I wrote this to answer a different question regarding Sheel keels but I think that it answers your inquiry:
Scheel keels are a very specialized version of a bulb keel. They are shaped in a very specific manner. The bottom of the Scheel keel is a semi-circle to reduce wetted surface and turbulance. The top of the bulb has tight fillets and a relatively flat top so that the bulb acts as an endplate, reducing tip vortex drag and pressure drop from the high pressure to low pressure side of the keel, tricking the keel into acting as a deeper keel span than it is. As a result a Scheel keel is more effective than a simple shoal keel and also more effective than a simple bulb keel.
As such a properly designed Scheel keel allows a reduction in draft from a deep fin with less of a penalty than would occur with either a simple shoal keel or a simple bulb keel. It should be noted that hydrodynamically nothing is more efficient that a deep keel without a bulb or wings. All bulbs and wings add drag without an offsetting lift
. Bulbs and wings came into popularity where a draft reduction is required either due to practical requirements or to take advantage of flukes in a racing
The result of the loss of efficiency of a Scheel keel vs a properly designed deep fin is that the Scheel keel will not point as well as a deep fin (more leeway, less speed, and a lower ideal angle to the wind) and will also be penalized at the extremes of light air and heavy air, as well as, when pointing dead downwind.
In a US Naval Academy study of groundings, it was found that round bottom bulb keels are the easiest of the keel types to free in a grounding. Deep fin keels were a very close second.
Lastly advertisements will often refer to a keel as a 'Scheel keel' that is not truely a Scheel keel. Scheel keels were invented by the late Henry Scheel and are patented. The true Scheel Keel was developed during very sophisticated tank test studies and require a very precise set of proportions, and profiles to work properly. Small variations can result in a keel that is less efficient than a simple bulb and which do not have the advantages of a properly designed Scheel keel. True Scheel keels can only be produced under licence to Henry Scheel's estate. Many of the so-called Scheel keels that were not built precisely to Scheel's designs offer huge disadvantages in terms of radically increased drag with no increase in lift
or even a loss of increase in lift.
In recent years further reserach into inproving shallow keel efficiency has lead to further development in bulb shaping with shapes referred to as 'beaver tail' keels often being more efficient than Scheel keels in cases where draft reduction exceeds the range that is acceptable for a Scheel keel.