Help on creating buoyancy in the Stern

Discussion in 'Boat Design' started by tonic78, Jan 13, 2014.

  1. tonic78
    Joined: Jan 2014
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Vancouver

    tonic78 Junior Member

    Hi all, hope you guys can give me some advice.

    I have a 23ft aluminum and I got the extension in the stern extended one foot to allow the motor to fit otherwise the top of the motor would hit the back of the boat when being raised. The weight of the motor and the kicker is making it stern heavy.

    I have trim tabs and that helps but i have to get at a high speed to keep the boat planing when its a little rough. The extension is under water at rest and when there's people in the back, the automatic drain hole are filled with water.

    I tried moving weight to the front and even adding exta weight but that hasn't done anything.

    We're thinking of welding a complete box under the extension where it conforms to the boat's shape to add buoyancy. It would be like floatation pods except it would would be one piece.

    Does anyone see problems with this or things that I should consider?

    Thanks.
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,632
    Likes: 252, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    It sounds like you have fitted a short "pod" that puts the engine further aft, but is not in contact (much) with the water underway. Naturally, this causes the boat to trim down at the stern a little further than before. It also sounds as though you intend to modify the pod so it becomes a continuation of the bottom of the boat. This should help your problem, but of course the engine will have to be lowered accordingly, so what effect that will have on it being tilted will have to be assessed. Another thing you could do is fit wedges between the engine bracket and the pod so you get more under-trim, if restricted in trim range. That might allow you to throttle back without the nose rising too much. If those alterations are still not sufficient, you could widen the pod, even using detachable sections, bolted on, that can be experimented with, size-wise. If the boat is not inclined to broach as-is, it should be able to accomodate a bit more bouyancy aft.
     
  3. tonic78
    Joined: Jan 2014
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Vancouver

    tonic78 Junior Member

    Thanks Mr.E, right now the motor is a little lower in the water than we'd like due to the boat being stern heavy and the extension was a little low to begin with.
    So all in all it doesn't sound like there would be any issues with a modified pod that conforms to the boats bottom.
     
  4. Kailani
    Joined: Apr 2013
    Posts: 113
    Likes: 8, Points: 18, Legacy Rep: 59
    Location: Hawaii

    Kailani Senior Member

    You didn't say if the boat is already fitted with trim tabs.
     
  5. Village_Idiot
    Joined: Oct 2007
    Posts: 380
    Likes: 13, Points: 18, Legacy Rep: 138
    Location: USA

    Village_Idiot Senior Member

    What type of boat is this? Sounds like it wasn't set up correctly to begin with. If the motor was hitting the back of the boat when trimmed up, it should have been mounted higher to begin with, possibly requiring an extended shaft motor. Your proper "fix" would have been to go up, not back, and that would've eliminated the additional problems you're running into now.

    Regardless, lengthening the boat should help remedy your problem at hand now. It should be a smooth transition along the hull bottom. You can add trim tabs beyond that, if needed. Seems like a lot of work for what should have been a relatively simple fix...
     
  6. SamSam
    Joined: Feb 2005
    Posts: 3,817
    Likes: 153, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    If the pod is made an extension of the hull bottom, it wouldn't seem hard to add some boxes to both sides and thereby extend the whole boat. You could then move your existent trim tabs to the back of them.
     
  7. tom28571
    Joined: Dec 2001
    Posts: 2,469
    Likes: 113, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Adding the whole flotation extension is probably the best solution and I don expect any other problems other than fitting the scuppers through the new extension. Sometimes the scuppers can be routed out the sides and avoid a pipe through the pod.

    You need added buoyancy both at rest and underway to make the boat better. Just lifting the stern underway is only a half solution. Adding weight is almost always a poor choice to a handling problem, even if it works. Setups like you have now also lead to prop ventilation in waves. Its the moment arm of the motor moved aft that is the problem not its weight. Brackets are all the rave now but many lead to problems.
     
  8. tonic78
    Joined: Jan 2014
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Vancouver

    tonic78 Junior Member

    Thanks for your comments everyone. I think VillageIdiot was right about mounting higher than going backwards. I don't why we didn't even consider that. Hindsight sucks. I'm guessing 700lbs extended 1 foot aft makes a huge difference in weight distribution. Anyone want to chime in?

    I think its the same amount of work to go back to the original extension and raise it up versus extending the hull.
     
  9. SamSam
    Joined: Feb 2005
    Posts: 3,817
    Likes: 153, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    700 lbs?

    Can you post a picture of the boat? If the motor was hitting the back of the boat, it sounds like it was rigged up to begin with, and not an OEM situation.
     
  10. tonic78
    Joined: Jan 2014
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Vancouver

    tonic78 Junior Member

    I'll try to get pictures this weekend. The boat was custom make about 12 years ago. Unfortunately we just got the motor last year and i don't think the boat was designed for the new 4 strokes found in the current market.
     
  11. tonic78
    Joined: Jan 2014
    Posts: 5
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Vancouver

    tonic78 Junior Member

    Here's a pic of what in talking about. It has trim tabs but we just took them off.
     

    Attached Files:


  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 7,632
    Likes: 252, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    The pic doesn't really show the bottom of the boat that well, but the pod does seem to be a long way aft of the transom. Is it just bolted on, or does the structure continue back into the boat itself ? It must be pretty hefty, 'cos it is not very deep, to get strength that way. How much clearance is there between the tilted motor, and the old transom ? Whatever that is, it is probably how much unwanted length you have got. I think I'd try padding out the bottom of the pod with something temporary so it is just a continuation of the hull, and see what difference it makes. Naturally you will have to lower the motor down, which may be a problem if it fouls anything. Another thing worth considering is transom wedges to increase the rake and the in-trim, these are inexpensive and easy to fit. In fact, I would try wedges before doing anything else, even if they don't solve the problem, you would just leave them there because they might go part of the way to a satisfactory solution.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.