self righting stability

Discussion in 'Stability' started by valvebounce, Jul 1, 2012.

  1. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    I have a 60's speedboat which I am converting to a pleasure fishing boat,I have replaced the stringers and I am about to fit a new deck.I intended to use 3/4 glass faced ply for the deck,which I have already.I thought this would be a little heavy,but help with the self righting effect.I am adding a small lightweight cuddy.I could use 9mm ply for the deck.But on a gut feeling I would prefer the heavier deck.I will be using a 18hp evinrude longshaft outboard.Top speed is not a crucial issue but getting out of the hole,and planing at low speed is my aim.Fore to aft trim can be obtained by adjusting the seating and cuddy arrangements,and a locker in the bow.

    Any input will be gratefully recieved.
    Best regards V
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,168
    Likes: 395, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Unless you are planning to drop an anvil on it from a great height, 3/4" ply is way too heavy. If there is any sort of structure to offer reasonable support, half that will be more than adequate. I am not sure how you see the weight of the floor bearing on "self-righting" ability though. If you want easy, low speed planing, restricting weight is vital.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 13,950
    Likes: 486, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    That is way too heavy for a deck. It will detract for the stability. Speedboats are not selfrighting as a rule.
     
  4. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    Hi mr E and Gonzo,
    Thanks for the input,its much appreciated.
    What I meant by self righting was not complete self righting,as we get in British lifeboats [RNLI] but a similar effect that a yacht has with a weighted keel.The boat is only 13ft long and 5ft at its widest point.The thickness of the fibreglass hull is only 8mm.This is the first boat I have actually owned,and is a project now that I have retired.
    The boat seems very light in weight,which makes me wonder about its stability whilst crossing heavy swells etc.In effect I have gutted the boat from all the old timberwork,including the complete transom[hull glass and timber]The back of the boat is an open space.I have added height to the stringers just enough to enable a flat deck to be fitted[graduated from the centre stringer outwards.]The lower futtocks will be let in flush with the top of the stringers.I am using mahogany for the upper futtocks,which will be fitted into the new gunnels and used as new seat supports.
    My concerns are that the boat might be top heavy and go topsy turvy even in a slight sea.Hence the pendulam effect of a heavier deck.The hull is almost flat from 3/4 down,and there is not much displacement.I suppose because I lack the experience I could be over concerned.I have attached pics of the boat.
     

    Attached Files:

  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    As we've previously discussed, you need to think light on your boat. It's way too easy to over build which will do two major things; first it'll shave your preformance down, which kills top speed, makes maneuverability more sluggish, decreases crew and equipment capacity, etc. and secondly typically raises the CG, which should be avoided at all costs in small craft like yours.

    You soles (what you walk on inside the boat) should be 1/2" (13 mm) plywood, not 3/4" (19 mm). Your decking should be 3/8" (9 mm) or 1/4" (6 mm) depending on beam spacing. Do not be tempted to "beef" things up as this most often results in a heavier and weaker (amazingly enough) structure.

    Your boat is relatively light to get the most out of the outboard that hangs on her butt. Since she's a small boat, you just don't have a very large margin to play with in regard to additional weight, so keep thinking light and tight, not hefty and massive. The hull needs to ride at a fairly narrow range of immersion to preform well. If she's forced down by more weight, then she'll get all pissy and ***** at you when you crank open the throttle.
     
  6. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    Thanks PAR,I suppose that was the answer I was looking for really.I'll go for 9mm decking and coat both sides with epoxy two part paint and epoxy it in.
    I got some Desoto two part epoxy paint.[2 gallons] Had to opt for light grey.Probably paint the boat inside and out with it.I got 1ltre of Emerson and cuming [Henkel] ecobond epoxy resin 104.part A 0.610 kg--part B 0.390 kg.Thinking of using it around the new transom etc.
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You boat's stability is largely based on it's shape, not the CG location. Simply put, it's fairly wide and flat, so has high initial stability. This doesn't mean you can make it much heavier, just that it's stability (up to a point) is shape related and you can't really improve this, without hanging a big hunk of iron or lead a few feet below the boat or outriggers, like a trimaran..
     
  8. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    That info has given me more confidence in the boats stability,thanks PAR.
    Be lucky to get lead here these days,most of it has been shipped to China.[Still got some for casting fishing weights though]
    I had no idea that they stabilised cats with that method,I've watched them racing on the tv,they seem to flip very easily.The height of their masts looks out of proportion.
     
