Auxiliary engines have become an permanent part of cruising boats. Having too large an engine can be as awkward as having too small an engine.

The HR 41 was built and shipped with a Volvo MD 21 (the Hallberg-Rassy builder’s sheet lists it as 75 HP while Volvo lists it as 65 HP) naturally aspirated diesel engine. We also think there was an option for a larger Volvo engine (perhaps installed by the dealer?). Maringret was shipped with an MD 21 which was subsequently replaced with a Mercedes 603A 6-cylinder turbo charged engine rated at up 135 HP.

Each hull design has a hull speed which is a factor of the hull shape and length of the water line along with the laws of nature as they apply to fluid dynamics. Beyond that hull speed the amount of energy may increase dramatically without the speed of the hull through the water increasing. For the hull shape of the HR 41 that speed is approximately 7.5 knots and the optimal size of engine (i.e. amount of power) needs to be calculated. Beyond that extra power can not be converted into additional speed through the water, it can only be converted into heat, noise, extra power to the water (as in pulling power) assuming a large enough propeller can be fitted (which of course is limited by the HR 41 hull design itself). Of course if sufficient power were available then any hull will plane but the amount of energy required for a long keel to plane is somewhere near infinite.

The Mercedes 603 that had been installed was a lovely engine, it was probably the prettiest, smoothest and quietest diesel engine we had ever seen. The installation job on the other hand was less than optimal:

  • the engine was too long for the engine room so the installers had cut a hole through into the saloon under the countertop;
  • the engine was too wide for the space available within the engine room and so the woodwork had been reduced to allow the engine to squeeze in;
  • due to the close tolerances of the installation the service parts were extremely hard to access and remove;
  • as the forward bulkhead and its sound proofing had been cut through, the engine noise was in the saloon (read “and the rest of the boat”), the engine heat was in the rest of the boat and the smell from the hot engine was in the rest of the boat.

Although the engine was a marvel of German engineering the Mercedes 603 was still the “wrong” engine for a HR 41. Maringret couldn’t use it’s power so the engine spent its whole working life idling – not what a diesel engine likes. When the propeller shaft was engaged with the engine at idle the speed through the water immediately went to 5 knots. So much for maneuvering at low speeds in a marina.

On a couple of occasions where we were moving through still water we opened up the throttle. Normally the engine idled at about 800 RPM when in gear, the red line was up around 3,800 which is almost a 400% increase! The speed through the water would increase to its designed limit under power (about 7.5 knots) and then the stern would go down. By opening the throttle we could lower the stern toe rail by about 6 inches (15 cm) as the hull tried to climb onto its bow wave and start planing. It made a good attempt but the required amount of power for an HR 41 hull to plane is astronomical. Nice try but no cigar.

Finally it was the extreme difficulty of accessing service parts along with the knowledge that the engine would never be able to work (diesel engines hate to idle or run under light load) that made our decision to seek out a replacement.

Engine Selection

As we looked for a replacement engine during 2005 we became aware that emission regulations were changing at the end of that year. Emissions targets had been published by various western governments and went into effect on January 1st 2006. There had been 2 responses from the marine diesel manufacturers: some such as Perkins simply exited the market below 100 HP while the others added computer circuitry to their engines in order to meet the guidelines. One thing we were concerned about was having any surplus electrical or electronic bits pertaining to the engine. we had heard enough horror stories of cruisers out “back of the beyond” when an obscure “black box” fails, one which can not be repaired locally. All of a sudden the boat owners have to get intimately familiar with the local parts network, duty and importation procedures and local travel arrangements as the people you need to deal with are almost certainly not located where you break down. We didn’t even want an electronic fuel pump and planned a day tank (click here for details) to bypass the chance of the electric pump failing.

As an aside, we had an 800 cc BMW cruising motorcycle which covered all of Europe, Asia Minor, North Africa and N America. The most exotic thing about it was where it chose to break down. When the breakdown was mechanical in nature (e.g. broken push-rod) then it might take a few days but the parts could be got, fitted and you were on your way. When the failure was in the electronics then you may as well go home. One time it broke down in Arctic Finland, went through various repair facilities on its way to Helsinki where the BMW dealer couldn’t repair it. Then onto the boat to Stockholm where the BMW dealer also couldn’t fix it. By this time the holiday was over and it was time to return to work so the motorbike was sent home in a box as freight. We had no wish to re-live such a scenario with a marine engine in a more remote part of the world.

Luckily there had been an article in Sailing Today magazine shortly before we started looking for an engine, the article gave us a list of manufacturers with engines in our horsepower range and their contact details. We just needed to “size” the new engine.

