17 degree transom angle, where did it come from?

[jasonmcintosh]

Stern transoms are at an angle, I think I read at one time it's 17 degrees.

I'm wondering why there's an angle at all. I'm guessing that its so a motor can thrust upward a bit which will help keep the bow down. But I noticed that even the small displacement hulls have this angle. I suppose that this 17 degree angle has become standard for motors, so any transom meant to have a motor mounted on it will have that angle.

Any other comments on this angle?

jason

[ks8]

crop circles?


I made my console sides angle up at exactly 7 degrees. Why? It looked right. Maybe its that mundane?

Oddly, if I remember right, the equal tempered western chromatic scale, when engineered on a fretted instrument, comes out to each successive fret away from the bridge being 1/17 th longer than the preceeding one...

... perhaps if we apply that into a sine function, and see if there are hydrodynamic harmonics of drag energies in the backside of a bow wave, or the separation velocity as a function of the viscosity of a tri fluid boundary limited by a fixed range of chine radii...

crop circles.

Or something to do with the Banach-Tarski paradox as applied to spherical solid objects?

Maybe there is a better answer. Maybe not!

ks

[Rick]

crop circles?

I made my console sides angle up at exactly 7 degrees. Why? It looked right. Maybe its that mundane?

Oddly, if I remember right, the equal tempered western chromatic scale, when engineered on a fretted instrument, comes out to each successive fret away from the bridge being 1/17 th longer than the preceeding one...

... perhaps if we apply that into a sine function, and see if there are hydrodynamic harmonics of drag energies in the backside of a bow wave, or the separation velocity as a function of the viscosity of a tri fluid boundary limited by a fixed range of chine radii...

crop circles.

Or something to do with the Banach-Tarski paradox as applied to spherical solid objects?

Laugh if you will, but I've designed a table using the Fibonacci series.

[jasonmcintosh]

ks: What can I say? I like the crop circle idea...

Rick: Love to see a picture of your Fibonacci table if you still have it...


One of the reason I've been thinking a bit about the transom angle is because I thought a good boat for a lot of the homes on south florida that are along a canal would be a smallish 8' boat like the PK78, but with a 90 degree transom. The idea would be to store the boat vertically in your garage. The 8' length means it would fit standing up, and the square transom would allow to be stored on it's "butt". A two-wheeler (hand truck, whatever you call them) could be used to move it to the water. Or you could permantly mount some small wheels on the top of the transom.

Can't say that this would be a big seller, but I'd sure make one if I was on a canal. (The homes along the canals in my neighborhood can't leave the boats out. They've got to be put away, out of site.)


jason

[ks8]

The 90 degree and wheel thing sounds good.

I believe I've seen this on commercial dinghys already... roto mold jobs, wheels and all.

Nesting Dinghies have 90 degree transoms (I think). I don't think you'll have any problems with your project.

But maybe someone else will chime in with a more boaty bit of technical reason for 17 degrees, but the sort of boat you are talking about, I don't think it will make a major difference.

sincerely,
ks

[ks8]

Hello Rick,

Laugh if you will, but I've designed a table using the Fibonacci series.

Does it look like a big leaf? Or a Nautilis shell? You know you have to post a picture, even if it isn't a boat, because now it has become relative to boat(ers). Maybe float it, and others may not object so much. But more importantly...

Are the legs screwed on at a 17 degree angle? If so.... why?

sincerely,
ks

[Boomer]

A square transom would be o.k. for a small displacement boat. I think that if you look a t commercial displacement baots they have essenially a squre transom (square sterned canoes for example)The 17 degrees is an industry standard.

You need the tuck to get the lift to get the stern up on a planeing hull, especially with smalle amounts of power.

Also an angeled transom is traditional on some rowboats (and I guess sailboats) to increase the load carrying area/capacity, but to minimaze the water line, and therefore resistance.

[ks8]

The topic, I think is exhausted already. But since no official reply gave exact science of the 17 degree number... I'll toss out some ignorant speculating, just for fun...

Also an angeled transom is traditional on some rowboats (and I guess sailboats) to increase the load carrying area/capacity, but to minimaze the water line, and therefore resistance.

I wonder if it helps buoancy. As the hull is further submerged (or in a following sea), the buoancy increases (since the volume of the hull is greater as you go up the angled topsides and transom) and lifts the stern to minimize lesser poops? Just wondering. As for decreasing waterline to decrease resistance... it may in one sense reduce wetted surface area and hence resistance (in one sense), but it also plays in conjunction with the hull speed waterline length formula (depending on the hull form of course), so that at times you will get better displacement *speed* performance with a longer waterline rather than a shorter one.

