Fishing vessel stern shape

Discussion in 'Boat Design' started by Bob M, Jun 30, 2013.

  1. Bob M
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    Bob M New Member

    I have followed your thread on sail assisted small fishing vessels and have reached the same conclusion as George Buehlera that sails are more as a get home device rather than as a power source.

    I would like to steer the discussion more to a discussion of stern shape and draft. My son currently fishes a Olaf Brastad designed & built 38’ troller and fishes salmon in Alaska and tuna off the west coast. In terms of se keeping ability, he likes the boat and the way it handles heavy weather but he is ready to move into a larger vessel in the 50 to 55 foot size.

    His interest is in an easily driven hull with a get home sail plan and before he commits himself to a specific vessel design, he is looking for expert opinions on the effect of stern shape on a vessels operating efficiency. He realizes that a fantail and horseshoe stern will allow for a larger aft deck but how do they compare with a canoe stern in terms of efficiency.

    He is also concerned that modern designs in the 50 to 55 foot range with 15 foot beam have drafts of only 6 to 6.5 feet where as the Fram at only 38 has a draft of 6 feet and his Father-in-Law’s boat at 45 ft. draws almost 8 ft.. How does this affect their sea keeping ability.
     
  2. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum Bob.

    Sea keeping is a much more complex subject then just one or two design elements, such as stern shapes and draft. It also tends to be a bit subjective, with some being willing to accept different handling characteristics then others.

    If discussing strictly displacement mode vessels, the shapes become more standardized, but it's still a pretty convoluted set of design decisions.

    I have a "bill boat" with sail assist, designed in the late 50's. It's a box keel, double ender, that is quite efficient, barely making a wake at WOT. The box keel offers hold space and an engine compartment down low and improves efficiency considerable too. This isn't the end all of efficient shapes, but is one solution. The sail assist is stickily down wind, naturally, as there aren't any additional appendages to permit upwind ability, but ghosting a trotline (or whatever) on a broad reach is a lot cheaper than cranking up the diesel. I can make 7 - 8 MPH in a stiff breeze with the sails.
     
  3. Bob M
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    Bob M New Member

    Thank you Par for your response.
    I am internested more in the relationship of draft to overall sea kindliness and I guess it has as much with the where the CG is located as any thing else, I.e. is a shallow CG better than a deeper CG?
    In terms of stern shape, what to look for when trying to maximize working deck area and hull efficiency.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Everything is inter-related Bob, CG, draft, hull form choices, it's all interconnected. You can design a boat with deep draft and a low CG, that has great sea keeping abilities. Conversely, you can design a shallower draft boat, with a higher CG, that still has great sea keeping manners.

    In terms of efficiency and deck area, the two aren't as directly related as you might think. You can have flare aft, to provide good deck space, yet still have efficient "quarters" below the LWL, so she has a good sea kindly nature. You can take this to extremes and have too broad a stern, in a following seas for example, but everything is a trade off, so you look at each element, of a design's SOR goals and address them in a prioritized way, giving in to some more desirable, so you can gain something else.

    Simply put, there's nothing in broad terms that can be offered, unless a well established SOR is worked through the design spiral, even if just hypothetically.

    Since you're looking at a fishing vessel, some things will just have to take a back seat to capacity and working practicality. This limits other aspects of the design in general terms.
     
  5. DMacPherson
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    DMacPherson Senior Member

    This is hardly the whole story on stern shape and seakeeping, but you might find this article interesting. Click to http://www.hydrocompinc.com/library and then click the April 2007 link for an article about CVP.

    Don MacPherson
     
  6. Easy Rider
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    Easy Rider Senior Member

    My $.02 this morning,

    See the stern shot of the Willard and the Fisher. VERY similar boats. But the "quarters" or "aft cheeks" are way different. The W is very full (deep and wide) aft while the F is very "pointy". The W should be more efficient near hull speed whereas the F would be closer to over driven. Especially if the F is pointy in the bow. The F should be less adversly affected by following seas than the Willard. The amount of control afforded by the rudder is another important factor. The "prismatic coefficient" (PC) is the engineering term that expresses this element of design and gives it a numerical value. Sailboats have very low PCs and are very efficient but only at very low speeds.