Engine Sizing

To aid our understanding and check if any “new” knowledge had entered the decision process since the mid 70s, we followed some different approaches to engine sizing – if they all agreed then we had even higher confidence that we had chosen the best path.

First we surveyed sister boats using the internet for engine size (the Yacht World website is invaluable) and came up with:

Engine Model HP
Yanmar 62
Volvo MD 21A 55
Yanmar 55
Volvo MD 22 75
Volvo MD 21A 55
Perkins 60
Volvo D2-55 55
Volvo MD 21A 55
Volvo 80
Volvo 75
average 63 HP

Second we checked with Nigel Calder who has written a number of books and articles on the technical aspects of yachts. Note that his formulas are specified in imperial measurements so certain steps below are simply converting metric to imperial measurement. His chapter 9 in “Marine Diesel Engines” which is titled “Engine Selection and Installation” is well worth reading. We have reproduced 3 of his sizing calculations below using the data for the HR 41:

First Calder derives an engine sizing based on a ratio of a 1/4 ton displacement per horsepower (which he lists as given in the book Propeller Handbook by Dave Gerr). Using a displacement of 15 tonnes (fully fueled, watered, provisioned and with full crew) he reaches a value of 60 HP.

Secondly on page 173 of “Marine Diesel Engines” Calder has an extended table titled “Determining the Horsepower Requirements of an Auxiliary Sailboat Using the Formulas in Skene’s Elements of Yacht Design”.

Quantity Value Units Explanation
LWL = 10.4 meters length of water line
= 34.12 feet
SLR = 1.34 assumed value
hull speed = 7.83 knots = SLR* SquareRoot(LWL) this is the theoretical max speed of the hull
displacement = 18 tonnes (metric)
= 18.29 tons (long)
water resistance = 55 lbs/ton this is from a reproduced graph in the book
total resistance = 1006 pounds = 18.29 * 55 the friction of the water, so to speak
EHP = 23.6 HP = total resistance * speed * 0.003 effective horsepower
propeller rating = 50% efficiency of power transmission by propeller
required HP = 47.2 HP = EHP / 50%
adverse allowance = 33% allowance for adverse conditions
target HP = 63 HP

Thirdly Calder in his book “Boat Owner’s Practical and Technical Cruising Manual” has a spreadsheet-like approach – we have transferred those formulas using the data for the HR 41 in the table below:

– step 1 – determine half loaded displacement (HLD)
displacement = 18 tonnes (metric)
= 18.29 tons (long)
1 tonnes
HLD = 17 tonnes
– step 2 –
LWL = 10.40 meters length of water line
= 34.12 feet
DLR = 460.40 = displacement long tons / ((0.01 * LWL)^3) DLR is displacement/length ratio
– step 3 –
SLR = 1.23 = 8.26/(DLR^0.311) SLR is speed/length ratio
– step 4 –
hull speed = 7.17 knots = SLR * SquareRoot(LWL) this is the theoretical max speed of the hull
– step 5 –
required shaft HP = 57.95 = HLD * (SLR ^3)/1,213 HP required to drive the hull at hull speed
– step 6 –
alternator loss + 1 HP = 50 amp * 14.4 volts
= 720 watts
= 1 HP
power consumption based on a 50 amp alternator
HP shaft = 59 HP = 57.95 + 1 rounded off
HP SAE = 62 HP =shaft HP + 5% allow for transmission losses

As a final summary we have:

manufacturer supplied engine size 65
average engine size in sister boats (Yacht World survey) 63
Nigel Calder calculation 1 60
Nigel Calder calculation 2 63
Nigel Calder calculation 3 62
average 62.6

It seems that the different methods essentially give the same result.

The engine sizing approaches above indicate that for propulsion purposes only the engine should be sized at about 62.6 HP. We short listed manufacturers of this size, then removed engines where computer chips had been fitted in anticipation of the upcoming emissions control legislation. We then removed manufacturers who did not have world wide availability of parts. In the end we were left with the American manufacturer Westerbeke who manufacture a 64 HP 4-cylinder engine based on a Kabota block. We ordered it in time so it entered the EU prior to the end of the year.

Engine Installation

We never saw the original Volvo MD 21 and so are not sure how the Westerbeke 64A compares in physical size. New engine feet had to be created as it was shorter than the Mercedes 603. The Westerbeke also fit through the cockpit sole hatch. The Westerbeke requires a rather large exhaust hose and there was some problem running it through the bilge to the transom.

