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In these pages we look at technical things.......... 

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Lithium the facts. 




“The only constant in life is change”-Heraclitus.




In the years we have been sailing many things have changed. When I started sailing in the mid 70’s Red diesel was .20p a litre, less than a £ a gallon. DECCA navigation was in it’s infancy and all sailors used BBC radio 4 for their main weather reports. The weather fax did not appear for a good few years. Compass, Charts and a sextant were the skipper’s main tools along with one of the new quarts watches that kept perfect time. The average boat owner had 25 feet of hull and a year in Southsea marina, Hampshire UK was a mind blowing £250.00 a year!

When it came to batteries, we had just two. Both lead acid, they were a start battery and a caravan battery known as a deep cycle. Our Domestic battery was just 100 Amp hour. The main sources of charge were the engine alternator and a simple battery charger. The latter being rated at 15 Amps and the former a massive 45 Amps!


Our cool box was charged with frozen bottles of squash and drinking water that slowly melted. Again it was many years before thermo electric fridges became cheaper. Cheap enough to fit one in our boat. The main drain on battery life was night sailing, and the NASA depth sounder. Yes ,we were fortunate enough in the 80’s to have a depth sounder, with moving parts and flashing lights. We were often asked about this new technology and often boarded by curious sailors for a demonstration.

Battery technology had not moved far in the last 50 years and neither had the average boats daily energy consumption.


Fast forward to today. We have 520 watts of solar panels, a fridge a freezer GPS, AIS, Chart plotting. Our own wireless network, 3D contour mapping sonar, a Smart TV, Blue-ray player, two Mac’s (one dedicated to charting and navigation).  And then there are the smart phones and iPads. All of which demand power in various quantities.

We are currently using AGM spiral cell domestic batteries in a bank of 400 Amp hours.   When we bought Impavidus 5 years ago these batteries were state of the art. Able to be charged quicker and maintenance free. They were over £175.00 each at the time. But fundamentally they are still a wet cell battery (lead acid) and as a result should not be discharged below 50% capacity. What’s more like all lead acid and wet cell batteries (including GEL batteries) the have a relatively high internal resistance. Taking the charge from 80% to 100% takes time even with all our solar and our 120 Amp alternator.  As live-a-board’s we put quite a demand on these batteries and even though the AGM ‘s have a better duty cycle and higher life expectancy they are limited to around 1000 charge cycles. Perhaps 1200 if you treat them carefully, you can only discharge them to 50% or use 200 Amp hours before the batteries would become damaged or the life expectancy maybe dramatically shortened. 


So what has changed in the last five years? Well to explain that we need to do a bit of history and some homework….


To help you understand I will clarify the terms of reference;


AGM = Activated Glass Matt (Lead acid battery)

GEL = Suspended gel acid batteries

Lithium = Lithium Iron Phosphate (LFP or LiFePO4)


Amps = Measurement of current (IE flow of current)

Volts = Measurement of Voltage (IE electric pressure or electric tension is the difference in electric potential between two points.)

Watts = Measurement of energy (IE how much work is done or used)

Resistance = Opposition to the flow of electric current. (Measured in Ohms)     

NB; Watts = Amps x Volts. Volts = Watts /Amps   Amps = Watts/Volts.

BMS = Battery Management System.  





For many years, nickel-cadmium had been the only suitable battery for portable equipment from wireless communications to mobile computing. Nickel-metal-hydride and lithium-ion emerged In the early 1990s, fighting nose-to-nose to gain customer's acceptance. Today, lithium-ion is the fastest growing and most promising battery chemistry.


Pioneer work with the lithium battery began in 1912 under G.N. Lewis but it was not until the early 1970s when the first non-rechargeable lithium batteries became commercially available. Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest energy density for weight.


Attempts to develop rechargeable lithium batteries failed due to safety problems. Because of the inherent instability of lithium metal, especially during charging, research shifted to a non-metallic lithium battery using lithium ions. Although slightly lower in energy density than lithium metal, lithium-ion is safe, provided certain precautions are met when charging and discharging. In 1991, the Sony Corporation commercialized the first lithium-ion battery. Other manufacturers followed suit.


