Why do longer boats go faster?
This whole issue confuses me. I have seen a simple formula which goes
something like this: Hull speed is equal to 1.4X square root of the length of
the vessel's waterline. Maybe by chance, it pretty well explains why my CHB34
will only go about 7 knots. Also, I have been told that the porpoise nose on
some larger vessels uses this device to increase the waterline length to
increase the hull speed.
Maybe the formula for hull speed is just a rough estimate, but it seems to
ignore the shape of the hull, particularly its beam.
I discussed this at my local lap pool and concluded that disstending my
stomach would probably not reduce my lap times.
Can you explain this issue to me please?
Thanks Tom
Hi,
My simple answer is that a displacement hull cannot go faster than the wave
it creates.
Todd Mains
Hi Tom, Try this, get a friend with a boat, go out with him and run side
by side, increasing speed, in steps, from about 3 knots to max speed.
Have your friend take profile shots of your boat at each speed step.
Now, look at the sequence of the pictures as speed increases.
Keep your eye on your bow wave. You will notice that the first wave
peak will move aft as the speed increases. Hull speed is where the wave
peak stops moving aft, and it is at this point your boat is trying to
climb out of a hole. Your stern does sit down a little when you put the
spurs to her does it not? Therefore, the max speed of a displacement
hull is related to waterline length (i.e. that is where the wave peak
stops), because the place where the wave peak stops moving aft is always
the same, and the value for this relationship is Hull Speed = 1.4 x sq
rt of lwl in feet.
You are right that other things have a BIG relationship on how fast we
go. BUT, if you have the correct prop, clean hull, etc. You can get
close to the theoretical hull speed. Remember we are talking about a
pure displacement hull. Any hull form with flat rear sections isn't
included in this discussion. If I remember correctly, (If you talk to
the Admiral, that statement is up for challenge!) the initial
experiments that established the relationship (among others), towed a
flat vertical plank with a load transducer that measured resistance. It
gets a lot more complicated after this. :-) Trust me, it works.
What you refer to as a porpoise nose is called a bulbous bow. The first
use was as a ram in the galleys that you see in the old movies, You ever
heard the expression "Ramming Speed" while some sweaty over-weight guy
in a toga pounds on a drum? Well, ....., battleships even into the
1900's were built with rams, out of habit.
When someone realized that current tactics didn't have a ramming option
in the play book, Navies started talking the rams off. Guess what, the
boats slowed down! Come to find out, the Rams had been creating a wave
on their own, so when two wave forms (BOW AND BULB) meet and combine,
some things get canceled out and other enhanced. The effects of the
bulb are now pretty well understood and one can be designed for any
hull., however, it will add some resistance and benefits only overcome
the penalties when speeds pick up. From what I remember (see above)
unless you get above 50 feet lwl or so. Maybe someone on the list can
chime in here. I'd love to see if a bulb would work on my boat.
John Blackburn
44 DeFever "Yak Rack"
Deale, MD
Thomas Averill wrote:
This whole issue confuses me. I have seen a simple formula which goes
something like this: Hull speed is equal to 1.4X square root of the length of
the vessel's waterline. Maybe by chance, it pretty well explains why my CHB34
will only go about 7 knots. Also, I have been told that the porpoise nose on
some larger vessels uses this device to increase the waterline length to
increase the hull speed.
Maybe the formula for hull speed is just a rough estimate, but it seems to
ignore the shape of the hull, particularly its beam.
I discussed this at my local lap pool and concluded that disstending my
stomach would probably not reduce my lap times.
Can you explain this issue to me please?
Thanks Tom
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The 'hull speed limitation' applies to ordinary displacement hull forms with
a beam to length ratio of 1 to 3 or 1 to 4 . My trawler is about 12 ft wide
by 31ft long at the waterline. Her ratio is 1 to 2.58. and she won't go
over 8 knots. My trawler hull form will not plane so that's it for speed.
However, there is an exception to the 'hull speed = 1.34 x Square root of
the WL length' rule.
If a hull has a beam to length ratio (both waterline measurements)of 1 to 10
or more, then that formula doesn't give you a 'hull speed' that limits the
boats speed.
Many trimarans and catamarans have displacement hulls that greatly exceed
that speed ratio. I built a Marples 44 trimaran(sailboat) that had
displacement hulls with that 'fineness ratio'(1 to 10) that could operate at
20 to 25 knots.
The obvious drawback is that interior volume and weight carrying capacity is
limited and a monohull that narrow would roll like crazy, but if you put two
such hull under a 'wingdeck' the boat can be fast, stable, and have
adequate(not generous) accommodations.
Check out PDQ power cats that operate at 15 knots, while getting 4MPG.
Larry H
Puget Trawler 37 "Jacari Maru'
Tom, while you're at your lap pool, try extending YOUR waterline
length and reducing your beam. Glide on your side while keeping your
lower arm extended straight in front. Then, BEFORE you start your
next pull quickly switch sides and bring your trailing arm all the
way forward to "catch up" with your forward hand. Then pull. This
keeps your waterline length fully extended at all times, and keeps
your beam narrow. You will notice that you are faster with fewer
strokes per length. Same for canoes and trawlers.
dave
On Jan 14, 2008, at 3:30 PM, Thomas Averill wrote:
This whole issue confuses me.
snip
I discussed this at my local lap pool and concluded that
disstending my
stomach would probably not reduce my lap times.
Can you explain this issue to me please?
Thanks Tom
Dave Friedrich
Monk 36
ELIZABETH
Clayton, NY
dave.friedrich@gmail.com
'hull speed = 1.34 x Square root of
the WL length' rule.
Reply:
This formula always comes up in these theoretical discussions. In practice,
I have wondered for a long time why there seems to be an 8 knot efficiency
hurdle for boats in the 45 to 80 foot range. The formula would say hull
speeds of 9 to 12 knots but yet I talk to a lot of skippers who say the same
thing: their best cruising speed is around 8 knots no matter what size their
boat is in this range. If you cruise the Inside Passage for days and days
you see boats of all sizes doing around 8 knots. A slow guy is doing 7.5
and a peddle to the metal guy is doing 8.5! Why is it that the 45 footers
aren't pulled over to the right and the 80 footers are steaming past? When
we compare notes regarding fuel consumption it always seems that there is a
linear increase based on size. You would think that the 80 footer would be
burning less fuel going that speed.
This is just based on years of casual observation and conversation. I think
that the simple formulas may be deceptive...
Eric Thoman
Abyssinia
'hull speed = 1.34 x Square root of
the WL length' rule.
Reply:
This formula always comes up in these theoretical discussions. In practice,
I have wondered for a long time why there seems to be an 8 knot efficiency
hurdle for boats in the 45 to 80 foot range. The formula would say hull
speeds of 9 to 12 knots but yet I talk to a lot of skippers who say the same
thing: their best cruising speed is around 8 knots no matter what size their
boat is in this range. If you cruise the Inside Passage for days and days
you see boats of all sizes running around 8 knots. A slow guy is doing 7.5
and a peddle to the metal guy is doing 8.5! Why is it that the 45 footers
aren't all pulled over to the right and the 80 footers are steaming past?
When we compare notes regarding fuel consumption it always seems that there
is a linear increase based on size. You would think that the 80 footer
would be burning less fuel going that speed.
This is just based on years of casual observation and conversation. I think
that the simple formulas may be deceptive...
Eric Thoman
Abyssinia