New Question, What foredeck shape makes the least drag?

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Bill Roberts
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New Question, What foredeck shape makes the least drag?

Post by Bill Roberts » October 30th, 2018, 8:07 pm

In the past several years we have seen all sorts of shapes on fore decks.
The foredeck shape makes aerodynamic drag while normal sailing.
1. Consider the wind flowing perpendicular over the hull upper half and deck. sideways to the wind.
2. Consider the wind flowing over the top half of the hull and deck at 45 degrees off the bow centerline.

The foredeck shape makes much hydrodynamic drag when totally submerged with boat moving forward at max speed, early stages of pitchpole.
1. Consider the water flowing along deck centerline at max speed and bow approximately 2 inches underwater. Has there been much hull drag increase?
Skipper and crew as far aft on the boat as possible.
2. The bow is 1 foot underwater. Has there been much hull drag increase? Can the boat be recovered from pitchpole?
3. The bow is 2 feet underwater. Has there been much hull drag increase? Can the boat be recovered from pitchpole?

Summary: Which foredeck shape is the cleanest aerodynamically, lowest wind drag?
Which foredeck shape is the cleanest hydrodynamic drag when submerged, early stages of pitchpole??
Last edited by Bill Roberts on November 6th, 2018, 3:53 pm, edited 1 time in total.

T Peterson
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Re: New Question, What foredeck shape makes the least drag?

Post by T Peterson » October 31st, 2018, 9:54 am

Howdy

I am unclear which part of the bow you are referring to as underwater - the bottom, the waterline, or the top (foredeck)?

I'll also say that if I was on a fast broad reach, and the top of my SC 17 bow was 2 feet underwater, I would be screaming like a girl and maybe airborne.

Bill Roberts
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Re: New Question, What foredeck shape makes the least drag?

Post by Bill Roberts » October 31st, 2018, 10:14 am

The bow is formed at the intersection of the sides of the hull at the front end of the boat. That intersection is called the stem.
The part of the bow that I am referring to is where the bow and the deck intersect. It could also be called the top of the stem.
To catamaran sailors, it is that part of the front end of the boat that when it goes underwater, you get real serious about easing the mainsheet.

Bill Roberts
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Re: New Question, What foredeck shape makes the least drag?

Post by Bill Roberts » November 1st, 2018, 8:02 pm

Hi T. Peterson,
It looks like you and I are the only ones interested in foredeck aerodynamic drag.
With air, a smooth continually curving surface is low drag from the aerodynamic point of view. Centerline ridges and corners are a no, no. An external hull to deck flange is draggy. It traps the airflow under the flange and forces this airflow to flow back against the incoming flow and then change direction and move upward to get around the flange and then change direction again to flow across a clean deck. So an external hull to deck connecting flange is a bad thing aerodynamically with the airflow moving across the deck at either 45 degrees or 90 degrees. This is why flag poles are round, not square.

Hydrodynamics:
Now let's put the forward tip of the foredeck underwater a few inches with the boat sailing forward at speed. Here comes liquid water shooting up the foredeck. A narrow deck is less draggy than a wide one. A curved deck side to side tends to shed the water off the deck quicker than a flat deck. A raised deck, roof top, with a centerline ridge will divide the water also but it is draggy with wind blowing across the deck. Again a smooth curved deck across the top of the hull is low aerodynamic 'top of the hull and foredeck' drag.
Now let's put the bow and foredeck under water about a foot. Here comes a wall of liquid water shooting down the deck and it has a negative angle of attack against the deck. When the deck is flat, this produces a down force which quickly pushes the foredeck underwater deeper increasing the negative angle of attack even more, the downforce increases even faster and in less than a second we have "PITCHPOLE". When a flat deck becomes the bow, the thing dividing the water, the drag increases several times, at least ten times, and the boat stops in an instant; the people keep going forward; I hope no one gets injured.
On the other hand, if a foredeck has a high crown shape to it, something like the bottom of a catamaran hull, when submerged it will continue to divide the foredeck water with only a small increase in foredeck drag and 'no downforce'. This is a signal to the skipper and crew; "back off on the mainsheet a little". Also the high crown foredeck shape puts more air volume in the forward section of the hull. This greater hull air volume means it takes more force to push the bow/foredeck underwater in the first place. This is a very important safety feature. There never has been a cleaner, safer, beach cat hull design than a SuperCat or Aquarius Cat, same boat. Ever think about why a submarine has the hull shape it has? That is a multi million dollar very low drag hull shape designed to operate totally underwater sometimes at Turnpike Speeds. See any similarity between a WWII submarine front end hull shape and an Aquarius catamaran front end hull shape? The elliptical deck curve is the lowest drag catamaran foredeck shape. The elliptical curve shape adds strength and stiffness to the foredeck also. Flat deck sections are not inherently stiff to axial loads like an elliptical curve shape catamaran foredeck.

WWII submarines had diesel power and used diesel power when they ran at sea surface to recharge the batteries. Below sea surface they ran on the batteries. The front end hull sections of these submarines looked very similar to SC front end hull shape for the same reasons, minimum drag.

Bill Roberts was a monohull sailor, a Flying Dutchman, when he and Steve Edmonds, a Hobie sailor, designed the first SuperCats. Bill lived on a lake where much cat sailing was done in SE Florida. There he saw Hobie and Prindle Cats turn over and could not be righted by the sailors on the boat. It always took two big guys. Hobie 14s would turn over and stick their mast tops in the bottom of the lake. Sometimes the boat was left mast stuck in the mud overnight. There was also much pitchpoling done by Hobies and Prindles on windy weekends. This was Bill's exposure to Catamaran sailing characteristics prior to SC designing. Also the Hobies and Prindles tacked so poorly that they ruined classical tactics sailing to windward.
Last edited by Bill Roberts on November 6th, 2018, 3:55 pm, edited 8 times in total.

T Peterson
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Re: New Question, What foredeck shape makes the least drag?

Post by T Peterson » November 5th, 2018, 9:54 am

Yup. I tell people that the Hobie 14/16 designed bows have two problems - One problem is that the pointed banana-shaped bows put the least amount of buoyancy where you need it the most going downwind - up front. The second is that if you do dip the bow in the water, the flat-topped deck (widened by the flange) accelerates your dive the instant it goes under.

As for easing the mainsheet - If I am out on a windy or gusty day, I tend to manage the wind by steering more than using the mainsheet - heading further up on a close reach or heading down on a broad reach if I am temporarily overpowered. I am not sure if I am smart or lazy...

Bill Roberts
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Posts: 515
Joined: November 17th, 2003, 9:13 pm
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Re: New Question, What foredeck shape makes the least drag?

Post by Bill Roberts » November 12th, 2018, 4:01 pm

Hi Folks,
I saw a Nacra 15 the other day and the foredeck shape for all functional purposes is the same as the SC 15. The N15 foredeck is made up of three flat surfaces that would be tangent to the elliptical foredeck of the SC 15. How about that, we have Marino and Melvin coping the 30 year old SC15 foredeck shape.

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