    The second boat (Dixie ll) is deeper than both the W & F and carries much more disp per foot of length. Dixie II is probably (more or less) what you're looking for. Notice the large deck area aft. But draft and disp do not promote efficiency. The common way to estimate the power loading of a FD hull is consider or evaluate her disp and power. Two hp per ton of disp is considered very low and 5hp per ton is fairly high. Fishing boats need to consider their gross disp. But it can be clearly seen that the more a vessel weighs the less efficient she will be ... all other things being equal. So for efficiency a low PC and a low disp will be most efficient. And very roughly speaking a very deep hull not excessive in length that has a big aggressive rudder would be very seaworthy. Long hulls are very seaworthy too but don't respond to the rudder as well so need to, to a greater extent take care of themselves. It's a matter of control v/s directional stability.
     

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  7. peter radclyffe
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    peter radclyffe Senior Member

    buehler is joking, build a sailing trawler if thats what you want, with a variable pitch prop and 400 hp
     
  8. Tad
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    Tad Boat Designer

    Length (on waterline) and weight (displacement) are by far the largest factors in the efficiency (resistance or drag) of any hull. The stern shape will make almost no difference at trolling speed but will make a small difference at cruising speed. As you mention modern boats are usually wider and shallower, giving more deck, interior, (and with higher topsides) hold space. The wider form means not just more drag from the stern but also from a blunter entrance and the additional beam (As PAR says above, it's more than one thing). Wider and shallower hulls tend to follow the surface contours more closely, so they roll quicker but not as far as the older and deeper hulls. And a wider stern is more apt to be pushed around by a sea (when running off).

    White Mist shows a really nice compromise, designed and built in the early 1960's. Her mid section is long and clean, with the blocked up cruiser stern.

    Whitemist.jpg

    A Sister Ship, Ocean Troller, showing another view of the same hull. Both are 41' by 12'

    OceanTrollerstern.jpg

    The older style double-ender with stern post, built in the late 1950's.

    Fullmoonstern.jpg

    And Richmond Star is a more modern (Late 1960's) wider and shallower hull. Probably 2-3' wider than the White Mist/Ocean Troller hull.

    RichmondStar.jpg

    Fearless is a larger (54' by 14') Auxiliary Schooner, designed by Frank Fredette in the early 1970's. This would be close to the most efficient stern of the bunch, but I don't particularly like the look and it would be complex to build. These deep hulls of moderate beam are very comfortable in a sea.

    Fearless.jpg
     
  9. nzboy
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    nzboy Senior Member

    my 2 cents also

    Interesting topic. In New Zealand there was a typical trawler built in the 60s into the 70s deep forefoot rising to a broad flat stern .House forward, motor then hold with fuel tanks each side of hold .Most trawlers cruisers are a take on this design Nordhavn engine rooms have tanks each side .Idea is you go out with fuel as ballast and come back with fish .How ever if you go out with empty tanks, the whole stability equation is upset and it is a design that doesn't handle a following sea .A Friend of mine built a number of boats around 40ft, single chine steel not to deep a forefoot but that depth continuing the length and rise steeply at the stern .To be almost out of the water designs not unlike George buehlers except beam narrows toward stern. It in some ways a chine version of dashews boats .These boats are still fishing 40 years on in some of the most dangerous waters in the world.
     
  10. Easy Rider
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    Easy Rider Senior Member

    I enjoyed your post and like most sailboats the 54' auxiliary has a very easy run aft but still enjoys a wide stern above to work from.

    I'm curious about the extremely raked stem and relatively non existent forefoot. If she had a vertical stem and the resulting deep forefoot I would think she'd turn reluctantly but enjoy great directional stability. Under sail the auxiliary would need to be able to turn fast enough to catch the wind but the powerboat has no need for that. How would these elements stack up in rather large following seas?
     

  11. Bob M
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    Bob M New Member

    Thank you all for your input. I think you have given my son the information he needs when reviewing plans and he talks to his designer.
     
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