One more thing to look at with the numbers you have and the dimensions
of your prop. Take the engine RPM and divide it by the gearbox ratio to
get prop RPM. Multiply prop RPM by the pitch in inches to get the
theoretical maximum speed with no slip. (inches per minute)

Now convert the boat speed in knots to inches per minute by multiplying
by 1,216. Divide that number by the theoretical speed and subtract from
one to get the propeller slip. The slip for a displacement hull should
be somewhere close to 35-40%.

For example, let's assume you have a 15" prop and a 2.3:1 gear ratio.
At 2100 engine RPM the prop is turning at 913 RPM. Times 15" pitch
makes 13,695. Now 7.5 knots times 1,216 gives 9,120. 9,120 divided by
13,695 and subtracted from 1 gives 33% which is pretty good so if my
assumptions are correct your hull is a good shape and clean. But we are
still not getting full RPM and the mystery gets deeper.

OTOH, the natural hull speed for a 20' LWL is only about 6 knots and
anything faster requires a lot more horsepower and results in more slip.
As your assumed slip is still low, we must assume that the prop
diameter is large. Ideally you want the prop dimensions such that the
engine will just barely get to its rated maximum RPM and that the boat
reach maximum cruising speed at 80-85% of maximum RPM. That is about as
far as we can get (maybe to far) without some real numbers and making
sure that the engine can actually put out it's rated power.

David Flew wrote:
Glen
I know it doesn't add up - and it's hard to separate the opinions from the
facts. I've now found out who made boat ( 1959) and plan to visit them in a
few days. Very local and still making boats, but as the former owner put it
" with sticks" - please take no offence, it was his prejudice, not mine.
The boat was supposedly dies-used for in excess of 10 years prior to the
former owner buying it, he had it for about 4 years. The 80 HP V8 petrol
engine was seized, he installed a 40 HP lister, but it would not even make
way against a stiff breeze and current. So it was converted to a nominal 80
HP truck diesel. The above hearsay.
Further inspection this weekend disclosed the remains of a second exhaust,
consistent with a V8 installation. But I've no idea if the reputed 80 HP in
it 15 years ago was the original, or what the 80 HP represented in terms of
boat speed or shaft RPM. Nor do I know if the Lister was in good condition,
or how it was matched to the prop.

I played around with speed vs. RPM yesterday - the speeds are accurate, GPS
based. RPM is numbers from the tacho - just added borrowing a tacho from
work and confirming how far the boat tacho is out to the job list .. (
oops - one way speed runs - could be out say +- 0.5 kt for tides.) Add
fitting the receiver for the mechanical log and calibrating that too .....
But getting back to your numbers. I found little speed increase between
2000 RPM and 2100; and essentially none over 2200. Given the age of the
"new" engine, exhaust system restrictions, losses to drive both the engine
and sea water pumps and the alternator, not to mention the gearbox losses
..... if it's putting 23 HP into the shaft I'd say it's doing well. It will
be interesting to see if the original maker has any records.

All of this has just gone down the priority list a little - making a new
engine cover is now No 1. I was going to just re-insulate it with loaded
vinyl/foam, but whilst measuring I found it needed some old insulation
removed. I'll leave it to you to work out the favoured thermal and fire
insulation material in 1959. I'm thinking that adding some lead to the
existing engine cover might be a good idea. Just enough so that it sinks.

Add a couple of P3 masks and some ear plugs to the stuff to "borrow" from
work.


Regards
David





"Glenn Ashmore" wrote in message
news:qa6rb.10844$62.5538@lakeread04...

Wait a minute, something does not add up. This is a 23' hull with an
80HP Nissan diesel getting 7.5 knots? Assuming a full displacement 20'
LWL and 6000 lb displacement it should theoretically take only about 23
HP to get 7.5 Kn.

As far as the exhaust is concerend what you describe is asking for
problems. I assume a relatively short run from the injection point to
the transom which works in your favor but if the outlet is level with
the exhaust manifold there is defilitely a risk of flooding the engine.
You probably don't have to worry about seawater getting in from a
following wave but while cranking that engine will probably need to put
3 or 4 gallons of water someplace. That plug of water you see coming
out when you start up is what was pumped into the hose during cranking.
If the engine is a little slow about waking up and the exhaust is
trapped there is a very real danger that the hose will fill up. When
you stop cranking to rest the battery at least one exhaust valve will
remain open. The water will flow back and fill that cylinder.

