Dynamic lift or not with transom bracket

Discussion in 'Powerboats' started by jfblouin, Sep 19, 2006.

  1. jfblouin
    Joined: Sep 2004
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    jfblouin Senior Member

    I tried to find information about use of dynamic lift or not with outboard transom bracket without succes.

    What is the pro and con of this two type of design?

    The bigger one with full bottom planing surface will add more buoyancy and aft lift but also add more drag.

    The smaller one help to clean water flow for propeller.
     

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  2. JR-Shine
    Joined: May 2004
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    Location: Vero Beach, FL

    JR-Shine SHINE

    I am sure there is no dynamic lift from the bracket. It angles upward from the edge of the transom, if water flows along it that way, its will actually cause a great amount of suction and boat will not be able to get on plane. Picture an airplane with its flaps down, but upside down

    There is normally a vertical gap between the end of the transom and the start of the bracket, this allows the water to "break free" and not follow up the bracket

    Hope that makes sense.

    Any added buoyancy is net zero gain due the engines being placed further back.
     
  3. jfblouin
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    jfblouin Senior Member

    What can we think about this kind of bracket?
     

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  4. LostInBoston
    Joined: Aug 2006
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    LostInBoston Junior Member

    The design is couter productive. Its a straight shot back, so the drive needs to be mounted lower, which gives it more bow lift, which is couteractred by the extra planing surface.
    JR-Shine, got it right, you want the water to break free of the hull and flow up into the drives without causing suction. Looks at the big, faster CC's the all have a notch before the drive. This allows the water to break free of the hull, airates the pocket to avoid suction and mount the drives higher to reduce bow lift and ride better.
     
  5. jfblouin
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    jfblouin Senior Member

    Because the bracket plane is in-line with boat hull, why I cannot assume that it is just like a longer boat?
     
  6. tom28571
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    tom28571 Senior Member

    I think I understand your english ok and nothing wrong with you reasoning either. Everyone's thoughts seem to be correct in a proper context.

    I find the use of transom brackets a bit confusing. Is there a really good explanation of the working of these devices somewhere? I mean a thorough discussion with backup data of the different types of brackets on different types of boat hulls.

    Just sticking one on an existing hull, and obtaining improving performance, seems to me to indicate that the boat was not designed properly in the first place. In one ecounter with a boat equipped with a bracket, the prop had a distressing tendency to ventilate at low speed in waves. This one thing would cause me to take it off regardless of any other good attribute it may have had. When it occured, the throttle had to be chopped until ventilation ceased and speed slowly built back up - a real nuisance.
     
  7. LostInBoston
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    LostInBoston Junior Member

    Yes, you can think of it that way. But adding a longer running surface liek that pushes the center of gravity further foward, but then the drive is mounted low to compensate for that. Like I said, they counteract one another. A bracket should allow the drives to be mounted high so the boat rides more level.
     
  8. Zac Penn
    Joined: Nov 2015
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    Location: Jacksonville, FL

    Zac Penn Junior Member

    Foreword...
    Please don't look down on me for bumping such an old thread especially as my first post here :D

    I have been searching and can't seem to find a long winded, really drive it into my brain, explanation on lift vs drag on a flush mounted transom bracket.

    I am working on a 1969 Seabird 23' project boat and I am starting from a bare hull. I am going to have a single outboard on a bracket off the back and I am trying to understand the causes and effects of different bracket designs. I personally have never been on a functioning Seabird 23 but from what I have read it is stern heavy and has a tendency to porpoise.

    There are many ways to counteract this but I am interested in trying to create stern lift from the bracket. I am interested in extending the total LWL by flush mounting a bracket to match the existing hull bottom surface (fare it so there is no turbulence at the joint). Here are two screen shots of my idea for your evaluation...
    Flush Outboard Brackets.png

    Flush Outboard Brackets1.png

    Hopefully we can have a good discussion about this.
    Zac
     
  9. JR-Shine
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    JR-Shine SHINE

    the stern heavy problem is probably due to people mounting heavier outboards than what the boat was initially designed for, a bracket with a flotation chamber would fix that.

    The porpoising is more than likely a weigh distribution problem, but I dont know that, just speculation based on experience

    The main worry I would have with extending the running surface that far is the added volume may make the boat pitch too much forward in a following sea. Funny things can happen when running fast in a quartering down sea if you have a lot of volume in the stern.

    I would be willing to bet there is someone out there with a 23 seabird who has installed a regular flotation bracket (you can get more volume by using a bracket with a twin motor pontoon).

    those are my thoughts and I would be interested in following the performance of whatever you decide to do
     
  10. BMcF
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    BMcF Senior Member

    I took a Rampone 23 Deep-V and removed the two inboard Volvo 140s (that were mounted well forward, under an oversized center console) and put two Yamaha 150 outboards on a typical transom bracket (i.e..narrow as possible and well above transom bottom). I also added two 16" Bennett trim tabs, mounted to either side of the transom bracket. The result was pure joy...

    So were it me, I would not add a transom bracket that extended the hull bottom.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    The best plan would be to look at, and speak to owners of, boats of this model that have been podded. You can't really judge what is best for an individual boat without knowing the characteristics of it, a set-up that enhances one boat, could be detrimental to another.
     
  12. Zac Penn
    Joined: Nov 2015
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    Zac Penn Junior Member


    I am glad you chimed in. I am following your new boat design/build and look forward to your progress.

    I am not experienced enough to fully understand what you are talking about. With the "wide body" bracket it seems to me like I am just building a 25' boat instead of a 23' boat. If I adjusted the weight distribution to account for the extra LWL by maybe adding a larger fuel tank and livewell in the leaning post, and maybe bring the console further back, then to me it should behave like a 25' boat would.

    With the "narrow body" bracket I would just be trying to add volume to the stern for a little more lift, both while at rest and at cruise. I love the way that you designed a good sized pad at the rear section of your new boat design, and don't understand why more boats do it. The last 1/4 or so of the hull will always remain underwater (except in really fun conditions) so you don't need the V back there. The Seabird has a small pad in the back but I think I would have to be going 40+ mph for it to really effect lift very much. That is why I was thinking of carrying it further back with the transom bracket.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    You can always put your small pod continuous with the bottom, into effect, and then raise it a couple of inches if it is having the effect of holding the nose down. running down-sea. Allow for that option in building it, imo, you won't need to raise the engine on the pod, or put extra holes thru the hull. if you do it right.
     
  14. Easy Rider
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    Easy Rider Senior Member

    I'm not a fan of these extensions.
    It's more work of course to just lengthen the hull at the stern and it appears far better to me. All the problems associated w the extensions disappear or become a positive instead of a negative. CG, seaworthiness, lateral stability, pitch stability ect ect. Even efficiency probably gets better than the tack on extension.

    And if you do a good job you even get a better looking boat. I have yet to see an extension that isn't hard to look at. I vote for lengthening the whole boat.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    Depends on the boat, I think. Without knowing the peculiarities of a hull, it is too hard to say. By extending the whole boat, you have created a new boat, and I'd say the chances of it being a better boat, are far from guaranteed.
     
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