sportfishing boats with high towers?

Discussion in 'Boat Design' started by jav, Aug 28, 2007.

  1. jav
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    jav Junior Member

    I have a general question about adding weight up high on a smallish sportfishing boat 30-35'. Sorry if some of my terms are in-correct. I often read that top heavy boats roll "more" and I wish to understand this as it differs from what I've observed. I added a small hardtop (not a tower) to my 32' x 13' beam sportfish. I was concerned about the weight up high, researched and even spoke to a few naval architects about this. Basically, I was told that without knowing the design parameters on the hull, an analysis of the additional weight up high would be very difficult (read quite expensive). In general though, I was told that it would change the roll period and it would roll "more".


    I have found that it actually rolls "less"- which is to say that it feels like the weight up high seems to act like a damping moment in most conditions. My top is relatively short in comparison some of the towers that I've seen that are another level above the roof line of a flybridge. Do the builders of these pipe towers really investigate the effects of the high weight? I presume the risk is that IF the vessel heals over enough, the high weight makes the vessel resist self righting? How does one determine when that effect is too great for a given hull?
     
  2. charmc
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    charmc Senior Member

  3. Texasgaloot
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    Texasgaloot Yacht Design Student

    If you consider center of gravity exclusively, your premise would be true. However, the design parameters other architects are thinking of include things like the increase in displacement of the entire boat (does "squatting" the hull a bit more increase or decrease stability relative to the particular hull form in question,) reserve bouyance (again, hull form is necessary to that analysis,) the location of the center of gravity of your top along the waterline, simple inertia (things at rest...) etc. My guess is that your hull is benefitting from the additional "ballast" although it's a bit higher than one normally associates with ballast.
     
  4. jav
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    jav Junior Member

    Charmc- thanks for the great article! It was very easy to grasp the concepts!


    Texas- I think your inertia comment best describes what I percieve the top to be doing with respect to roll motion/period. As far as increased displacement and it's effects, in my pea brain this would seem minimal since the top only added approx. 300 lbs. to a 15-16,000 lb boat (small change as a function of precentages??).

    Your both spot on with the effects I'm trying to describe- I don't know if I should have posted this to the stability forum but the crux of my question is, when an end user has a certian hull, the design parameters (cg/cb) for that hull are quite often not avialable. How does one go about assessing the implict of the added weight up high of a tall pipe tower etc..? Do you think a stibilty analysis is done rarely, some times , every tiime?
     
  5. messabout
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    messabout Senior Member

    Jav;
    I think there are some carefully designed sportfisher boats with towers included in the original calculations. I think there are plenty of similar or not so similar boats whose owners decided to instal a tower without regard to the wisdom of doing so.

    I shudder when I see so called flats boats with a tall pipe tower way aft. There are a gob of those here in Florida. Some of them have the control station installed six or eight feet above the waterline and the operator drives from the elevated position. Better to find the fish of course, but some of those things are clearly a disaster looking for a place to happen.

    Jav, you are one of the smart ones. You are asking the right questions.
     
  6. jav
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    jav Junior Member

    Messabout,

    thanks for the kind words but I'm not sure how "smart" I am. I suppose I'm smart enough to ask and be concerned, but having added the top without any real data with which to understand the limits of acceptability is more lucky than smart I suppose.

    I have had it on for 3 years with and have been generally happy with how the boat has handled it but it's always in the back of my mind that in the right set of conditions... who knows.
     
  7. charmc
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    charmc Senior Member

    Jav,

    Just a thought: you didn't mention the manufacturer of your sportfisherman, but I've noticed over the years that manufacturers with a good reputation have offered options on the same hull that have affected weight and CG/CB. 30'-35' cruisers have been offered as express cruisers (open helm, windshield only), sedans (covered helm), and with a flying bridge (usually with upper and lower helm). Pacemaker, Luhr's, Chris Craft, and several Downeast Maine builders come to mind; Rooney's Blackhorse 32 is currently available in those 3 configurations.

    My point is that many good boatbuilders probably do those calculations and design the hull to be safely stable with any of the optional configurations. There will be some differences in roll motion, but all within known parameters. Contacting the manufacturer might answer all your questions. In any event, your description of your modification and the result seems to conform to this theory.
     
  8. Poida
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    Poida Senior Member

    Jav did you call me? Sorry I thought you were calling for pea brain.

    My question is, do you really need a flybridge on a boat?

    I understand that you do need a bird's eye view, so to speak when navigating around reefs and searching for fish.

    However satallite navigation and fish finders would surely reduce the need for them and, reducing the cost of duplicate controls.

    In boat safety courses they recommend sitting down as low in the boat as possible if the weather gets rough, so obviously standing on a flybridge would not be advisable.

    Wind resistance would also be a factor in fuel consuption.

    There yer go

    Poida
     
  9. jav
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    jav Junior Member

    Charlie,

    the boat builder in this case (Trojan) has long since gone out of business. I did try to obtain some of the hull design parameters from some sources but struck out.

    Poida -

    No... calling myself pea brain. As far as needing a flybridge... I suppose it's personal preference especially since ours is rarely used to spot fish. But we, like many recreational boaters, really enjoy the flybridge and the benefits it provides. Your right though, there definitely some down sides to havng one as well.

