Atkin "Ripalong"

Discussion in 'Powerboats' started by frank smith, Dec 1, 2010.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    As is well known, planing hulls typically have a speed point of high resistance after which resistance falls, to then rise again steadily. Would the "ripalong" have that drop in resistance at any point of its speed/ resistance curve ? I doubt it.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Considering Ripalong can cruise comfortably at S/L of 4 and over 5 at 150 HP, I'd say your assumption is incorrect Mr. Efficiency.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    The vessel is alleged to hit 25 mph flat out with 150 h.p.........that would put its "comfortable cruise" no higher than 20 mph for economy sake, if nothing else. I like the idea of this thing as a mid to high teen cruise with a turboless diesel....provided the axis of prevailing winds and sea was favourable. Imagine motoring along at 15 knots with a following sea of similar wave speed......surf, plough, surf, plough some more, the ploughing taking up most of the time. The waterways would be full of boats of this ilk but for that.
     
  4. PAR
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    PAR Yacht Designer/Builder

    At 15 knots Ripalone is motoring at 3.2 S/L which by about everyone's estimation is a full plane.
    Anyone can travel at the wrong speed or take an unfavorable course, in any configuration hull form. This isn't the fault of the hull shapes the designer selected..
     
  5. frank smith
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    frank smith Senior Member

    The monohedron hull would have the advantage in that that you could trim the boat so that the attitude of attack would be the same , i assume that it would be more predictable in terns of impact with the back of wave at speed . but we still have the problem of using more power and having a deeper hole to clime out of. Not sure if I am using the right terminology here but I hope my point is coming across.
     
  6. PAR
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    PAR Yacht Designer/Builder

    I think I understand what you're saying Frank. The monohedron hull choice can work with lesser deadrise then what's common currently. The warped bottom can work as well, both of course assume a reasonable set of sections forward, which Ripalong doesn't have. It's now possible to design powerboats to "get up" comfortably, without a lot of bow rise drama and cruise pretty much as level as they do a displacement speeds (within a degree or two). We've also been designing power craft that corner fairly level and predictably as well in recent generations. Ain't modern engineering cool.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    In practice, it is not realistic to think you can compensate for the characteristics I mention by fooling with the throttle settings or zig-zagging around to get a more favourable angle to the seas. It doesn't work as a sensible solution.
     
  8. PAR
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    Having literally many thousands of miles driving every imaginably type of yacht you can think of, the above comment is truly lack of experience. Some boats require a lot of skipper input in certain conditions and it's prudent to accept these realities. The tedium of these inputs can be mitigated to a large degree by throttle and course change inputs. In other words, if you've taken a boat out that has crappy following seas habits and are in following seas, then a course change is the sensible solution or you can stand there at the helm playing "catch up" for how ever long it takes you to figure this out.
     
  9. tom28571
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    tom28571 Senior Member

    As Porgy said to Bess, "It ain't necessarily so".
     
  10. Easy Rider
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    Easy Rider Senior Member

    I think Ripalong is a fine design and efficient (but only at 10 to 15 knots. The only thing Atkins may have gotten wrong (probably gotten wrong in my opinion) is the 150hp part and the 25mph part. I really love Atkin's boats. I don't know everything about how correct they are but they are so beautiful I browse in the Atkin & Co Catalog on a regular basis. The Catalog is strange though. Helms are always to port and speeds are expressed in mph and not knots. Displacement is listed but not ever provided. On the same page as Ripalong is 26' Chum, whose aft section is totally full displacement (with a transom fully out of the water) with a steep QBBL and it's speed is listed as 9mph. Other boats have speeds listed that shouldn't be attainable. It never says so but I believe the speeds listed are at WOT under ideal conditions. Operational speeds would seem to be much less as in 15 to 20mph (for Ripalong (not knots)). But w it's full keel, easy rounded forefoot and very soft chines PAR is right ... it's anything but efficient at 25mph but at 12 to 16??? The engine and gear box probably weighed 1000lbs and the hull is full of white oak and has heavy planking. Get some people aboard, fuel ect and one may need 100hp for this 26' boat. As to the "hook" in the run aft ....it's so small it's hard to say for sure it's really there and the chines amidship are hard for a rounded hull form.
    "have that drop in resistance at any point of its speed/ resistance curve".
    Not profoundly but, yes, to some degree ...in my opinion.

    Easy Rider
     
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  11. PAR
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    PAR Yacht Designer/Builder

    Ripalong's hook is the proper length and height for her target speed. At 10 to 15 knots she's plowing pretty good at first, but then settles down once over say 13 knots as the hook starts to toss it's effect into boat trim. At 15 knots the hook is working great, but this would be the low end of the spectrum for this boat's target speed. She's an 18 to 22 knot boat, before drag becomes a considerable issue. She' could be forced to 30, but the power requirements would rise exponentially.

