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  #1  
Old 12-14-2006, 12:29 PM
meren meren is offline
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Chine and strake design

Hello!

Are there any formulas or basic rules for starke and chine design for planing hull? I´ve heard that chines and strikes combined shouldn´t be wider that 20% of BWL.

There are also some hulls with good seakeeping abilities without strakes at all. So are strakes just trying to correct a poor hull design?
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  #2  
Old 12-14-2006, 06:33 PM
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Willallison Willallison is offline
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There's a great deal of art that goes into successful chine and spray rail design.
In general, chines shouldn't be more than 3 - 5% of the wl beam. Ideally they should be angled down slightly, but no more than 5 degrees. They should sweep up to the bow, usually exiting the water at around station 4.5.

As fro spary rails, there are a dozen different styles and a thousand different opinions as to which type is best! For higher speeds, it is generally accepted that 3 per side is ideal, with those closest to the chine running all the way aft and the other two somewhat shorter. The one closest to the keel being the shortest.

They are most definitely not simply a means for correcting bad hull design. They are used to reduce wetted sueface area - and thus drag - and thus power requirements.
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  #3  
Old 12-18-2006, 07:27 AM
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Pericles Pericles is offline
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Meren,

May I suggest you look at Tom Lathrop's site http://bluejacketboats.com/ as he has developed a planning hull design that uses less power to get on the plane. He uses wider flat chines to promote early planing. It's a very good site with instructive photographs

Pericles
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  #4  
Old 01-04-2007, 05:40 PM
Shorebreak Shorebreak is offline
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Meren,

Below are a few comments for reference, but I recommend that you do not build anything based on this information without first consulting a professional.

The rule that I use is that chine width should be about 1.5 to 2% of the maximum chine beam, per side. However, I have designed many boats that exceed this value. With wider chines the boat will roll less at idle or rest, and gives added stability at speed, however, the boat will ride a little rougher with wider chines. The chine can be angled down between 0° and 15°. Its all about finding the balance you want for the boat's purpose, and only experience will help you identify exactly what is needed.

Strakes follow similar rules. Boats with a volumetric Froude number less than 4.0 don't need lifting strakes... for example, a 55-foot boat weighing 60,000 pounds should only need strakes if it will go faster than 42 knots. Location and shape, again, is about finding the balance of the design.

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Darron
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  #5  
Old 08-22-2009, 10:30 PM
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HakimKlunker HakimKlunker is offline
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Quote:
Originally Posted by Willallison View Post
1. In general, chines shouldn't be more than 3 - 5% of the wl beam.
2. Ideally they should be angled down slightly, but no more than 5 degrees.
3. They should sweep up to the bow, usually exiting the water at around station 4.5.
4. ...with those closest to the chine running all the way aft and the other two somewhat shorter. The one closest to the keel being the shortest.
After three years ( ) I have questions:

1.: As a planing hull lifts out, the WL beam will decrease; does this in your eyes justify a reduction in chine width ?
2.: I talked to a designer who after tank testing found that angles down did not improve the performance; so the angle seems to correspond with speed, hull shape and probably trim angle?
3. & 4.: I again think of a planing hull, lifted higher than in resting condition: It seems to me that the chines all should end at the transom; the one furthest from the keel ending at or close to the stem, the others ending further back. Station '4.5' becomes less important here?
Anyhow, a chine also stiffens the skin and so perhaps should be continued further towards the bow (in this case for structural reasons and not so much for hydrodynamics)?
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Old 08-23-2009, 02:30 AM
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Willallison Willallison is offline
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Ok - to start with, all the above are rules of thumb. There are always exceptions and the ideal angles, widths and locations of spray rails, chines - and everything else in boat design for that matter! - will vary from vessel to vessel.
To take your points in order...
1. Do you mean the width of the chine flat, or a reduction in overall beam at the chine? Chine flat's are most beneficial on planing hulls. They provide lift, increase dynamic stability (and to a lesser extent initial stability) and reduce spray. If they are too wide, however the boat will suffer from a harsh ride. The 3 - 5% that I referred to was for planing hulls. Again - this is a generalised rule of thumb - plenty of boats get by with no chine flat at all.

2. Talk to ten designer's about chine's and spray rails and you'll probably get 20 different answers!! However, it is generally accepted - and quite logical if you think about it - that a slight down angle will reduce spary and increase lift. Sometime's that latter is beneficial, sometimes not. Too much down angle at the chine can force the spary down with such force however that it actually creates more spray. In my experience, the same can be said for excessively wide chine flats.

