Trawler questions

Discussion in 'Stability' started by alrod, Dec 5, 2008.

  1. joz
    Joined: Jul 2002
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    joz Senior Member

    Thanks for that Tad
  2. tranmkp
    Joined: May 2002
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    tranmkp "wherever you go. there you are"

    still a nagging question in my head. Concerning long keels in trawlers. I am under the impression that one does not add ballast in the keel for stability. That the sability comes from the hull form and that the keel is there for directional stability. ok I can sort of see this and how it relates to semi-displacement hulls. But what is the difference in really heavy full dilplacement hulls? They have keels too but because they are immersed so deeply that the actual hull gives it the stability? I am becoming befuddled as I know that sailboats are displacement hulls and so are tugbats. Yes one is more shallower and uses external ballast and the other is way down there - but boat are limited to hull speed. Sooo, if a semidisplacement boat (sisu, duffy...) gets most of its stability from static hull form why not add ballast in the keel to control roll?
  3. Tad
    Joined: Mar 2002
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    Tad Boat Designer


    Lot's of factors at work affecting the stability characteristics of a particular boat. Usually a conflicting requirement gets involved in limiting stability. In the case of semi-displacement boats, more ballast is not added because that's adding weight, weight limits speed, speed is more important than stability, and so it goes.

    Transverse stability is not really related to whether a boat is sail or power, displacement or planing hull. That is to say that every hull (no matter it's form) is acted upon by the same factors. A key factor in transverse stability is the length of GM. Raising M means a longer GM and thus more stability, lowering G also means a longer GM and more stability. M is dependent on hull form (BM = I/V), G is dependent on the vertical CG of all items that make up the boat.

    B is the center of buoyancy (center of underwater volume)
    M is the metacenter (theoretical point about which the hull rotates)
    I is inertia of waterplane (a barge has a different waterplane from a kayak)
    V is immersed volume (displacement)

    Change any of these factors, or change G, and stability changes.

    In a shallow semi-planing powerboat B and I are high because the waterplane is wide and full (higher CP), V is low, but BM ends up fairly long as B is not far under water. If G is not too high (big deckhouse) GM is short (low ultimate stability) and roll is snappy.

    In a modern sailboat (shallow canoe body) I is low (fine waterplane) and B is lower than the semi-planing boat, V is moderate and BM is not that long. But G is much lower than the powerboat thus GM is longer (Higher ultimate stability) but motion much different.
  4. kraftee
    Joined: Dec 2009
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    kraftee Junior Member

    Hi, I'm new to the Boards - and I know that this is an OLD thread - but after reading it and seeing the misinfomation, well, I just HAD to respond.

    I am Sales Manager for Mirage Manufacturing, builder of the Great Harbour Trawlers and Mirage Sportfish boats. I spend a LOT of time aboard - in fact, I log about 5,000 miles per year aboard the various Great Harbour models - inshore, nearshore and in multi-night offshore passages. I CAN talk enthusiastically and knowledgably about our products.

    The gentleman who started this thread asked a simple question. He wanted to know if a Great Harbour trawler was suitable for use in the Caribbean and to traverse the Panama Canal to head up the West Coast. Well, instead of correct information from posters who might have been aboard a Great Harbour offshore, he gets UNINFORMED CONJECTURE from posters with EXACTLY ZERO EXPERIENCE!

    Rather than throw stones here, let me make a few simple statements of fact. Great Harbours are not "...high sided box(es) floating on top of the water..." They are VERY HEAVY, FULL-DISPLACEMENT, UNSINKABLE, OFFSHORE-CAPABLE yachts with simple, owner-serviceable systems, HUGE engine rooms, reliable home appliances, VERY livable spaces, TONS of storage, TWIN small diesels, shallow draft and easily-maintained, NO-WOOD exteriors. Our hulls are SOLID fiberglass (two-inches thick at the bows) - not cored. We only have coring from the rubrails up - and that coring is plastic honeycomb, vacuum-bagged NidaCore. No maintenace, no rot, excellent sound and heat insulation. The coring in the decks and superstructure - combined with no ballast is what gives us our unsinkability. We will float even when fully swamped. Physics.

    Best of all, our Form-Stable hulls are naturally stable both underway and at rest and do not rely on expensive, complex, dangerous stabilizer systems. Believe me, we build HEAVY, FULL-DISPLACMENT boats. We displace exactly as much water as other full-displacement boats of similar size. We just take our displcement OUT rather than down. Again. Physics.

    But that's enough ranting from me. If the original poster still logs on to these boards, I am more than happy to discuss the benefits, features and limitations of our boats. Alternatively, I am happy to arrange for him to correspond with any of our long-term owners - or Ken Fickett (owner of our company) or even Lou Codega (our hull designer). Finally, the answer to his original question is a resounding YES. Our boats were DESIGNED for exactly the type of trip he envisioned.

    Sorry if I ruffled any feathers here, but I really think that people should not answer posts unless they have something FACTUAL - or at least within their experience - to add to the conversation.

  5. Sheepy
    Joined: Apr 2009
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    Sheepy Junior Member

    All boats ARE sinkable. End of story! I don't care what you say otherwise.
  6. apex1

    apex1 Guest

    YOU did not add anything factual here! Just a rant and salesmen drivel!

