Stepped transoms

What's the benefit of this design? I'm particularly interested how in functions with a small tunnel/ prop pocket.

 

Basically, these sort of cutouts or "prop tunnels" are to facilitate a larger prop, a shallower shaft angle, or reduced overall draught.

On many boats, this is the only way to get an inboard engine's propeller to fit; however, they do nasty things to the water flow into the prop, and even nastier things to the planing efficiency of the hull.

IMHO, prop tunnels like these are best avoided at the design stage. They can work, but more often than not seem like a bad hack to fit a straight-shaft inboard engine to a hull that's better suited to other drivetrain options.

 

In addition to the propeller tunnels, I think tkinak is also asking about the effect of taking the hull and stepping it up for the last couple feet. It must add lift when getting up, but then the planing surface is moved forward once over the hump. I don't have the knowledge to provide an answer but it seems like an interesting topic to discuss that I haven't seen here before. I'm not sure if the two need to be answered in combination or whether considering the two separately is the way to go.

 

Ahh, OK - so as seen in pictures 2 and 3, where we have a bit of hull that is submerged at rest but clearly will not be part of the planing surface one up to speed.
Usually, when I've seen things like this, it's to keep the boat from riding too stern-down at rest, often due to a swim platform, etc. cantilevered out past the transom. As soon as the wake begins to break away from the transom, such an extension will have little or no effect. Its main function seems to be to keep the stern from sinking too deep in a stern-heavy craft; many planing hull forms like the CG to be fairly far aft at high speed but do not balance well at low speed in this condition- some extra buoyancy and/or large trim tabs are required at lower speeds.
Not to be confused, of course, with a true stepped running surface, which can be a very fast and efficient solution for higher speed craft.
Again, personally, I don't think this is a hallmark of good design- but it seems to be a somewhat common, and somewhat workable, solution for some boats that would otherwise ride too stern-down at rest.

 

Obsession,
The tunnels Give a slight shrouded propeller effect and less draft. A single engined inboard deep V could use a reduction in draft. The tunnel sucks water up and aft .. aft is fine, up is not. Water goes up, stern goes down and the small step in the picture is bad as that boat needs all the flotation and lift it can get back there .. especially just under the hump. I think these tunnels should only be used when draft is the #1 design priority.

Easy

 

Sorry, I think you guys are right the first pic doesn't have a stepped transom.

After thinking about this for a while I think the SeeVee 32 in the second picture is probably originally an OB powered boat design that may have been converted to inboard diesel for the economy minded boater. The motor is forward mounted so the extra buoyancy may not be needed.

The third picture is of a Pearson True North. Obviously not a conversion. Hmmm?

I always understood it that a longer planing surface is usually better. So why not use it?

http://www.seaveeboats.com/320-boat-options.html

http://www.tnyachts.com/yacht/true-north-38

 

"I always understood it that a longer planing surface is usually better. So why not use it"

I find the whole subject of planing very interesting. I wish I understood it better. The gross theories, lift and resistance I have a decent grasp of. The subtle minutia of optimization, not so much.

 

Tolly,
I think it's like airplanes .. lift over drag. If you get the weight too far fwd you get too much wetted surface and not enough lift. Too far aft you get high angle of attack and the lift is largely lost escaping around the chines. They call that the wing tip vortice on airplanes. That's why they have the wing tip fences (upswept tips) on some aircraft now. We could plane at lower speeds as aircraft can fly at lower speeds w tip fence's if we had fins sticking down along the chines. Wouldn't be a good thing if on the face of a wave broadside to same. A hull's ability to carry weight at specific and optimum speed and angle of attack are probably much like aircraft. I think there's too many variables with boats to nail down a specific angle of attack. Also (like w aircraft) the angle of attack for min power to keep on plane and the AOA for best miles per gallon will be different ect ect. What do YOU think Tolly?
By the way .. Do you have a Tollycraft and where is your Fox Island?

Easy

 

Easy,
Most of what you are saying I think I get. I think that buoyancy plays a part in boats that it doesn't play perhaps with planes in establishing initial attitudes of attack. I'm sort of splitting hairs but the weight and the buoyancy play off of each other in a way I don't think you get with planes.

In a plane it would seem that gravity is always pulling you down while in a boat gravity is still pulling you down but buoyancy would be pushing you up and holding you on the surface until such time as sadly your weight exceeded the weight of the water you displace and you sank. When moving lift would add to the buoyancy and help keep you on the surface.