3. The length, shape and location of spray rails will elicit even more opinions than chines.....
However, the primary function of a spray rail is to separate the water flow from the hull bottom. They do other things as will, but that is their primary goal. Thus, adding a spray rail where it can't achieve this does nothing but add to the wetted surface (and hence drag). On all but the fastest of planing hulls, the bottom is immersed to almost the full beam at the transom (if not the full beam). Therefore adding rails close to the CL will only slow the boat down.
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  #7  
Old 08-23-2009, 04:44 AM
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HakimKlunker HakimKlunker is offline
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I think, I got the idea. The optimum solution looks like a couple of models tested in a tank. Unfortunately, I have no such opportunity.
My present object is an updated 13.65 m sportscruiser with twin surface drives and a target speed well above 50 kts; the original is from the 1970's and did well in Italy.
The bottom is slightly rounded.
I only have photos and a not too exact drawing, but well enough to re-draw the general lines.
Bcwl is ca. 2.5 m and 3% makes 75 mm
The inner chine (in my simple world) will be effective at top speeds only and with the hull quite far out. I expect higher water pressure in this condition which requires less surface?
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Old 08-23-2009, 06:29 AM
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Willallison Willallison is offline
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Yes - the higher the pressure, the less the surface area required to support it. You have pics?... please post them, we're always up for looking at a new project...
But do yourself a favour - a boat like the one you describe is not for the amateur designer. I urge you to use an appropriaely qualified designer.... it will almost certainly save you money and anguish in the long run
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  #9  
Old 08-23-2009, 06:47 AM
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HakimKlunker HakimKlunker is offline
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Thank you for the details. I am a certified master of boat building with 23 years on the job. But you are right: I am not as arrogant as many others and do know where I have my limits (this is why I try to learn here ). I am actually in contact with two experienced designers and before I start mixing the resin, they will be consulted.
I have done some raw sketches in jpg (sic!),
and Rhino and I do the details now...
The other photo is the original 'Drago' (I was told that Sonny Levi was involved )
Attached Thumbnails
Chine and strake design-flash-2010-mk-ii.jpg  Chine and strake design-flash-2010-interiour-2.jpg  Chine and strake design-drago-5.jpg  

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  #10  
Old 08-23-2009, 07:19 AM
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Alik Alik is offline
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Hakim, I would suggest to move fuel tanks forward - under bed or in bottom, slightly aft of middle.
This way You get more favourable LCG with full load.
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  #11  
Old 08-23-2009, 09:08 AM
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HakimKlunker HakimKlunker is offline
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Thank you Albert.
I'll consider it. I appreciate very much your support.
At the moment I am still working on the lines and still need to find the centre of buoyancy. The tanks in the sketches are just assumptions at the moment. Once I know the detail weights later, I am going to re-arrange the weight distrubution. When I have my general lay-out finished, I would like to have it counter-checked by you anyway
And by the way: I like your idea of naming pictures 'bulky corners' ...
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  #12  
Old 08-23-2009, 09:12 AM
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Alik Alik is offline
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Quote:
Originally Posted by HakimKlunker View Post
Thank you Albert.
And by the way: I like your idea of naming pictures 'bulky corners' ...
Yes, You got it!
Sounds like some company name, isn't it?
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  #13  
Old 09-27-2009, 10:33 AM
StealthAssassin StealthAssassin is offline
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Anyone have any pictures of strake shapes? I am wondering about a triangle vs. a wedge shape with the outer edge of the strake at 90 degrees?...
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  #14  
Old 02-18-2010, 03:57 PM
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u4ea32 u4ea32 is offline
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From the NACA testing reports, it sounds to me like the effectiveness of strakes or chine flats is very limited. the only thing a strake does is release the spray, cutting the wetted surface from the bow wave. The only thing a strake at the surface, or a chine does is reduce the effective deadrise from the surface athwarthsip angle to the angle from the keel directly to the outside of the chine.

Extra strakes under water are more drag.

Wide chine flats are worse than smaller ones because its (1) more surface area, but worse (2) its like a concave bottom, which pounds more that a simple V or convex bottom.

Think of the deadrise like a spring, you want the spring to be most firm at first, and then get LESS firm with more compression. A concave bottom, and wide chine flats, is the opposite, so far less comfortable, far more structural loads.

This was tested EXTENSIVELY in test tanks and with full scale (100') vessels in the early years of WW2. The result is what we see: either flattened V near the centerline, as on the world's fastest V bottom powerboats (Fountain), or straight V constant deadrise as on nearly all other boats, with small chine flats -- just enough to ensure the water flow does not climb up the topsides when moulded in Fiberglass. And strakes that end about where you think they go under water when planing.

Several builders have found it easier to leave off the strakes completely for easier building, or to just use them for styling purposes.
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  #15  
Old 02-21-2010, 11:25 AM
Wayne Grabow Wayne Grabow is offline
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"either flattened V near the centerline, as on the world's fastest V bottom powerboats (Fountain),"

I assume that the "flattened V" is what is also called a keel pad. Such boats as the Rascal, Rifleman, Diablo and others also incorporate a keel pad. Does anybody have guidance for the width and shape of such a feature? I am guessing that the keel pad should be parallel to the aft chine and straight for its entire length? Any particulars on its optimum width as a % of total chine beam? or its width possibly affected by the overall deadrise angle?
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