Angled transomes?

[duluthboats]

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

Gary

[tom28571]

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.

[duluthboats]

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

[CDBarry]

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.

[woodboat]

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?

[duluthboats]

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

Gary

[woodboat]

You mean ask the guys that wear rain gear when it's not raining about a little water over the transom?

[tom28571]

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.

[woodboat]

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.

[duluthboats]

It’s not a conspiracy; 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

[Corpus Skipper]

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?

[Tim B]

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.

[Willallison]

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.

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.

[nevd]

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

[Willallison]

Originally Posted by nevd

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.

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)?