  9. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Cats are much like your little boat and rely heavily on "form stability" to remain upright. Their stability curve is very impressive up to about 30 degrees, then it falls off a cliff. Some ballasting can be useful in cats, but it's a fairly small role compared to the generous beam (width) they employ. Narrow mono hulls can be quite a contrast, with little initial stability as a hull shape, but a big hunk of lead hanging several feet below the yacht, to keep it upright. I just finished a design that had 139 degrees of positive righting stability, which is pretty darn good and there are some (not very many) that just can't be capsized, without it popping right back on her feet. A typical fin keel production boat will be in the 110 to 120 degree range, with centerboarder's and trailer sailors being somewhat less (often a lot less)
     
  10. philSweet
    Joined: May 2008
    Posts: 2,258
    Likes: 150, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    I had a 16' that I decked with 4mm and a drop of 6 oz glass. Spent the next 25 years wishing I had used 6mm (I ended up waisting a lot of time adding tiny carlins between frames). No way you need 9mm (and it wouldn't take much of a bend, either). You do need to get your frame spacing and carlins and deck crown correct though.

    Can't you just replace exactly what was there? It apparently lasted fifty years. I'd go with a hardwood ply. Marine mahogany or meranti. Holds up to dings much better than softwoods, particularly if you are putting a thin skin of glass on top. If you go with softwood, you want at least 10 oz cloth skin, and two 6 oz cloths are even better.
     
  11. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    Hi Phil,
    Thanks for the advice,maybe 6mm is the way to go.When I got the boat it was in a bad state,it had no deck only rotted duckboards placed on the hull between the stringers.I have raised the centre stringer,which is now the deck crown,and graduated the other four stringers level with it.This is now the deck base height.There will be four lower futtocks let into and level with the top of the stringers,with clearance underneath for any shipped water.I intend to use stainless steel brackets on top of the deck to secure the upper futtocks to the lower futtocks,that way the deck will be a complete unit without holes for joints.I will pay attention to the frame spacings and carlins with the 6mm deck in mind as you advise.I intend painting the underside of the deck, stringers and hull with epoxy two part paint.I intend putting aluminium oxide grip strips where required onto the deck,not sure whether to fit them on top of the epoxy,or mark out for them before the deck is coated,and fit them directly onto the ply afterwards.
     
  12. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    Hello again PAR,
    I dont understand where the datum is when you say 139% of positive righting.
    Horizontal being 180% [Flatline] Vertical off horizontal being 90%.[right angle]
    I'm obviously missing something here. V
     
  13. valvebounce
    Joined: Dec 2010
    Posts: 496
    Likes: 10, Points: 18, Legacy Rep: 124
    Location: manchester uk

    valvebounce Senior Member

    PAR,I was thinking you meant a cat.
    I understand now,you mean the main mast has 41% left to horizontal,thus 139% of lean on a narrow hull boat.
     
  14. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,168
    Likes: 395, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    I am wondering where the crew are when the boat is heeling 139 degrees.
     

  15. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 473, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A stability curve is calculated through 180 degrees of rotation. At some point during this rotation the boat will "go over the top" and flop over. The point she flops over is the AVS (Angle of Vanishing Stability) or often called the PVS (Point of Vanishing Stability) and this is where her "intrinsic" stability ends for the most part. The mast being level with the water is 90 degrees, the mast top touching the water a few more degrees, say as much as 95. 139 degrees AVS has the boat with it's mast pointed at about the "5 o'clock" position, underwater. The crew would be lashed or tethered in the cockpit, or preferably below decks, wedged in tightly. I've been rolled a few times and it's not something you want to experience. You never come away from this without damage, but fortunately 99% of sailors never see conditions like this, particularly with modern weather forecasting and navigation. A multi hull typically has a very steep or "tall" stability curve, meaning the initial portions of the curve, from 0 through 30 degrees is a towering line, rising very quickly, but then it flattens out briefly, typically from deck structures and other encumbrances, as it round over the top an falls sharply, usually as steeply as it rises. Your little powerboat's stability curve is shaped like this and perfectly normal, considering what she is.

    [​IMG]

    This 139 degree AVS curve, is an ocean going sailboat. The bumps and dips are when cabin sides, appendage immersion and other stuff affect the stability trend.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.