Help With Economical Semi-Planing Designs

I ran across this aggregation site the other week and it seems to me that you might find some value here.

Additionally, it appears to me that any one of these boats might fit your needs to a tee. YMMV and all that.

Best,

Leo

 

Thanks, Leo. The Dyer does indeed seem to come very close to what I am looking for, except that I am looking for a boat to build, not buy. That's why I asked about the Atkins and Devlin. As far as what is different about the seabright skiff, it's my (limited) understanding that the box keel and reverse deadrise are thought to create lift at cruise speed, and delay the point that the hull begins to dig a hole in the water. The only thing I can see that is advantages about my semi planing hull is what I mentioned earlier; it maintains a smooth crusing speed as the seas get up better than a pure planing hull. I think it's partly because it's still got it's forefoot wet, and partly because it remains closer to it's optimum design speed (for that hull) in the lumps. My old Mako was a pretty nice boat and was reasonably efficient on plane, but it was hard to keep it on plane in a chop because of the pounding. It just needed too much speed to plane. I eventually put big tabs on it so I could run in between it's waterborne speed and it's planing speed. It was expensive, but it was a lot better running 42 miles home at 12 knots than it was at 5.5. Of couse I was fishing the gulf stream then and swells WERE a consideration there

 

From everything I have read about tunnel-stern Seabright skiffs, I believe this is true. Robb White said his rendition of Rescue Minor lifted its aft end higher as more power was applied. This naturally pushes the bow down and insures that it stays in the water instead of rising up and allowing waves to hammer on the bottom of the boat in choppy conditions.

I think these boats have zero tendency to squat because they simply do not dig a hole in the water like many other boats ...

As more power is applied the aft end rises up because of the downward thrust of water created by the hook in the aft most section of the tunnel portion of the hull. Since most of the boat's weight is being carried by the long, slim, double-ended flat-bottom box keel anyways, the boat simply accelerates smoothly from displacement to planing speed with little or no pitch change.

Robb White said he thinks the reason these hulls are so fuel efficient and easily driven is because the aft tunnel section of the hull traps the energy created by the stern wave of the box keel, so the boat basically surfs on its own stern wave. Interesting theory ... as this would also explain the small wake created by these hulls.

 

I'd still like to see some real data that suggests that the SB skiff is any more efficient than a comparable vessel of 'normal' form.
There's no doubt that the downturn in the tunnel creates upward pressure, but does it simply offset the loss of lift created by having what is in effect a giant prop tunnel? And the added wetted surface of the box keel must be of some detriment...

Hey - I like the boats, but some of the apparent efficiency must come from their light weight.

 

Me too, but even if tunnel-stern Seabrights are not any more efficient than comparable 'normal' vessels they still have other virtues that make them attractive:

1- Extreme shallow-draft that only a jet or airboat or "tunnel hull" (as opposed to tunnel-stern) can match while under power.

2- Easily beachable and sits upright and stable on its flat box keel bottom.

3- Stability that far exceeds the boat's size and weight while running -- this according to Robb White who had no reason to embellish the truth like the original designer might have.

4- Relatively flat running at all speeds -- they never have to climb out of a hole to get on plane, instead they perform consistently throughout their entire design speed range.

This is possible of course, but I don't think so. This is an inboard boat with the engine near the middle of the boat so it doesn't need the broad flat immersed bottom that most ouboard require to support all that weight aft while at rest.

Robb White said the hull sits a few inches high at the bow while the boat is standing still (with people in it I think, but maybe when empty too) ... and then as power is applied the stern gradually lifts, pushing the bow down and keeping it in the water. I suspect that this slightly bow-high attitude is partly the result of the aft portion of the boat not having a broad flat bottom that is already in the water until after the boat is moving, and partly where the people are located in the boat.

One thing Robb said is that when the boat is underway it doesn't matter where the people are located because the boat rides flat all the time and feels like it weights 10,000 pounds -- which I took to mean that it feels far more stable when underway than a boat this size should feel -- and that's a "good thing" in my opinion.

