Angled transomes?

Discussion in 'Boat Design' started by duluthboats, Mar 25, 2004.

  1. duluthboats
    Joined: Mar 2002
    Posts: 1,585
    Likes: 43, Points: 58, Legacy Rep: 779
    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    :?: Are power boat transoms angled because of tradition, appearance, or function?

    Gary :D
     
  2. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    Gary,

    It has always been my assumption that transoms on outboard powered boats need to be angled to allow for adjusting the thrust angle of the engine. On inboard powered boats, some are angled forward or aft and some are verticle. Matter of taste I guess since the angle serves no function there.
     
  3. duluthboats
    Joined: Mar 2002
    Posts: 1,585
    Likes: 43, Points: 58, Legacy Rep: 779
    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    Thanks Tom,
    That’s much the same as my thoughts. With outboards it seems like a chicken and egg thing, why doesn’t the motor have a bracket that allows it to be hung properly on a plumb transom? In my case the outboard will be in a well. I want to use a plumb transom just to simplify construction, and it looks fine to me. I have noticed that the transoms on inboards and outboards under 40’ are mostly angled, but I have found very little written on this subject. I want to make sure I’m not missing something.

    Gary
     
  4. CDBarry
    Joined: Nov 2002
    Posts: 791
    Likes: 30, Points: 28, Legacy Rep: 354
    Location: Maryland

    CDBarry Senior Member

    12 degrees is a standard angle because outboards and IOs are built to that angle. Why they are built to that angle, I don't know, maybe it's because most transoms are at 12 degrees.
     
  5. woodboat
    Joined: Nov 2003
    Posts: 312
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Baltimore MD, USA

    woodboat Senior Member

    Could it be as simple as wave deflection. One would think if the transom were angled forward at anchor a wave would come up and over much easier and then now they have adopted this 12 degree angle as a standard?
     
  6. duluthboats
    Joined: Mar 2002
    Posts: 1,585
    Likes: 43, Points: 58, Legacy Rep: 779
    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    If that is a valid reason, then we have to ask what those sail boat designers are thinking.

    Gary :D
     
  7. woodboat
    Joined: Nov 2003
    Posts: 312
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Baltimore MD, USA

    woodboat Senior Member

    You mean ask the guys that wear rain gear when it's not raining about a little water over the transom?
     
  8. tom28571
    Joined: Dec 2001
    Posts: 2,474
    Likes: 116, Points: 63, Legacy Rep: 1728
    Location: Oriental, NC

    tom28571 Senior Member

    If you look at the geometry of the outboard mounting, you will see that to have a vertical transom and still allow a sufficient range of thrust angle would mean that the mounting bracket would need to provide for such an angle anyway. This would add unecessary torque and require both a stronger transom and more extensive and therefore heavier and more expensive bracket. Much simpler, cheaper and stronger to build transoms at 12 degrees. It has nothing to do with waves or other hydrodynamic factors.

    Of course they could make the angle between the drive shaft and ring gears in the lower unit at 78 degrees instead of 90 and get the same results. Again, more expensive and probably more trouble prone, especially for shifting gears. Since I'm about to tell more than I know, (unless I already have) I'll stop here. :D
     
  9. woodboat
    Joined: Nov 2003
    Posts: 312
    Likes: 1, Points: 0, Legacy Rep: 14
    Location: Baltimore MD, USA

    woodboat Senior Member

    I don't buy it. Not that I know the answer cause I don't. Jack plates are strong, complicated pieces that hold up well. In your theory the boat builders got together with the Outboard Motors builders and developed a way for engines to be made cheaper.

    My first two posts were a little tongue in cheek per say. I really have no idea the history of the outboard.
     
  10. duluthboats
    Joined: Mar 2002
    Posts: 1,585
    Likes: 43, Points: 58, Legacy Rep: 779
    Location: Minneapolis,MN, USA

    duluthboats Senior Dreamer

    It’s not a conspiracy;:eek: it just ended up that way. In all things boaty, (well almost all) things that work, survive, and things that don’t work, disappear. I think Tom has as good an explanation as I could come up with. At least to the point that it became standard and now the fact that there may be a better way doesn’t matter. It is amusing that I have many references to tell me what the angle should be but none that discuss why. Hey someone has to ask these dumb questions. ;)

    Gary :D
     
  11. Corpus Skipper
    Joined: Oct 2003
    Posts: 606
    Likes: 8, Points: 0, Legacy Rep: 173
    Location: Corpus Christi TX

    Corpus Skipper Hopeless Boataholic

    Just a thought from an offshore fisherman, I think there may be something to the wave deflection thing. I fished a boat once that had a plumb transom, and got drenched when backing down. My boat, although an inboard, has a 12 degree transom, and you don't get drenched when backing down. Also, possibly provides a tad of reserve bouyancy?
     
  12. Tim B
    Joined: Jan 2003
    Posts: 1,438
    Likes: 59, Points: 0, Legacy Rep: 841
    Location: Southern England

    Tim B Senior Member

    I suspect the answer is in an engine-mounting vein, but it would be worth asking the question as to whether a transom angled forward at the deck is more efficient at controlling the flow off the aft part of the hull. I don't know, it would be an interesting area to research, but why do almost all sailing boats do it these days? I can only think it must be of benifit in some way.

    Just a thought.

    Tim B.
     
  13. Willallison
    Joined: Oct 2001
    Posts: 3,590
    Likes: 130, Points: 0, Legacy Rep: 2369
    Location: Australia

    Willallison Senior Member

    :D :D :D

    As far as fwd sloping transoms on stick-n-rag boats goes, I always understood that it created a longer waterline length with minimal additional weight.
     
  14. nevd
    Joined: Nov 2003
    Posts: 99
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Australia

    nevd Junior Member

    Transom angles

    I normally use 22 degrees transom angle as it allows faster backup without bow waves over the transom. Motor is also mounted high so the leg is still above the water when trimmed right up.

    I also agree with Will as to why so many sailing boats use the "wrong" transom angle and remember they normally can't go very fast backwards when under sail.

    Regards,

    nevd
     

  15. Willallison
    Joined: Oct 2001
    Posts: 3,590
    Likes: 130, Points: 0, Legacy Rep: 2369
    Location: Australia

    Willallison Senior Member

    :?: What happens when you trim th o/b right in? I could imagine some fairly alarming handling at speed. Also, doesn't that restrict the amount of outward trim available (as opposed to tilt)?
     
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.