Of Sharpies, Skipjacks, and Carolina Schooners

Bernouli may very well exist but it is not worth the headache in contemplating the phenomena. The venturi sections of a carburetor cause fuel to exit the discharge nozzle, a paint spray can sucks paint out of the can via a venturi arrangement. Carburetors deliver fuel from the discharge nozzle, paint cans deliver paint. No argueing those observations. But a boat.... well that may be a different deal altogether. To begin with, the boat is not operating inside a closed area as is the carb nozzle and paint can shroud. If any suction exists on the boat, and I suspect that it might, the negative pressure leaks out sideways. Still we might have expended some energy in the process. Bernoulli/venturi is based on the premise that a particle must accelerate when traversing a curved surface. Velocity and pressure are inverses. Sure enough it must be so in the case of a curved boat bottom. next paragraph please.................

Not long ago there was a spirited discussion, on this forum, about whether there was positive or negative pressure on a moving boat bottom. Drill a hole in the boat bottom and you will see that the pressure must be positive ,cause the damned hole will let water into the boat so as to wet your feet, ankles, knees, etc. Seems like the positive pressure overwhelms the negative. We might have lost some energy in the process, alas.

Beyond certain very low speeds, wave drag is the dominant influence. Take a look at the marina queen bopping along somewhere near hull speed. You will see a good sized wave at the bow and another very slightly smaller wave at the stern. In the middle of the boat, where the maximum section size is located, there will be a trough that falls below the static waterline. The fattest part of the boat is cheating. It ain't holding the boat up as much as it could. If the speed increases a little bit (it probably wont) the aft wave tries to lag behind the back of the boat and the aft end of the boat is running in a depressed area. When the boat begins to move the waves will begin to generate. At slow speed the waves will be close together. As speed increases the waves move farther and farther apart.That is the whole picture of hull speed. Did you know that if you could measure the distance between the bow wave and the transverse wave that occurs after the transverse bow wave, you could do an accurate estimate of boat speed? Physics do not lie. The distance between the waves is a function of the boat speed. Longer waterlines let you go faster before the second or following wave reaches the stern. Marchaj explains this stuff quite eloquently.

The size, and therefore the influence, of the following wave is closely related to the displacement of the boat. Heavy boat, big wave, light boat, little wave. Little wave, not so influential. Moral: Light boats are potentially faster. Accelerate way better too (see Newtons second law as in F=Ma)........ Anti-moral: heavy boats are a better deal in rough water, and heavier boats can carry their way while tacking and a very light boat can easily get caught in irons (been there, done that).

Are we there yet? Suggestions: Get the transom up to clear the water but only slightly clear. Weight placement is important, obviously. Keep the boat light if you want spirited performance, Hard chines have a little bit more initial stability than rounded ones. Little bit, maybe not a lot, depending on the radius of the round chines. Hard chines are likely to make more eddy drag than rounds ones so it is a trade off. Hard chines require a bit less energy in the transition to planing mode, again the useful difference depends on the radius of the chines. Try not to apply too much windsurfer knowledge to dinghys. Different breed of cat. Windsurfers are planing, and sometimes flying, boats. Dinghys and cruisers do not fly except in hurricanes.

 

How about this???

Stephen,

I'm just trying to get my mind around a theory of performance at the stern, irrespective of dimensions.

Re Mariner:
Looks like a cut off niche in the stern on the CL.
What happens when the Hull is heeled,? (which is how it will run most of the time.) The Cut off may rotate up above the WL, or may provide an off center vortex if some of the niche is dragging ?
Most cumbersome though, it looks like there is a bulbous after body that is cutoff underwater which would create all kinds of eddies.
Wouldn't you think there would be a considerable difference between a vertical cut off underwater, and a Cutoff that would be aerated.

Maybe that's it!!!!!!