David Flew wrote:

Jim
The installation is fairly typical of small wooden boats in my area - at

23

ft this is at the upper end of small. From the exhaust manifold piping
angles down. Water injection point is in this downward sloping metal
section. At some convenient point the system becomes rubber hose, which
then connects to more metal components where the system penetrates the
transom. The outlet is "about" the same level as the exhaust manifold,

and

the centrifugal cooling water pump is directly driven by the engine. A

few

boats have water lifts or water locks.

I've not heard of any backing up problems as you describe, I'd guess

that

even during extended periods of cranking and failing to start the

engine,

there is enough air going out of the engine to keep the exhaust from

filling

with water - and there is little out put from the cooling pump at

cranking

speeds anyway.

I suspect the hose acts a water lock, if it's big enough it certainly

acts a

crude silencer. I went from 1.75 inch to 2.5 inch hose on a 10 HP

diesel in

my "old" boat ( hope it will be my "former" boat rather than my "old"

boat

in a day or two ) and whilst I have no figures, subjectively the engine
could be run to much higher revs before it became unpleasant to sit on

the

stern thwart. In the quieter mode, there seem to be discrete plugs of

water

ejected from the exhaust, along with continual spray. Once the RPM and
exhaust velocity goes up, it's just a gas/water mix. So I suspect that
water accumulates in the low points of the hose, and it's a water

baffled

silencer, if you get my drift. The water locks I've seen advertised

suggest

this is part of their function - but don't give much data to support the
claim.

I measured the hose on the "new" boat today - it's 2". I'm really

looking

forward to replacing it with 3.5". I think minimum bend radius goes up
something like the cube of the diameter ... I disagree with Glen, it's

not a

job for a circus contortionist, but for an animal trainer. I'm really
looking forward to taming the new hose.
I can see the attraction of just piping the exhaust straight from the
engine through the sides - even if you burnt some dangly bits every time

you

walked past the engine ....
This time I'm going to take some noise readings as I go along, also
performance details. Although the difference between 7.5 and 7.6 knots

at

full throttle is not exactly important!
Regards
David

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com





--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Glen
I've just added checking the gearbox ratio and the actual tailshaft RPM vs
engine RPM to my list of things to do. When the weather warms a little I
can easily jump in and measure the prop dia, but how does one measure the
pitch? Boat's not due on the slips for a year.
I shall also measure WLL. Looks like this one will be around for a while;
I'll advise progress.
David

"Glenn Ashmore" wrote in message
news:hLNrb.13587$62.1025@lakeread04...
One more thing to look at with the numbers you have and the dimensions
of your prop. Take the engine RPM and divide it by the gearbox ratio to
get prop RPM. Multiply prop RPM by the pitch in inches to get the
theoretical maximum speed with no slip. (inches per minute)

Now convert the boat speed in knots to inches per minute by multiplying
by 1,216. Divide that number by the theoretical speed and subtract from
one to get the propeller slip. The slip for a displacement hull should
be somewhere close to 35-40%.

For example, let's assume you have a 15" prop and a 2.3:1 gear ratio.
At 2100 engine RPM the prop is turning at 913 RPM. Times 15" pitch
makes 13,695. Now 7.5 knots times 1,216 gives 9,120. 9,120 divided by
13,695 and subtracted from 1 gives 33% which is pretty good so if my
assumptions are correct your hull is a good shape and clean. But we are
still not getting full RPM and the mystery gets deeper.

OTOH, the natural hull speed for a 20' LWL is only about 6 knots and
anything faster requires a lot more horsepower and results in more slip.
As your assumed slip is still low, we must assume that the prop
diameter is large. Ideally you want the prop dimensions such that the
engine will just barely get to its rated maximum RPM and that the boat
reach maximum cruising speed at 80-85% of maximum RPM. That is about as
far as we can get (maybe to far) without some real numbers and making
sure that the engine can actually put out it's rated power.

David Flew wrote:
Glen
I know it doesn't add up - and it's hard to separate the opinions from
the
facts. I've now found out who made boat ( 1959) and plan to visit them
in a
few days. Very local and still making boats, but as the former owner
put it
" with sticks" - please take no offence, it was his prejudice, not
mine.
The boat was supposedly dies-used for in excess of 10 years prior to the
former owner buying it, he had it for about 4 years. The 80 HP V8
petrol
engine was seized, he installed a 40 HP lister, but it would not even
make
way against a stiff breeze and current. So it was converted to a
nominal 80
HP truck diesel. The above hearsay.
Further inspection this weekend disclosed the remains of a second
exhaust,
consistent with a V8 installation. But I've no idea if the reputed 80
HP in
it 15 years ago was the original, or what the 80 HP represented in terms
of
boat speed or shaft RPM. Nor do I know if the Lister was in good
condition,
or how it was matched to the prop.