    And calling the contributors here unexperienced...............
    well, it makes your product look poorer than it might be!

  7. kraftee
    Joined: Dec 2009
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    kraftee Junior Member

    Well I guess you are entitled to your OPINIONS, but when I said "inexperienced", I think it is quite plain that I meant inexperienced with this particular brand/type of boats. And you cannot argue that.

    ALL boats are sinkable? Well, I guess you are right. If you loaded TONS of lead aboard, you could probably get it to sink. Otherwise, No.
  8. apex1

    apex1 Guest

    Inexperienced with the brand, you may be right in some cases. With the type of boats, the contributors are not!
  9. kraftee
    Joined: Dec 2009
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    kraftee Junior Member

    Look, I understand that there are MANY posters on this board with DECADES of boating experience and with HUGE amounts of design knowledge. I was referring specifically to the posters on this thread who offered sheer conjecture about our hard-chined, full displacement trawlers. They may have tons of boating experience and they may even have displacement trawler experience, but unless they have cruised extensively aboard a Great Harbour - or a modern hard-chined work/fishing boat - then I stand by my statement that they are "inexperienced".
  10. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    Eric (kraftee),

    No need to be quite so quick to anger, mate.

    Regarding construction: At the time this thread was started, the Great Harbour website and marketing materials were really hyping the light cored composites. I see that at present, the fact that the hulls are >1" solid features prominently on the website, and a reasonably detailed list of the materials and construction techniques used is also there. Not many builders are so open about what goes into their products. Still, I have spent enough time working with exotic composites to know to insist on a thorough inspection by a very good surveyor, even for a brand new boat, if such materials are involved in any way.

    Regarding hull form: Perhaps Tad's initial observations about the GH line might have been inaccurate, but I can't find much wrong with anything he's posted here. Shallow draught, plus wide beam, plus unballasted, plus a light superstructure, does suggest a sharp, snappy motion in a seaway. Then again, at 21 tonnes, the GH37 / N37 is very much on the heavy side for a boat of her length. And it has been well known for at least a century that longer, narrower hulls are more efficient and tend to maintain better VMG in rough conditions than do shorter, wider hulls of comparable displacement.

    Regarding systems: "Owner maintainable" can mean a lot of things. To an engineer like me, a lot more things are owner maintainable than would be the case for someone less mechanically minded. Great Harbour- like most luxury trawler builders- lists a fairly comprehensive inventory of fairly complex systems for all their boats, and the maintenance costs and time will scale accordingly. House-like spaces and appliances are sure to be appreciated by coastal cruisers, but will leave the crew cursing if they hit a blow offshore and start getting tossed around.

    I would love to spend some time on a GH trawler- from what I've read about them and from what owners and reviewers have written about them, they seem to be very well suited for coastal cruising and island hopping, plus the occasional longer passage in nice weather. Since GH never shows up at the boat shows around here and I have not seen a single one on Lake Ontario to date, I doubt I'll get the chance just yet.

    Richard (Apex),
    I think it's OK to turn that German temper down, my friend.... the point has been made and I think Eric sees where you're coming from.
  11. kraftee
    Joined: Dec 2009
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    kraftee Junior Member


    I am sincerely sorry if I offended you or any of the other posters on the board. But, you would not believe the misinfomation that is bandied about - especially online - about our boats due to two facts. 1) Our vertical hull sides give the (not altogether true) impression of more windage than similarly sized trawlers. 2) Our shallow draft leads many to assume that we are somehow displacing less water than other deep-keeled, ballasted "traditional" trawlers.

    Although it would probably not be productive to get into an in-depth (pun intended) discussion on these issues, I am happy to discuss anything specific that might puzzle you about our boats. Suffice to say that I have been in all kinds of conditions offshore in our boats. I am generally on a tight delivery schedule and I don't ever have the luxury of waiting for "favorable" weather. They truly live up to our hype about their comfort and stability.

    If you would truly like to experience a Great Harbour trawler underway, we have an excellent charter program available on our N37 and N47 models in the Abacos. Contact me if you are interested - I'll even throw in a discount! I am also always looking for crew on passages to the Bahamas or to boat shows, etc. We don't pay you for crewing with us - but we'll feed you!

  12. apex1

    apex1 Guest

    Nice offer Eric thank you!
    But for me boating starts at 88´ or 27 meter!

    I did not want to give you the impression that I have any negative comments on your boats. I do´nt know them, so how would I have even a opinion.

    Just the way you valued the experience of some of our well respected members here was a bit too harsh imho!

    There was no such "German temper" Matt! That looks (and feels) quite different.:D

  13. kraftee
    Joined: Dec 2009
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    kraftee Junior Member

    88' - Wow! That's not a boat - that's a ship! We do have a brand-new 74-footer on the drawing boards - and with its 24' beam, it's darned near as big as the typical 80-footer! If you're at all curious, take a look at this:
  14. apex1

    apex1 Guest

  15. kraftee
    Joined: Dec 2009
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    kraftee Junior Member

    Best of luck with your project.
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