The energy density of lithium-ion is typically twice that of the standard nickel-cadmium. There is potential for higher energy densities. The load characteristics are reasonably good and behave similarly to nickel-cadmium in terms of discharge. The high cell voltage of 3.6 volts allows battery pack designs with only one cell. Most of today's mobile phones run on a single cell. A nickel-based pack would require three 1.2-volt cells connected in series.


Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, making lithium-ion well suited for modern fuel gauge applications. lithium-ion cells cause little harm when disposed of.


Despite its overall advantages, lithium-ion has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current on most packs are is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.


Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.


As a result of a move towards electric power cars and transport, the need to store power from renewable energy sources, such as solar and wind. Lithium battery prices have fallen dramatically in the last 5 years. The real cost of lithium over 5-10 years has made them cheaper than the best lead acid batteries.


Now that’s a sweeping statement!  But lets look at the maths and the pro’s and Con’s of switching to Lithium and the massive advantages Lithium can give.


Unlike with lead acid batteries, it is considered practical to regularly use 90% or more of the rated capacity of a Lithium battery bank, and occasionally more. Consider a 100 amp hour battery – if it was lead acid you would be wise to use just 30 to 50 amp hours of juice, but with lithium you could tap into 90 amp hours or even 100Ah (100% DoD).




Laboratory results indicate that you could expect to see 2000 to 5000 cycles out of a well cared for Lithium Iron Phosphate battery bank. Both C-Rate and Depth of Discharge (DoD) affect expected lifespan. Some recent measurement shows that a LFP battery will still deliver more than 80% of it’s capacity after 2000 cycles at 100% DoD or even 5000 cycles at 65% DoD. All these tests are done at 1C-Rate Cycles.


These cycle life results are much better than NMC or NCA chemistries, massively used in electric vehicle industry.


In contrast, even the best deep cycle lead acid batteries are typically only good for 500-1000 cycles.


Picture below shows the expected number of cycles for Lithium-Iron-Phosphate batteries at different DoD at 1C-rate.



The discharge curve of lithium batteries (especially relative to lead acid) is essentially flat – meaning that a 20% charged battery will be providing nearly the same output voltage as an 80% charged battery.


This prevents any issues caused by the “voltage sag” common to lead acid as they discharge, but does mean that any battery monitor or generator auto-start dependent upon voltage levels will likely not work well at all when monitoring a lithium bank.



Lithium-ion batteries can be “fast” charged to 100% of capacity. Unlike with lead acid, there is no need for an absorption phase to get the final 20% stored. And, if your charger is powerful enough, lithium batteries can also be charged insanely fast. If you can provide enough charging amps – you can actually fully charge a lithium ion battery just 30 minutes.

But even if you don’t manage to fully top off to 100%, no worries – unlike with lead acid, a failure to regularly fully charge Lithium-Ion batteries does not damage the batteries.


This give you lots of flexibility to tap into energy sources whenever you can get them without worrying about needing to do a full charge regularly. Several partly cloudy days with your solar system? No problem that you can’t top off before the sun goes down, as long as you’re keeping on top of your needs. With lithium, you can charge up what you can and not fret about leaving your battery bank perpetually undercharged.


So let's look at the cost or do the maths as they say….


Modern AGM Lead acid;


400 Amp hours  £480.00

Usable energy  @ 50% = 200 Amp hours .

Charge cycles @ 50% = 1000

Charging time from 50% based on 50 max charge rate. 7 Hours  

Expected life at 50% duty/discharge 2-3 years at best.  Longer if a de-sulphate cycle charger is used.

Typical weight 40 Kgs.

Lithium Iron Po4 with built in BMS


400 Amp hours £3000.00   

Usable energy @ 80% discharge  = 320 Amp hours

Charge cycles at 80% = up to 5000

Charging time from 80% discharged based on maximum charge rate =  30 minutes (in theory)

Expected life at 80% discharge = up to 10 years possibly more.

Typical weight 13.6 Kgs.