While the engine was out we replaced all skin fittings in the engine room and rebuilt the bench that holds the bilge pump etc. The duel Sepa fuel filters were relocated to the aft wall of the engine room, also the raw water intake filter was placed there. Below them was mounted the grease pump for the rudder bearing. The T-connection on the raw water intake was re-installed (click here for details).

Alternator Sizing

The Westerbeke 64A shipped with a 50 amp alternator. A lot of electrically intensive boats would have had a larger alternator installed. Each appliance that is connected to the engine drive shaft takes off some power whenever the engine is running. The 50 amp alternator takes away about 1 HP (1.5%) off the engine output:

50 amp * 14.4 volts = 720 watts = 1 horsepower (there are 746 watts in one horsepower)

1.5% isn’t that much really but we didn’t feel we needed to raise that amount. We did look at mounting a second alternator of the same size but the company installing the engine couldn’t figure out a way to do that. A second alternator would have given us twice the charging power but also redundancy should one of the alternators (or their drive belt) fail. A way to increase the effective charge from the alternator without swapping in a bigger alternator was to use an intelligent alternator controller.

Intelligent Alternator Controller

When we bought Maringret she had a Sterling intelligent alternator controller installed. This small box served 2 main functions: it reduced the alternator voltage to the slightly lower charging voltage that gel cell batteries require; and secondly it administered a 4-step charging process. 4-step charging maximizes the battery charging while minimizing the time taken. The 4-step charging curve is specific to each battery technology and can even be fine tuned to a specific manufacturer’s product. Standard alternators produce a standard voltage which is applied directly to the batteries. Battery science has found out that as batteries are charged the amount of current they can accept without being damaged is constantly changing – it starts out very high and gradually decreases to a small amount that is acceptable. Also the voltage that can be applied to them while charging is not constant. By managing the charging voltage and amperage the 4-step chargers optimise the charging in terms of time required and percentage of charge achieved.

Once the Westerbeke was installed then the alternator control wires were connected to the Sterling alternator controller and it controlled the charging of the gel cell batteries from the very first start up of the new engine.

Propeller Sizing

Specifying a marine engine installation is a complex process. The engine power, the rate of revolution, the propeller pitch and diameter must be coordinated using various formulas to get a proper installation. Too much pitch and you get cavitation on the blades of the propeller where energy is wasted as  vacuum pockets are created, too little pitch and you get slip and the engine is not loaded sufficiently along with little energy transfer to the water. Too small a blade diameter and not enough power is transferred to move the boat, too big a diameter and the propeller will not fit into the hull aperture. The efficiency of a propeller is abysmal – ranging from 10% for a folding 2-blade propeller through to 50% for a fixed 3-blade (what Maringret has fitted). It should be remembered that for a sailing boat the propeller is a double-edged sword – the bigger the better when under motor as they transfer more power, but when sailing the smaller the better to reduce drag.

There are a variety of propeller calculators, we used one located at “surfbaud” but as of 2011 the website did not exist any longer (although it was redirecting to someone’s personal blog site and later to Amazon UK). The calculator was titled “Surfbaud Freeware Propeller Calculator for Excel” and although it was not at it’s previous site any longer, numerous other sites had made copies of it that can be downloaded. Search for “Surfbaud Freeware Propeller Calculator for Excel” in Google and most likely there will be some sites. It is an Excel spreadsheet of about 120 lines although quite a few of those are just instructions on what to enter in which cell. It will take the engine size, gearbox specifications, boat displacement along with other measurements and give you a sizing for a double, triple of quadruple bladed propeller.

Propeller Shaft

When the large Mercedes 603 engine was installed it seems that Maringret’s propeller shaft was replaced with a heavier one. The present shaft has a diameter of 34.5 mm. As all the stern tube fittings were already sized to that diameter it was not changed. Having a thicker propeller shaft is not really a problem because should something entangle the propeller then the thicker shaft is less likely to bend.

Because of the skeg, the propeller shaft could not be fitted from the rear without risking bending the new shaft. While the engine was out the prop shaft was fitted from the engine room and even then it was long enough that it stuck through the bulkhead of the engine room. Luckily the hole from the old 603 Mercedes engine was in the right place for it to stick through.

Dry Shaft Seal

The HR 41 like most boats has the most inaccessible seal on the prop shaft. The original stuffing box had been replaced with a dry seal unit. Unfortunately we could not find parts etc. for it and decided to replace it. We installed a SureSeal Dripless ShaftSeal made by Tides Marine of Florida.

