Stability at Rest Improvement for 28 Bertram

Discussion in 'Stability' started by Keelboater, Apr 8, 2015.

  1. Keelboater
    Joined: Apr 2015
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    Keelboater Junior Member

    Hello fellow boat junkies and designers,

    I'm new to this site and stumbled upon it while searching for information on improving stability at rest for my 28' Bertram. I have been running a downeast boat with a keel for a few years while completing a major refit of the Bertram. Needless to say, the rock solid drifting characteristics of the downeast hull have really grown on me. As I was getting ready to wrap things up in the bilge area of the Bertram project, I began to think about stability and the possibility of adding a relatively shallow keel to dampen the harsh roll. My concerns are how it will perform at speed - especially while turning, and if keel walking or following seas would become a problem all of a sudden. We usually cruise at 21 knots, so I'm not planning to build a very thick keel for that reason, maybe just 1.75" would work. I was also thinking of adding water ballast instead of a keel (for the drift only), which might help reduce the roll, but I'm not sure how much this would actually help the issue or increase any snap roll. The proposed keel would run from the lowest point of the hull and continue back until it reached a max. height of 6" to 8" which would be about where the shafts protrude through the bottom of the boat - well ahead of the props. Overall draft of the hull would not change, with the bottom of the keel being about the same depth of the bottom edge of the rudders. And speaking of rudders, I did make oversized rudders by extending the trailing edge back by 2" which should help drastically in the steering dept. Can anyone shed some light on this for me? Thanks in advance for any useful information. This seems to be a unique forum. As a Mechanical Engineer, I can tell that I'll be spending some time reading here. :)
     

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  2. JSL
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    JSL Senior Member

    Leave as is and see if you can adjust to the boat. There are a lot of them out there and they have been serving people well for many years.
    A small keel (skeg) will probably not dampen the roll but could lead to chine walking-as you mentioned.
    Increase in rudder area could overload the stock & steering but if you are sticking to slow speeds you might be okay.
    Water ballast could work but it depends where it goes, the amount, and shape of tanks. Done improperly, you could reduce your stability
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    If this was going to be beneficial, maybe after all these years they would have made them like this ? The basic problem is the 25 degree vee-bottom, which means there is not much boat, and bouyancy,in the water, at the sides, to resist heeling, initially. The boat you praise as rock-solid is no doubt much flatter in the bottom. You will get a damping effect, but it won't be much, imo. Water ballast can work, but you have to find space to accomodate hundreds of gallons of it, situated near the centre of the boat, which will be a problem, that space is already spoken for by engines, fuel tanks, etc. Also you get a change in steering, it will be slower to turn, hopefully no interference to the propellors. It might even want to lay over in strong cross winds with the change in the underwater profile. You wonder about the wisdom of a deep vee with a modest cruise speed of just 21 knots, there seems little point to it. Oh, and that skeg will slow you down with additional wetted area drag.
     
  4. Keelboater
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    Keelboater Junior Member

    Thanks guys. These are the kinds of replies that I was expecting. I've owned the boat for 11 years already. Normally we use it as a pocket cruiser. Point A to point B is great, and in the anchorage it's rarely a problem. Aftermarket rudders are sold to replace the tiny stock high speed rudders. I just opted to modify mine instead. I'm sure they will be a big improvement over the stock design, and don't think torsional fatigue failure of the shafts will become a problem. Relatively speaking she is no speed demon at 21 knots, but she gets through the slop just fine at that speed. I was hoping to come up with a solution to help reduce or at least dampen the snap roll on the drift. Adding the somewhat smallish keel will shave some speed, but if that is the only ill side effect it would be fine by me. Sometimes I tend to think too much, but as an engineer it's what I tend to do. Generally speaking we tend to believe that "if it would work that way to begin with, don't you think they would have built them all that way?" However, the view point of "the consumer" and the view point of the designer come from completely different angles and it's not always that simple to answer. Cost cutting has ruined many a good product over the years and the aftermarket is alive and well with viable improvements for just about anything.
     
  5. JSL
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    JSL Senior Member

    the rudders
    make sure they are not foil cross section - not good on fast(er) boats. A couple of boats we put a 'fishtail' at the trailing edge. A big help in low speed steering.
     
  6. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You can't do anything with this that will do much good, imo, but you could devalue it with a skeg that really should not be there. It is the nature of the beast that it is, to do what it does, flopping around at rest, dictated by the distribution of bouyancy below the waterline.
     
  7. Keelboater
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    Keelboater Junior Member

    Agreed on the resale devaluation. As the old saying goes, there is an *** for every seat. But some seats can become tricky or next to impossible to fill if modifications are done incorrectly or just plain don't look right. It was wishful thinking there for a while, but I think the keel idea has just been keel hauled. ;) This boat has already survived 35 years without a keel, and it may have another 35 to go after this refit - and by then it will no longer be my *** in the seat!

    The cross section of the stock rudder is actually wedge shaped with a thin leading edge and a thick trailing edge. The modified version is not a true foil, but it does flatten out aft of the weld joint. I'm not sure if you can make it out in the photo or not. If anything, it's a reverse foil. We'll see how it handles before making any changes. Epoxy fairing compound can work wonders if need be.
     

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  8. rasorinc
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    rasorinc Senior Member

    Why not hang outriggers out off the sides with heavy weights hanging down into the water 5' deep for anti-sway or role. I forgot what these are called.
     
  9. Keelboater
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    Keelboater Junior Member

    You must mean those rocker stoppers. Kind of like hanging mushroom anchors off the side of the boat :eek: That's just not my style and it certainly can't improve the fishing, but I appreciate your input.
     
  10. rasorinc
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    rasorinc Senior Member

    OK similar idea but in reverse, put on removable out riggers with small floats on the ends
    that lock in the down position and lift clear of the water and lock in the vertical. the floats can be calculated for size. Trying to find away to avoid adding a skeg
    which I don't think will help you much with roll.
     
  11. Keelboater
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    Keelboater Junior Member

    Similar to the float and mast of an Aqua Cat sailboat but horizontal? The way this boat rolls so violently at times would most likely tear it apart. Also when rafted up to other boats, this concept would not work. I prefer not to go with the add on route. If I do anything at all, it will become a permanent part of the hull with nothing to store when not in use. Thanks again though.
     
  12. Keelboater
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    Keelboater Junior Member

    Here is a stock Bertram 35 with keel, similar to what I am proposing for the 28. It's relatively small, yet helps dampen the roll at rest. The 25, 28, and 33 Berts did not have keels. The hull shape of the 28 is similar to that of the 35 with the deep forefoot. I have heard that the 35 handles much better than the 33, so something must be right about this concept. Any comments?
     

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  13. JamesG123
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    JamesG123 Senior Member

    Small chine runners on the hull sides beneath the water line that will be out of the water at speed on the step?
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Can't really see that well from the pic, but is that skeg more about grounding, and propellor protection ?
     

  15. Keelboater
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    Keelboater Junior Member

    No. The skeg provides no prop protection at all. It is a twin screw deep V design and the props are wide open for impact with floating debris. As for grounding, if this hull beaches itself on the skeg, it will most certainly take out at least one of the props in the process. The intent must be for either tracking, roll damping, or both. Apparently it works fine according to those who have operated this hull. I will try to obtain a better picture to post when I get the chance.
     
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