I played around with speed vs. RPM yesterday - the speeds are accurate,
GPS
based. RPM is numbers from the tacho - just added borrowing a tacho
from
work and confirming how far the boat tacho is out to the job list .. (
oops - one way speed runs - could be out say +- 0.5 kt for tides.) Add
fitting the receiver for the mechanical log and calibrating that too
......
But getting back to your numbers. I found little speed increase between
2000 RPM and 2100; and essentially none over 2200. Given the age of the
"new" engine, exhaust system restrictions, losses to drive both the
engine
and sea water pumps and the alternator, not to mention the gearbox
losses
..... if it's putting 23 HP into the shaft I'd say it's doing well. It
will
be interesting to see if the original maker has any records.

All of this has just gone down the priority list a little - making a new
engine cover is now No 1. I was going to just re-insulate it with
loaded
vinyl/foam, but whilst measuring I found it needed some old insulation
removed. I'll leave it to you to work out the favoured thermal and fire
insulation material in 1959. I'm thinking that adding some lead to the
existing engine cover might be a good idea. Just enough so that it
sinks.

Add a couple of P3 masks and some ear plugs to the stuff to "borrow"
from
work.


Regards
David





"Glenn Ashmore" wrote in message
news:qa6rb.10844$62.5538@lakeread04...

Wait a minute, something does not add up. This is a 23' hull with an
80HP Nissan diesel getting 7.5 knots? Assuming a full displacement 20'
LWL and 6000 lb displacement it should theoretically take only about 23
HP to get 7.5 Kn.

As far as the exhaust is concerend what you describe is asking for
problems. I assume a relatively short run from the injection point to
the transom which works in your favor but if the outlet is level with
the exhaust manifold there is defilitely a risk of flooding the engine.
You probably don't have to worry about seawater getting in from a
following wave but while cranking that engine will probably need to put
3 or 4 gallons of water someplace. That plug of water you see coming
out when you start up is what was pumped into the hose during cranking.
If the engine is a little slow about waking up and the exhaust is
trapped there is a very real danger that the hose will fill up. When
you stop cranking to rest the battery at least one exhaust valve will
remain open. The water will flow back and fill that cylinder.

David Flew wrote:

Jim
The installation is fairly typical of small wooden boats in my area -
at

23

ft this is at the upper end of small. From the exhaust manifold piping
angles down. Water injection point is in this downward sloping metal
section. At some convenient point the system becomes rubber hose,
which
then connects to more metal components where the system penetrates the
transom. The outlet is "about" the same level as the exhaust manifold,

and

the centrifugal cooling water pump is directly driven by the engine.
A

few

boats have water lifts or water locks.

I've not heard of any backing up problems as you describe, I'd guess

that

even during extended periods of cranking and failing to start the

engine,

there is enough air going out of the engine to keep the exhaust from

filling

with water - and there is little out put from the cooling pump at

cranking

speeds anyway.

I suspect the hose acts a water lock, if it's big enough it certainly

acts a

crude silencer. I went from 1.75 inch to 2.5 inch hose on a 10 HP

diesel in

my "old" boat ( hope it will be my "former" boat rather than my "old"

boat

in a day or two ) and whilst I have no figures, subjectively the engine
could be run to much higher revs before it became unpleasant to sit on

the

stern thwart. In the quieter mode, there seem to be discrete plugs of

water

ejected from the exhaust, along with continual spray. Once the RPM and
exhaust velocity goes up, it's just a gas/water mix. So I suspect that
water accumulates in the low points of the hose, and it's a water

baffled

silencer, if you get my drift. The water locks I've seen advertised

suggest

this is part of their function - but don't give much data to support
the
claim.

I measured the hose on the "new" boat today - it's 2". I'm really

looking

forward to replacing it with 3.5". I think minimum bend radius goes up
something like the cube of the diameter ... I disagree with Glen, it's

not a

job for a circus contortionist, but for an animal trainer. I'm really
looking forward to taming the new hose.
I can see the attraction of just piping the exhaust straight from the
engine through the sides - even if you burnt some dangly bits every
time

you

walked past the engine ....
This time I'm going to take some noise readings as I go along, also
performance details. Although the difference between 7.5 and 7.6 knots

at

full throttle is not exactly important!
Regards
David

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com





--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com