Dynamic lift or not with transom bracket

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

  1. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    There is a great tendency to over-think things like this. A transom extension is, from the point of view of the water, just a longer boat. A transom bracket (raised above hull bottom) is a cantilever beam with the engine further aft.

    Which works best is dependent on a variety of factors and one is not necessarily superior all the time. Simply adding a bracket can make a good running boat a poor one with a tendency to porpoise and/or ventilate the prop at low speed in waves. A bracket can also improve running characteristics of a boat. Balance of weight and hydro factors is important. I don't buy the theory associated with the raised bracket unless the boat is designed for that in the first place, either on purpose or accidentally.

    In many, if not most cases, Easy Rider has it right and that includes some boats of my design. In every case, lengthening the boat or extending the transom/bottom with a platform work exactly the same, provided the hull balance is taken care of.
     
  2. tom28571
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    tom28571 Senior Member

    This can be true but just adding the cantilevered bracket is at least as much of a risk. The bigger and heavier the boat, the less of a risk but that is moving in the wrong direction in my small boat thinking.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    I think it better to err on the side of having too little lift aft, as a result of a podding, you can always use tabs to increase it, underway, but no easy way to reduce it. I recall a small boat that was very successful, then being extended 2' from the transom, with no other changes, and becoming a wetter and "boggier" boat as the bow failed to rise as easily as the shorter boat, when ploughing in to the back of a wave. You can use engine trim to some extent to counter that, but it is not as effective as tabs in the opposite case.
     
  4. Zac Penn
    Joined: Nov 2015
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    Zac Penn Junior Member

    One thing to keep in mind here is that I am rebuilding this boat from the hull up. It has been stripped completely so I can maneuver the weight anywhere I need to to counteract the extra lift in the back.

    I have already experienced the ventilation during rough seas on a 20' Hydra-Sports 2000 transom mounted 25" shaft outboard already and it happened at a very bad time. The waves were SERIOUSLY confused and coming from all directions. I had to shift into neutral then forward to regain control and it was very annoying to say the least. I do not plan on going out in really rough seas like that again but sometimes the weather will sneak up on you and I want my boat to be able to handle that situation better next time. Granted I am starting with a 23' boat this time, and I am hoping that if I can extend the transom to make it say a 25' boat it will not be an issue.
     
  5. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Prop ventilation can take away control of a boat just when you need it most. Does not happen often on a transom mount but a raised and extended bracket absolutely makes ventilation more likely. A transom extension does not have that problem any more than a boat of the same hull length.

    My theory is that brackets are raised above the hull bottom primarily for marketing purposes. A one size bracket can fit a variety of boats which makes marketing far easier and profitable. A bracket to match a hull bottom would be very difficult to produce in numbers that would turn a profit. The sales team then comes up with a performance advantage that boosts sales. Not saying that some advantage can not be realized with a standard bracket but the envelope of that advantage is limited and there are disadvantages.
     
  6. leaky
    Joined: Sep 2008
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    leaky Senior Member

    Adding a V'd bracket is basically extending the length of your hull.

    Speaking of typical sealed non-V'd engine brackets. The biggest advantage I'm aware of with a bracket is you can take a boat that originally had an outboard engine well or cutout, an inboard, I/O, or sea-drive package and very practically, quickly, and sanely transform it into a closed transom w/ outboard(s)..

    This gains deckspace, moves engine rigging outboard, and allows one to close off an engine cutout (which is a bad thing in a saltwater boat for sure)..

    The technical aspect of how the boat changes I think is very hard to figure out and gets a lot of false hype IMHO. I'm no expert on the subject but have added a couple brackets to my own boats and known others that have done the same.

    You hear things like they make boats too heavy in the transom or that the bracket adds floatation - pretty much all BS. Move an outboard engine back 30 inches with a bracket on a outboard-powered 25 foot 6000 lb hull and it's going to float about the same - the difference is now you can have no engine cutout, so it's going to be more seaworthy in that sense and maybe it's sitting a touch lower in the transom, a little less seaworthy in that sense.. It's not going to cause waves to crash over the transom or solve a problem with a deck set too low.

    You also hear they backup better.. I'm pretty sure after being on a bunch of boats with brackets they do not generally backup better and this is BS hype by the bracket builders. The water seems to just pound against the transom and you get a lot more cavitation in reverse than a standard setup.

    It is going to change the weight distribution of the hull and add a step though. Given a stepped hull and engine setback is real well proven to increase top end, I can't see how a bracket can't help with that - getting the prop back into cleaner water, but it's going to be dependent on what other issue you trade off.

    Holeshot - god knows how that changes, can get way better or way worse.

    I will say it seems to make a boat particularly touchy to engine height and trim with the setback. I put one on my 26 foot seawolf 10 years ago and it took a bunch of playing w/ props and engine height to find a happy medium - if I ever did it again I'd add a set of hydraulic jack plates. The difference in top end between what I get on a happy medium (to not spin out in chop) and raising the engines to an ideal height for topspeed is like 35 versus 40 knots.

    Jon
     
  7. leaky
    Joined: Sep 2008
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    leaky Senior Member

    As has been pointed out, you can also get into a situation w/ the bracket where effectively lengthening the hull may create a stronger tendency to torpedo a wave, ie run through the seas and not over them (better ride but wetter)...

    Most likely your problem was just your prop and/or engine height with the ventilation. The engine height and prop that gives you the best top-end is not the best to stay locked down in non-ideal conditions (ie top end you want the engine high, smaller diameter and less cup, in the chop you want the engine low, prop bigger and more cup which is usually less efficient at full speed).

    After have been through this - the safe bet in my experience is since you like the bracket idea is add a non-V'd bracket and budget $800 for a hydraulic jack plate. This will make the whole setup easier and give you options to adjust for sea conditions.

    Down the road if you decide you wish you had a V'd bracket, you can have yours modified to suit fairly easily, just don't mount the thing with 5200 (which is not at all necessary anyway).

    Something else to mention is usually with a bracket, because of raising the engines higher due to setback, you end up going from a 25 inch shaft to a 20 inch shaft. I think this is not usually necessary and you may get more benefit from mounting the engine higher and keeping the mechanical parts away from the water, versus lowering a little bit of the weight 5 inches.

    Jon
     
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    A boat that has a fulsome chine beam at the transom is a more likely candidate for podding than a slimmer one. This is especially illustrated by cats, where boats that performed extremely well with transom-hung engines turned into a crock when motors were stepped back, without any extra support under them.
     

  9. JR-Shine
    Joined: May 2004
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    JR-Shine SHINE

    as far as loosing bite, prop selection and motor height has a whole lot to do with it. A 4 blade prop goes a long way in helping. My SC23 never lost bite with the 4 blade, while a standard 3 would brake loose.

    If it were my boat, I would go with a regular bracket with the added buoyancy of either a larger (twin) pontoon or one that has extra volume (like the hermco)
     
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