These figures are approximate but lets look at some detail. To get the same usable energy from a Lithium battery you need a lot less batteries, which brings the cost down. Typically you could have 300 Amp hours of Lithium and still have more usable energy.  Bringing the cost down by £750.00 or so. More if you went down to 250 Amp hours.


Then there are the massive weight advantages. Up to three times lighter. The same 400 amp bank in Lithium would be 54 Kgs compared to 160 Kgs again, if you wen down to 300Amp hour of Lithium this would be just 40.8 Kgs. A saving of 119 Kgs .


Finally there is size to consider and everyone knows boat space is at a premium whatever boat you have. Lithium’s are 1.8 times smaller or just over half the size.

If you add this to the faster charging rate and the fact that they hold the voltage curve longer  (sag, operating at a higher voltage throughout the discharge) only dropping of in the last part of the discharge they are more efficient. A lead acid battery will drop it’s voltage as it discharges from 12. 6 Volts down to 11 Volts or less as it approaches 50% discharge.  As the voltage has a direct relationship to the current draw in Amp’s  IE the voltage goes down the Amps go up to provide the same energy in Watts. This is a direct result of the bigger internal resistance of Lead acid batteries and of course this again one reason Lithium can be charged quicker.



So, looking at the facts and the massive advantages Lithium has it is no wonder more and more boats are changing their batteries as their old ones die.

It is not just the rich and famous, Like SV Delos, or Follow the boat. A few weeks ago our friends Magnus and Tina from Sweden changed out their 500Amp hours of failing AGM’s for 300 Amp hours of Lithium Iron. The cost was £2100. After Magnus got a good discount on his batteries.  Sofia, a 2007 Bavaria 42 now has more space in the battery box. Is considerably lighter on the Port side. Magnus/Tina can run an electric hob through their inverter. They are now looking into an electric oven and doing away with gas altogether!


So will we be fitting Lithium soon?

Well the short answer is no, not just yet. We still have a good life in our AGM’s. Lithium is still new (relatively) and like the changes we noted at the beginning of this article, the cost of this technology will come down as it become the norm rather than the exception. Just like chart plotters and ultrasonic depth sounders it will not be long before every boat has them and the initial cost is no longer a barrier to those of us that are so tight we have brass hinges on our wallets.


Quick points to note. Not all Lithium batteries on the market have an internal BMS. Not all Lithium batteries are LiFePO4 you will need to check this. You will not need to change your charger, alternator or solar regulator if the battery has internal BMS but it may be necessary to alter the parameters of the charger and solar regulator. Most modern ones have this facility. Maintaining a lead acid battery for your start battery protects the alternator should the Lithium BMS shut down all the Lithium cells.


Anthony Kirkby.


Credits. Power Tech systems.




Two good videos on this subject are made by our friends on YouTube Ryan & Sophie. Well worth taking a look.


Thanks to Ryan for his expert advice in Almerimar this winter.

You are welcome to copy, share or reproduce this information within the sailing community for non-profit. Please credit us if you do.


Varnishing.    (Companion waysteps).

Here is a question from one of our subscribers. We thought the answer we gave would be useful. 

Bruce Jamer



Ant and Cid, Hello from Western Canada.  I really enjoy your videos and have learned a lot from them to apply to my Bavaria 34C.  I am wanting to tackle refinishing my companionway steps which are very similar to yours and have a couple of questions as you have recently done this:

1.  Where your steps glued and screwed to the supports, and if so how did you approach their removal?  Mine are quite reluctant to come off even when the screws are removed, feel like they might be glued as well as screwed and I don't want to damage them while removing.

2. How did you prepare the step surfaces for polyurethane?  I'm assuming there was previously a factory 2 part lacquer finish-that's what mine has.

3. Any other helpful advice that you might have would be greatly appreciated on this task.

Kind regards,

Bruce Jamer  SV Chimera

SV Impavidus reply 


Bruce Hi. Our steps were screwed to the support under the rubber anti slip strips. 

You have to be careful removing these screws as the heads catch on the edges of the slots as they are removed.

The steps are also glued but this is only thin veneer on the top of the step support. Use a craft knife at the back of the step to slice through the veneer as close to the step as you can. 

Remove the screws and tap the step upwards with a rubber mallet or nylon hammer. 