I don't know about planes, having only passenger not pilot time, but I don't think optimum attack angle would vary as much for a boat. As I understand your implication that they might with a plane I guess I think that if you have too radical an angle of attack it increases lift but also increases resistance so economy would suffer. The excess lift not really giving you anything for the fuel burned to get it. A boat once it gets over the hump seems to keep a relatively constant angle. Flying you have thousands of vertical feet to fool around with, in a boat it's all within a few feet or less of the surface. I'm not sure that's very clear but hey.

One of the guys that often posts here, Tom Lanthrop has given me a lot of insight into what I think I know about planing. He started building boats after he retired from sort sort of engineering backround I think. His niche, boats that plane at low speeds with low horsepower and have a smooth transition between displacement and planing speeds. He mentions the concept of bottom loading which I take to be the weight of the boat divided by the area of the planing surface. Lower loading numbers let you plane with less lift needed and less horsepower. Sort of like a supercub being able to pop up easy and then fly slow and low.

All of this is what I THINK not what I KNOW if you get what I'm saying. I'm fascinated by all of this and want to put a lot more into the know column.

Yes I have a Tollycraft, Wally is the name of my dearly departed dog. My lat/lons are

47 16 335
122 39 117
about 4 feet above sea level on high water.

 

Tolly,
Many similarities, many differences. Wing loading is just weight and square ft of wing but the aircraft is held up mostly by the upper surface of the wing whereas the boat is held up 100% by the bottom and the surface area supporting the boat varies considerably while underway. Maximum load carrying ability probably will occur at a different speed than max mpg. Top speed will probably occur at a specific angle of attack for both boat and aircraft. As far as buoyancy goes I think it's a boat thing. Planes (excluding lighter than air) only have lift as a comparable element to buoyancy. I think I also think and know and also dream. The great philosopher Kant said "I think and therefore I am". I'd hate to think of what would happen if we stopped thinking so I'm going to accept thinking and knowing. Since it's easy to think and hard to know I think (there's that word again) that our existence is largely thinking and a bit of knowing.
The Tollycraft boats are without exception good to look at and that has been rare since the 60s. Prolly good otherwise too.
My "Easy Rider" is the name of the only boat I have designed and built. I thought is was appropriate on the Design Forum and then I started getting called a biker. But many know me by that now.
Your GPS numbers put you right in the middle of the "Industrial Waterway" in the Port of Tacoma.

Easy Rider

 

"the aircraft is held up mostly by the upper surface of the wing" - I think that this has recently been successfully refuted, Easy. In fact, I couldn't remember the name for the theory that posits this effect (Bernoulli), so I Googled my query. This fellow has a much firmer foundation than I; http://wiki.answers.com/Q/How_does_an_airfoil_work
Cross the Tacoma Narrows from where you located Tolly and turn left at Gig Harbor - He must have rounded the coordinates... Good day.

 

That's pretty scary, I didn't check those numbers out, I just pulled an old pocket rocket garmin out of a drawer and fired it up. I did have to put fresh batteries in it. LOL, there is a disclaimer screen on the gps about not being used for navigation etc.

I can find my trap buoys in the dark with the furuno so at least the repeatability is good on that one. Mark's got it figured out, I could describe my driveway and he'd be able to stumble on in.

 

It was Rene Descartes

From Wikipedia:

"He is best known for the philosophical statement "Cogito ergo sum" (French: Je pense, donc je suis; English: I think, therefore I am; or I am thinking, therefore I exist), found in part IV of Discourse on the Method (1637 - written in French but with inclusion of "Cogito ergo sum") and §7 of part I of Principles of Philosophy (1644 - written in Latin)."

I really like your commentary, Easy Rider, but I don't want you unintentionally spreading misconceptions. Philosophy and boat building do go together in some weird way.

 

Wayne,
I thought I stated Kant correctly??? Since it's off topic it dosn't matter .. to me anyway.
Tolly,
Your'e right near the bridge on Fox Is. I got a better map .. or read another one properly!
Mark,
70% top 30% bottom .. not so anymore??? I looked at the link and I've heard all that stuff before. There are many theories on lift. I even have one of my own based mostly on inertia. It's not important here but if you want to talk about it give me your phone # and I'll call you. I type w one finger.

Easy

 

Easy,
Well I guess I'll put a little more trust back in the little pocket rocket!

I never use the little garmin anymore. For what it's worth, I doubted the machine not you! And I was too lazy to walk over and look at a chart.