The keel design for the Offshore 42 has by far gotten the most attention when it comes to engineering. We all know that a keel on a monohull is one of the most important aspects when it comes to safety and performance but judging by the failures we have seen in recent years, keels seem to have become a neglected part of production boat design.
The keel of a monohull has three major tasks:
1. To keep the boat upright when sailing,
2. Turn the boat right side up after it has been knocked down by a wave,
3. Create lift to make it possible to sail upwind.
In order for a keel to perform optimally, one would want a keel that is as deep as possible, with all the weight concentrated at the very tip and as narrow as possible in order to not have too much wetted surface area. As one can imagine, that puts a huge strain on the keel-hull attachment and this seems to be the area where modern production boats are having issues. The amount of serious keel damage and personal injuries including death reported in the last 10 years alone is very worrisome. As we wrote in last week’s post regarding the prototyping, it is possible to prevent this from happening if the right engineering assumptions are made, and not just the basic assumptions required by law.
These minimum safety rules are not written specifically to take into account the higher strain put on boats that are intended to be sailed tens of thousands of miles or more.
Common practices and minimum requirements for keel design:
- Safety factor of 6 on keel attachment,
- Allowable loading of material does not take fatigue cycles into account,
- No grounding calculations are required.
Assumptions used in Offshore 42 Calculations:
- Safety factor of 16 on keel attachment,
- Allowable loading of material is selected to withstand hundreds of groundings and knock-downs,
- Grounding loads are calculated at a full stop in 0.2 seconds at a speed of 8 knots fully loaded and 10 knots when sailing unloaded.
This can all be calculated and it leaves no room for errors.
Now that we have that out of the way, we can talk about…
Options for the keel:
It is the intention for this boat to sail as well as possible. That means that we want to choose the draft that most people would consider the maximum practical draft. We have gathered that this is around 6’ or 1.80 meters. We also want the resistance to be as little as possible and the generated lift as high as possible. Therefore a modern shape fin keel with the proper NACA profile (reverse teardrop shape) is required.
The most standard fin keel options for production boats are cast iron and lead. Both have advantages and disadvantages.
Advantages for Lead:
- Very low maintenance.
Disadvantages for lead:
- Relatively high center of gravity as the fin itself is very heavy as well,
- It is a very soft material that deforms easily, wrong storage, grounding and even sailing for extended periods of time on 1 tack can deform the keel shape.
Advantages for Cast iron:
- Cheap to build in large numbers,
- Easy to maintain.
Disadvantages for cast iron:
- They rust if you don’t maintain them properly,
- Relatively light material, so a larger volume is required to get the desired weight. This results in a higher resistance through the water.
We could make a fiberglass keel and fill that with lead. The disadvantage of this method is that it is difficult to make it part of the hull if a fin keel shape is desired.
Another option is to make a lead bulb, cast some very long keel bolts in there and have the fin made out of wood, foam or fiberglass. This has disadvantages as well. Long keel bolts result in more elongation, therefore they are much harder to keep tight, which will result in a keel that flexes and will ultimately let water seep in between the various parts. It also means that the keel bolts are relatively close together, resulting in much higher loading of the hull-keel joint. That means more bolts and a heavier bottom structure for the hull. But that is not all, as these filling pieces are prone to water absorption over longer periods of time. It is possible to monitor it, but it requires being meticulous about your maintenance which can be difficult when sailing to remote places or in bad weather. It also means that if the water absorption is neglected, that the consequences could be quite severe as the material will compact and will give the bulb as well as the bolts room to wiggle around, slowly destroying the construction of the boat. This could happen over the course of an ocean crossing. Even though we are of the opinion that a boat needs to be maintained properly, it does not mean that we advocate building keels that potentially end up falling off if not maintained. We want more safety than that.
A fabricated steel or aluminum keel, with lead, poured into the bottom section, is another option. The steel version will have similar issues with rust as the cast iron keel, while the aluminum version may have electrolysis issues if the electrical installation is not done properly. Fabricated keels are also more expensive. To properly compare each keel option, we have created a table below which ranks each option on a numerical scale.
We have ranked the various characteristics of the possible materials from 1 to 5, 1 being most optimal and 5 being the least.
From the above summary, based on our own long-term experience with sailing, boatbuilding, and boat maintenance, we conclude that a cast iron keel or an aluminum fabricated keel with lead ballast is the most advantageous choice for the Offshore 42. The added performance and lower maintenance of the alloy keel, in our opinion, justifies the added cost.
Photo: Bagheera’s keel, which has a similar geometry to the keel of the Offshore 42.