Nordhaven, and???

[scoobert]

silly question. when you say full displacement...
do you mean NOT planning, semi-planning.
or do you mean displacement speeds?
because some of the boats i am looking at are planning hulls, but nothing stops me from changing out an engine and going slow...

[lorenzo b]

read Voyaging Under Power by Robert Beebe, he will explain it all to you.

[scoobert]

Originally Posted by lorenzo b

read Voyaging Under Power by Robert Beebe, he will explain it all to you.

if i had time to read a book, i would be on the boat, and i wouldn't have to read the book.

[lorenzo b]

OK, as a boat move through water it has to push water out of its way and creates a wave as it does. A full displacement hull is deep and rounded and moves most efficiently at the same speed as the wave. A planing or semi-planing hull is designed to go faster than the wave and uses a great deal of energy lifting itself over its own wave and skimming along on top of the water. It may be fun, but you use 3 times as much power getting from A to B.

[scoobert]

i realize that, but what about when your going very slow?

[lorenzo b]

Even if you slow down, a planing hull will still direct a good portion of its energy to lift the hull and will not be as efficient.

[scoobert]

is your boat a converted fishing boat?

[twistedtree]

Originally Posted by scoobert

i realize that, but what about when your going very slow?

In my experience a planing hull running at displacement speed gets about the same mileage as a displacement hull of comparable weight. There is probably some difference, but it's not huge. Ive traveled on and off with displacement trawlers (mine is planing) and we compared mileage along the way. The differences in our mileage were all attributable to the weight differences in our boats.

And at least technically it is incorrect that a displacement boat is most efficient at displacement speed. Efficiency for a boat is measured in miles per gallon just like for a car. As you increase speed from zero to hull speed, a boat's mileage drops steadily along the way. Boaters' obsession the GPH blinds them to this.

When someone says their boat has a sweet spot at xyz, what i think they really mean is that that speed represents their preferred tradeoff between economy and speed.

Ok, let the flaming begin.......

[scoobert]

that does not apply to gasoline boats. i tried mine oneday, i went 5kts all day vs my typical 30kts, i got 20% better mpg. i bet if i put a 25HP diesel in mine i could get 10 mpg.

[chrisjs]

I thought the origin of this thread related to "crossing oceans". There is no way that a planing or semi-planing hull is suitable for that activity. Mind you, I guess the Kontiki was a flat bottom raft!!

[twistedtree]

Originally Posted by chrisjs

I thought the origin of this thread related to "crossing oceans". There is no way that a planing or semi-planing hull is suitable for that activity. Mind you, I guess the Kontiki was a flat bottom raft!!

Yes, you are right, and thanks for bringing the conversation back around.