Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

  1. FAST FRED
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    FAST FRED Senior Member

    I have been reading my back issues of PBB while cruising and came across an interesting concept.

    Seems the Navy uses a rudder input as a stability enhancer. Sure on any airplane kicking a bit of rudder will lift a wing , before slewing the hull around , but the thought of doing this on a boat is interesting.

    I had read of simply using a dagger board to lower roll, and have asked (with no luck) about a centerboard with a flap on the back as a stabilizer .

    But an anti roll input to the rudder would be even less work ,if it works on a single rudder (the Navy reference was to "rudders").

    This would be a simple addition to a cruising boat , could be light AND not too expensive.

    Comments or references?

    FF
     
  2. longliner45
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    longliner45 Senior Member

    the staelth bomber has a sophisticated on board computer ,that makes it able to fly ,,unlike boeings wing in the 60s,,what if this technology is applied to vessals?
     
  3. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Having a computer operate roll stab is easy enough , the hassle is the COST!

    Also boats have been holed and sunk from active stabilizers on running aground.They were "supposed" to sheer , but the hull was built poorer than advertised.

    A board or center board in a case would preclude this disaster.

    FF
     
  4. Excalibur
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    Excalibur Junior Member

    As a separate system, yes making a rudder into a computer operated roll stab would be costly. But if you built it into a gyro'd autopilot, the cost would be limited to the additional software. The aircraft version of what you are talking about is called Dutch Roll. There is both a roll and yaw component, so you will get a heading change along with the roll adjustment. Differential power inputs would be needed to counteract the yaw input and not the roll. I can see this working as a stability enhancement on a vessel with computer controlled rudders and variable pitch props but not otherwise. Now your thought of placing a trim tab on the rear of a centerboard is interesting. Especially if the centerboard is balanced and allowed to rotate along the vertical axis, similar to the way wind vane autopilots are designed. You would certainly want to design in self centering tho, so that it would not be forced up into the casing cocked at an angle if you hit bottom...
     
  5. tom28571
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    tom28571 Senior Member

    Last week my wife and I were returning to Poole, UK from St Malo, FR on one of COndor Ferries 300 foot fast wave piercer catamarans, otherwise known as the "Vomit Comet". This was in the remains of the storm that banged up the Fastnet Race. I noticed that the wake had some rather sharp oscillations back and forth.

    I thought that this must have been a steering stabilizer system. Anyone know about these cats, probably made in Aus? The ride was not too bad but some did get sick. Propulsion is through waterjets from 28,000 Hp (I'm guessing diesels).
     
  6. Willallison
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    Willallison Senior Member

    Tom - Unless they've changed, the Condor Ferries are built by Incat, here in Tassie (I went to school with the founder's son). http://www.incat.com.au/
    I think most of their cat's are fitted with foils these days, but I'm not aware of any roll stabilisation system as such.
    Earlier (and smaller) Incats had steering mechanisms that could account for the wake disruptions you mention. The were fixed 'rudders', set at an outward angle that were proegressively lowered to produce increasing rates of turn. A little like interceptors, except for steering rather than trim.. The bigger boats, however I think are all fitted with steerable water jets.
    Trust you had a pleasant trip.....
     
  7. tom28571
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    tom28571 Senior Member

    Will, The trip was a 50th anniversary present from our son and his wife. The whole family (with another son) had 10 days on a canal narrowboat in the west midlands of England and then Liz and I went to France for 4 days. The trip was great but travel is really hard sometimes. Had to spend a whole night in Gatwick and had two flights canceled in succession. Over 40 hours in airports, boats and trains. Getting too stiff for that kind of stuff. I think the French never put in a straight road if they can possibly fit in one with curves. Beautiful countryside.

    I would think that the wake excursions that I saw were too great to be acceptable unless for a purpose. There was no way to learn anything on a ferry packed with over 700 passengers and cars though. Much better than flying anyway. Wish we had a similar train service in the USA.
     