Maybe cutoff the stern on the heeled WL deadrise angle desired.
That way the wake would be aerated and reduce the wave build-up under the counter at any speed.
Then, with some kind of pattern on the surfaces that would be wetted when heeled, ( to enhance boundary layer detachment), maybe that would reduce dragging up a following stern wave at speed??
Design steps:
1) Start from the bow to provide the desired entry, and provide lift .
2) Then begin from a cut-off Stern at the desired heeled Dead-rise WL.
3) meld between bow and stern to hard chines from the heeled deadrise angle beginning from the fore-quarters to the outboard edge of the stern.
( Note, I'm working from the bow and the stern toward the middle.)
4) with as little rocker as possible along the half sections ( while still providing buoyancy) .

( I'll not further confuse the issues with keel or CB, or ballast, or Rig. )

Is this at all rational?

Bill PKS

 

The end of the end?

Mr. Messabout,

I recall a Sunfish with a little drain in the bottom, that would pull water out at higher speeds, and it would plane on a close reach, even with such small sail.
I wonder if the whole " Displacement Hull" bow wave to stern waves effects may be the result of Bernoulli? more wind = more speed = more suction = larger the waves , until the power used in wave making exceeds
available power.. I head some clipper ships sailed under when they couldn't get sail in fast enough. Maybe the vacuum exceeded buoyancy.??? (( and water is more viscous than air. ))

As commented, I want to try to sail on top of the water, not swim thru it..

At the end, your opinion to end the stern at the waterline ends up pretty much where I ended up.
Do you agree??

Thanks for the mental gymnastics,
Bill PKS.

 

This may not be the sort of thing you were looking for with your question, but the upswept sterns in the old oystering sharpies had one very important purpose: reserve buoyancy. They were designed to settle at the stern as they were loaded, while the bow stayed just clear of the water.

When the transom and water met, the boat was fully loaded and it was time to head in.

When riding light, an oystering sharpie had less waterline to drag and moved easily under sail, especially in light airs. When it picked up speed, the length of the waterline increased as the boat settled into the trough, giving it a higher maximum hull speed.

The traditional sharpie builders apparently thought the shape of the bottom was very important. Instead of being rockered from end to end, most of the New Haven-style sharpies had a straight entry, rocker amidships, and then a straight run again aft.

If you follow the link below and scroll down to post #220, there's a profile shot of a model I'm building. It shows the general idea on the shape of the bottom.

http://www.boatdesign.net/forums/boat-design/30-plywood-sharpie-30029-15.html

 

Your model certainly makes it clear what you're talking about. So judging from your model it looks to me that the bulk of the rocker corresponds with the bulk of the hold and the load. I wonder l if area of the straight run aft is like a lever being used to pry the bow up so she trims nicely fully loaded. I understand what you mean by reserve buoyancy and am certainly not quibbling over semantics. In my mind I see the boat loaded up and then having the crew toss oysters into the *** end till the bow came up enough to not be squirrely and then head for the barn.

Those old timers were great about optimizing designs to suit very local conditions. All of this is just idle speculation that came to mind as I fooled around with your picture with a straight edge looking for the charactoristics you mentioned and imagined my self in the oyster biz way back when growling at the crew to get some more weight aft so she wouldn't root around. Maybe this was commonly a 1 man fishery and I wouldn't have anybody to ***** at and would have to do it myself.

I know you're into these boats. Any more tidbits of insight?

 

Well, they standardized pretty quickly into two sizes: a 25-28', one-man boat, and a 35' two-man boat. So you'd have needed the bigger one if you wanted to play Captain Queeg....keeping in mind, of course, that you'd be growling at a guy with a pair of oyster tongs in his hands.

I don't have any special insight into sharpies; I just know they're relatively easy and cheap to build, and fun to sail. But I imagine an academic-minded person with design experience and a computer could have all kinds of fun analyzing characteristics of the breed, particularly the old working boats.

For example, the masts were normally installed plumb, judging by the plans I've seen. But when you loaded the boat down and the stern sank, you'd wind up with a noticeable amount of rake in the tall masts....would that do anything measurable to performance?

Since they're flat bottomed and hard chined, they ride a chop better if they're heeled. And since they're narrow beamed, they heel fairly easily. Phil Bolger probably spent hours banging his head on his drafting table and redrawing lines, working toward hulls that formed a relatively symmetrical vee when heeled a reasonable amount. But heeling beyond the optimum will probably leave you fighting weather helm anyway, even in his sharpies...

