Creating lift in displacement hulls

For better or worse I added 4 feet to my Ed Monk Roughwater. Increase waterline, add buoyancy on stern for jetski, swimstep etc. Remarkably the result was surprising. Vessel has Newer Perkins Saber 225HP. Before at full throttle it got up on step, 14 knots, hell of a wake and lots of fuel. After extended it get up to 10 knots so easy....at very little throttle but Ill be damned if the stern doesn't disappear underwater when you give 1/2, 3/4 or full throttle. Overall it's probably more efficient to 9-10 knots but wow she squats. So....next step is add a few ridges side to side across the new extension to try breaking that water plane/stern suction. Much like a interceptor?.....I hope... we'll see...If that doesn't work maybe reverse the curve the last 2 feet to create lift. Any suggestions besides cut it off?....lol....anybody?

 

My goal was to work towards some kind of "double ender" like those I see slipping through the water effortlessly trolling for salmon in Alaska. Or closer to the "wineglass" ease of the Bruce King Columbia 45 I sailed for years, or the simple shape of the 1968 Bill Lapworth Cal 28 I sailed to Alaska as a youth. But making the Ed Monk hull into a double ender may be difficult without adding another 4 feet and tapering inward and up into a point mimmicking the bow. I see the issue now...the flatness, the force pulling down as power is applied. After reading here about the "Interceptor" tabs creating force in front off itself and upward, I'm going to add them to the stern.
The dynamics of waterflow are fascinating....I've done some collaboration on ASD Tug designs with Jonathan Parrot and Robert Allan Hulls....A SWATH ship with Guido Perla and explored a multitude of design features in my 42 years as a vessel master but I was still surprised when the stern went under on this Roughwater 41 extension....I will overcome it somehow.

 

Coanda was right!A few years ago I read Kotaro Horiuchi's book "Locus of a Boat Designer" vol 2 and he described his attempt to solve a similar problem.Put simply he added an extension,but attached it to the main hull in such a way that it left a step,which meant that the flow detached from the original transom edge at speed.whether this approach would work for you boat,who knows?The attached image hopefully illustrates the principle.

 

Obviously, same principle as steps on seaplane floats, effective at low velocity when step is fully wetted, and planning on the step at higher velocity !
Should be effective

 

It doesn’t look like she makes her waterline at rest. Based on the red paint; she’s stern high. But that only accentuates the bigger problem.

When you throttle up; it looks to me like she’d squat looking to equilibrium since not enough buoyancy aft, but wait, there is more.

For me, just a casual observer without a formal boat education; the mistake was making her into a sailboat or double ender shape because the boat is trying to find her center of buoyancy and to do so means more of the back of the boat must be in wetted area..and then the shape of the extension creates massive amounts of drag because the water doesn’t release cleanly. This is called the Coanda effect. My boat has a wee bit of it. Yours is worse because your boat is also changing at varying rates of speed too much.

The Coanda effect is displayed when you take a spoon under a faucet and low water flow. The water will follow the spoon. If you then use say a butter knife; the water will fall off the end. Your hull is very spoon shaped; sorry to say.

So, my advice is to hire a professional naval architect and rebuild the extension, probably along the lines I drew in 1 minute.

No rocker intended; probably some stern angle to prevent flooding the extension backing.

This may result in the stern sitting too high at static rest; this problem may be corrected by a step or perhaps even flooding the extension with a dynamic ballast, both way above my paygrade.

 

The title of the thread is ‘creating lift’, but probably ought to be eliminating drag.

I don’t think the problem can be fixed by guessing. Someone needs to model it with software…

 

Your issue is the shape of the duck tail (the extension)...the water flow is inconsistent with the speed. in relation to the hull, simply.
You need a vertical transom and no rocker/hook.
As noted HERE and HERE.

