faster backwards

Just floated my 1/10 scale model of my 14 meter catamaran design.

The hulls have a 1/13.85 BWL/LWL with a .65 cp.

When pulling and pushing it across the water, we noticed that the boat glided farther going backwards than when pushed forward.

The bows are rounded and the stern is faired to a point. Would this account for the noticable better performance going backwards?

Or is there a direct relation with airfoils that are always 180 degrees to what boats use?

Ideas anyone?

 

Usually the bow is pointed and the stern is rounded or squared, so yes, it's definately the hull shape. It's not a 180 deg effect, it's something else, probably related to the water's surface. I'm not sure what, in fact I would also like to hear, why is it good to have a hull's max beam aft of amidships, with a pointy bow, whereas foils are the opposite?

 

Nero,

From your picture it looks as though you have decribed it opposite to what I can see. i.e. bows pointed and stern rounded, like normal boats, which is the opposite of an aerofiol. Could you post a picture looking down from the top so we get a better view of the stern.

I have never found anyone who could properly explain why a pointed bow is good on a boat hull but bad on an aoerfoil, be it a wing or a dagger board.


Gareth

 

air compresses, water not. my uneducated guess is thats why the pointed bow should beat a airfoil shape in resistance.
"glided farther going backwards than when pushed forward" like to see the "back" and bottom yes..
pic is the bow yes? say the trailing edge of the foilshape right? but faster backwards hey? hmmm..
how fast, how did you notice? i can imagine stationairy hullwaves riding backwards but i'm no expert and like to hear others.

 

Okay, I'll make a guess. A submerged object pushes a region of entrained fluid ahead of itself, resulting in a smooth, gradual, low-drag boundary layer. With a boat at the water's surface, the water will pile up vertically in front of a rounded bow, increasing the total force that resists the hull's progress. So the boat is forced to separate the water actively with the pointed bow. A similar effect occurs behind a slow-moving stern, which is why canoes are double-ended. At faster speeds, air behind a blunt stern can rush in quickly to relieve the low pressure that would otherwise occur. The dynamics of the fluid entrained behind a submerged object must be unstable, necessitating the long tapered tail.

 

I think it s mainly to do with wave resistance, and here the bow shape. A blunt bow will produce larger wave resistance (smaller angle of entry reduce wave resistance). At low Fn, a pointed stern is better (less pressure drag , see canoes), as Fn increases a blunt stern enables a longer length (between waves), and therefore reduces wave drag (powerboats ).
There is an article in one of the Cheseapeake Symposium about a 12' being towed in the towing tank backwards, and as expected at small Fn (low wave drag compared to viscous), it produces less resistance than going forward

 

Axing a good question?

Think of the boats hull as an axe. An axe shung backward may be more aerodynamic than one swung forward. But try chopping wood that way.
A boats hull has to 'chop' through waves. Or, if you like, chop through a chop.

Bob

 

Quick reply before I go take some photos to post. There is one clear problem with the bow. It is too blunt. At slow speeds it seems to work okay. But at faster speeds (a medium walking pace) there is a hugh wave that rides up the hull and spreads out forward of the boat. It made me sick when I saw it appear. So first up I have to fair the bow to a sharper point. Since the boat glided well both directions, I think this will help the most. I posted three other pictures in the works under progress gallery. Going to take photos now There are some renderings of the boat in the multihull sail gallery. "14 meter catamaran"

sharpii2, excellent metaphor.
yipster, the boat was pulled by a string that ran around a cleat on the dock. The person pulling the string walked at slow and normal walking speeds. (not to helpful or scientific?) This gave a steady pull. The boat was only pulled in the forward direction. Then we went to the end of the pier where there was the waves from boats passing and from wind. We gave the boat a gentle push to get it out away from the pier. It was then we noticed the difference when going backwards and forwards.

thanks for the information.

 

This is a very interesting thread for us as we have been finding exactly the phenomenon that you have been finding.

We are building a four hulled boat, the hulls are deliberately aerofoil shaped as we want to get lift without a dagger board, and had similar results.

We towed full size hulls at 10 knots and measured the force with a spring balance.

Like Nico I beleive that the pointed bow is to reduce wave drag.

The thing to bear in mind is our hulls are only 2.2m long, so these effects are more pronounced as we are operating at higher Froud numbers, and Wave resistance is only dominant over a relatively narrow speed range.

As Nico points out at small Fn viscous drag dominates, but this is also true at higher Fn, as shown by the Miss Nylex C class catamaran data. At 10 knots the wetted surface drag was 15% of the total while the wave drag was 22% of the total.

Gareth
www.fourhulls.com

www.fourhulls.com

 

additional photos

Here are some close ups of the bow and stern. Need to point out that the bottom of the blue marker line is the dwl. The model is 8 kg the design displacement is 7,200 kg. The model floats a bit low as designed. Overall model dwl is 138.5 cm.

Yes, the fairing is pathetic!

 

more of stern

The last photo is of the underside of the bridgedeck. During all the time the boat was in the water it did not get wet from a wave. In the center at the front which is the lowest point it has 9 cm of clearance. In the arches it has 11.5 cm. Hope this double arch will help to meet large waves smoothly.

 

Gotta go back to the advice Ted Turner gave the designer Britton Chance, geez Britt even a turd is pointy at both ends !

 

I am very excited to see these pictures, far more developed than my rough model mock-ups which never see water. Then again they are hovercraft and the stage to operating model is more involved because of motorization.

 

grob: "the hulls are deliberately aerofoil shaped as we want to get lift without a dagger board"

That's what many catamarans & proas do. They get lift out of the hull(s) by making them deep and narrow. You still need to deal with surface waves though, and the pointy bow won't prevent you from getting lift.

 

the bottom shows sharp (should be a sharper) double ends. but why faster backwards is to me stil a mistery. under sail it shure will go forward. a slow walk pulling the model can on scale easely exeed topspeed. can anybody here say some more on tanktesting, stability, waves, power what else can be tested?

i like your model, the "lightbulb" shape hulls made me think of the dogstar, did you read those articles?
dogstar: http://www.john-shuttleworth.com/Dogstar50-article.html