Planing on the Chine

Interesting phenomenon being discussed on the WoodenBoat Forum: http://forum.woodenboat.com/showthread.php?130270-My-Simmons-is-a-One-Trick-Pony. A recently built Simmons Sea Skiff 18 sometimes rolls in moderate waves while at planing speeds and adapts a new, stable attitude at a considerable heel angle. It remains at this attitude until the throttle is cut and the speed slows. The Simmons Sea Skiff 18 is 17' 1" long with a 5' 7" beam. It's a V-bottom but the deadrise is small and the chines go the the base of the stem. Perhaps best thought of as a flat bottom skiff shape which has been modified with a little bit of deadrise in the bottom. Website for the Simmons Sea Skiff is: http://www.simmonsseaskiff.com/ Note there are two sizes of Simmons Sea Skiffs and the 18 is the smaller.

Discussion and responses about this particular boat are best done on the WoodenBoat Forum. But I have some questions about the phenomenon in general:

What can be done in a new design to prevent this phenomenon from occuring?

Is it generally limited to flat bottom skiff type designs or does it occur with the chine curving up and greater deadrise towards the bow?

Anyone know of any research into this phenomenon?

 

The Simmons is a well documented and respected design. The issues the one fellow is having may be associated with his particular build, probably a CG issue, unless of course he made "changes".

There are so many possible permutations and variability that hazarding a guess as to his issue(s) is just fodder. I've seen similar problems with over powered, hook equipped boats and backing off was the only fix, other then flattening out the hook. In this case, he's running what is called for, which suggests it's a trim issue.

The Simmons likes to run with her bow clear or she'll root around and bow steer, which will cause her to adopt what ever "trim attitude" she find least resistive. A skeg may be helpful, as may a ventilation plate addition, though getting her trimmed out properly will likely fix it.

No, this problem is not indicative of flat bottom boats, but boats with some rocker, narrow bottoms, hooks and other design "devices", intended to "improve" one aspect of the SOR, usually effects other areas.

 

This phenomenon is officially called "non-oscillatory dynamic roll instability", meaning that a boat will lay over to one side an stay there unless you cut back on speed. It is caused by too much hydrodynamic suction on the forward half of the hull. This, in turn, has a number of causes:

• The boat is to heavy for its size
• The LCG is too far forward relative to the longitudinal distribution of chine beam
• There is hook in the hull bottom near the transom
• In profile, the buttock curvature is too extreme near the bow and extends too far aft
• Rudderpost ventilation
• Rudder ventilation
• Improper rudder toe-in/toe-out
• Rudder trailing edge too close to, or extends aft of the transom

Just to dispell any indication that you think I am really smart, this list is actually taken from a very good article in Professional Boatbuilder magazine, issue #84, August/September 2003, called "Correcting Dynamic Roll Instability" written by Donald Blount and Dean Schliecher. Don Blount is one of the world's premier high-speed powerboat design professionals. As you can imagine, each of the causes above has a solution: Lighten up the boat; Go slower or change to a smaller engine; Move the LCG aft; Change the hull design to reduce curvature of the bottom; Attend to any of a number of rudder issues.

There have been a number of articles, technical papers, and books done over the last 50 years that address high speed powerboat design and performance by the likes of Blount, Savitsky, Du Cane, Koelbel, Hadler, Hubble, Fridsma, etc. There is a very good document, Technical and Research Bulletin R-42, "Seakeeping of Hard Chine Planing Hulls" by Daniel Savitsky and Joseph Koelbel, November 1992, published by The Society of Naval Architects and Marine Engineers, through which you can buy a copy. For those of you in the UK, you may have access to a very good short article published in The Naval Architect, from The Royal Institution of Naval Architects, March 1979, also by Savitsky and Koelbel, titled "Seakeeping considerations in design and operation of hard chine planing hulls", page 55. This is the short version of the SNAME T&R-R-42 (above) published 13 years later.

I hope that helps.

Eric

 

Thanks for the replies and the insight. The boat in question is reported to have some hook in the bottom aft. It has an outboard and the owner reports changing the trim of the motor so that the bow rides higher, and in doing so he picked up speed.

The boat is open with a center console, and based on photos the center console is amidships, and when a passenger is aboard they generally ride beside or ahead of the console so the CG may be too far forward.

 

Eric,
Several months ago I was told by one of my clients (boatyard) about such a non-oscillatory dynamic roll instability in one of his models (not my design, luckily). He suspected lack of symmetry between bottom sides being the cause. I have not done further investigation as he didn't ask me to do so. If things are like that, here you have one more cause to add to the list.

Cheers.

