In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

I thought I mentioned this before. Hope I'm not repeating myself.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

I thought I mentioned this before. Hope I'm not repeating myself.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone
Newburyport, MA

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.

The physics says that extra-long-wavelength swells just propagate more
slowly. The c-squared = lambda*g/2pi is pretty fundamental for surface
waves (as opposed to deep pressure waves, like tsunamis).

There's a passage in John Craven's "The Silent War" where he gleefully
chants the formula from the sail of a nuclear submarine as he watches
her bow and stern waves demonstrate the validity of something he had
drilled into his head in grad school. Maybe you have to be a geek to
appreciate it g.

However, the oversimplified nature of my "push-the-stern-wave-aft"
explanation is, of course, quite true. IANA.

If you haven't already read it, Jere, van Dorn's "Oceanography and
Seamanship" has a pretty good discussion of the speed-power curves for
planing, semiplaning and displacement hulls - as well as, among other
things, a nifty nomograph for predicting sea-state from duration or
fetch of sustained winds.

Good sailing.

Al
s/v Persephone

On Mon, 25 Apr 2005 20:16:09 GMT, Albert P. Belle Isle
wrote:


Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!


Jere -

As a quick calculation, 24ft DWL would yield a hull speed of about
6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the
effective DWL at 1.34 would be a little over 27ft - not 35ft.

My previous boat had a DWL of 28ft, for which a speed-length
coefficient of 1.34 would predict 7.1kt.

I easily got 7.4kt on beam reaches, which would say my real
coefficient was almost 1.4 (or that my effective DWL at 1.34 was a
little over 30ft - a two foot "push-back" of the sten wave, which was
roughly how the peak of the stern wave looked from my cockpit.

Regards,
Al

In article ,
Albert P. Belle Isle wrote:

As a quick calculation, 24ft DWL would yield a hull speed of about
6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the
effective DWL at 1.34 would be a little over 27ft - not 35ft.

We've sustained much higher speeds, but don't feel like opening myself
up for someone saying that it's impossible, that I'm surfing, my
knotmeter's off, there's a current, or some such.

I would agree except that I eliminated all those things. We really have
done "impossible" things. Would love to know how, but gave up and simply
enjoy.

--
Jere Lull
Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD)
Xan's Pages: http://members.dca.net/jerelull/X-Main.html
Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/

Jere Lull wrote:

In article ,
Albert P. Belle Isle wrote:


As a quick calculation, 24ft DWL would yield a hull speed of about
6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the
effective DWL at 1.34 would be a little over 27ft - not 35ft.


We've sustained much higher speeds, but don't feel like opening myself
up for someone saying that it's impossible, that I'm surfing, my
knotmeter's off, there's a current, or some such.

I would agree except that I eliminated all those things. We really have
done "impossible" things. Would love to know how, but gave up and simply
enjoy.


It isn't a trick. The rule isn't a natural law, it's a general
guage, and it is inaccurate in most cases. It might hold true for
any one hull shape and weight, but it does not imply any limit, it
is only an estimation of a constant of some sort we call hull speed.

It's really all a question of fuel or propulsion force mileage. If
you can ignore current and wind, the distance per gallon would be
less at high speed than at low. There might be a "resonant" speed at
which you could get the best mileage for your hull form and weight.
You could plot a few speeds and fuel economies, and calculate a
number which might express some value for, call it formatic drag,
for want of a higher education...

Each boat would have it's own value.

A good comparison might also formulate the "answer" as an expression
of fuel cost per mile / hour, or in effect how much it would cost
per voyage in terms of dollers per hour wasted or saved in transit.
In other words, to express the cost in fuel efficiency as a term
that implied impatience, or something. Me, I like to spend as much
time as possible on the boat, so don't count "slow" as a cost. A
floating raft is almost as much fun, and you can fish!

The more time wasted, the better fuel economy? How much per hour to
save each hour wasted? How do you value your time?

Terry K

I use english measure for the sense of familiarity. Old rules of
thumb last just as long as the thumb, I guess.

On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:

In article ,
Albert P. Belle Isle wrote:

On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:

In article ,
Stephen Trapani wrote:

Hull speed is the absolute maximum that boat can travel through water.
All your examples have the water moving forward also so the boat is not
exceeding hull speed through the water.

Hull speed is a suggestion for our boat, not the law. Though our
theoretical hull speed is 6.65 knots, we regularly exceed that with
aplomb, close hauled, close reach, broad reach, whatever point of sail.
Spent a wonderful afternoon with 6 other sailors last season. As long as
I was on the tiller, pushing her to where she likes to be, we were well
above the theoretical hull speed. As we pinched to get back into the
harbor, she insisted on doing over 7 knots directly into the wind (okay,
about 15 degrees off). That last was our lovely lady showing off, of
course, as what we did was clearly impossible.

1.34 was derived from observing boats about a century ago. Depending on
the hull, that constant can be quite a bit different. As I recall, some
multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp
transom keeps her driving towards a 1.7 or so constant.

Jere,

It sounds like your speed-length parameter is higher than 1.34 - a
testimonial to your hull designer.

Full agreement.

The 1.34 comes from the fact that speed-squared of a wave = g/2*pi
times wavelength.

Yes, I agree with the derivation of the formula -- as long as we include
that wavelengths can differ. Swells have wavelengths 100s of feet and
periods many seconds from crest to crest, while wind-driven waves have
quite a bit shorter wavelengths and periods. And wind-driven waves have
different periods and wavelengths.

If your hull's stern really places the stern wave a distance back from
the bow wave equal to your design waterline length, then 1.34 is
pretty accurate as the point where the curve of additional HP to yield
additional speed for a displacement hull becomes almost vertical.

However, with sweet butock lines, stern reflexes and other
sophistications of hull design, the stern wave can actually be moved a
bit aft of your transom. The wavelength thus becomes greater than your
DWL.

Our resting WL is 24'. To maintain a 1.34 constant and get the speeds
we've verified while definitely not surfing, our effective waterline
would have to be greater than 35'. That's a LONG way behind our transom!

Since speed-squared is proportional to wavelength, and since your boat
speed and the wave speed must match, you get a speed-length parameter
that's higher than 1.34 as the effective multiplier times the square
root of your DWL (since DWL is now less than the wavelength).

It feels like you left a bit out and mixed a couple of things here.
Again, I agree that it probably has something to do with the wave speed,
which has a certain value when the constant is 1.34. Change the wave's
speed and you change the constant, and wave length.

At least that's my simplified understanding of a very complex subject.

Al
s/v Persephone

After having chased several NA's explanations for a few years, I finally
gave up trying to explain and simply accept that it's more complex than
1.34, since other hull shapes have much higher observed constants.


It isn't a different constant. It is just that many boats nowadays are
light enough so they aren't limited to the length of the wave they
make.

If you observe a tug traveling without a tow, you will see very easily
what wave we are talking about.



Rodney Myrvaagnes NYC J36 Gjo/a


"Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab