Round chine vs. Hard chine... Basic differences?

Discussion in 'Boat Design' started by thill, Mar 4, 2014.

  1. thill
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    thill Junior Member

    As I posted on Aluminator's thread, I am in the process of restoring a round-chine aluminum boat, a 14' Crestliner Viking 710, 1957. Again, here is the catalog page for this hull. Mine is/was the "710 decked" model pictured top right and bottom left:

    [​IMG]


    This is the first aluminum boat I've ever been in that had round chines, and I am VERY impressed at how soft and dry the ride is, through chop and even waves. Most aluminum boats would beat you to death in those conditions, but this one has a truly amazing ride.

    This boat was originally equipped with a 60 HP Evinrude (or 40 HP) but I have put a 25 HP on the back of mine, and with the inside having been gutted before I got it, mine jumps to 25 MPH with either 1 or 2 men aboard.

    I find it interesting that extra weight not only does not slow it down, it actually makes it accelerate faster and run better. Maybe it's because the front deck, seating, carpet, flooring, etc., were removed, and it needs the weight to balance things out properly? As empty as it is, and with the light motor, it sits very high on the water. Maybe it needs more wetted surface to plane properly, and to engage the lifting rails?

    In researching round chines, I came across some Govt. guidelines that allow for MORE horsepower on boats having round chines, but otherwise being the same. WHY is a round-chine boat allowed more horsepower?

    When further researching the Crestliner site, I learned that they offered a number of different chine types. In the 1961 catalog, they label round-chine hulls as "exceptionally soft riding" and offering "outstanding stability and seaworthiness, suitable for offshore cruising and large inland lakes." But the hard chine, they label as, "fast planing, full power cornering without speed loss, preferred for ski-towing boats.

    They also show a "modified soft chine" which may be what mine is, with those big rails, not sure.

    So this goes back to my main question... what are the fundamental differences between a round chine and a hard chine?

    Thanks!

    -TH
     
  2. DCockey
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    DCockey Senior Member

    I assume the "guidelines" you saw were the US Federal regulations on determining maximum allowed horsepower ratings for outboard boats under 20' in length. http://newboatbuilders.com/pages/33CFR_HP.html Boats with a flat bottom and hard chines without "Remote Steering and at least 20" Transom Height" have a lower allowable maximum horsepower than other boats.

    My understanding is that when the regulations were formulated over forty years ago some boats with flat bottoms and hard chines, particularly aluminum jon-boats, were found to behave adversely with power which was satisfactory on other boats. The regulations used the distinction of flat bottom, hard chines and without "Remote Steering and at least 20" Transom Height" because that was simple to interpret.
     
  3. PAR
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    PAR Yacht Designer/Builder

    The formulas employed by the USCG to determine the outboard size you can safely use are very outdated and frankly dangerous on many small craft.

    An example is my Digger 17 design, which according to the USCG's formulas suggest, you can hang a 50 HP outboard on. Well, the boat will just barely hold this size engine up and speeds, will be in excess of 60 MPH. Just imagine a flat bottom clamming skiff, with a 50 HP outboard and scooting along in a modest chop, at 65 MPH! It's absurd to consider, so my recommendation is 30 HP as the max, with a 25 HP being preferred. With the 30 HP you can run in the high 30 MPH range, with a light load and glass like water. Of course, it'll pound your fillings out in any kind of chop, so the 25 HP is my strong recommendation, which makes her run in the low 30 MPH range, lightly loaded and a much better ride and fuel efficiency.

    My point is you have to understand what is reasonable, as opposed to what the USCG or other regulations and guidelines might suggest. There are lots of variables that govern "what is reasonable" and these old warp bottoms (like that shown above), need to be seriously looked at for power suitability.
     
  4. thill
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    thill Junior Member

    Thanks for the quick reply.

    That page is a more simplified version of the one I was referring to, but is basically the same. And it applies, because I am curious about small aluminum boats in particular.

    What adverse behavior are you referring to? It seems to me that guys love to overpower jon boats. Very curious about the possible consequences.

    -TH
     
  5. thill
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    thill Junior Member

    PAR,
    Thanks for the reply. I agree that regardless of what the supposed rating is, common sense must be applied.

    Just really curious about these round-chine boats.

    Actually, I DID have a round-chine boat once before. It was an 18' downeast-style boat. A Midland 18, if I recall. Big and roomy. It came with a longshaft 25, but I decided a 85 HP would fit it better.

    Well, with the 25 HP it did about 20 MPH, which was impressive for such a big boat. With the 85, it did... maybe 25 MPH, and it got VERY dodgy and hard to control, until you dropped below 20, then it ran fine. The increased HP did NO good at all!

    I pulled the motor and put it on Craigslist, and sold it the next day to a very eager guy who said it was rare, and that he'd been looking for one for years. Both of us were happy with the deal.

    So it seems to me that something about the round chines makes them very efficient and seaworthy. Almost like a semi or full displacement design... efficient, but with certain speed limitations.

    That experience is another reason I'm so curious about this boat planing so solidly, and handling 60 HP and pulling skiers, and so on. Could the strakes, keel and those big rails be the difference, despite the round chines? That Midland had a perfectly smooth bottom, besides the center keel, which was only 1- 1-1/2" high.

