Flat Bottom Skiffs- Design ?

[holzbt]

Almost everyone agrees that the heel of the stem of a flat bottom skiff should be clear of or just kiss the water. But almost every Atkin skiff design has the stem heel firmly planted below the waterline. The Atkin's (father and son) designed plenty of flat bottom skiffs and should know what works. Who's right? Anybody have any thoughts or experience with these two design philosophies?

[Jack Heinlen]

My limited experience is that having the broad ass above the waterline with a typical load is much more important than the location of the root of the stem. Talk about drag from turbulence! But you knew that.

Hmm. Billy, particularly, thought long and hard about these boats. I'll be interested in the replies.

[AngWood]

Perhaps with the heel of the stem below the waterline tracking is better, though you'd think on a sailing skiff you'd want to tack on a dime. Also, entry is sharper with the heel of the stem submerged--divides the water.

[Dennis M]

There is some disagreement on this. In his "Good Skiffs" Stambaugh notes that an immersed stem increases lwl and thus potential speed while making the boat less subject to pounding. Bolger, on the other hand, argues that in skiffs, the curve of the bottom should match the curve of the sides to decrease turbulence and resistence and increase performance.

I owned a bolger skiff designed on that latter principle and it handled like a sports car. Tacked on a dime, pirouetted on its bottom in a turn as graceful as a ballerina, and was fast! The absence of a skeg increased its liveliness and maneuverability, but made it awkward to row without the leeboard down a bit to aid tracking.

I've always wondered about this, being a lover of sharpies, noting that they (Ohio pound net boats, and New Haven sharpies) did not immerse their stems. However, the Chesapeake has all kinds of examples of skiffs where the stem is immersed.

[ 03-20-2005, 01:30 PM: Message edited by: Dennis M ]

[John Bell]

My power sharpie would steer badly if the stem got into green water. The bow would go one way and the stern the other, with an alarming outside lean. All the sailing sharpies I've ever been aboard had their stems clear and I found them to sail well that way. I've no experience the other way around, but it must be OK too if done right. I've never had the guts to draw one like that myself for fear of embarassing results.

[Dennis M]

This is from Jim Michalak's website where he discusses Bolger's theory mentioned above.


AFLOAT ON SEAS OF PEAS...

This is Phil Bolger theory. I suspect he fell asleep hungry one night and dreamed he was sailing on a sea of frozen peas. As he sped along he saw the individual peas crashing into the hull and getting pushed out of the way. They slid along the hull in a direction perpendicular to the section lines as shown in Figure 2. He could see that the more blunt the hull, the harder the crash of the peas and the greater their resistance.

His first dreamboat was shaped like Figure 2A and it was a nightmare. The bottom was dead flat and the curvature all in the sides. As she pushed through the water the pressure of the peas was higher on the sides as the sides pushed the peas out of the way. The high pressure peas on the hull's curved sides wanted to move toward the flat bottom where the pressure was lower. They swirled around the hard chine in a drag producing vortex. At the stern the opposite happened as the vortex reversed direction to allow the peas to flow from the flat bottom to the lower pressure where the sides pinched inward.

For all the swirling peas she was a slow mean handling boat. The faster she went, the more she wanted to lift her stern and bury her bow.

Phil rolled over and started dreaming again. This time the boat had evolved to lool like Figure 2B. Her sides were flat and all the curvature was in the bottom. The peas still wanted to swirl around the chines but in the opposite direction as the first dreamboat. In this case the bottom had the high pressure in the bow and the low pressure in the stern and she trimmed bow high at speed and she was a better handling job. This shape hull has been used in working scows for a long time and is still universal for working barges. It has maximum deck space and volume of its length. But a short wide one can be a mean thing in rough water.

Phil rolled over again and dreamed the third boat which had its sides and bottom shaped to the same curve as shown in Figure 2C. The front view of this hull shows the line that represents the chine bisecting the angle formed by the bottom and sides. That insures that the sides and bottom have the same curvature, something you can tell with a quick glance at the end view of the lines. Now as the hull pushed through the peas there was equal pressure on the sides and bottom and the peas had no interest in swirling around the chine. Instead they simply flowed straight along the sides and bottom with minimum drag. She still had a trend to trim bow up at speed but that was better than burying her bow.