I think the prop is sucking water into the tunnel section from forward then pumping it out of the tunnel section fast enough to create a suction between the hull bottom and the surface of the water. A suction like this would probably be so strong that it would easily resist the movement of people in the boat -- and keep the hull vaccumed tightly to the surface as long as the tunnel is full of water -- which of course depends up the prop spinning.

Some writer said something like this too, but I think just the opposite is true. I think the box keel acts like a super-efficient displacement hull (like a catamaran or kayak hull for example) that supports most of the weight of the boat and its contents. This portion of the hull would naturally be easily driven through the water.

I think the boat smoothly accelerates from a standstill all the way through the range that most people refer to as "semi-displacement speeds" without trying to bury its aft end because the box keel part of the hull is effectively still running in displacement mode until the boat is very near its top speed.

During this acceleration, long before the boat begins to close in on the box keel's "hull speed" and starts trying to squat (assuming it ever does try to squat), the water has filled the tunnel and is gradually contributing more and more support or 'lift' to the aft bottom section of the hull, thus counteracting any attempt to squat.

I also suspect that this is the reason why these hulls do not feel safe when you overpower them. I think the box keel section gets unstable when you push it too fast. This section of the hull can probably be pushed a bit beyond its theoretical hull speed just like any other displacement hull can ... but if you push it too far it will likely begin to behave badly, and then you're no longer running safely. Better to run these boats with only as much power as they need, after all they are not speed demons to begin with.

The Atkin boats were never designed to be "light" but William Atkin claims they are efficient. Then again he's a designer and we all know that designers will brag about their boat's capabilities even though the boats themselves may not live up to the hype. Unfortunately the only recently built tunnel-stern Seabright skiff I'm aware of is the one built by Robb White, and with him gone we really have no other similar boat to look at until someone else finishes one.

I'm building one now, but it won't be done for a while, and even when it is completed and on the water I don't know how it might compare with other similar boats because I still have a problem defining what is 'similar' to these boats.

Note that I'm not building it primarily for the fuel efficiency either, although it would be nice to see the numbers some people have reported. My favorite feature of this boat is its extreme shallow draft combined with its reportedly excellent seakeeping ability, and the fact that the boat can be beached with no worries about prop or rudder damage. It should be able to run over things in the water more safely than an outboard too because nothing extends below the flat bottom box keel. To me this suggests excellent potential durability and reliability -- with fuel economy a big plus if it is actually there.

 

There are other way fully studied ways to avoid bow up and associated resistance.
http://www.scielo.org.ar/pdf/laar/v34n4/v34n4a11.pdf
http://www.daviscoltd.com/public/En...ency Improvements to the USCG 110 Ft WPB .pdf
http://www.amc.edu.au/system/files/Paper-1.pdf

And the best seems to integrate the stern lifting effect directly in the hull form (See page 28)
http://www.na.chalmers.se/~bathfiel/CYR/clementine_thesis.pdf

Note the drawback of bow down is wetness and some times instability. Spray rails seem to go with flaps.


On the design forum, Fast Fred says that Robb White boat is 550 lbs empty. That make a D/L of 20 empty and of 50 when 2 200 lbs people on board. I think this is certainly the key factor for performance.

 

""I think the prop is sucking water into the tunnel section from forward then pumping it out of the tunnel section fast enough to create a suction between the hull bottom and the surface of the water."

I think the efficiency comes from the water already accelerated by the forward part of the boat, not suction. A prop is a pump, and all pumps push better than they suck.

If the water drug along by the hull is 3 or 4 inches deep, almost ALL the water at the prop cavity will contain energy , to be reused when more power is added by the prop.


