The power of a rule of thumb – takeaways after 27 years in the automotive business and 27 days in the maritime business

Eight weeks into my new role as Head of R&D at Oceanbird, I’m reflecting on the differences between the automotive and maritime industries, as well as the striking similarities. The most noticeable difference for me personally is that I am now a fish in a much smaller pond. Oceanbird is a two-and-a-half-year-old company with 30 employees that has set out to decarbonize the shipping business by offering wind propulsion. Scania is a 50.000+ employee behemoth with 133 years under its belt. Although the company Oceanbird is a small, speedy and agile fish it swims in the big and conservative pond of shipping. But even more noticeable is how engineering is very similar, no matter the business. One such similarity is how a few number of good rules of thumb can help you navigate the fuzzy everyday life of an engineer. Here, I elaborate on the similarities and differences to finally land in the importance and usefulness of rules of thumb.

Takeaways after 27 years in automotive and 27 days in maritime

After just a few weeks into my new role, one thing that has become clear: at Scania, we often told ourselves that the market was challenging. But keeping the company going was fairly straightforward - just sell a new truck to an existing customer, and you’d keep the business running. Even if that truck had to be electric, the core task remained: transporting people and goods from point A to point B on wheels. For Oceanbird, it is more challenging. We have to convince every new customer to buy something they have never bought before i.e. a wing sail. In that sense, Oceanbirds modus operandi is all about “winning new customers” while Scania is a “not lose existing customer business”. If electrification at Scania is to replace “diesel revenue” with “electrified revenue”, at Oceanbird, every sold wing sail is a new “revenue pool”.? That difference can be sensed in the atmosphere in the two companies R&D organizations.

At Scania, it was more a fear of missing out and destroying 133 years of success, at Oceanbird the attitude is more “we have everything to win”.

While winning the market is different there are of course similarities.

One similarity between the two businesses (automotive and shipping) is that they both stand in front of a green transition. Automotive is half a decade ahead however, right now, suffers from a slight puncture of the inflated hype with stifling demand. Shipping on the other hand is just starting its transition. Despite that, I hear the same cocksure statements about which green fuel that is right for shipping as I heard in heavy trucks 10 years ago. The future will tell and most likely it will be many different fuels that prevail in both businesses.

Another similarity is that product development is the same everywhere. Being a development engineer is about constantly having problems. In fact, solving problems, hopefully the customer’s, is what engineering is all about. At Oceanbird, just like at Scania, the time plans seems squeezed, the suppliers underperform, and the market seems to move faster than you can keep up with. What I have learned is that, when the situation feels overwhelming, the only thing you can do is to trust that the process, the culture and the people eventually will deliver solutions. Therefore,

leading R&D is more about building the “development machine” consisting of people, culture and processes (in that order), than it is about you taking smart decisions in every tricky situation.

The fourth thing that stands out for me, after just a few weeks in a completely new business, is how much I relied on existing knowledge, heuristics and shortcuts in my previous role. There were few questions in my former automotive job that I could not answer, or at least judge the realism for. Questions like “what is a reasonable consumption of a certain operation”, “how does the cost split look for an operator” and “what is a reasonable cost for a certain component”, all those questions I could answer just by simple rules of thumb I have gathered over the years. If the question was harder, I could usually find a good-enough-answer by a quick back-of-the-envelop calculation. In my new business (maritime), I don’t know how much fuel a container ship takes during a day (it could be up to 50 m2 per day) or the cost of a new Ro-Ro vessel (which is in the range of 100m$) or how much of a ship’s cost cake fuel make up for (up to 70%). That has set me out on a quest to hunt and collect my five to ten most important rules of thumb for my new business, the maritime business. And I need to do it fast to survive another couple of years in my new role. Therefore, I find it appropriate to elaborate a bit on what a rule of thumb in the R&D context really is.

What is a rule of thumb?

Someone has said about the difference between science and engineering that:

“engineering is useful, but not necessarily accurate while science is accurate, but not necessary useful”

That quote capture much of the essence with a rule of thumb. So, let’s dig in deeper to what marks out a good rule of thumb.

A rule of thumb is a heuristic that is useful, easy to remember and can be fetched from memory to answer questions good enough when time is limited. They are an “engineers life hacks”.

Many rules of thumb are invented as a result of previous failures. In that sense they harvest wisdom. In the best of cases, they are also rather generic, meaning they can be used in many situations to answer various questions.

How to use rules of thumb?

An engineer should be able to answer any question on any subject by just doing in-your-head-arithmetic’s and end up in the right magnitude.