    Billy was old school and educated by teachers who drove to school in a horse drawn buggy, unlike John who had teachers that likely drove in a car (huge difference mentally). Displacement to both was a constantly variable number and not much regarded with most of these "articles", which for the most part are hijacked directly from a DIY feature in the magazines they were published in. In this era, powerboat speeds were given in MPH, still are to a large degree in the USA.

    The helm is properly located to port on these designs for a simple reason, the skipper isn't expected to help in ground tackle or docking duties. He is then oriented with the action directly ahead or to his starboard, which is the natural location for a person in a left hand drive world. The helm to starboard is employed when the skipper is expected to reach over the rail or out a port to grab a dock line or fend off. This is way a bass boat for example will have the helm to starboard, the skipper will be involved in docking and ground tackle operations, possibly all by himself, while in a 26' cruiser, the crew would preform these tasks.

    Back to the hull, which is essentially flat from just aft of midship to the transom. This lets her plane off early, but beats you up in a chop and limits top speed considerably, especially with the hook. Lastly, it's a heavy beast, especially compared to more modern warped bottom hulls. Her nicely radiused bilge turns will skid nicely in turns, but also prevent her from "carving" very well, so high speed turns need some room to swing her butt. In a moderate chop and properly throttled back to suit conditions, this hull can be an enjoyable ride, but very few modern skippers back their throttle to suit the sea state and the boat pounds out your dental work.

    In the end it wouldn't be especially difficult to "fix" this hull. The beam would be narrowed slightly to compensate for a modern build method's weight savings. Next a straight line from station 5 to the bottom of the transom drawn and the deadrise from station 6 aft maintained as best as possible, which would result in about 7 degrees or so at the transom. This would also increase draft and displacement some, further reinforcing the beam alterations. The result would be similar to a more modern warped bottom and a much better riding and maneuvering boat. Then again, this is now a total redo of the design and there are plenty of good examples of these modified shapes, in which to place Atkins styling on. In fact it's not hard to make a boat look like it's an Atkins as their styling was fairly pronounced and easily duplicated.
     
  12. tom28571
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    tom28571 Senior Member

    Easy, It does not take very much hook to be effective. Many boats have such by accident and have to be corrected. If it can be seen on such a small scale (and I can see it) it will be effective.

    It is quite possible to have a boat that planes before it reaches theoretical "hull speed". On such a boat, the presumed peak in resistance followed by a drop in resistance may not be detectable.
     
  13. Easy Rider
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    Easy Rider Senior Member

    Thank you PAR and Tom,
    I agree w all except that I think the stbd helm was mostly a result of boats to stbd of the helmsman and the right of way. Perhaps it was also influenced by left hand propellers being popular at the time the stbd helm became standardized. In the navy I remember a civilian ferry called the Nickel Snacher in San Deigo Calif was operated by one person. He landed always to stbd, reached out the window and put the bow line over the cleat w a bit of slack, reversed the boat almost to a stop, then put her in fwd gear at an idle w some port rudder and waited for unloading and loading. The boat was a 45 or 50' by about 18' flat bottomed wood boat much like an old row boat but w very little (if any) rocker and a DD 6-71 right in the middle. I don't ever remember a boched landing. Some of "Billy's" flat bottomed boats, Marcia, Wader, Esther, Marigold, Little Effort and Easy Goer interest me. I have a theory that short and wide w much rocker will amplify pounding whereas long and narrow w minimal rocker (or none) would produce the softest riding boat. I also think lots of "Billy's" flat bottomed boats were designed to keep the forfoot submerged much of the time. Billy claimed to be a master of flat bottomed boats and I think he was and I suspect much of what he knew was not passed down ...things he/they knew from experience. PAR "Ain't modern engineering cool." Yes but all modern engineering becomes old engineering (quickly now) and if we don't build on what we know we will never know much. That means we should try and remember much of the past so we actually increase the knowledge base. Reading Pete Culler reminds me of this.

    Easy
     
  14. tom28571
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    tom28571 Senior Member

    Easy,

    I think you are right about holding the stem down on flat bottom boats giving a softer ride. It just makes sense that if the bow is held down, pounding will be minimized. That may not satisfy the wish for highest speed or efficiency but comfort is often the most important factor and at low speed, the efficiency of this technique ain't bad either.

    If I did a flat bottomed powerboat, it would have minimal rocker with aft hook or wedges to force level running.
     
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  15. EStaggs
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    EStaggs Senior Member

    A question was raised early on about contemporary thinking in the semi-displacement hullform and how Atkin's boat is premier in efficiency for the speed. As it turns out, Bieker Boats did a project in the size range ( 25' LOA ) that has a little different shape and achieves better numbers in economy with a lower horsepower requirement:

    [​IMG]

    Unfortunately there's no lines drawing, but you get the general concept. 60hp, cruise of 16 kts, able to be forced faster, but this only uses more fuel and initiates pounding, changes trim, etc.

    E
     
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