Is a chine flat necessary on planing hull?

Discussion in 'Boat Design' started by BayouDude, Jul 16, 2012.

  1. BayouDude
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    BayouDude Junior Member

    I have been working on a hard chine planing hull that i would like to eventually build for myself out of aluminum. My background is in steel work boats (mainly tugs and OSV's) so I haven't messed with planing craft theory since college. The hull in question has a 21 deg transom angle, 26 deg at midship and 52 degree at the bow. I have been torn between adding a 3-4" wide chine flat to the design. Also the dead rise angles aren't set in stone. This will be an outboard powered 30' LOA x 8'6" beam fishing/family boat. I am wanting something capable of running offshore comfortably in a 3' chop. Anyone with planing craft experience care to chime in on the effectiveness of the chine flat i would like to hear from ya.
     
  2. drailton
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    drailton Douglas E. Railton

    A flat chine is not necessary. I was part of the design team that developed the Israeli Super Dvora patrol boat – Aluminum boat, 20 deg dead rise aft, no flat chine, no lifting strips.
    What speed are you looking for. In general chines are beneficial. The chine will add to lateral stability, reduce the preplanning resistance somewhat and tend to dry the hull off faster, but at high speed the vessel could ride harder in chop.
     
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  3. BayouDude
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    BayouDude Junior Member

    I am looking for a 25-30 knot cruise with maybe 45knot wide open. Hoping to power it with twin 130-150 hp outboards if i can keep the weight down to around 3500-4000 lbs on the hull.

    Was spray an issue without the bottom strakes?
     
  4. sottorf
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    sottorf member

    You dont need to use a Chine flat per se. As alternative you can use a interceptor /fin placed longitundinally aong the hull as shown in the picture. This is easier to construct and serves the same purpose.
     

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  5. Silver Raven
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    Silver Raven Senior Member

    Gooday 'B-D' 1st/ I don't agree with 'Dairl' & neither does history. Google is your friend - with this 'rails' subject. Look at the off-shore Bertrams & especially the greatest off-shore game fishing boats ever made - Hattaras. Ther are 100's of other examples for you to check.

    Your strake should be about 2 1/2" wide - at the transom - it should be angled down by 15 to 20 degrees, but you'll see all that when you do your homework.

    No you don't "have to have them" but then neither do you have to have 4 wheels on a four-wheel drive (3 might work) but it will sure hepl. Oh - by the way - I've been building - redesigning - repairing & racing them since the mid 60's.

    Go for it & do enjoy both your fishing & your family. Ciao, james
     
  6. drailton
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    drailton Douglas E. Railton

    What is the all up weight you expect the boat to see during normal use.

    One of the most misunderstood areas of small high speed boat design is the application of chines and lifting strips. For over 25 years my partner in my Naval Architecture firm of H/RI was C. Raymond Hunt Associates. Ray Hunt and Dick Bertram were drinking buddies with Bertram taking the Hunt hull form to commercial heights with Bertram Yachts. Add my years at Arneson, I have literally seen thousands of problem applications. My experience is that generalities tend to make for general failures.

    You might need to put lifting strips on the boat if you are going to go 45kts. At 25kts, I would expect that the chine will be immersed. At 45 kts I would expect to see the bottom drying off, with a negative chine immersion. The lifting strips placed at the correct width will give you a clean break for the water as well as add some dynamic lateral stability. I can advise this once I have a boat weight. Your 21 deg deadrise angle will give you a decent ride.

    As an example our 38’ Cougar race boat that we won the world champion in Offshore 1 for two years had chine width of 2.5”, with a boat speed of +150mph. Our Arneson 44’ demo boat using a Hunt Hull with 18 deg of deadrise had 4.25” chine width at the transom and a boat speed of 45kts. Tom Fexas used wide chines in excess of 10” on some of his designs. I know of no oversimplification method to add chines or lifting strips. We added a 5” chine at the transom tapering to 2” at the bow to the Dabur class patrol boat and increased the speed slightly, but more importantly, we added stability and dried the boat off at 30kts.

    The 2 ½ wide chine will perform the same as a wider chine for knocking down spray, It will also ride smoother at speed than a wider chine. At 25kts, the wider chine may provide better performance, depending on boat weight. The fin that Sottorf posted is a good solution to add the lifting area that a chine would provide. These fins tend to be somewhat susceptible to damage. They also do not add buoyancy as a traditional built-in cine would provide that may be beneficial if this vessel is used for fishing.
     
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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Having no chine flat in such a boat will give somewhat increased heel in turns, more likelihood of porpoising, and a more nose up attitude, all else being equal. Cross wind lean will increase, and planing threshold speed will go up, load carrying ability down. Wave impacts should be lessened, depending on whether the trim angle is not corrected for, by e.g., engine trim angle.
     
  8. BayouDude
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    BayouDude Junior Member

    Thanks for all the input.

    Drailton,
    I expect the boat will be around 6k to 6.5k pounds loaded with motors, fuel , persons and gear. This high speed stuff is a little new to me as my clients are mostly interested in fuel capacity and bollard pull. I know i could buy a proven hull off a show room floor but i would much prefer to have something of my own design. Not to mention the cost savings of building my own hull.

    Just thought i would add a shot of my current lines for the proposed boat.
     

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    Last edited: Jul 17, 2012
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  9. drailton
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    drailton Douglas E. Railton

    Nothing wrong with staying with your design, it is a nice simple deep vee configuration. You are running virtually constant dead rise from station 7 aft, so at top speed the vessel will have a tendency to run a little flat, but at cursing speeds of 30kts or so, you will have a more efficient application. Don’t let your LCG creep forward.
    I would run a full length lifting strip 2 to 2 ½” wide on each side located 2.5ft off centerline at transom. Follow your chine profile to get the height forward of station 7. This boat will start to dry the chine off at 32+ kts, so the lifting strips give you a clean break for water separation.
     

  10. BayouDude
    Joined: Feb 2005
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    BayouDude Junior Member

    Thanks for the input. Good to get some positive feed back. Once i get my layout nailed down i will try and put some more accurate weights together and get this hull into GHS
     
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