As the glue is only between the bottom of the step and the veneer it will lift the veneer from the step support. But, the cut you have made at the back of the step through the veneer, it will not show and stop the veneer splitting further back where it could be seen. You can then clean the underside of the step removing any glue or veneer. 

We used a polyurethane varnish which was a soft sheen not gloss. 12 Coats starting with a 50/50 mix of thinner and varnish. reduce the thinner with each coat. the last coat use 90/10 thinner varnish mix. Sand each coat when it's hard. 

Sand the first coat with a medium grit paper and then work up the grades so the last but one coat is sanded with 800 grit. 

This will give you a better than factory finish. If you have sun damage use a light mahogany stain before the varnish. Be aware the steps will darken as varnish is applied. to get an idea of how dark they will go, just wet a small area with the tip of your finger. This will give you an idea if you need to stain or not. We did not re glue our steps. 

Hope this is useful. Sail Safe. Ant.

All about your boat; 

Part one.  Written for learner and New sailors FB group.



Getting the best from my boat. (Why is my boat slower than similar boats?)  


Probably the two of the biggest questions that new and learner sailors ask us. There are no definitive answers. However, there are things that will significantly affect your boat/yacht. Things you may not think would do so. Let us try to answer some of them without getting overly technical or giving you information overload. So here are a few thoughts, observations and facts that you may like to read or contemplate.


This may not be the best place to start but it’s about the size ☺    


The maximum speed of your boat in flat water will be approximately 1.23 times the square root of the waterline length in feet. (Not planning boats) The waterline length is not the hull length, or put another way:


A vessel's waterline length (abbreviated to L.W.L) is the length of a yacht or boat at the level where it sits in the water (the waterline). The LWL will be shorter than the length of the boat overall (length overall or LOA) as most boats have bow and stern protrusions that make the LOA greater than the LWL. 

As a yacht becomes more loaded, it will sit lower in the water and its ambient waterline length may change; but the registered L.W.L is measured from a default load condition. (We will come back to loading in a later or in another article.)


This measurement is significant in determining several of a vessel's properties, such as how much water it displaces, where the bow and stern waves occur, hull speed, amount of bottom-paint needed, etc. Traditionally, a stripe called the "boot top" is sometimes painted around the hull just above the waterline. Or there is a defined level at which the boat will sit given still water and even loading. 

Specifically, it is also the name of a special marking, also known as an international load line: Plimsoll lines and water line indicates the draft of the ship and the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy.


So having said that, there is with almost all mono-hull yachts an optimum “heal” or angle from the perpendicular (90 degrees from the water) at which the boat will have a slightly increased waterline length, (hull shape dependent), the yacht may, or may not, perform better at certain angles from the waters surface at certain degrees of heel. 

This changes from one hull shape to another. It also changes according to the angle of wind both True and Apparent. It will also change with the load distribution or centre of gravity on the boat. 


How clean is your bottom? 

One big factor in a yachts performance is the cleanliness of the hull. A clean bottom, a clean prop, a folding or feathering prop and a good low friction surface will make a significant difference. Fouling or a dirty bottom may not be high on your train of thought. But if your 3 knots slower due to fouling it makes a huge difference. Not just to your speed, but they way you use your yacht and even where you go. 


How So? 

Well think of it like this; You have a 25-27 foot yacht with a fixed keel and a standard Bermudan rig. Like a Westerly Griffon or a Trapper 500 something in that sort of size range. Your waterline length is going to be around the 23 to 21 foot range. It’s this that dictates your hull speed. (Displacement boats not foils or planning boats)


The math to calculate this on a 23 foot waterline would be 1.23 x √ 23. = Hull speed in knots. 


Or put another way. 4.795 x 1.23 = 5.89 Knots.  


That in theory would give you a range of 58 miles at optimum speed in a 10-hour sail or 140 miles in 24 hours. However, lets knock of a knot and a half because of a dirty bottom and half a knot because your prop is dirty or not folding correctly. That alters the range to 44 miles in a 10-hour sail, or just 105 miles in 24 hours. In 24 hours that’s 33% less range! Conversely that’s 33% more range if you get things perfect!


