Water surface roughness vs hull roughness vs boat speed

Power Boats only. Please include the boat type in your question.
rick berrey
Active Poster
Active Poster
Posts: 382
Joined: Sat Jun 20, 2009 2:16 pm

Re: Water surface roughness vs hull roughness vs boat speed

Post by rick berrey »

I read an an article years back about a golf ball finish applied on the bottom of a boat to create air bubbles to generate lift , I don't remember if it worked or not . But a rough bottom creates drag , if you you were lucky enough to get a finish that traps air you might be getting some added lift .

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: Water surface roughness vs hull roughness vs boat speed

Post by fallguy1000 »

The surface dimples of a golf ball create lift when the ball spins. This lift exceeds drag created by the dimples and the surface. The dimples basically make the ball fly higher, but not faster. If you top the ball, the spin of the ball is the wrong way and it is driven down!

In a boat, lift is created by foiling shapes or other factors that lift the vessel up.

And, in the case of slightly rougher water; your boat gets a bit of lift from the sea or wind or both which reduces the drag created by the rough surface.

If rough surfaces on boats were better; do not flatter yourself in believing a new discovery. Simply not valid.

The reality is you can calculate or measure and test the coefficient of friction or drag for a surface. Rougher surfaces have more drag. A fine and glossy paint finish has the least drag.

Less drag and the same amount of lift means faster boat; not faster boat with more surface drag!
My boat build is here -------->

viewtopic.php?f=12&t=62495

User avatar
OrangeQuest
Very Active Poster
Very Active Poster
Posts: 3944
Joined: Tue Aug 28, 2018 1:14 pm
Location: Houston, Texas

Re: Water surface roughness vs hull roughness vs boat speed

Post by OrangeQuest »

This may help explain what is happening with a rough surface in ripples verses smooth water.

https://phys.org/news/2014-01-smooth-ro ... ic%20flows.

NOTE:
The researchers modeled both laminar and turbulent flows, and unexpectedly found that the drag-reduction was larger in turbulent conditions. The irregular fluctuations and swirling vortices in turbulent flows on smooth surfaces generally increase drag, Kim explained. However, the air cushion created by the superhydrophobic ridges altered the turbulent patterns near the surface, reducing their effect, he said.
"that it isn't just an ordinary sort of boat. Sometimes it's a Boat, and sometimes it's more of an Accident. It all depends." "Depends on what?" "On whether I'm on the top of it or underneath it."
A. A. Milne

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: Water surface roughness vs hull roughness vs boat speed

Post by fallguy1000 »

The rough surface is the water!
My boat build is here -------->

viewtopic.php?f=12&t=62495

mhd
Active Poster
Active Poster
Posts: 354
Joined: Tue Feb 15, 2022 10:41 am
Location: New Mexico

Re: Water surface roughness vs hull roughness vs boat speed

Post by mhd »

The physics of boundary layers (e.g. between water and air) remains one of the toughest areas in the whole of science. Turbulence, laminar flow, parabolic Navier-Stokes equations. Sure, there are models, but most of them aren't very accurate when you get to the details, due to the fact that solving Navier-Stokes equations in anything except ideal situations is practically impossible. All we can do is approximate with fast computers and modelling. For boundary layer physics, much still gets worked on by using experimental testing, scale models, computer models, trial-and-error, etc. etc. We can send people to the moon or Mars or spacecraft millions of miles out of the solar system and that is easy in comparison. But still there is no easy way to accurately predict, from theory, exactly how a boat will move through the water until it is actually built. Everything gets done with "what worked before", "trial and error", "rules-of-thumb", etc. etc. etc. and apart from faster computers to give better model results, the fact is that until better physics comes along, we're stuck with uncertainty. What works for one boat might very well not work for another.
FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

mhd
Active Poster
Active Poster
Posts: 354
Joined: Tue Feb 15, 2022 10:41 am
Location: New Mexico

Re: Water surface roughness vs hull roughness vs boat speed

Post by mhd »

And for golf balls, the spin is only part of the story. Yes, there is lift created (Magnus force) by the spin, but a non-spinning golf-ball will still go further than a spinning golf ball. This is because the dimples actually are there to intentionally create turbulence around the ball...completely counter-intuitive as usually we think of laminar flow as better than turbulent flow for speed, etc., but who said physics was easy? The dimples do create some additional drag but also *intentionally* create a turbulent layer (in the boundary layer) around the ball. This is so the separation of the boundary layer occurs further towards the back of the ball, and this reduces the drag due to pressure. This reduces overall drag...and the ball flies further.
FB11 (Designer Evan Gatehouse)
VG23 (Designer Jacques Mertens)

A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects. Robert A. Heinlein.

pee wee
Very Active Poster
Very Active Poster
Posts: 2276
Joined: Sun May 01, 2011 8:29 am
Location: Georgia

Re: Water surface roughness vs hull roughness vs boat speed

Post by pee wee »

I remember reading about attempting world record speeds for a boat, and that they picked up a little speed when the water had little ripples on it. I'm sure the hull was super smooth, though. Also, there may not be a direct correlation between speed record runs and the type of boats and speeds we are using here.
Hank

Matt Gent
Very Active Poster
Very Active Poster
Posts: 600
Joined: Sat Jan 05, 2008 7:54 am
Location: Treasure Coast, FL

Re: Water surface roughness vs hull roughness vs boat speed

Post by Matt Gent »

I have many hours operating small speed boats and can conclusively say they are all faster in a small ripple (say up to 6") than on glass smooth water. Up to a few mph different on faster boats.

Your boat would likely be a little faster in both conditions if the bottom were smooth.

There are many hulls with steps included to aerate the running surface, and they are faster than without steps. But these are all backwards-facing. There is also a designer with data that in the right conditions an interceptor (transverse strip mid way across running surface) increases speed and efficiency. These are all intentionally designed features. I'd say a general roughness will generally increase resistance.

31mph isn't bad on a 20hp Jon boat anyways.

Post Reply

Who is online

Users browsing this forum: No registered users and 0 guests