Of course, Fintry is different from your thoughts. That's why I said
"Broadly speaking ...."
I disagree with the thrust of this:
A deep V sails better through heavy seas, but you have to be prepared to
spent more
energy to make her push the water aside and make her way through it. A
planing
or semi-planing hull needs less power and is faster, provided the sea is
calm. Once the sea gets rough, this advantage less power/speed is lost,
you have to slow down to reduce the pounding and, if things get worse,
then you probably think that your choice of a semi or planing hull was not
right.
Later you say
In every case I will be spending more fuel/mile.
With which I agree.
At a displacement speed (S/L ratio below around roughly 1.2 or so) the
displacement hull will use less power than the planing hull, assuming speed
and weight are the same. As you go faster, eventually the planing hull does
better, because it planes -- gets up and out of its wave train which
otherwise is making it go uphill. You feel this in the average outboard, as
you go faster, the boat starts to push a lot of water, then suddenly it gets
up on plane and you can actually throttle back, using less power, and still
stay on plane. There is only a fairly narrow speed range where the planing
hull on plane actually uses less power than it does when it is not on plane
and even in that range, it may use more fuel than the displacement hull at
the same weight and speed.
This is why the so called "trawlers" that will do twelve knots with a 36'
boat need a lot of power and at seven knots will use more fuel than their
full displacement sisters -- you can't have it both ways in "normal" hulls
(no Swath, cats, or other newfangled modern things here). This is one of
several reasons why we eventually turned down Tarapunga (see
http://www.mvfintry.com/boatsnotbought.htm) -- she was designed as a patrol
boat and used too much fuel for a really long distance cruiser. For an up
and down the East Coast (of the USA) boat, she'd be great.
-------
Now, ballasting down could be a great idea if you wanted to be able to work
in difficult, choppy water -- Lake Erie for example. It could make you a
lot more comfortable as the weather piped up. And yes, ballasted down would
use more fuel, so it could be good to pump it out for flat-water work.
There are, indeed, multispeed marine transmissions. You raise an
interesting question. In ordinary boats, you choose the prop to use the
maximum horsepower available from the engine,usually at its top speed. The
propeller law (hp required varies approximately with the cube of
revolutions) then protects you at every other speed, because the required
horsepower almost always is less than the available horsepower at any shaft
speed.
If you chose the prop (or props) for top speed when your light/heavy boat is
light and you have, say 1000hp, to get her going on plane while light, then
the horsepower needed for eight knots or so when she's heavy is only around
50hp. Since that's so much less than the max, you'll probably have trouble
getting her to go slow enough, particularly while docking. A "trolling
valve", which essentially allows the transmission clutches to slip for long
periods could solve this problem for, I think, less money, than a two speed.
TwinDisc also has a new "QuickShift" as well as their "Omega Control" which
address these issues. Tarapunga (see above) was built with Omega Control so
she could run at very slow speeds for long periods for survey work.
As you say, free surface is an issue, but you'd probably always have the
tanks either full or empty.
As to
(1) my concept is wrong (2) my concept is right but
only few prospective buyers would make such choice- so no target group.
I think it is (2). Weight, complication, space taken away, are all killers.
Remember, too, how few 45 foot power boats ever leave the marina for more
than a few hours.....
--
Jim Woodward
www.mvFintry.com
"AP" wrote in message
...
Jim, thank you for your posting.
What you have between Fr. 2-6 and Fr. 41-45 is sort of fore peak and after
peak tank, which
as far as I understand the effect is to make a pure displacement vessel
even
heavier/deeper or
to change the trim. (this possibility was reduced since you converted the
fore peak to a bow thruster
compartment.
Indeed, what I am talking about is a different thing.
I take for granted (if I am wrong please correct me) that
A deep V sails better through heavy seas, but you have to be prepared to
spent more
energy to make her push the water aside and make her way through it
A planing or semi-planning hull needs less power and is faster, provided
the sea is
calm. Once the sea gets rough, this advantage less power/speed is lost,
you have to slow down
to reduce the pounding and, if things get worse, then you probably think
that your choice of a semi or planing hull was not right.
If we place the displacement hull on a scale on zero and the lightest
planing hull on ten, any hull
you find on the market will cover two points one the scale. No more. Your
mv Fintry is 0-1, a
low powered Hatteras goes 3-4, a Sealine goes 6-7 and a Baja sport boat
goes
8-9 and competition boats go to 10. A prospectvie buyer will make his
choice according to his needs.
And the manufacturers of mass production try to make the best hull form
for
each (narrow) range.
What I want to do is to ample the range for a hull (lets say from 3 to 6
or
7) and I am prepared
to pay the price, which to my understanding is that I will have to be
overpowered and give the engines more fuel than that of the Sealine and I
wil never peform like a Sealine in calm seas. Neither like a Hatteras in
rough seas. In evey case I will be spenting more fuel/mile.
Of course I will lose space for bigger engines and the ballast tank(s) let
alone I will need good
tranfer pumps which are heavy and additional manhours and cost to make
transverse and longitudinal separations to reduce the free surface
inertia.
I will probably need Gear Box of two ratios (I have heard that ZF started
producing something like that)
To cut a long story short, I want to be fast in calm seas and not to start
praying in heavy seas. I will slow down, ballast and be closer to the zero
on the "scale" I described you earlier. I would sail
not exactly like a Hatteras or a Bertram, but closer to them
I do not think that I have invented (or re-invented) the wheel. The fact
that manufactures of those
moving on the water machines have not made the slightset effort to ample
the
range using ballast/extra horsepower etc make me think (1) my concept is
wrong (2) my concept is right but
only few prospective buyers would make such choice- so no target group.
What makes me post this is my hope that somebody will reply and tell me I
am
wrong because this
and that or will tell me : you maybe right but you are "minority"- most
people do not like it, or whatever..
I havent found any boat plans with ballast tank on the net.
Is my dream to build such a ballasted/deballasted boat an " utopia"?
Thanks for giving me your time to read this. Somebody has something to
say?
Regards
AP
"Jim Woodward" jameslwoodward at attbi dot com wrote in message
...
Broadly speaking, it's done all the time in larger vessels.
Fintry was built with tanks for 50,000 pounds of seawater ballast. We'
re
putting a bow thruster in the forward one, so we'll replace the weight
there
with lead, but the aft pair (14,000 pounds each, p&s) allow us to get
her
up
to a draft of less than seven feet for sheltered waters or down to over
eight feet at sea.
Actually doing it in a boat that will go between planing and
displacement,
is another thing. The hull forms are quite different and it might prove
to
be a challenge. And, of course, you're talking about a lot of water --
you
might not like what it does to the interior in a 46 foot boat. You can
see
Fintry's tanks between frames 2-6 and 41-45 at
http://www.mvfintry.com/details.htm -- the scale on all the drawings
shows
frame spacing, which are 20" apart.
--
Jim Woodward
www.mvFintry.com
"AP" wrote in message
...
I want to build a cruiser with the following characteristics
46 foot with very deep V foreward. 25-28 degrees deadrise at transom
Overpowered.
And I want to have a " ballast tank."
The idea is :
If the sea is calm I will have the ballast tank empty and I will make
her
plan using my extra horsepower.
If the sea is rough (or I meet heavy weather on the way) I will
ballast
my
tank, increase the displacement, will slow down the engines and I will
be
sailing like a deep V boat, good for
rough weather.
Does that make sense??
Comments/suggestions are kindly invited.
Regards
AP