Another characteristic of the New Haven style sharpie is that they used tall, flexible, unstayed masts. That might hurt ultimate performance, but it also meant that in a sudden gust the masts would initially take up some of the force and spill wind -- a nice feature for a narrow and shallow boat.

What do I mean by tall and flexible? Fig. 38 in Chapelle's American Small sailing Craft shows the sail plan of an early 27' sharpie -- with a foremast stretching 28' from the deck, and a 26' mainmast. Those masts were probably 4 1/2 inches at the partners, and only 1 1/2 inches at the head.

Chapelle describes the 35' sharpies as having foremasts 28-36' tall, with 19-20' sprits. He says mainmasts were typically two feet shorter, and the main sprit one foot shorter.

In practice, the sails of the two-masted sharpies were handled like those of a sloop with a large jib. Another interesting feature of the masts is that they were designed to rotate easily. Since they were also unstayed, that meant the crew could loose the sails and let them blow downwind, without the sprit boom binding on the mast -- which they often did, while drifting and tonging. And if running directly downwind wing and wing, they could let the sails swing somewhat foreward of the masts, which stabilized the ride.

The New Haven sharpies had huge balanced rudders; according to Chapelle, many of them were six feet long. But the boats were mainly steered with the sails, anyway. I imagine that big rudder was mostly for close maneuvering, especially in case someone wanted to kick the stern around and spin the boat on its centerboard -- which they were capable of.

Hoo boy....think that's enough rambling on for one night. Didn't mean to get so longwinded.

 

So right about the steering of the shallow rudders. When in the up position, the rudder was near to useless at high speed and steering often required some sail adjustment. Reserve buoyancy may have played a part in the long overhang of the stern. Perhaps the main reason for it was that, in combination with the shallow rudder in the up position and a raised centerboard, the boat had draft equal to hull draft. This would have been most important in the thin water of the Carolina Sounds.

Don't know why so many need to be in denial of Bernoulli. It is a fairly simple principle that is exhaustively proven in practice. The fact that there is a NET positive pressure in the flow over a curved surface is no proof that negative pressure does not exist.

 

Sharpies worked.

Troy 2000,
It all sounds reasonable.
Your Model looks really good.
(( Although you might no want to see it, go to You Tube > Sea Skiff 22 to see a bay& inlet boat I built, ( just in time for the sinking of the boating business.) The fore deck crown was set by pulling a string from stem to the stern, while the gunnel cap followed the sheer,,,, to my eye that worked well.. With your sheer though, crowning the deck to a string line might be a bit much.))
A while back, I proposed to build a 44' Carolina Schooner ( Like the PDF at the first post ) and give it to a group of local museums, for joint use. I wanted to use a traditional hull assembly, but to use treated boards that were water resistant, to avoid rot. Anyway, the project fell apart before I found a satisfactory treatment.
I think the genius of the Sharpie was ( notwithstanding the fact that it was so simple and cheap to build), the hull form became a displacement V on heeling, burying the surface of the side to resist leeway, and used virtually no ballast to hold the rig up.
Somewhere in Chapelle or the Commodore's writings it said, ' a Sharpie would pound at anchor, while a Skipjack would pound underway.'
But then, those guys didn't care as long as the hull did it's job of hauling stuff.
Bill PKS
PS: Lumber is cheaper than plywood right now.

 

All Thumbs

Troy 2000

To your second post,,
Carolina Boats used a " Thumb " cleat ( I don't know about others),
where the sheet runs over top of a loop in the tail.
A quick jerk of the sheet would spill the main to a flaw wind.
Many smaller boats used tiller lines to a yoke on the cap of a relatively small shallow rudder. The main advantage was, as nets were kept in the stern, there was no tiller in the way when flaking them down,, also the boat could be steered by a fellow sitting on the end of a spring board amidships.
Bill PKS

 

I agree that the ability to tuck the rudder up under the stern when necessary was an advantage in thin water. But if that had been the major reason for carrying the stern above the waterline for several feet, there are easier ways to go about it. For example: since they were already building pivoting centerboards, I'm sure they could have come up with a pivoting rudder.

