how big a sea can my boat handle?

Discussion in 'Boat Design' started by tugboat, Sep 25, 2012.

  1. tugboat

    tugboat Previous Member

    On the "small tugs" website-there is a "whitepaper" on the seaworthiness of flat bottomed displacement hulls..(rockered)...

    in it- he mentions this: "The SmallTug hulls are all quite seaworthy and capable in any sea into which a vessel of their approximate LOA might wisely venture." (this is an excellent read btw, and might shatter some misperceptions on flat bottomed boats!!)

    ok cool!..the only thing is--I dont personally have a reference point for this..I have been out in 12 ft seas in a 26 ft sailboat off the coast Port Elgin Ontario on lake Huron.

    that boat didnt have too many issues--mind you it was built for the North Atlantic. and was a deep keeled sailboat hull. and we were running with the wind...with 12 ft following seas...
    it was heavily ballasted...

    but i really dont know what "reasonable" is for my 26 ft tug??
    obviously im not going out in heavy seas that anuy idiot could tell is going to kill me -but in between those types of seas and flat calms lie the ones im worried about...

    The "lakes" usually have on average about 1-2 meter waves(3-6 ft)not too bad-

    but they can get big and ugly in a hurry- and as was said on a previous post by one member that I probably haven't seen a real gale--pffff-
    i beg to differ!!.

    october and november here-produce hurricane force winds and they can come up and sweep over those lakes almost instantly..they are devils those lakes when the seas pick up- the motion is different than the oceans--the waves crest sooner and the wavelengths are closer- many a sea captain who has never been seasick before come onto the great lakes and have a puking session because the motion is so abrupt and different.

    sooo my tug- ( about 26 ft long and around 10 wide),

    what winds and wave heights are reasonable for this size vessel?...displ. 8500-10000 lbs aprox.

    what if im out in the middle of Superior and a gale picks up?? (no seriously!)

    then what?...yea i know-i know, check the weather channel, and forecasts etc. but..they have been really unpredictable.

    i also have glen-l's websites info on thier tugs

    "...They feature deep-draft, heavy-displacement hulls for the motion and feel of little ships. With their oversize props, big rudders, and deep keels, these boats can turn on a dime and punch through seas that would stop or flail about less-able "flyweights" of similar length"

    ok again--??? does that mean they handle 4 ft waves easily-or would that sink it?

    so-what factors would be used to determine what size sea's any given 26 ft vessel can reasonably handle? or is there really no rule of thumb?
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Make a model of it and put it in the bathtub, then stir vigorously. See what it takes to roll it over. Repeat as required.
  3. tugboat

    tugboat Previous Member

    it might get sucked down the drain...!!:eek:

    seriously--where are all the experts?

    yes a model helps- i get that- but scaling a bathtub wave is not easy...
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Do a roll timing test on her and you'll quickly find out.
  5. tugboat

    tugboat Previous Member

    ill look into that --thanks Par. im assuming this is done with the model?
    i built two models of my tug 1:1 and 3/4 : 1 (builders scale)

    and they were both really least in waves at the beach... problem was i built it them 16 ga. steel. scaled up that was like .16 x 4--so the shell was like 1/2 inch in real world. this made the model sink way below her lines--so she didnt react like a lighter hull would... it wont roll...even when broaching..
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    What I worry about in small boats is breaking waves, particularly hollow breaking waves, the kind you find on barred inlet entrances, there is a lot of energy in water that is moving forward in such situations, whereas offshore in deep water the forces are much more benign for any kind of conditions you would be likely to be out there for in the first place. Forget the bloody equations and err on the side of common sense caution.
  7. tugboat

    tugboat Previous Member

    It would seem thats the right way to go about it...simple common sense.

    i was thinking maybe the N.A's can provide relative conditions that a certain boat can handle?..maybe include it in the plans package?...doesnt have to be complicated..something like- "this boats handles waves in the 6-8 ft range and beaufort 3.?" or "do not operate on seas greater than 1 meter in height"? --some plans state--for protected waters -or coastal cruiser-or passagemakers..etc but this is highly subjective.

    i guess ill find out in the end way or another. hopefully not the "wrong" way.
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Making stability calculations for small boats where non-fixed weights (also known as "crew" and their gear) represents a significant proportion of the total weight, are problematical at best. A boat that has a strong righting moment at high angles of heel (in theory) will behave quite differently if the team on board have all fallen over to the low side as she tilted.
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  9. messabout
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    messabout Senior Member

    Do the roll timing test as Par suggests. You can find the details for this simple test, on the full sized boat, in Gerrs book; The Nature Of Boats. The roll frequency rate and the distance between beam seas is a matter of considerable interest I believe.