"And the added wetted surface of the box keel must be of some detriment...
Some writer said something like this too, but I think just the opposite is true. I think the box keel acts like a super-efficient displacement hull (like a catamaran or kayak hull for example) that supports most of the weight of the boat and its contents. This portion of the hull would naturally be easily driven through the water."

I believe the submerged box keel will not generate surface waves , as a submerged submarine does not.

IF The 60% or 75% of the displacement in the box does'nt create waves (and loose energy) , all we are "paying for" is moving 25% of the hull weight .And I'm not sure the reversed V has much more wetted surface than a stock V boat.

"I think the boat smoothly accelerates from a standstill all the way through the range that most people refer to as "semi-displacement speeds" without trying to bury its aft end because the box keel part of the hull is effectively still running in displacement mode until the boat is very near its top speed.


During this acceleration, long before the boat begins to close in on the box keel's "hull speed" and starts trying to squat (assuming it ever does try to squat), the water has filled the tunnel and is gradually contributing more and more support or 'lift' to the aft bottom section of the hull, thus counteracting any attempt to squat.

I also suspect that this is the reason why these hulls do not feel safe when you overpower them. I think the box keel section gets unstable when you push it too fast. "

The two write ups on Atkin hulls that were overpowered said the steering got wonky at top speed .
Atkins cure was a simple lifting strake to allow the bow to rise to the hight of the stern.

MY cure would be to use OTS trim tabs to adjust the bow hight.

At "normal" speeds the tabs would be part down to allow the stern area to perform as the stabelizing lifting surface and carry weight at the proper angle.

At the highest speed the tabs might need to be thinned to reduce the aft lift ,stop depressing the bow , and allow the bow to run on the normal waterlines.

Yes, the Rob White boat was light , but 28nmpg at 10.5K is a claim I have never seen anywhere else.

The Atkins boats were claimed to be flat in turns at speed. Not sure if this is good or bad , but Rob White got to like it.

With tabs a small one sided trim could have the boat bank into a turn if that was considered desirable.

With a fancy joystick steering , it could be automatic.

FF

 

FF-
I actually think the SBS has great merit for your application. As you say, there are many other attributes that make it a worthy contender. I would expect that the box keel configuration ought to suffer less from the sort of overloading that becomes so common on board cruising boats too. I know there are a number of people - myself included - that have at least toyed with the idea.

But, there's been a great deal made of its apparent fuel economy And as it was designed (and I certainly believe it was designed to be light, as fcfc has demonstrated) I'm just not convinced that it's any more efficient than a simple lightweight boat of 'normal' form would be.

I can't wait till you have yours in the water - then we'll all know!!

 

You're welcome. Some of the links here - don't know which ones now - are advertising bare hulls for sale. That might be an alternative to building the entire boat. Find the hull that meets your performance requirements and finish it out as your needs and likes dictate. Certainly an alternative to be considered.

Best,

Leo

 

Sorry for the confusion here Fred. When mentioned suction I was talking about stability, not efficiency. Personally I don't think the suction has anything to do with efficiency, but I think it has everything to do with the "feels like the boat weights 10,000 pounds" stability Robb White experienced while under way.

I don't think it matters much if the box keel's stern wave breaks the surface or not. The box keel will still generate a wave, and that wave will be trapped beneath the hull -- which means the aft hull can "get a free ride" on this wave. Surfing your own stern wave is a "good thing" in terms of efficiency I believe ...

Looking at the hull I'm building now, I can tell you that *if* the inverted vee has any more surface area than a regular vee, it is certainly minimal.

I think I read this too although I don't remember where at the moment. This brings up an interesting point. If you take a good look at Shoals Runner in profile you will see that its chines are in the water by design:



Now take a look at Rescue Minor and note that RM's chines are above the waterline:



Why the difference? Maybe William Atkin discovered something about RM (design #500) that was "not so good" and maybe he thought he could fix it by keeping the chines in the water in future boats -- such as Shoals Runner which came along several years later as design #786. Probably not the case but it's a possibility. In any case here's one of my current theories on this issue:

If the aft chines are in the water by design, the tunnel won't need as much downward hook in it to counteract squatting because the chines in the water will help float the aft section of the hull all the time instead of only when enough power is added. This will help prevent squatting earlier. This also means the prop wash could be directed more horizontally by using less downward hook in the tunnel, and then the boat won't try to bury its bow as much.