The New Haven sharpies had iron-bar rudder stocks in pipes, with holes drilled in them. Rudder depth was set by inserting a pin into the appropriate hole, and letting the pin ride on top of the pipe. With a rudder stock length of 36-40 inches, I imagine that made the rudders a lot more useful in a seaway.

And I don't know enough about Bernoulli to join that discussion...

 

I used a yoke and tiller-line setup on a flat-bottomed, double-ended sailing canoe I built years ago. It worked very well, and allowed me to get my weight out of the stern.

The canoe started life as a simple Caddo Lake bateau, built from 1x12's. In a series of fits of insanity I removed the live well and added a strake of 1x6 lapped plank; a pivoting centerboard; a yawl rig consisting of a gaff main, boomed jib and spritsail mizzen; and the aforementioned rudder with yoke and tiller lines.

It was horribly over-canvassed, and I spent a lot of time sailing it jib and jigger with the mainsail laying in the bottom. I think I had more fun in that thing than any other boat I've ever owned, before or since. But I always made sure I had a change of clothes and a big, fuzzy beach towel in the pickup before I launched it....

 

Just in passing I'll note that I've read that a healthy rake to the mast makes it easier to use a halyard for a whip line for hoisting cargo etc. Less hassle than a picking boom. Not sure how pertinent that is to the boats in question, food for thought. These were working boats, always a bit of headscratching about making work a little easier.

 

I suppose they could have come up with a kick up rudder but they didn't, at least I never saw one on any of the traditional sharpies. My experience on larger sharpies is limited to one 36 footer built from Chappelle plans. The rudder was built as you say. There is scant counter transom on which to mount a rudder of any kind. On the other hand, Sprits'ls have transom mounted rudders but only a few modern ones have kick up rudders. The traditional ones all have shallow rudders that are not nearly as effective as the pivot kind. We have a fleet of such boats here in the NC Maritime Museum in Beaufort. They served their intended purpose as work boats just fine but are outclassed as pleasure boats by most all modern daysailers.

 

Good discussion, Messabout. I agree with most of it. But I'd like to stipulate that by "suction" what I'm really talking about, at least at displacement speeds, is reduction in static pressure. If you were to map the static pressure on the bottom at rest and consider that the datum, does the pressure go up or down at speed? In general the answer will be up in front and down in back, but the vertical component of the change in pressure will be strongly influenced by the curvature OF THE FLOW.

Larsson & Elliason have a graph of optimum LCB/LCG and optimum Cp at various speeds IF YOU CONSTRAIN THE PROBLEM BY KEEPING THE TRANSOM CLEAR OF THE WATER. They suggest that, while at a given (high) speed a boat with a lower transom might be faster, a boat with a high(ish) Cp and a transom clear of the water will be the better all 'round boat over the range of speeds most monohull sailboats travel.

I'm working on a 5 panel hard chine sailboat (you could call it a sharpie, but it looks a little like a Volvo Ocean Racer). I've based the upright static curve of areas on a paper published by the Stars & Stripes design team after the 1987 America's Cup, and on the Larsson & Elliason graphs. The transom is just barely above the waterline, as Messabout suggests.

 

Tom,

I wonder if the lack of comparative performance between Sharpies ( Spritsail Skiffs) and modern boats is the rocker.
But as troy 2000 points out, the original Sharpie hulls were designed with abundant reserve buoyancy to haul a load of fish, ( and convenient to work in a following sea ). Spritsails are the offspring.
The Commodore noted that a New Haven Sharpie with low camber ( low rocker) surprised everybody in Florida, beating them all to Windward.
It seems to me this may support the case for Bernoulli.
I have never seen a kick up rudder on a Spritsail, or Carolina Sharpie either,
Maybe they didn't want to mess with something that was likely to "ground" or break. Perhaps they thought it was easier to balance load, CB and sail, than to steer with a big rudder.
Bill PKS ( Over on Bogue Banks )