    Steering stability in rough going is another consideration that deserves attention. Will the boat pitch over a wave crest such that the rudder, or some part of it, comes free of the water? The depth of immersion of the forefoot will, on some boats make her twitchy, even dangerous, when quartering. That is a trait more comon in hard chined boats but plenty possible on a round chine boat. This happens more frequently with boats that are bow down on account of improper weight distribution. And there are a whole mess of other considerations that make me want to stay tied to the dock in stormy weather.
  10. Ace Dragon
    Joined: Aug 2012
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    Ace Dragon Polyurea and Spray Foam

    commercial drift boats of Cook Inlet Alaska known as Tin Cans are made of aluminum, they are 32 ft long and 10 ft wide, 5 ft keel

    They have a long history of 20 ft waves. The saying is, they can take rougher water than the skipper wants too.

    All the info I can help with. More than likly, the only way to find out is history, experience. A lot of it will be how you handle the boat, that might be the limiting factor before you get to the boats limits.
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  11. Bomlo
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    Bomlo Junior Member

    I am inquisitive how big of surf this thing can manage. I am new to sailing last season so I only have 1 complete period under my buckle. Close relatives likes it and am just inquisitive what the vessel can manage.

    Any guidelines for operating in difficult water? How about traversing a awaken, What's the reccomended course?
    Floor Scale
    Last edited: Sep 29, 2012
  12. tugboat

    tugboat Previous Member

    Hi Bomlo--welcome. Thats exactly the big question on my boat..the others here have given great input--im getting a clearer picture too that its going to be a lot trial and error and getting to know my boat. Im going to investigate the roll timing test...

    i apologize for any misunderstanding but whats an "awaken"
    difficult water im assuming you mean currents?..or cross seas? or maybe manouverability?
  13. PAR
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    PAR Yacht Designer/Builder

    Bomlo, again, it's about experience. Some tactics can be employed, maybe after some reading, but the fastest and easiest way to figure out what these are, is to start in modest conditions and get a feel for it. Eventually, you'll build up skills and can take on bigger water and worsening conditions. Simply put, a skilled skipper can handle most thing in most craft, by generalities about handling traits in any given sea for any given yacht is an arbitrary and quite speculative set of opinions only.

    Take your boat out and challenge chop, wakes and other conditions. Do so in relativity fair weather, so a mistake is just embarrassing. You'll be able to figure things out and tackle bigger conditions later. It would be nice if there was a set of guidelines you could read and then have the skills to take on a full gale in your Catalina 22 and survive, but there really isn't such a thing, as each boat reacts differently and so does each skipper.
  14. Richard Woods
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    Richard Woods Woods Designs

    A study of the ISO stability standards for recreational craft will help you determine which category your boat fits. These standards come in three parts, sail, power and boat under 6m, so chose the right one.

    "ISO (International Organization for Standardization) standard 12217-1 Small craft – Stability and buoyancy assessment and categorization" is the one for powerboats.

    Although initiated in the EU, pretty much all recreational productions boats world wide are now built to these standards

    The standards themselves essentially divide seaworthiness into stability (whether the boat will capsize) and the ability to withstand "downflooding" (whether the boat will sink if flooded from water coming over the side)

    You are right that waves in lakes tend to be steeper and of shorter wavelength than in the open sea

    Richard Woods of Woods Designs
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  15. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Ultimate seaworthiness has nothing in particular to do with length. Look at ocean rated lifeboats for info on this.

    You ask about certain sized waves sinking your boat? will those waves sink your boat? Downflooding is the first issue, water coming in and filling her up. This usually happens when a hatch, window, or door get damaged and open to the sea. Thus commercial boats have strong watertight access hatches and doors mounted on high sills.

    The need for stability information is so that you can understand when (heel angle) and how these openings are subject to flooding. Stability information of models is meaningless in regard to your boat, which I believe does not exist even in design form. Once you have a design you can do an accurate weight study and run computer hydrostatics to estimate approximate stability (which can be compared with existing standards for sea state or voyage class). But the estimate will be different than reality as real VCG will be something different than your estimate.

    Despite what Mal has written, a vee bottom hull of the same size and displacement will have a lower VCG due to machinery and tanks being lower. Thus it will be a safer vessel with respect to stability.
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