 

"Eliminating drag" is more accurate. Thank you all for the quick response and disection!
A stepped hull was considered as was a straight extension. But I'll add rest of the story here...I gained my of target of 1200 lbs. buoyancy aft and it actually trims nicely with 800 lb jetski and water tanks full. CB and CG righ where I want them.
But in order to turn and maneuver well with current prop/rudder position I need the tapered shape inward and upward to surface allowing clean flow under stern as the vessel gets longer. Those features are a huge success. The drag underway is not...but I'll overcome this with a workaround.
Thank you Ad Hoc for quantifiing the resulting forces from this shape. I understand why the fine shaped double enders achive wave speed so easily...the drag is mostly on the deep, rounded sides...as the water comes back together astern the force is horizontaly outward on hull instead of pulling the vessel downward. A clean, natural apex having achieved a near pefect wave form...evolved like a duck has over millennia.
Is it possible there's another dynamic that can affect water flow?....not the clean break transom or the "stepped" stern, not the square "Interceptors" creating lift forward or the foiled stern hydroskegs used on AML barges to create lift upward......but rounded ridges across the flow?...like a sluice box gathers gold behind it's strakes.
Pulling crab pots in the Bering sea we use a 3 buoy set up. The 12" hard foam diver buoy, (so the sealions can't pop them), a 24" inflated middle buoy, (to support the sinking line), and a smaller inflated trailing buoy to grapple and recover crab pot with. When you pull that round buoy fast of course it dives like a bowling ball or "squats" but the trailing buoys, semi-oblong in shape with 3 tiny ridges, literally skip across the water with no resistence.
The ridges are rounded, not square. Do they break that drag with the pressure in front?.....or the minuscule disturbance created behind them?...or is it that any disruption at all destroys the waterflow hence breaks the drag? I don't see 3 Interceptor tabs in row on any vessels although I do see multiple steps on big racing cats that must break the force loose or grab air.

 

And it doesn't help that my extension came out shaped more like a canal barge than a duck tail, flowing primarily upwards instead of inward. Just one of the limitations with bending Coosa Board into a curve....a compound curve on the sides.
With more stern point and side taper inward I'll get there. That will eliminate some of the drag and interruption of waterflow will eliminate more. I'll be done when I achieve 10 Kts. burning 2 GPH on even trim....and that's probably near the limits on a now 45' foot vessel with 13' beam with this shape. After operating a multitude of vessels for 42 years I now know the forces of hydro dynamics innately but it's interesting later in life to see the math and the "why" different hull forms behave as they do in various weather and sea state. If you want to throw a curveball into you tank model or computer program try adding a tow astern, offshore in tremendous weather tethered by 1500 feet of 2.5 inch steel cable....say an 80,000 bbl petro barge....lol.
I now understand what a "parabolic" bow is.

 

There are so many things wrong here, but I’ll start by saying lose your errant hypothesis and listen to the words of a qualified NA. Just forget all prior notions. I’ll try to share some reasons.

Your boat and a duck’s arse are not wise as the same shape. That which makes nature beautiful does not translate to a boat.

There are many reasons, but your idea of ridges is made up from the notion that a duck is constantly adjusting it’s arse to accomodate pitch, roll, speed, etc. And your boat cannot adjust anything. Ridges will make the drag worse than it already is and it is bad.

The second problem is your boat goes faster than a duck.

This is elementary stuff. Where a NA gets to have fun is explaining, if willing, how far off you are here.

Reynolds number for a duck.

Length 0.3M
Speed 0.5m/s

R = VL/viscosity of water

R = 0.15/1x10^-6
150,000

Boat
length 15M
Speed 5m/s or 10kts
R= 75/10^6
=75,000,000

you boat is not comparable to a duck’s arse in length or speed and the factors affecting it are drag and wavemaking

For your boat to be comparable to a duck, or the above duck; the speed would change to a new maximum we back into.

R(duck) = 150,000

V(boat) = 150,000•1x10^-6/15m (boat length) = 0.01m/s

This means for your boat to be comparable to a duck; it needs to be adrift in a nearly windless sea. Otherwise, it is not at all like a duck and cannot benefit by being shaped like one.

You needed professional design and still need it.

