 3.5 and 4.0 mph
respectively.

I would be pleased to hear from any of you who are also interested in
electric propulsion.

Bob Swarts


Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.


Take it with a pinch


Brian Whatcott Altus OK

Basically I agree with your analysis, but you assume equal over all
efficiencies for both motors, which negates both Minnkota's published
specs and my observations. Modern small electric motors can (not saying
this is actually the case)have efficiencies well above 90%. One of the
problems is that neither Minnkota nor MotorGuide will state efficiencies
for their motors or props. But IF this is the case with the Minnkota 50 lb
model, then an over all efficiency of 80% would not be impossible. In any
case, without being able to accurately measure the current or thrust at
speed, it is difficult to wring much more information from my results.
Also, there were probably a couple hundredths of a volt more drop along
the internal and external motor cable (I measured at the input end of the
factory cable). Note, though, that your theoretical values of available hp
(.3 and .43) are not in bad agreement with my observed results of .28 and
.53 hp.

I sure would like to see some more people contribute some actual numbers.

BS

On Fri, 28 Jul 2006 16:03:55 -0700, Bob S
wrote:

derbyrm wrote:
With a GPS and a spring scale, you could get a pretty good approximation of
the thrust. Attach a drag; e.g. a triangle of plywood, to a line and
measure both the reduction of speed and the pounds of drag. Assume as a
first approximation that the thrust/drag curve is about linear for a small
increment of drag. (We know it's exponential, but you're probably not up
near hull speed where it goes vertical.) If there's any wind or current,
make runs in several directions.

Add drag in increments and you'll generate a series of slopes. Find an
exponential curve that fits and you should have a pretty good guess at the
thrust curve.

Roger (or maybe not)

http://home.insightbb.com/~derbyrm

"Bob S" wrote

I am not sure this will work satisfactorily. The drag total drag force
is comprised of that from the plate and that from the boat itself. I
could measure the force on the plate as you suggest with a spring scale,
but unless it is significantly greater than the drag on the boat, I
can't solve for the thrust. And while it may be much greater than the
drag on the boat as its size increases and the speed decreases, that
will be at low speed, so I still don't have thrust at full speed.

BS


Work with him, on this one. You are going [let us say] at 3.44 mph
via a gps speed average of ten runs, and then you drag a plate which
drags at 4.23 lb for an average of ten readings, which reduces your
gps speed to 3.21 mph average of ten readings. Because drag and thrust
are in balance (else you speed up or slow down) you can say it takes
4.23 lb of thrust to contribute the extra 0.23 mph from 3.21 mph

Although this one set of data is not enough, in principle you could
say
T (thrust) proportional to 3.21 ^2 kT = 3.21 ^2
and also, T + 4.23 proportional to 3.44 ^2 k(T + 4.23) = 3.44 ^2

Supposing that the scaling constant k stays constant over this small
range, we can divide one equation by t'other to get this:
(T + 4.23) / T = 3.44 ^2 / 3.21 ^2 = 1.148

T + 4.23 = 1.148T
0.148T = 4.23
T = 28.6 lb. at 3.21 mph

Like that, but with more data points.

Brian Whatcott Altus OK

On Fri, 28 Jul 2006 02:36:00 GMT Brian Whatcott
) wrote:

Small electric motors can have 80% efficiency. Small water propellers
can have 80% efficiency.
If you take the product of voltage across the motor, AT the motor, and
the current through the motor, and multiply by 0.64 and divide by 746
you'll have an estimate of the net HP available for thrust.

V x I x 0.64 / 746 = HP for thrust.
Thrust at constant power varies with water speed, and is greatest at
standstill
(which is why troll motor makers specify thrust at standstill, where
it is meaningless)

Thrust depends on the propeller dimension. Bollard pull (Google gives
plenty hits) is commonly used with tugs, but conditions have to meet
minimal requirements to measure.

My previous boat was (minimal) faster with its 11 hp then a identical
boat with 16 hp. Bollard pull with my boat give a 150 kg, the boat with
16 hp 100 kg.

Let's work your numbers:
12.14V x 30A x 0.64 / 746 = 0.3 HP
11.96V x 42A x 0.64 / 746 = 0.43 HP

Rough, rough cross check:
if power required is proportional to v^3
then power required at 3.5 mph is 3.5 x 3.5 x 3.5 / ( 4 x 4 x 4) or
0.67 of power at 4 mph.
Power available for thrust at 3.5 mph = 0.3/0.43 = 0.88 of power at 4
mph

Thrust and boat speed don't behave proportional, why else bollard pull
for tugs.

This suggests to me (it could be a dozen other things) that the prop
is less optimal on the faster skiff.

Possible indeed. I think efficiency is best tested by measuring the
supplied power at different speeds, but requires an ammeter.

--
Richard
e-mail: vervang/replace invalid door/with NL.net
http://web.inter.nl.net/users/schnecke/