If the curvature is pronounced, the stem is up well clear of the waves. In light winds she'll sail upright with a minimum of wetted area. In stronger winds she heels to produce a longer waterline. Sail her heeled with one chine flying and she becomes a V bottomed boat cutting smoothly through the chop. This sort of hull has become almost a standard "Bolger sharpie". The simple box shape can deliver smooth water performance under oars, sail or motor comparable if not superior to some much more elaborate hull shapes. It can also be very easy to build have lots of useful volume and stability. It also can get slapped around by rough water.

Bolger claims the ideal sharpie has no flare in it's sides, especially for a pointy bow sharpie. The sides are at right angles to the bottom. I suggest that slight flare angles and small differences in curvature between side and bottom only degrade the theory by small degrees. A little flare or a little curvature variance will increase drag just a little bit. This is unlike using a little twist in the hull's planks which totally negates simple curved panel drafting and requires much more demanding drafting techniques.

Using Seas of Peas theory links together to a great degree the lines of some hulls since the sides and bottom will have the same curvature. If you draw the side view and any cross section, all the remaining cross sections and all of the top view are predetermined. Another approach might be to draw the main cross section and then the chine line in the top view.

All hulls can be examined with Seas of Peas theory in mind, even round bottom hulls. It is just a thought tool to visualize the flow of the water around the hull.

WELL, THAT WAS ABOUT 15 YEARS AGO...

... and I think we have drifted from the theory in attempts to get boats that are more presentable to the eyes of most folks. Boats that Phil did to this theory are like his Martha Jane and Mico (even there I'm not sure they are exactly to the theory) and you see you usually end up with a scow with a very high bow. (The high swept ends also make for quick turning.} I did a few back then to the theory like Pencilbox and Deansbox and Cubit and Jewelbox, Jewelbox Jr, Musicbox2, etc but I suppose like Phil I found folks expect a boat to have a pointy bow.

[RonW]

Bottom of stem above or below waterline, who is correct? They both are. It depends on the intended useage. The pacific dory a perfect example of this, with the bottom of it's stem sticking out of the water and the rest of the flat bottom, side to side is also flat to the transom, Is intended for beaching and with a outboard on the transom as a planing hull.But it is also a well known fact that the flat bottom will pound on anything other then smooth water. In fact even in choppy water it will pound so hard that you have to reduce speed to 12 miles or less to keep you from being pounded to death.
Now take a look at the Carolina dory, and the bottom is perfectly flat from front stem to transom and from side to side. They where made to run pleasantly as a dissplacement hull, as well as being able to get up and plane when the conditions are right. A universal flat bottom hull, they also beach quite well due to the fact that the bow area is so fine, but a fine bow area also cuts into waves more then rising up and over, so a little bad with a little good.
Then you have Atkin who does just the opposite and sticks the bow below the water and curves the bottom from the middle upward till the transon just touches the waterling as is so common in any easy powered hulls such as a launch or sailboat.
Atkin knew and accepted the fact that the worse planing hull possible was the flat bottomed hull, and did not waste his time trying to make one into a planing hull.With maybe the exception of the russelr which has a beam to lenght ratio of 4 to 1 and was intended for low power.Look at his hearts desire and read his comments, a easy to build, roomy and stable hull that will handle rough water, run at low speeds and be a pleasant and very economical boat to use and own. It also would be a good candidate to be turned into a electric boat in modern times.
So why did the flat bottom hull become and still is so popular. A quick easy and cheap boat to build that can be dragged upon the bank, a universal type boat hull reguardless of what type of power even rowing or paddling as is evident in a pirogue.I think the pacific dory is so popular on the pacific coast due mainly to the fact of it being able to be launched directly from the beach, and it is a seaworthy hull, rough ride is a compromise.
Pick your useage and there is a flat bottom hull that will fit the bill, even sailing. Look at the long and lean sailing scows of the great lakes that would flat out fly.
As to comfort and economy, Atkin was correct.

http://www.boat-links.com/Atkinco/Utilities/images/HeartsDesire-2.gif

[Captain Pre-Capsize]

And here I thought my skiff had the heel of the stem out of the water because of my weight. What a relief that now I can explain, "Actually it took a great deal of time on the part of the designer to make it lift out of the water just so..." :D