 

Your certainly correct on those points and seeking professional help is always good advise. I appreciate your tolerance and time in replying here...please bear with me. My math sucks and I often learn by trial, error and breaking large things on boats.
I've worked and collaborated with a multutude of NAs over the years.... one off designs, vessel modifications and newbuilds. I was part of the design commitee for our Lockheed Martin SWATH Ship Susitna and responsible for the 28 tons of ice armor in her bows as well as the positionong of intakes and nozzles for the 16,000 HP worth of Kamawa Jets to avoid ice jamming. Lots of NAs on that one. And it was certainly a NA at Bay Shipbulding in Sturgeon Bay that designed the original bow on the 7,000 ton VRO Barge Olympic Spirit which I dragged up and down the Pacific Ocean between Santa Barbara Channel and the refinery in Burnaby B.C.. Two years after launcing we had to cut the bow off and replace it with a parabolic so it could be towed at sea without killing everyone. It was flat, (looked like my stern extension actually), and it ran away like crazy under any speed. My point is the success of a vessel in operation goes beyod the design itself....it must go beyond the laptop and tank test and into the sea under conditions that are impossible to simulate. That being said it's a group effort start to finish to make a vessel function successfully at the task it's designed for. Bernard Moitessier wasn't a NA but after a couple single hand circumnavigations I'll bet he could tell us a hell of a lot about vessel design characteristics and features.
I digress but still haven't found an answer to why a vessel with perpedicular ridges would have reduced drag...except the interceptor affect, which would mean the pressure is upstream of the ridges. It doesn't appear to be.
For your interest, reflecting on "R = VL/viscosity of water" above. Here is pic of the viscosity of water I've been dealing with most of this winter. You see why most theory goes right out the window. While we appear immobilized in the ice, we are actualy drifting sideways at 6 knots in Cook Inlets 39 foot tidal currents....in shallow water. We only get through it with the brute fore of 5000 HP Azumithing Stern Thrusters on a 78' x 40' chunk of a tugboat. A Robert Allan design with a GM around 9...if that's even possible. Thanks again for your input here.

 

My question with pics.....for anyone with an answer. Tow this round buoy at 5 knots and it dives underwater. Pull the oblong buoy and it dives partially under....But tow the oblong buoy with rings around it and it skips over the surface. It can't be just the shape or the oblong without rings would skip.

 

This theory regarding external ribs or hoops would be easy and inexpensive to test with models. You might be onto something, or the observation might have an alternate explanation. Are you going to find out?

 

Are the oblong buoys roughly the same size and weight? It's an interesting issue.

 

Why do you insist on comparing dissimilar bodies in water? Even a casual boat lover can see the folly in it. Sure, an interceptor is a unique gadget, but they go on the end of the boat where no further drag can occur. If they worked all along a boat; then designers would put angle iron on the bottom of boats! But they don’t. My intuition tells me it is because the drag cannot be overcome and the pressure acting on the hull will result in more stern squat.

The Reynolds number of a buoy.

Length in water = 0.1M or less
Speed being dragged = 2 m/s or say 4kts if my math is right

R = 0.1 • 2 / 1x10^-6 = 200,000

Again; the buoy is similar to a duck. The speeds are higher and the ridges create so much drag the rope pulling the buoy stretches until it cannot and then lift occurs, but it isn’t an efficient process. The buoy cannot provide the propulsion to do it to itself. If the buoy had a propulsion system; it would force the buoy front down as the ridges create the drag. If the propulsion system was real powerful; the buoy would fly out of the water and its behavior would be erratic as varying degrees of drag would result if when and how it landed. But it would eventually fail or operate in a state of chaos as a chaos machine.

I’m going to bow out of the thread in hopes someone with a cv helps steer you away from duck tails.

I’ll offer a small carrot to egg you on. It is possible to gain some lift with a foil or a single interceptor at the back, but I doubt it will be enough to overcome the drag and it is also above my paygrade. I don’t know if a foil can help at lower speeds either. Again, beyond my wheelhouse, but perhaps someone else knows.