I could be questions like: “are there more bio-mass in humans than I the rest of the eéarth’s living mammals on earth”* or “will a wire spanning in a straight line across the length of the lake V?ttern touch the bottom”** or “how many houses can you heat with the electricity from a powerplant in the river Lule-?lv”*** (you can look up the answers in the end of the blog post). The way to overcome those challenges is to have a palette of rules of thumb. By pick and choose from your palette, and combine in a smart way, you should be able to answer any question and, at least, end up in the right magnitude,

There are many ways to use a rule of thumb. Sometimes they can work as a proxy themselves meaning the answer the question more or less right away. For example, if someone says that you will never make up for the CO2 needed to produce battery electrical vehicles by the CO2 you save in the use-phase, therefore it is more environmentally friendly to continue run your old petrol car. One easy way to answer that question is if you in your palette of rules of thumb know that it takes 50-100 kWh of energy to produce a battery with 1 kWh storage capacity. If you then assume you charge with 100% green electricity, it means that after 50-100 times of charging you will break even with the petrol car (since you then have swapped 50-100 kWh of fossil energy to green electrical energy). The rest of the drive is CO2 saving. 50 times charging might take some time for a pass car but in a truck, which charge two times a day, it is less than a month of driving. I know there are many ifs and buts to this reasoning, like the fact that the EV and the ICE-car have different efficiency and that I don’t take into account the CO2 needed to produce the rest of the car etc. Although, the answer might be exactly wrong, it is roughly right, just like an engineer wants it.

Another way to use the rule of thumb is to box in your answer. This is usually the approach when the numbers get big. When I interview candidates for engineering positions, I test this by have them answering a question. One that I often use is “How many trees can you see when you do the Vasaloppet (Vasaloppet is a 90 km long cross-country ski race in the north of Sweden)? It is always interesting to hear the candidates reasoning. One way to attack the questions is to divide the racetrack into 9000-meter-long segments and ask yourselves how many trees you can see left and right in such a segment. It is certainly more than one tree on each side. I’d guess you can see 10 but certainly not 100 trees in a one-meter-segment. Hence, the magnitude of the answer is 90,000*2*10 i.e. somewhere in the range between 1 million and 10 million trees, likely at the lower end.

Sometimes a rule of thumb is there just to remind you not to take the wrong decision. Like the “One Trip” Rule which says that when carrying stuff, like for example groceries, if you think you can take all the bags in one trip, you’ll probably drop something - or get a cramp in your hand halfway to the door. This is a category where many designers rules fall. One such example is that you should never mount two parts on a truck chassis with less than an inch in between because at some outcome of the tolerances, they will interfere. ?

A final thing about rules of thumb is that they facilitate engineering discussion. If you are a group that share the same accepted rules of thumb, they become a communication lubricant. For example, at my previous job, Scania, we had a rule of thumb for how much added cost of materiel we could accept, if the new solution at the same time reduced the fuel consumption (i.e. for the financial gain from fuel reduction would make up for the added cost). It was in the range of 300-500€/percentage of fuel saving. Since that rule was accepted by everyone, it was easy in a group discussion to judge a new solution by just refer to whether it adhere to that rule of thumb. It is even so that a shared set of rules of thumbs can be something that defines a sub community and ties it tighter together.

Summary

Rules of thumb are easy to remember, useful condensed pieces of engineering wisdom define engineeringness. It helps the engineer to be able to answer every question roughly right. It is a social bond in a group and, I would say, it sets the functional and effective engineer apart for from the academic and unproductive ones.

?So, if we summarize. A rule of thumb is characterized by:

• Useful not necessarily exact
• Easy to understand and remember
• Generic i.e. it can be used in many situations.
• Used to box-in the answer, as a proxy or just a reminder
• Work as a social bond within a subgroup

Finally, I want to share the secrete, golden master rule of all engineering rules of thumbs:

“if it moves and it shouldn’t—duct tape. If it doesn’t move and it should—WD-40”

An engineer’s way to answers the questions

Here are a rule-of-thumb way to answer the questions mentioned in the text.

“are there more bio-mass in humans than I the rest of the earth living mammals on earth”

The starting point here is to know that westerners eat around 100 kg of meet per year and have an idea about how big portion of livestock that ends on the plate. If you assume that half of earth population eat no meet and half are meat-eating westerners, and that only 25% of the animal end up on a plate, you need 1/2*100kg*1/0.25 = 200 kg of meet per person and year to feed the world. If you then also take into account that livestock, on average live, longer than one year it is clearly so that there is more non-human mammal bio mass (i.e. livestock) on earth than it is human bio mass. The sad thing however is that, if you deduct the livestock, there are then more human bio mass than wild life mammal bio mass on earth. That is what economy of scale and our way of living has done to the wild animal life.

**“will a wire spanning in a straight line across the length of the lake V?ttern touch the bottom?”

The key here is to roughly know the length (135 km) and depth (128 meters) of lake V?ttern, the radius (6370 km) of the earth and also be able to recall some high school geometry for calculating cord length.?

Some quick math will reveal that the lake must be deeper than 358 meters for a wire spanning in a straight line from to north to the south of the lake not to touch the bottom. Therefore, the answer is YES, it will touch the bottom.?

***“how many houses can you heat with the electricity from a powerplant in the river Lule-?lv”

This is a tricky one. You need to know the fall height of a typical power station and the flow in the river as well as average energy consumption of a house. Taking some rough numbers, we assume fall height 50 meter and that the average width and depth of the river is 20 x 5 meters and the water speed is 5 m/s. That will give a flow of 500 m3/s. Knowing that the potential energy is m x g x h (neglect losses in the turbine and generator), you will get a power of

500,000 x 9.81 x 50= 240 MW

If you have a house that consumes 20.000 kWh per year that is an average power consumption of 2,3 kW. Hence, you can power 104.000 houses with one power station in Lule-?lv.