I suspect that this could be one of the reasons for the "in the water" chines of SR. Of course we have no way of knowing this, but we do know that SR is a later -- and therefore theoretically or potentially better (?) design than RM, which is why I'm designing a new tunnel-stern Seabright skiff based more on the lines of SR than RM.

My design uses built-in spray rails to help lift the bow (and keep the spray down of course). The spray rails are designed after the Tolman Widebody and Jumbo skiffs which incorporate them into the hull itself rather than just tacking them onto the outer hull later. Not only does this strengthen the hull using less material, but it also creates more interior volume above the spray rail line:

I think this sounds like a great solution because it also means not building as much hook into the aft tunnel section of the hull bottom. Then again, avoiding the expense and requirement for trim tabs means a lower price and simpler operation with fewer things to go wrong ... and I like 'cheap and simple' whenever possible.

 

I do not understand. There is a direct relation between a wave system wavelength and its speed. v = 1.34 sqrt (L). or ( u= sqrt(gL/2PI) is SI units).
So at speed above 8.5 kts, the next crest is more than 40 ft aft the first one. If the hull is 20 long, it is totally on the aft slope of the bow wave. I do not understand how it can surf something.

If the speed is below the hull speed, the wave system is shorter than the hull, and there is something to surf or recover. That why the resistance is considerably lower for speeds under hull speed. (or expressed the other way, the resistance is considerably higher for speeds above hull speed).

And if you are speaking of staggered hulls, perhaps there is a speed at which wave system cancels, but you should know that there are other speeds at which wave systems do ADD. This technic is known for nearly one century with bulbous bow design. And although this technic is known and mastered for big ships, no small boat under 70 ft has really gained something measurable in real conditions with this.

 

I'm not sure I do either, but Robb White said he thinks his boat surfs on its own stern wave ... and if it does, maybe that's one reason why it gets such great fuel economy.

There is one hull on top of another in this boat. The bottom hull is long and skinny and ends in a sharp edge. Most people refer to as a "box keel". The other hull sits on top of this and ends behind the box keel, so it is the upper hull's aft end that might surf on the lower hull's stern wave.

Robb said "stern wave" not bow wave so maybe you and he are talking about different waves? Or maybe the boat is surfing down the face of the first crest, not the second? Or maybe the tunnel-stern "reshapes" the wave in such a way that it produces the feeling of surfing?

We do have a propeller thrusting water backwards aft of the box keel and forward of the aft end of the inverted vee hull, too. I don't think we can ignore the influence of this item, although I have no idea how to predict what unusual effects it may create. in terms of its interaction with the boat-generated waves.

I would guess that hull speed for a double ended box keel is pretty high since it is so long and skinny. Do you know what might be a reasonable guess for a hull speed here? I think it is probably about 2 feet wide and maybe 16 feet long or perhaps a bit longer.

If you mean one hull on top of another with the lower hull ending forward of the upper, that's exactly what we have in a tunnel-stern Seabright Skiff.

Maybe this is what's going on then. All I know is that there are lots of good things said about tunnel-stern Seabright skiffs and economy is just one of them.

 

Looking at the lines above, I am struck by how the curve of the hull aft of the box keel describes a kind of expansion section. Is it possible that the lift comes from the sudden lessening of flow rate aft of the keel as the water flows into the larger area? There is certainly incidental lift from the hooked stern, but there may be induced lift there as well, which is a more efficient way of generating lift.

BTW, the one thing I can comment on a little is submerged hulls. Submarines most certainly DO create detectable waves, and under certain conditions can be tracked by those waves, even though they are submerged.

