Auxiliary power system sizing

Vega has been lucky enough to have a power supply on her home jetty for the past few years and has only enjoyed short weekend coastal passages. As a result she has non of the auxillary charging systems often associated with offshore or cruising yachts. We need to fix that. Where we plan to go, it will be very useful to not need to connect to mains power or run the engine on a regular basis (even though in the Med we probably will…) so we are looking at solar and wind as options. Hydro is also an option, but it is very expensive and generally the domain of major sponsored offshore racing yachts.

In assessing what kind of set up we need, the first step is to work out approximately how much power we will use on a typical day. Here is our guestimate based on either coastal cruising or passage making:

power consumption estimate

Clearly these calculations are subjective, but we have benchmarked the consumption from a number of different sources so it should represent a reasonable start.

Vega currently has four gel batteries installed in 2013 that are in good condition, so we are reluctant to change them. The batteries are all identical so that they can be swapped around and charged together easily. They are all rated at 130Ah. Only two are available to serve the power loads calculated above as one is dedicated to the engine and another to the bow thrusters and windlass.

How many panels do we need to support our consumption? The EU has released a fantastic solar calculator which allows us to get an idea of how many solar panels would be required to provide enough power to meet our needs. Here is a screen shot of our inputs to estimate the panel size to manage our coastal cruising consumption.

online solar screen grab

We’ve assumed that to enhance the life of the batteries we would never want to discharge them to less than 50% capacity. We’ve also based the calculations on an area of average solar insolation for the Mediterranean (East coast of Sardinia) and that the panels are installed flat (assumed to be on a radar arch that we still need to build).

We would like the solar panels to initailly meet all our coastal cruising requirements from April – September. The power input from the engine will be a bonus and will hopefully offset any loses from shading by the boom, sails, backstay etc. Using the EU’s Photovoltaic Geographical Information System we were able to work out the percentage of days that our battery bank would be fully charged and fully discharged for different solar array peak output ratings. The graph below shows that we would need a solar array rated at at least 200Wp to meet our average coastal cruising daily power demands. Clearly this is based on peak loads and includes a bunch of assumptions, but it should be a good starting point.

solar graph

Installing a new Engine

When we purchased Vega, she was 25 years old. So was her engine. Now don’t get me wrong, there are plenty of 25 year old engines knocking around, but i have learnt that old engines can be fickle…very fickle. One minute they are fine, the next..well they are not. When they are sick, the bills can mount rapidly as can the time spend diagnosing intermittent faults. As the bills mount up it can be hard to cut your losses as you have invested so much in to the old donkey.

Vega’s original engine was a Thorneycroft T98 Diesel Engine, essentially a marinised version of the Ford XLD1600 42hp car engine. Very reliable – and the base engine is well proven and tried and tested. Spares are still around, but are getting increasingly hard to source. Our engine seemed in reasonable shape even if she shook and vibrated a bit, However, she needed plenty of TLC and we had no records for her hours or maintenence history.

We had a decision to make. Take a chance on her and if things went south put up and shut up accepting we may end up spending extended period’s in port fixing issues or replace her with a new generation, fuel efficient engine. We umed and are’d for a long time…a new engine is an expensive proposition. we looked at a whole range of engines, Beta, Yanmar, Vetus, Sole etc

What Horse Power?

We found it difficult getting clear data on the engine Horse Power that would be suitable for our yacht. But we came across a host of imperial data and rule thumb…

The Westerbeke Corporation suggests 2 hp for every 1,000 lb of displacement for coastal cruising, and 2.5 hp per 1,000 lb of displacement for offshore work. Vega displaces approx. 22000 lbs, that would mean 40Hp for coastal cruising and 55Hp for Offshore use.

In their book “The Gougeon Brothers on Boat Construction” the authors suggest a good rule-of-thumb is 1 hp for every 500 lb of displacement… that would mean a displacement of 22000lbs a 44 Hp engine should be suitable.

Other sources referenced 3 – 5 hp (continuous rating) per long ton … that would mean a boat with a displacement of 22,000 lb should have a 30 – 50 hp engine. For offshore conditions some people suggest the engine be larger than the size calculated for coastal cruising, possibly + 25%… so in this example the engine size would be increased to 60hp at the upper end.

So, based on a bunch of research and discussions with various suppliers it seemed that 40-60Hp was the right kind of size. We went with 50 HP. It seemed that once you had an engine of broadly the right horse power, the matching of the prop the the engine was of more real importance than the overall hp.

Retro Fitting

Non of the new engines on the market would fit on the original engine mounts. The width of the mounts was fine. The issue was in relation the the depth of the sump. The modern sumps were deeper and therefore our mounts had to be raised up. This was done with the fabrication of a pair of steel rails which sat onto of the original mounts, raising the engine up.

What Engine?

Yanmar and Volvo are probably the two most prevalent makes. We had a yanmar 3GM30 in our beneteau and it was not a bad engine, but with only light use ( 950 total hrs) we had issues relating to the exhaust elbow, water ingress to the cylinders, air intake and engine mounts. Replacement parts were very expensive. I understand Volvo parts are equally pricy. We Settled on the Beta 50Hp. They are british made and have an excellent reputation – they have been developed from a Kubota engine which is used extensively in heavy machinery such as forklifts and heavy duty generators where they can run for hundreds of thousands of hours!

The Beta seemed to fit many of our requirements, we could afford it and it had been installed in a number of similar yachts. We purchased the engine through TS Marine in the UK who also offered an installation and commissioninpackage (in Spain). The cost of the whole installation was just over 12,000GBP. Including Engine, 130 amp alternator, new engine mounts, new shaft, prop and PSS Seal. It also covered a host of other incidentals during the installation.

Was it the right choice?

Heather and i can disagree on lots of things..but on the new engine, we both agree it was the the right choice. I am aware of at least two Moody’s with old engines that packed in this season. From the outset, we had piece of mind when cruising, and when cruising with a young family, that was important to us. Once or twice we had to punch into 40 knots and a decent sharp wave set and the moody/beta combination worked a treat. Cruising at 7.5 knots if we need to we can get places quickly too 🙂

7 thoughts on “Technical

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  1. Dear all,

    Let me introcude myself. My name is Diana Kulchitskaya and I am a freelance journalist. Currently I am helping a Russian site about sailing create content. They are looking for stories about trips to the Mediterranian. The site is

    I spotted your blog and decided to write to you. Would you agree to give the permission to publish some of your posts on the Russian web-site? There will be a credit, saying the content belongs to you and it is translated from English to Russian.

    Thanks in advance!



  2. Hi guys, thanks for sharing your engine story. We are in the boat market and looking at similar boats with or without new engines. Your post is super helpful. Hope to see you on the water sometime! Lynita


    1. Sorry for the delay in replying – Rich needed his technical officer to reply and I’m not an easy person to pin down to a computer. We have 2 x 130Ah AGM batteries – this year they don’t seem to be holding their charge as well but they are now about 5 years old. I reckon that we are down to 130Ah capacity and considering we only want to drain to 50% capacity, we only have around 60Ah of storage left – time for new batteries soon. We have been absolutely fine all season while the sun is shining but can not get through an overcast couple of days without running the engine. Luckily in the Med, clouds are few and far between. We ended up getting 2 x 80W polycrystalline panels made by Viltron. We also have a Viltron MPPT controller. We have found that in terms of usage – all the estimates have been fairly accurate, only underestimate was the autopilot. In big seas it can draw 7-8 Amps so after a night passage under sail with the autopilot, particularly later in the season (with fewer daylight hours), the batteries are desperate for a top up by morning. Might be time for a new, efficient autopilot as well 🙂


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