    -TH
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    That rail running along the transition from bottom to sides looks pretty substantial, and effectively makes it a hard chine boat, take that away and the performance would alter noticeably, imo. Maybe it was done for style purposes, (the rounding) or it suited the method of construction, but there is no 'magic' in it that eases the boat's progress.
     
  7. PAR
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    PAR Yacht Designer/Builder

    It all depends on the hull shapes employed, regardless of its type (round or hard). A lot of early round bilges are Jersey Skiff hull types, which are essentially a flat bottom boat, with rounded bilges and forefoot. These are great little boats, get up on plane very quick and at low speeds, turn nicely without tripping, but still can pound if driven too hard and on some chop laden courses.

    Another hull form commonly seen in these types of boats is the Sea Skiff style of hull. These generally have a bit of deadrise at the transom (round or hard), a warped bottom, some will have a deeper forefoot than others, but most have generous flare forward, to knock down spray. These hulls can take more sea state abuse, before they start to ***** about it and a softer ride at speed (round), but require comparatively more power to get up on plane, because of the increased deadrise and wetted surface.

    As to what this particular hull has and/or does, we'll need a set of lines to see or at least a number of photos detailing the hull form.
     
  8. thill
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    thill Junior Member

    Mr Efficiency and PAR,

    Agreed about the strakes and those big spray rails. Surely those aren't there by accident.

    Here is another catalog page that shows the back of the boat and also a side view of the hull. This gives a view of the transom that the other pictures don't show. Funny how you can't see how round the transom is in the side shots, but in the bottom picture, you can see it clearly:

    [​IMG]


    The resolution is not great, but maybe this is helpful?

    Even so, I'm not asking you to waste a lot of time on this, was just curious of the basic differences between a round chine and a hard chine boat. Thanks.

    -TH
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    The bottom aft would be near enough to flat, rounding the change from bottom to sides offers some protection from tripping, it seems structurally complicated, but a mass produced thing would allow the tooling required. Plonking a rail on that radius offers lift in such a light boat, to keep it from dropping off plane.
     
  10. PAR
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    PAR Yacht Designer/Builder

    The upper rail is very likely a structural consideration, more than a spray knocker, though forward it would be effective. Judging by the way the lift strake doesn't rise very much along the last 2/3's of the bottom and it's general configuration and era, it's a Jersey Skiff model. You must remember, when these puppies where designed, a round bilge boat was the norm and hard chines, not necessarily as acceptable to the old schooler's, that would have considered purchase. Most tend to go with what they know, so a round bilge is a logical marketing choice, considering the era (late 40's, early 50's).
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Now that I see the latest pictures, the rail looks too high out of the water to be doing much of a hydro-dynamic nature, really these boats are so light and flat-bottomed they just have to plane easily.
     
  12. thill
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    thill Junior Member

    You are right about the rail being high, especially on that open model,

    But the rail definitely digs in on the hole-shot, and also when cornering. This is a great turning boat, very smooth and directional in the transitions, not skidding, like many I've been in. I'm guessing that rail has a lot to do with it, along with the strakes.

    What do you think of the side-view of the rear of the boat? I didn't realize it stepped up like that. Again, a very interesting hull!

    -TH
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Looks like an elliptical section, I'm a little intrigued by the actual process of making them, the shaping etc. In the 50's there would have been plenty of people with experience and expertise in aeroplane manufacture, and they took that into the leisure industry of boating, using similar techniques. Rivetted alloy boats today are virtually non-existent, at least where I live.
     
  14. thill
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    thill Junior Member

    I think you are onto something with the forming of these boats being related to the aerospace industry. It appears that these were some well-engineered hulls.

    On other forums, I keep hearing almost worshipful praise. Expressions like "these boats were before their time" and "some of the finest aluminum hulls ever designed, then or now" and other such high praises. I've also had several offers to buy mine. I think there is something very interesting about these hulls.

    When some of this snow melts, I'll try to get you some pictures of the inside of the hull. Here is one of the outside of the hull:

    [​IMG]

    In this case, there are thinner inverted arches across much of the bottom, but also thick, extruded I-beam arches as cross-members in the middle of the boat, about every 16" or so. These thick I-beams are what the floor is attached to. To these is attached a solid skin panel going most of the width of the boat, but ending below the round chine, which is mostly formed by the sides. About 2/3 the way forward, the bottom skin panel is split at the keel, and is then riveted to a channel, getting sharply steeper from there, forming the deep-V bow section.

    This design makes for a really solid boat. My boat was completely gutted- most of the front hood cut off and both bunks removed. She was just empty of any kind of support, yet when underway, she didn't flop around like a modern hull would under the same circumstances.

    Even so, I didn't like the idea of those two bunks being gone, so using the existing front bunk bracket as a connecting point, I built a front deck. This gave me storage and a place to replace floatation:

    [​IMG]

    But in the back, I'm not sure of what I'm going to do. Primary, in my mind, is to find a way to add more floatation to the back of the boat. So what I'm thinking of doing is adding a bunk to house the foam, and give me a place to mount a seat for using the tiller. Another view of how the boat looks currently:

    [​IMG]
     
    Last edited: Mar 5, 2014

  15. PAR
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    PAR Yacht Designer/Builder

    Two side boxes, one to sit on and the other just to balance, both filled with foam or an athwart bench like box, maybe spaced off the transom a bit, so behind it acts like a splash well.
     
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