 

Interesting concept, I never thought of lift being generated like that before. I wonder how the propeller -- being in the tunnel between the box keel and rudder -- might relate to the generation of lift in this manner?

 

Hmmm… Isn’t it interesting that a comment like this can set off a chain of thoughts. Mine follow.

Instead of flow rate, let’s think about pressure.

We know that liquids are (for all intents and purposes) incompressible. But we do know that we can apply pressure through various means to a closed system and cause at least a reaction, if not real work to be done, in a different part of the closed system.

Thinking back to my basic hydraulic instruction and differential areas of pistons in a closed system. If you’ve ever studied hydraulics you probably remember the diagrams. Essentially, they have a large piston moving a small distance causing a large movement in a smaller piston – or vice versa.

The key here is the differential in area of the two pistons. A 10 sq in area piston will move a 5 sq in area piston twice the distance the other one is displaced. – And vice-versa.

That’s all fine, well and good. But what about lifting capacity? If piston 1 exerts a force of 10 psi on a fluid, piston 2 also has 10 psi working on its area. But if the area of P2 is larger than P1, P2 will exert more force on an external load.

Seabright hull form. Water, being virtually incompressible, and in this case virtually immovable, the boat being both movable in the horizontal plane via a propeller and moveable in the vertical plane via lift.

Water moving around the box keel – which at its aft end comes to a point – gains some amount of pressure as it first passes by the hull form (box keel and immersed chines) and then by and through the turning propeller. As it passes the propeller, the higher-pressure water has a “cavity” with which to dissipate pressure into. Since water can’t expand or compress, all it can do is be affected by pressure.

Hypothetical numbers only…

Passing the end of the box keel and through the propeller the water column has had imposed on it a pressure of 10 psi. That 10 psi affects an area of – for arguments sake only – 314 sq in – which happens to be the area of a 20” diameter propeller.

Once past the propeller that 10 psi has the opportunity to dissipate into a cavity - bounded on the bottom by incompressible (mostly) stationary water (that the hull is passing over) and on the top by a concave area of the hull. The sides of this column of pressurized water is bounded by water that has had some velocity imparted by the moving boat hull, but still at some lower velocity than the hull is moving at.

So we have 314 sq in of 10 psi water moving into an area of lower pressure and therefore the 10 psi can begin to dissipate into the larger area. Some of said 10 psi is going to exit the rear of the tunnel providing some forward thrust and, via the hook in the hull form, imparting some upward component to said thrust, thus forcing the bow (the other end of a lever) down as observed by White with his RM.

Let’s speculate that the area of the 'tunnel' is 10 times the size (in square inches of area) of the aforesaid water column in square inches – 3,140. Let’s further speculate that this water column doesn’t lose all its pressure when being thrust out the back of the tunnel by the force of the turning propeller, but only 50%. So if my math is correct, that 10 psi column of water would dissipate 50% of its pressure into an area of 3,140 sq in, which at the remaining 5 psi, would exert a force of 15,700 pounds.

Logically, there is not 15,000+ pounds of force pressing the hull skyward. Some amount of that pressure is going into the surrounding water and some percentage of that is exiting out from under the sides of boat as it moves along. What percentages? I couldn’t even begin to take a SWAG – and I doubt that there is anyway to actually measure something like this without extensive and expensive equipment.

But even if 10% of that 15,700 pounds were exerted against the hull – that’s 1,570 pounds of positive lift.

All of this speculation means that if the Seabright hull form is creating some sort of pressure wave in a relatively small area and that said pressure then expands into a larger area, then the result would be the behavior that has been described by Robb White, i.e. the solid feeling and the bow assuming a nose down attitude as the boat increases speed.

The key must be the box keel some how produces some hydraulic pressure.

Convoluted. Yep. But let’s see if this very speculative hypothesis will hold together after some examination by the denizens of this keep.

Best,

Leo