I speculate that the positive angle creates a gradually increasing buoancy to help lift the stern *gently* in a following sea, but not lift it too radically fast lest the bow get buried. A transom angled toward the bow (negative) as it rises from the water will increase waterline but then some other way of making the buoancy effect more gentle would be needed, like pinching the stern at the waterline, so that the topsides still have a positive angle. Of course the buoancy does not increase like a vertical transom since even the negative angled transom is reducing buoancy as you rise (there is no boat above the waterline at the stern in this case!), but there is more vertical resistance to downward motion with the negative angle simply because there is a greater surface area to the bottm because of the increased waterline (unless its been pinched). The positive angle pushes water out of the way. The negative angle makes the waterline longer, and the consequently larger bottom *slaps* more with a more abrupt vertical resistance to motion (lessened by pinching). But I am no expert. I tend to think that a positive angle in choppy seas will be a little more forgiving and perhaps that is where the 17 degree angle came from? But any case with a beamy stern must not have too pointy (very technical term) a bow or it will get buried more often than any boater would like.

The *flatter bottom* and larger surface area and longer waterline of a vertical or negative angled transom may help the performance of a planing hull, that does not heel consistently. A flatter and beamier stern will also allow a bit more sail carrying for more power (on a sailboat), but again, it could bury the bow more often if something isn't done up front to add a bit more buoancy there as well (which unfortunately will increase wetted area and resistance). But I could be wrong. As I understand it, many factors are involved and the maxi developers spend millions to squeeze out a little more performance, but they also are not known for the gentle rides in rougher seas. A negative transom would reduce waterline when a sailboat heels, even though it increases it when the hull is not heeled. So a smaller boat that is not intended to heel much could actually benefit from the longer waterline, in speed, but the ride might be a little rougher. On what scale? I'm talking through my hat anyway so what does it matter???

Years ago, the racing rules for sailboat designs were such that designers put large overhangs on the bow and stern so that waterline would increase as soon as such boats heeled on their best points of sailing. It also made downwind runs their worst point of sailing because of the shorter waterline. Sailboats are not like NASCARs or hot rods going around 200 mph. There is a different dynamic. A little bit of speed, an extra 3 mph, could win the race for an automobile, if it has the horsepower for quick acceleration when strategicly needful, but with a racing sailboat, it may be a 20th of a knot that makes the difference overall, and light weight that gives the equivalent of instant horsepower when needed, all other things being equal. So hull shape becomes critical there. But in a longer distance ocean racer, the hull also must *survive* and also keep its crew alive and healthy manning the rig rather than manning the rail and the bucket while replaying lunch. The fastest hull in such races requires good health to endure the ride and more violent motions.

But a small rowboat for a canal, to *mess about in* is another matter.

A row boat, and a small yet wide one at that, is an entirely different thing, and already is somewhat boxy to maximize cargo and buoancy, not speed or efficiency (an ambiguous term). A vertical transom is probably not going change things much here. Shift your position in such a boat and there is a different hull presented to the sea anyway. But a performance rowing boat has factors purposed to maximize speed and maneuverability. You don't shift all around in such boats while under way. You try to get the best forward motion for each stroke. Just try to take a pulling shell out in rough seas! I couldn't tell you what is best in such a boat, but it sounds like the *canal* boat you speak of is not intended as a high performance rower, but something more to *mess about in*.

And then of course, boat design is not strictly science, because humans and usage are involved, and that necessarily brings in poetry and art to the realm of design.

I know there is ignorance in this post, but I'm hoping that if some here never thought of what is behind hull design before, now, perhaps, you'll enjoy looking at a hull as I'm learning to, considering how each line is affecting the performance of the hull. I find it fascinating.

Have fun putting on the wheels on your canal boat(s)! Put another on the bow, that swivels and steers, and you can do some down hill racing with it inverted! Boat box derby? Then, like a skateboarder, maybe as you fly off the end of the dock you can flip it in the air and splash down landing squarely at the helm???

You do that and we will have to have Jacques add a video section to the site!

ks

[stickystuff]

You want a straight answer.

Most all engine manufacturers mounting brackets are beveled at 14 to 17 degrees. I thinks 14 is the most common angle. This is why the transoms are angled. This allows you to tuck your motor in further which is needed sometimes to keep the hull from poirpoising. Some hull design will bounce or poirpois at low speeds. This angle allows the engine to tuck further inwards and keep the bow down. Make sense? I has spoken.

[ks8]

so, its a powerboat thing.

thank you sir sticky.

And when it comes to powered planing hulls, it just plain seems to work best at stopping porpoising with the hull shapes popular for planing. Obviously, if that is the sole reason for it, a slow moving displacement canal boat has no porpoising problems to tame.

Or does it?

ks