Or whatever it's called... Why are longer boats faster? It is well known in the paddling world that even small differences in the length of a boat make big differences in hull speed. This seems to be true for both planing hulls and displacement hulls. Please help me understand why a longer boat is so much faster despite what I assume to be increased friction/drag?

12/30/2007 4:01:37 PM

I dont know much about marine engineering, but it probably has to do with the weight of the boat being spread out over a larger area and an increase in its buoyancy

12/30/2007 4:05:43 PM

i have no idea and i dont have my reference to explain it

[Edited on December 30, 2007 at 4:08 PM. Reason : 

12/30/2007 4:07:12 PM

do those longer boats happen to be narrower too?

12/30/2007 4:13:33 PM

Not really. There's a fairly standard minimum width that a kayak needs to have in order to be stable enough to be safe and practical. The volume increases with the length.

12/30/2007 4:15:18 PM

friction and surface area and what not

12/30/2007 4:23:39 PM

it has to do with the amount of bow wave resistance generated. i don't really fully understand it, but i think longer hulls generate waves that propagate faster and therefore get out of the way sooner, so the boat can go faster before it starts trying to outrun its own wake too much (which generates a shit ton of resistance)

[Edited on December 30, 2007 at 4:25 PM. Reason : asdf]

[Edited on December 30, 2007 at 4:27 PM. Reason : it isn't skin friction drag...if that's increased, it's negligable.]

12/30/2007 4:23:43 PM

Boats with a larger surface area don't sit as deeply in the water. If the boat sits deeper in the water, then it takes more effort to push said water out of the way to move the boat. Boats are not made extremely wide because the water would have to be pushed further out of the way as the boat makes forward progress, requiring more work.

Imagine walking through water. It's a lot easier to walk through ankle-deep water than knee deep water. Same idea.

Quote :

"it has to do with the amount of bow wave resistance generated..."

12/30/2007 5:08:50 PM

Wow, that was published in 1893 and it looks like a modern book minus the colorful insets and graphics.

12/30/2007 5:12:11 PM

i think it has to do with wave propogation and overall water resistance, not just surface friction.....but that's kind of a guess.

12/30/2007 6:07:02 PM

Quote :

"Boats with a larger surface area don't sit as deeply in the water."

that's an oversimplification at best.

Quote :

"depends on the boat and how fast it's going. many boats have hulls with ridges or other geometry that helps divert air underneath as it travels forward, reducing drag. drag is proportional to the square of the velocity. while the difference in drag from 0 to 5 mph is probably negligible, the difference between 100 and 105 is much greater. "

of course, but what i'm saying is that the skin friction drag on a boat hull is just about negligable relative to other resistances (and in fact would be even more relatively negligable at high speeds)

12/30/2007 6:08:00 PM

The boats I'm talking about don't achieve speeds greater than what you can paddle except if they're surfing. So I'm guessing drag isn't the biggest factor.

12/30/2007 7:19:34 PM

What's easier to push through the water? A brick or a stick? There's less resistance on the bow. Boats with a wider beam, I think, have more loading capabilities than those with narrower beams, but that additional loading comes with a decrease in its speed.

12/30/2007 8:05:28 PM

I understand that there's a difference. I'm trying to understand the nature of why there's a difference. If all other dimensions are equal why is a longer object easier to push?

12/30/2007 8:08:22 PM

dear dipshits who don't know shit about anything engineering/hydro dynamically/in life its self, SHUT UP IF YOU DON'T KNOW.

It is because the longer the vessel the faster it will go before trying to come on plane. A displacement hulled boat shouldn't have enough power to come on plane whether paddled sailed or powered. Now a hull that will come on plane can go as fast as you can keep it stable for.


displacement hull :
1.34 X the Square Root of the Boat's Waterline Length
(This is Theoretical Maximum Hull Speed)

12/30/2007 8:20:32 PM

So does resistance rise proportionally or exponentially or what as you approach a hull's theoretical max?

12/30/2007 8:24:22 PM

well its really neither... it goes up exponentially vs speed until the boat actually comes on plane.
because its pushing a wall of water, as soon as a boat planes it almost starts over from scratch cause it trades lift from the water for lift from the air.

12/30/2007 8:27:03 PM

So what ifcoming on plane wasn't an issue. What about an object like a torpedo or submarine (minus the sail) they would also have a theoretical max, no?

12/30/2007 8:30:31 PM

sort of i mean power required goes up exponentially so it would have an asymptote somewhere

12/30/2007 8:51:44 PM

I think you're looking for a Chief Hydrological and Hydrodynamical Engineer

[Edited on December 30, 2007 at 9:14 PM. Reason : looking =ed missing]

12/30/2007 9:13:58 PM

12/30/2007 9:34:05 PM

http://playak.com/news.php?idd=104164335026

1/30/2008 11:27:04 AM

Exactly... Unless you make it come out of the water.
then its an entirely new story that aerodynamics even gets to play a roll in.

1/30/2008 11:41:29 AM

weight of boat = constant.

therefore, displaced volume = constant.

Objective is to design a certain volume in the water to have the smallest resistance.

Amount of water "pushed aside", essentially what has to be accelerated out of the way is proportional to the area when looking from in front or behind. Therefore, we want to reduce this area. Reduce that dimension, and then the only remaining dimension is lengthened - the length.

Same with cars, submarines, and most things. If it were just a mile long and a hair's width wide it would be displacing almost no mass as it moved. At a point though, friction becomes a competing factor.

1/30/2008 12:16:35 PM

Quote :

"Same with cars, submarines, and most things. If it were just a mile long and a hair's width wide it would be displacing almost no mass as it moved. At a point though, friction becomes a competing factor."

not to mention the increased turning radius

1/30/2008 4:55:30 PM