 

Hi Guillermo,

Thanks for that: Asymmetry. It may sound kind of funny that the two sides of the same boat could be different! But you are right, it does happen. I consulted in a case some years ago in which an aluminum offshore supply boat, about 50' long I think, had been converted to a tour boat and had suffered some damage during a haulout. When the boat was measured with digital measuring equipment, we found that the planing surface one side was about 4" (100 mm) wider than the other side. This fact did not materially affect the outcome of the matter, but it was interesting to see. This boat was built on the US Gulf Coast in Louisiana where some shipyards have a tendency to build commercial vessels with hardly any plans at all--i.e. no design! And that is probably why this discrepency occurred.

So you are right, asymmetry can be a cause of bad behaviour.

Eric

 

Hydrodynamics of High-Speed Marine Vehicles by Faltinsen discusses roll stabiliity in sections 7.7 and 9.4. There are reports on work by others with examples of changing stability as Fn increases, including cases of boats at speed with non-zero stable roll angles. I was slightly surprised on how brief the discussion is, and the lack of anything on the mechanisms which cause it.

 

A similar thing to what Eric is talking about occurred with the USCG 30 foot Surf Rescue Boat in the 1980's. Because it was a single engine boat with a rather large prop and a lot of torque the designer deliberately put a hook in the aft planing surface on the port side to counteract the torque. This combined with a rather forward CG and Convex bow result in the boat nosediving to the starboard side. Not a good trait in a rescue boat.

 

The above case is reported in "Marine Technology, April 1987, pp143-163". It includes a valuable list of references on a subject that pops up in the world of planing boats now and then. A number of design twists with little or no scientific value are used by some designers lacking a basic understanding of the forces developed by a planing hull, often resulting in instability of the moving hull.

Eric's list of items covers the issue very well; it is often a combination of two or more factors beeing "off-line" that manifests in a bad behaviour. Sometimes a builder deviated from original design for some reason, producing a bad example of a boat design that otherwise has a good reputation.

 

Eric's list is good but does not address why the Simmons is especially vulnerable to this behavior. That is because it is a narrow bottom dory with a lot of flare in the topsides. Like another very popular boat, the C Dory, it is more subject to this "dynamic roll instability" than other hull types. Actually, we should probably not call it unstable as these boats become very stable once on their sides. I found that cutting the throttle was the only quick way to get the boat back up.

Because the Simmons has such a small planing surface, it likes to run with the nose high and Simmons probably put in the hook aft to get the bow down. As Eric points out, driving the bow down on such a narrow boat can create bow steering issues and cause the instability under discussion. Its a good boat for specific purposes but not the best for general use. For running inlets and fishing the near shore, it is very seaworthy but its best to know its limitations.

 

I have never heard of a Newengland type dory having this problem.

the Simons is a V bottom boat and not what I consider a dory.
I built and worked on many flat bottom power dories while working for Pert Lowell Co and Lowells Boat shop of Amesbury ma. and never heard of this problem.

it is not a "Dory characteristic"... sounds down right dangerous

 

What is a New England type dory? The original Banks Dory type rowboat has been bastardized by countless people enamored with its reputation for seaworthyness. I would agree that a flat bottom dory might be less prone to this behavior than a V bottom, especially if there were more deadrise warp forward. There are a great many boats that exhibit this behavior. As Donald Blount said in the referenced article, "its a dirty little secret". You should read the article Daniel.

 

When talking about Simmons Sea Skiffs keep in mind that three different designs are known by that name, 18, 20 and 22. The 20 is considerably beamier than the 18. The beam is 2' 4" wider with the OAL 2' 3" longer. Small scale lines drawings of the 18 and 20 can be found by pressing the "InfoPack" button at http://simmonsseaskiff.com/SSS plans/index.htm

 

To me a dory has a relatively narrow flat bottom (though with rocker in profile), a very narrow transom, and flaring sides. Tradtionally build dories also have some specific construction features including fore/aft planking on the bottom, sawn frames and lapped planking.

"Semi-dory" has been used to describe motor boats created by widening the aft end of a dory and taking some of the rocker out of the bottom profile aft. However I suspect many folks prefer to think of their boat as a "dory" rather than a "semi-" anything.

"Dory skiff" has been used to refer to boats with more skiff like proportions, wider aft and lower sides, than a traditional dory.

 

In addition to the excellent references Eric provided, Professional Boat Builder magazine, number 31, has an article by Lou Codega on non-oscillatory instabilities, The Dynamic Stability of High Speed Boats. Codega discusses the effects of hull shape mostly. He says that non-oscillatory roll instability is probably more common than suspected. Codega discussed the USCG 30' surf rescue boat, The problem with that boat was somewhat different. It had two stable trim angles, 6 deg and 1 deg. At the lower trim angle the boat was unstable in roll, and if it rolled it when broach.

Codega, and Blount and Schliecher in the article Eric reference, both suggest that non-oscillatory instabilities can be forecast by examining a trim angle vs speed curve. Bouble humps or other unusal behavior of the curve may be an indicator of a tendency to non-oscillatory instabilities.

Both articles are worthwhile reading for anyone designing or modifying planing boats.