    As an engineer, I tend to question things and this "high weight" thing is something I've never really gotten a good answer on. It looks like it should be a concern but with all the towers you see, it seems like the concerned are in the minority? Heck - I look at some of the modern cruise ships and visually, it looks like the CG is way too high above water line and given the hulls width and draft, it doesn't look like it would take much get the CG outside of the CB.... but what do I know.
     
  10. charmc
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    charmc Senior Member

    Jav,

    Trojan was the mfr at the back of my mind that I could not recall. They built many excellent boats, many with optional superstructure configurations. Here is one example. It's a review which mentions that a flybridge is a factory option on the F30 express cruiser. Much more information available at this site: http://www.trojanboats.net/
     

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  11. lewisboats
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    lewisboats Obsessed Member

    Going back to your original question and situation...I think by adding a bit of weight up high you reduced the apparent stiffness of your hull, which can seem to reduce the roll but actually increased it a bit. Think of roll as a pivoting motion along the centerline of the hull relative to the surface of the water. If you have a stiff boat, the hull rides flatter relative to the surface of the water...regardless the angle that surface is relative to true horizontal. If the starboard side of the boat is nearing the crest of a wave and the port side is in the trough...the boat is tilted to port relative to horizontal. With a softer motion, the Starboard side is digging in and the port side is almost out of the water but the hull is in a flatter position relative to horizontal so it seems to roll less, when in fact it is rolling more (relative to the surface of the water)...just counter to the waves. Once the inertia is overcome, then the boat rolls the other way but you are probably on the down side and the apparent roll is dampened there too. There is a point tho when the roll and the waves will cooincide and then you'll really feel it. This is called cooinciding periods...and usually applies more to sailboats and soft chined displacement boats but is possible in all boats. I hope I made myself clear and don't sound too finger tied. In your instance I think you added some inertia which would initially slow down your period but as the water got bigger might come back to bite you although...you didn't add much weight relative to the displacement and it isn't really high...so I bet you are just fine...just don't go looking for the "Perfect Storm".

    Steve
     
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  12. charmc
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    charmc Senior Member

    Steve,

    Thanks, that's a good explanation of one of the benefits of making a vessel a little less stiff. I've heard offshore crew (check some threads on stability and the Bourbon Dolphin capsizing) speak of boats having too much stability; i.e. they roll only a little, but the motion is very fast. Crews actually prefer a boat that will roll more; in part because the motion is slower, and in part because of the effect you described.
     
  13. jav
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    jav Junior Member

    Steve,

    it took me a few times reading your post to "get it". I think I understand and it appears the terminology is counter-intuitive. In my mind, "roll" was measured relative to true horizontal, and not the waters surface angle. By adding the mass up high, I get the sense that the boat responds slower to changes in the water surface angles- which in looking at how you described it, actually means the relative angles between the boat and water surface is now greater. My preception and description of "roll" was a based in the true hroizontal and this may have been a misunderstanding of how the term "roll" is used in a nautical sense.

    So boatds that have stabilizers (like the Naid system) to reduce roll actually increase it despite the greater comfort of less perceived motion?

    I think I understand the cooincident period... In laymans terms - is that when the roll period of the vessel matches or couples with certain wave periods and the two feed off each other in esseance amplifying, instead of damping motion?
     
  14. lewisboats
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    lewisboats Obsessed Member

    Yes and no...Those are active stabilizers...basically flywheels that greatly reduce rolling (relative to true horizontal) and can be tuned to conditions to prevent coincidences from occuring; they stop the boat from moving with the waves...or slow it tremendously(try spinning a 10 speed wheel and then turning it sideways). Who you talk to will determine the definition of roll so you might prefer to establish the relativity to what first.

    You got it :)

    Steve

    PS: A quote from Dave Gerr: "...If you own the average powerboat with a roll time greater than 1.1 times the beam it would probably not be prudent to operate her in conditions that will roll her deeply; she could pass the point of no return. On the other hand, if your roll time works out to well under 1 times the beam, you may want to ask yourself if your vessel is as comfortable as you'd like. She probably has a corky snap roll-more stability than you want. One way to fix this is to add weights up high- just don't add enough to slow her roll time to more than 1.1 times the beam."
     

  15. jav
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    jav Junior Member

    Steve-

    thanks for that quote from Gerr! I've read his prop handbook but none of his other works. Thats the first refernce I've seen that provides some guidance as to what is and isn't accepetable. Since no reference to terms is given, am I correct in presuming that beam refers to -beam at the water line? And x 1.1 would be equal to seconds per foot? Example -10' waterline beam means roll period should not exceed 11 seconds? Also, when I measured roll period, I rocked the boat rather substantially, let go and then timed 10 full rock cycles (port gunwale high point to port gunwale high point equals 1 full cycle) then divided the total time by 10... as that correct? Is there any definition of a low limit roll period for power boats? I seem to recall my roll period was well below this rule of thumb ? I want to say my roll period was on the order of 6 seconds (but it's been a while.).. thats well below the beam x 1.0 but my boat doesn't appear to snap roll?

    Thanks for all the great info!
     
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