  8. Willallison
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    Willallison Senior Member

    Yes - it's somewhat ironic that we come home from a holiday in need of a rest!:p

    I'll probably be out at Incat before too long... we do a bit of work for them now and again.... I'll ask the question....
     
  9. FAST FRED
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    FAST FRED Senior Member

    Found another good reason for a beachable boat.

    On the "loop" from Chicago south the Illinois river is running 5 ft beyond flood stage . Trees , picnic tables, chairs you name it is in the water.

    Of course the rapid current is helping the fuel consumption , 12 mph at 850 rpm is really nice for the wallet
    But the constant eyeballs needed to avoid junk is a pain.

    FF
     
  10. tom28571
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    tom28571 Senior Member

    Fred,

    I suppose you will be going north up the ICW next Spring. When you get to Oriental on the Neuse River, you will be only a mile from my dock. You are welcome to stop and stay awhile.
     
  11. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Tom , Thanks for the kind offer , but FL is end of the loop.

    Dont know if we will head to New England (CT) in boat , or take a summer of motor home jaunts.
    I hope to have the plans and model of STROLLER done by then, and hope to find a Canadian yard to work with.
    If the US buck keeps sinking , it may be cheaper to have Boeing do a one of!.

    Perhaps we could stop by in our GM 4106 ex Greyhound.

    FF
     
  12. u4ea32
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    u4ea32 Senior Member

    Beam over 8.5 feet for trailering:

    On the road, you really won't have a problem being over 8.5 feet. Freeways, probably 12 feet is no problem.

    At the launch ramp, you may well have problems being over 8.5 feet: My boat is 8.5 feet, and MANY ramps have some entrance constraint that leaves me with bare inches on each side.

    Also, I have very little extra width where I keep my boat alongside my house, so I've got no choice.

    But there is another reason for narrow beam that should be of interest on this board on this topic, even if you don;t want to trailer anywhere: narrow beam makes for much lower vertical accelerations -- a smoother ride.

    This means the semi-planing (or better, planing) but light (and therefore efficient) vessel can still be comfortable underway. And isn't comfort and efficiency the hole point of a semi-planing boat?
     
  13. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    "And isn't comfort and efficiency the hole point of a semi-planing boat?"

    Sure , but efficiency is in the eyes of the owner.

    A super efficient boat like a Herrishoff Marco Polo 55ft long 10 ft wide , will be really fine to operate under way.

    But as many boats are used as a cottage (US) or "gin palace" (UK) the concept of having minimum room and paying for 55 ft , is no where as "efficient" as a 35 ft box 3 or 4 stories high.

    The few hours spent underway will never pay for the extra 20 ft of dockage, and small volume.

    Efficiency needs a daffy-nition to work.

    FF
     
  14. ALowell
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    ALowell Junior Member

    Over the years of reading about and observing boats I have come to a few conclusions about semi-planing boats, one of which is:

    A hull that has a hull-speed type run aft (up-swept) will squat when pushed harder than hull-speed as everyone knows. However, a boat with the same overall shape but significantly narrower beam will squat less (though still more than might be optimal) and go reasonably well above hull-speed, up to about 18 kts when performance deteriorates.

    Is the relationship between narrow beam and the ability to exceed hull-speed something that has been written about?

    ~ ALowell
     

  15. kengrome
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    kengrome Senior Member

    I don't know if this particular topic has been written about, but it makes sense to me. The narrower boat will be much easier to propel forward so the available power will move the narrow boat "out of the hole" faster and more efficiently than the wider boat, and therefore it will not squat as much.

    There are several threads here and in other forums that discuss the theoretical and practical advantages of tunnel-stern Seabright skiffs. These boats never squat, in fact their aft ends lift when power is applied. If you are at all interested in these boats you can learn a lot from these forum threads as well as from the Atkin and Co. web site:

    http://www.atkinboatplans.com

    Billy Atkin designed lots of these boats several decades ago, and from his reports and a few others including Dave Gerr they seem to do everything well, including run very efficiently on low power.
     
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