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Old 24-02-2009, 18:05   #1
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Hull Leading Edge

At the 2009 boat show">Miami boat show, catamaran sailboats could be seen with leading edges at the waterline that ranged from about 1/2 inch radius to 2 inches or greater. Lengths of these boats ranged from about 36 feet to 60, but there was no correlation between boat length and slenderness or bluntness of the bows. Boats with the larger leading edge typically were making quite a fuss, with water sprouting up the bow as much as 12-18 inches.

Wickipedia, under the topic "Wave making resistance" says Fine entry
A hull with a blunt bow has to push the water away very quickly to pass through, and this high acceleration requires large amounts of energy. By using a fine bow, with a sharper angle that pushes the water out of the way more gradually, the amount of energy required to displace the water will be less, even though the same total amount of water will be displaced. A modern variation is the wave-piercing design.

But it doesn't say what is blunt and what is fine.

Somewhere in the past, I saw a formula that related the leading edge radius to the Froude number (which includes the length of the boat and the speed). I cannot find it now. Does anyone have any knowledge of this?

Thanks, Calculator
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Old 24-02-2009, 19:35   #2
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Where the water is separated at the bow is a small percentage of the boats overall resistance. Its not worth fussing over it too much. As a boat gains speed a boats wave making resistance becomes a huge factor. The downside to a fine entry is its lesser resistance to damage should the boat hit something. Also, you have less buoyancy forward with fine entries, therefore more inclination for the boat to pitch or hobbyhorse.

Fine entries work better for catamarans because a hulls beam is proportional to its stability, cubed. But with a cat, you have the stability advantage of two hulls. A monohull of course has to start flaring out at a greater angle much sooner to achieve the same stability as a cat.

Take a look at some swath hulls. I think you will be pretty impressed.
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Old 25-02-2009, 08:46   #3
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I think an over-riding consideration is the difficulty of construction. A sharp bow mold, especially a deep one is brutally difficult to reach into to achieve a bubble free layup. A large radius is much easier to craft, as well as being more resilient and stronger. An alternative is to square off the bows, and add a sharp, glued on, bow piece and call it a "water tight crush zone" or some other dazzling bafflement!

Some people claim that a rounded bow produces more spray amidships, but I can't help but wonder; if a sharp bow didn't serve up some performance advantage, would any builder would go to the trouble?
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Old 25-02-2009, 12:43   #4
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David and Sandy - You both offered reasons why the bows tend to be rounded off. Thos are probably good reasons. I am looking for the formula that expresses the ideal from a resistance point of view.

David - The distance between hull centerlines (not boat beam and not hull beam) is in the formula for stability, but not cubed. Right?

Cal
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Old 04-03-2009, 01:09   #5
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I would think that the objective is to have a easily driven hull, thus only needing smaller moters (less fuel) and less sail area (to handle) to push it.

How exactly this is achived it what I want to learn, I think that back of the hulls have a lot to do with this also.

can someone explain?

thanks C-man
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Old 04-03-2009, 15:35   #6
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Originally Posted by C--man View Post
I would think that the objective is to have a easily driven hull, thus only needing smaller moters (less fuel) and less sail area (to handle) to push it.

How exactly this is achived it what I want to learn, I think that back of the hulls have a lot to do with this also.

can someone explain?

thanks C-man
Search/Google for Steve Dashew designs, Sundeer & Beowulf are considered to bo some of the most easily driven hulls. Fine entry angle and narrow beam are indeed some if the requirements.

cheers,
Nick.
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Old 04-03-2009, 18:49   #7
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Search/Google for Steve Dashew designs, Sundeer & Beowulf are considered to bo some of the most easily driven hulls. Fine entry angle and narrow beam are indeed some if the requirements.

cheers,
Nick.
I did, thanks, but only could find a nice power boat
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Old 04-03-2009, 19:29   #8
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Old 04-03-2009, 19:38   #9
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amazing boats!

just not cats
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Old 04-03-2009, 22:49   #10
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Look at the fine entry angle and the length-beam ratio!

For much information, incl. formula's and examples, I can recommend the Dashew book "The Cruising Encyclopedia".

cheers,
Nick.
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Old 04-03-2009, 23:41   #11
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Of course the most efficient designs are just the opposite - they use a big underwater bulb at the bow. Look at any modern tanker that is not in ballast to see what the underwater shape is; it looks like a submarine or a whale. It's not just monohulls; the high-speed catamaran ferries have them on both hulls. Apparently that shape is best applied to hulls over 45' and can result in 12-15% less drag. See Bulbous bow - Wikipedia, the free encyclopedia for details. I wonder how long before this idea makes it to the boat shows?
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Old 05-03-2009, 02:58   #12
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Of course the most efficient designs are just the opposite - they use a big underwater bulb at the bow.I wonder how long before this idea makes it to the boat shows?
Look at a Dean 365 they already have this.
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Old 05-03-2009, 03:41   #13
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The bow bulbs were popular in some of the Crowther cats in the 80s, but gradually changed to a more integrated system for underwater bouyancy. There are a lot of poer cats with long underwater bulbs. There was a fishing cat out of Lakes Entrance Victoria Australia that had metre long underwater bulbs and quite a few working round the Great Barrier Reef on fast ferries. For a sailing cat, the change in bouyancy under sail makes it hard to optimise the bulb. Reverse stems are a way to get round this problem. Have a look at the A class cats or the G force Schionning Cat. This reverse stem system has been used very successfully in Scandinavian work boats.
I have drawn up reverse stems for a 15m/10m Harrproa to reduce, even further, loss of drive through pitching.
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Old 05-03-2009, 04:17   #14
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I think bulbs are meant to improve performance of higher spped vessels in larger waves with long waterlines. I don't think (guess) that there is much benefit when you scale the bulb and boat down to yacht size when you consider typical waves size and yacht speeds.

Finer entries have less resistance it would seem, but a displacement hull will produce a bow wave which causes "friction" and this seems the problem - wave making from displacement hulls.
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Old 05-03-2009, 05:11   #15
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Look at a Dean 365 they already have this.
That looks like a big mistake to me

Bulbs look like an afterthought to reduce slamming on flat forefoot and to try and get "some"extra buoyancy to mitigate that pod that drags through the water.

It even has antifoul on it

Dave
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