The buoyant bow and bow steering in heavy weather

Discussion in 'Boat Design' started by kach22i, Jan 4, 2007.

  1. kach22i
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    kach22i Architect

    I trust that I should be posting this topic here and not in the "Stability" section.

    Tell me what I need to know about bow design for a lightly loaded fast craft in rough seas.

    Is there a safe proven shape?

    What are the advantages and disadvantages of each design/shape?

    1. Flat angled so you ride the swell - which angle is best?

    2. Round or blunt curve so water does not enter over the top (old whaling boat-ish/like).

    3. Pointed or curved shape to cut the waves.

    4. What about RIB's? Is there a different rule set for an impact absorbing bow?


    For example this - explain it to me:
    http://ourworld.compuserve.com/homepages/fishwick/boats/classic.htm
    [​IMG]
    http://www.allinflatables.com/shopping/paint/test2.html
    [​IMG]

    [​IMG]
     
  2. Loveofsea
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    Loveofsea New Member

    Have you considered using a tiller handle on your engine? The tiller allows for instant lock-to-lock steering as well as the most articulate throttle control possible--like a dirtbike one the water... As you know, in rough seas control of the vessle is your most valuable asset.

    If you can articulate the throttle, you control the pitch of the bow. A drop of as little as 50 RPM can be the difference between burrying the nose or riding up over the swell.

    There is no way that a standard throttle lever will afford you that degree of control, particularry in rough seas.
     
  3. kach22i
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    kach22i Architect

    Thank you for answering this thread "Loveofsea". It was your words in another thread which gave me the term "bow steering" in the first place, fitting that you should come up with an answer.

    A boat, like an airplane is moving in three dimensions (not including time). It's difficult to imagine an airplane "torque steering" but they do this at take-off all the time. A boat uses torque too just controlled through the throttle, right?

    I offer this image as a partial description/explaination:
    [​IMG]
    http://www.grc.nasa.gov/WWW/K-12/airplane/torque.html

    All that said, and pretty diagrams glazed over, there will be a bow shape which asks less work of the operator.

    I'm not claiming there must be a hands free design vastly better than all the rest under all conditions.

    I'm looking for the bow you would put on a non-weighted shallow draft and most likely flat bottom boat which may be your only means of transportation to the safety of shore or a mothership in bad weather.
     
  4. Loveofsea
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    Loveofsea New Member

    Torque steering: i imagine an aircraft experiences this effect mostly when it breaks free of the runway and enters reality :)

    As you know, outboards have a tab adjustment on the anti-cav plate that you can adjust to counter the torque from the prop. i use a tiller and i want that handle to maintain a slight bias toward my body so i don't have to hold it. (also, it will swing the skiff to port if unmanned:( I cast an epoxy wedge on the skeg to augment the tab)

    For a sea worthy bow configuration, look at the dorys. I prefer the ones with a sharp bow and a minimum of flair.
     
  5. kach22i
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    kach22i Architect

    [​IMG]
    http://www.pearsonunlimited.com/DorySpecs.html
    The classic pointed high bow, might of guessed.

    How about something designed higher speeds? Mono-hull to keep it simple.
     
  6. Loveofsea
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    Loveofsea New Member

    Hello kach22i~!

    Lets talk dory with a planing hull:

    Here is what i did with the Good Skiff. I scarf spliced 2, 5' X 9' X 3/4" sheets of marine plywood, end to end. The bottom ended up being 17.5'. The widest part of the bottom was determined by the width of 3 portable fuel tanks nesting laterally 2/3 forward from the transom. Two 12 gal tanks with a wedge shaped 6 gal between them. The widest part of the hull is 59" across and it tapers on a flat plane to the transom where is is only 51" wide. The widest part is what i call the impact point, that is where all of the fuel/ weight is.

    Because the hull is wider toward the bow, it creates lift. To counter this lift i fabricated a SST strake across the transome with a cutout for the lower unit. The tab extends 11" from the transom and the corners are bent down to function just like a trim tab. So the hull lifts at the impact point 2/3 fwd from the transom, at the same time the strake on the transon creates exagerated lift. [that strake is tuned to augment the lift from the trim of the engine...]

    There is a slight penalty for the two conflicting lift points, but the performance in seas more than makes up for it. The weighted bow crushes the water and the exaggerated lift from the strake keeps the bow pressed to the surface; if it doesn't lift, it will not slap back down.

    There is a great advantage to using portable tanks. i averave about 175nm per trip so i am able to shift my empty tanks around in order to maintain the best weight distribution. Plus, you eliminate venting and contamination problems with portable tanks---and i can fill up the skiff without having to move the boat:). Also, every time i empty a tank, i use that as a milestone event for a log entry... i log engine hours, fuel consumed, lon/lat, time, conditions, etc..
     
  7. kach22i
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    kach22i Architect

    Great, from what I'm reading it's safer to keep the bow down and cutting into the water than to let it ride up and slap down.

    I'm sure there is a threshold where the bow may plow-in, thereby turning you into a submarine - but I'll worry about that later.

    I'm not building a boat, just trying to learn why bows look the way they do.

    The truncated bow (see first image) is most puzzeling.
     
  8. Loveofsea
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    Loveofsea New Member

    Burrying the nose or "sticking it" mostly happens in heavy following seas.

    When i first built the skiff, i had purchased 2 17' X 3/4" mahogany planks, one hard, one soft...

    I ripped them to 1.5 inches wide and laminated 4 four on each side of the hull to form a continuous splash rail. The rail is 1.5" thick and projects out 3", the soft wood sandwiched between the hard pieces. The rails are thru bolted at every BHD with 3/8" SST althread. Originally, the splash rail ended about 2 feet from the bow. I had to go back and splice new wood to the original in order to extend the rail all the way to the bow. Before that mod, the bow did stick it a few times, no water in the boat, but it shorn green water from the bow. Now she rides up and over rather than sticking---

    Plus, you can now walk all the way around the skiff on the outside:)

    I have seen truncated bows on a planing hull, but it was for a very specialized application. It was on a ranger's boat and it was hinged so they could drop it down and get on the beaches at Catalina. i would want one on the Good Skiff because i usually go out for two nights per trip and it would not be fun to be anchored with the wind and seas slapping the bow rather than spliting it. For a rowboat, it looks like a good idea....
     

  9. FAST FRED
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    FAST FRED Senior Member

    Torque steering: i imagine an aircraft experiences this effect mostly when it breaks free of the runway and enters reality


    Actually the biggest problem is nursing the power onto a stopped taildragger. The engine can easily make enough torque to overpower the rudder , at low speed.

    Very slow power application till the tail feathers have enough air speed to work , then T/O power , and away you go.

    FF
     
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