OK before the keyboard warriors get their knickers all twisted. 


These are theoretical numbers and you will never sail continuously at your optimum hull speed unless your part of a well trained and probably professional sail crew, but even so, the difference is a direct result of drag and being able to increase your range by 30% or more will get to your destination hours earlier. It will change the way you use your boat and where you go?


There is another factor. The faster your speed over ground/through the water, the faster your apparent wind speed will be sailing upwind. Upwind the apparent wind angle will also decrease (Towards the bow). Down wind the angle will also change towards the stern and the slower your apparent wind speed will be. 

Generally (roughly) you simply add the boat speed to the wind speed upwind to give you the apparent wind speed. Or deduct it downwind. 

So upwind at 5.8 knots hull speed in 17 knots of wind your apparent wind speed will be 22.8 knots. Downwind the apparent wind will be 11.2 knots. (Obviously this changes if the angle is off the bow or off the stern.)


Clearly there is a huge difference between an 11-knot wind and a 22-knot wind. The loads on the rig, sails, helm, keel etc; Will be entirely different both in direction and force. The balance of the yacht will be entirely different. Beating into the wind on a passage and running downwind will likely be at a completely different level of comfort too. 


Hull shape or design aside, a longer waterline length will have a faster hull speed and therefore the effect is relative. Given that the amount of fouling has a direct effect on the % of hull efficiency due to the coefficient of friction, you can understand that more energy is needed to drive a vessel at its hull speed. (Either power or sail). Hence the commercial vessels spend a lot of money keeping their bottoms clean and the boat more energy efficient.  


So in summary, longer waterline length and a cleaner bottom will change the way you sail and the distance you travel in a given time. Be it that you are just cruising, enjoy club racing or just pottering about….. 


Training and experience will help you to understand how your particular boat performs in any given condition, sea state or wind angle.  


Moving on… Now while this is important there are other factors; Lifting keels, dagger boards, hull shapes and designs, displacement (boat weight to hull volume) sail shapes, rig types, stability factors and much more. Such as, how to set your sails in different conditions, the age, amount of stretch in your sails and sail material/type/cut.   As well as hull build materials and the all-important nut behind the wheel or helm ☺ 


Two things we would suggest.


Have a certified competent instructor (preferably one that races, has raced or has in depth sail trimming experience) The best have good reputations and recommendations so ask around.  Employ him or her to do a days sailing with you on your boat.  Or a group of you with similar boats to keep the cost down. Get a competent sailing club member to come out and give you a few tips. Every club has hundreds of years of sailing experience in it and many have good instructors that give discounted rates to club members. Make use of the club members knowledge. 


If you have not got one we would suggest that you try to get a copy of your boats VPP. 

What’s that?

A velocity prediction program (diagram) (VPP) is a program or diagram which provides information for the performance of a sailing yacht in various wind conditions by balancing hull and sail forces. VPPs are used by yacht designers, boat builders, model testers, sailors and sail makers. You can make your own or get various programs, even some navigation software will give you one by inputting the data for your yacht.  You should also try to get a polar diagram for your yacht. Both these will give you a good indication of your boats performance ability and you can then look at how to get to these figures. 

I will add examples to this post so you can see what they look like.




In the next post we will look at upwind sail trimming, as this seems to be a constant source of questions. We can then look at basic downwind sailing, motor sailing and other things that will help new and learner sailors get the most from their boats and this group.


Hope you find this useful, please suggest any future post coverage or subjects you would like to know more about or would find of interest.


Sail Safe. Ant, Cid and the pooch crew.  



You are free to cut, paste use or forward this post for yourselves, club members or other groups. Please credit us on SV Impavidus and mention our YouTube channel and Facebook page. Thanks.  


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We have recently upgraded to Lıthıum batterıes. Here ıs the lınk to the vıdeo:  There ıs also more ınfo on the downloads page 

This article was written for the Bavaria owners magazine in 2015. We have added it as a PDF so you can download or share. 

Stainless Steel

Stuff you need to know 

You can download this document from here.

Still under construction. Please call back. 

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