Transom extension on plate boat

Discussion in 'Boat Design' started by stocko, Feb 12, 2021.

  1. stocko
    Joined: Feb 2021
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    stocko Junior Member

    Hi all chasing some thoughts on extending my transom on my 5.5m plate boat. Boat has a half transom for motor and extended chines,. Boat is bum heavy and throws the bow offshore and porpoises, I’ve tried lifting lowering the motor, 4 blade prop, foils etc. I’m thinking the boat needs more planning surface and getting the area closed in from extended chines to existing transom pod and I will be able to get the tabs further back Thoughts
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, so what you have there is a pod that is a simple continuation of the hull bottom, I am at a loss to understand the thinking behind that, if it is indeed the original configuration of it, losing all that bottom either side of the hull extension. My initial thought is that larger tabs might be the answer, but that would be an expensive experiment, what you could do is rivet on a wider (across the width of the boat) plate to the existing tabs, and see if that does the job. But that would likely put too much strain on what are fairly small tabs, for ongoing use. But at least you may find out whether bigger tabs would do it. If that isn't enough, then filling in the voids is seemingly an option. However, if the boat is inclined to broach at all, filling those in might possibly make it worse. Although it is not likely to be the case, there is a reasonable amount of vee aft. Just my opinion, but try growing those tabs, they do appear to be small, and especially considering they are not right aft.
     
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  3. ondarvr
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    ondarvr Senior Member

    That isn't an uncommon design for aluminum boats in that size range, and occasionally they have some running tendencies that need to be fine tuned.

    These hulls frequently can be purchased with several different power options, including flotation brackets.

    Sometimes they suffer from a heavy stern, squatting, and porpoising due to the bracket surface being too small.

    And sometimes the bracket isn't welded on at the correct angle.
     
    Last edited: Feb 12, 2021
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    There certainly are plenty of podded boats, but most don't use the level-with-the-bottom idea these days, and in this case if the boat is squatting and porpoising, full-width transom hung would have been better. 5m with any pod is getting a bit marginal, and I assume the extra 0.5m in this case is the pod.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Your trim tabs are waaaaaaaaaaaaaaaaaaaaaaaaaaaaaay too small:

    upload_2021-2-13_10-13-52.png

    They should be a near tight fit across the width of the section. Whereas yours.... hmmmm :(
     
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  6. ondarvr
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    ondarvr Senior Member

    Short boats tend to benefit more from the extra running surface of the bracket. But as you said, it should be larger.
     
  7. Barry
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    Barry Senior Member

    Where is the 5.5 m being measured from, Bow to the transom or bow to the back of the pod?
    What is the beam and deadrise?
    Fill in the pockets, ie make the pod and pockets as one, as in extending the hull.
    Move the trim tabs back, you can probably use your rams/drive and get as wide as tabs that can comfortably fit on the back.
     
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  8. stocko
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    stocko Junior Member

    Boat is 5.2m from how to back of transom plus 500mm for the pod
     
  9. Barry
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    Barry Senior Member

    And deadrise and chine width, I originally said beam but am recovering my error here
    This is important as you mentioned porpoising
     
  10. stocko
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    stocko Junior Member

    20deg dead rise . 160mm chines
     
  11. Barry
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    Barry Senior Member

    chine width at the transom or did you mean 1600mm?
     
  12. stocko
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    stocko Junior Member

    Chine width at transom 160mm

    beam 2.25

    thanks
     
  13. Barry
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    Barry Senior Member

    We are getting there but the interpretation or wording may not have been clear
    The beam of a boat is taken normally at the gunwales and hence my post saying that I wanted the beam was incorrect, which I corrected and said that I want the chine WIDTH, at the transom.
    The Chine width at the transom is the distance from the outside of the chine on one side to the outside of the chine on the other. 160mm is not the chine WIDTH that I am looking for.

    Perhaps when you put the beam as 2.25 and without the units, I would assume that you mean meters. I do not want to assume that this reference to beam is the CHINE WIDTH defined above
    So a Table

    Length of the boat from the bow to the transom ie before the pod 5.2 meters 17 feet
    Length of the pod from the transom to the end of the pod 500 mm 1 foot 7 inches
    Chine width at the transom, outside to out side ?????
    Deadrise 20 degrees

    The chine width is important as wider boats of the same length tend to be more prone to porpoising than narrow boats. Perhaps when you mentioned beam above you were referring to chine width
    Another question, At what speed does the porpoising start

    Regarding the trim tabs. After the pockets are filled and they extend to the width of the chine width at the current transom or close to, you should be adding trim tabs to reduce the chance of porpoising.
    The trim tabs should be mounted about 1/2 an inch to 1 inch above the hull bottom as compared so that the hull immediately ahead of the trim tabs has the chance to ventilate. The alternative is to have them right at the level of the hull bottom which will work but not be quite as effective. With the hull ventilating, when the tab interacts with the water, a new stagnation line will occur providing a max lift for wetted surface.
    As others have said, make the tab wide. This increases the stagnation width and provides the most amount of lift for the least amount of drag. MOST brackets that are used to mount tabs, have this
    condition exist, but not all. ie some transom mounted brackets mount to the transom (yes I know, obviously) but the bracket hinge configuration often will put the tab and hull bottom on the same plane.

    So two questions,
    Chine width out side across the boat to the other outside the chine, at the transom
    and the incipient speed of porpoising

    Note that with the outboard, power trimming adjustments will often reduce porpoising issues.
     
    Last edited: Feb 15, 2021

  14. stocko
    Joined: Feb 2021
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    stocko Junior Member

    Hi Barry

    beam at chine = 2.0m

    The area I am looking to infill is 0.5m x 0.44m on each side

    Current tabs are 10mm up from bottom at leading edge ( installed as per lenco instructions) but are a long way forward so it reduces the efficiency.

    most similar plate boats in Australia have a full width transom.
     
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