Keel Design

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:

  • Indestructible,
  • 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.

Hybrid materials:
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.
keel table.jpg

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.


6 thoughts on “Keel Design

  1. Francis Livingston says:

    I have been following the evolution of this boat for several years now – I note with dismay that the concept of this boat has moved from simple and affordable to “competitive” in price with other boats of its size – if it is going to be competitive in price it will have to be competitive in features as well – great another boat for the 1% – just what the industry needs – I am no longer interested if that proves to be the case


    • Stein Varjord says:

      Hi Francis.
      I’m also a supporter and participant on AAC and have several decades of sailing experience of all kinds. I’m a multihull fanatic, 🙂 and even prefer them very fast, so I’m not a potential customer of an Adventure 40 or Offshore42, but I very much like the thinking around it, so I pay attention and learn.

      To me it seems quite premature to complain about price before anybody can know anything about that. One could object if priorities seemed to drift towards luxury solutions rather than high quality functionality and durability, but that seems to not be the case at all.

      My impression if the core idea of the Adventure40 on AAC is that it’s a boat where all solutions are very reliable for long distance cruising with a lowest possible REALISTIC retail price. All designs and equipment is chosen to fit that target. The price point is reached by making the list of included gear complete but fairly minimal, so more can be added to fit the preferences of the owner, later.

      I see nothing here that doesn’t totally indicate that they want to adhere to those targets. These guys do however, put in the effort also to make some money. There’s no way to make a 42 foot good long distance cruiser available for everyone, even if it was for pure enthusiasm. It is possible though, to build it way more affordable than most fancy cruisers, and get an inherently better boat too. That is indeed the obvious target here. If we want the best possible result, some optimism and enthusiasm is more suited, exactly like Mike does just below here.


    • offshoresailboats says:

      Hi Francis,

      We have not disclosed a price for the boat yet, as we feel that there has been too much guessing for too long. At this point, we believe that we will be able to make the Offshore 42 available within a close margin of the original target price, but we will not make any promises until a full budget analysis has been made. We are not willing to make any concessions when it comes to reliability or quality, just as the original core principles of the Adventure 40 specified. We will shortly have our design priced by a yard with a great reputation for quality and efficiency. Together with the yard, we will work out a budget for the Offshore 42 that we feel has the best ratio of reliability, quality and affordability. Thank you for voicing your concerns and we look forward to your thoughts once the Offshore 42 has been quoted.



      • Francis Livingston says:

        Fair enough – glad to hear that the core principles of the “Adventure 40” concept are still being adhered to.


  2. mike595959 says:

    Erik and Kip,

    Wow, after the interior and deck redesign, which I really like, the O42 will now also have an alloy keel instead of the lead keel as proposed on the AAC blog. So far the amicable split from AAC has produced very positive developments. After months without any news on AAC I also really appreciate the cadence of the updates on this site, thank you. How will be keel be attached to the hull? Will there be a substantial flange? What material will be bolts be made of?




      Hi Mike,

      Thank you for the encouragement! Your questions are right on point and will be answered in detail in one of the posts that we have scheduled to be published in the coming weeks.



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