The SureSeal is a dry non-lubricated shaft seal. One of the great features about it is that extra seals can be fitted closer to the engine so when a seal wears out the next one is simply slid down into place.

Rope Cutter

On the Maxi we had got a polypropylene line caught around the prop shaft and had it bend the prop shaft. That cost us a new prop shaft, the bent one had eaten out one of the cutlass bearings in the mean time and the engine had become unaligned. We replaced the prop shaft on the HR 41 and had a rope cutter fitted at the same time. Hopefully this will prevent a repeat of the Maxi experience.

Main Fuel Tank

While the engine was removed it was the perfect time to clean the main fuel tanks. There are 2 access hatches on the floor under the engine, bolts into captive nuts and a neoprene gaskets seal both access hatches. While the hatches were off we cleaned out the tank, a tar from asphaltenes in the diesel had accumulated. It was solidified onto the bottom of the tank and we used a scraper and a strong detergent to break it down and remove it. The tank is so narrow at the aft end that we had to use rags on sticks to clean it out. We renewed all the fittings on the access hatches as well as the float measurement mechanism.

We recorded the internal dimensions of the tank and calculated the total volume of the tank to be 300 litres. The fuel gauge does not read the last 70 litres in the tank (i.e. when the gauge indicates empty there can be up to 70 litres of fuel in the tank).

Epoxy Paint

While the engine bay was empty and after the fuel tanks had been cleaned, the hatch fittings replaced and then the tanks resealed, we painted the whole engine bay with an off-cream colour 2-part epoxy paint called GelProtect from Blakes Paints. It is intended for sealing the wet area as part of an osmosis treatment. Being an epoxy paint it is very hard and being creamish in colour it shows any oil or other substances that fall on it. The finish is very smooth and so little dust etc. sticks to it. It has turned the engine bay from a greasy and dark area to a bright and airy area.

Sound Proofing

The sound proofing in the engine room was sheet foam that had been glued in place – it was better than nothing but probably provided as much thermal insulation as it did sound insulation.

We replaced it with a 45 mm thick foam with 2 synthetic lead layers in it. The new insulation was quite heavy and supporting it was a mechanical issue. A system of pins on flat bases was used, the base was screwed onto the walls and then the pin was pushed through the insulation and a friction lock plate was slipped over it to hold the insulation sheet in place.
Fitting the insulation on the ceiling of the engine room was a bit more challenging, as the weight is about 13 kg per square meter. The pin system we used for the walls would not support  the sound proofing without sagging. The cockpit sole lifts off (which is the access path for removing the engine) and across this we fitted 4 stainless steel bars that support the pieces of sound proofing. The ceiling sound proofing also runs over top of the wall sound proofing on the walls so they support the outer edges. We had holes drilled into the 4 bars so things could be hung from them.

The insulation was put up before the engine was finally fitted.

Engine Room Blower Fan

While sound proofing the engine room we replaced the broken engine room blower. While not as necessary in colder climates, exhausting the heated engine room air is crucial in warmer locations. The engine draws in air for its combustion, the cooler the air the higher the oxygen content which turns into a higher combustion rate. Additionally the cooler air reduces any fire issues.

After a year of operation we connected the engine cooling lines to a water heater (calorifier) (click here for details). A couple of years after that we changed the fuel supply to a day tank (click here for details).

  • too large an engine can be a problem
  • engine specification and installation is very specialised, we retained people to make the installation

  • The engine isn’t used very heavily with 300 hours on the engine in 4 years. The 64 HP is adequate for moving the boat, the 50 A alternator is adequate given the solar and wind charging on Maringret. There is ample power in the motor and prop for an “emergency” stop where the engine is put into reverse and the boat is stopped as quickly as possible.
  • The Westerbeke “approved” dealer in the UK used bolts that were too short to fix the drive plate to the couple on the propeller shaft. These came out as we were crossing the busy ferry route between Helsingborg and Helsingor. At rush hour no less! We hadn’t chosen the “Westerbeke approved” dealer in the UK but were informed by Westerbeke that if we didn’t then they would not honour the warranty. As they never provided any warranty service it would have been cheaper to use the mechanic we had originally chosen. At least we would have got the installation done correctly (the “Westerbeke approved” dealer “wasn’t able” to install the engine oil removal pump – so they just left it off!) And the “Westerbeke approved dealers” all dumped Westerbeke and went with other engine brands a few years after we cleared through.
  • After 8 years of use, a mechanic servicing the water pump broke the belt pulley (we are not sure how he did that). He replaced it with a pulley of the same diameter, but a different boss on the shaft. This difference in the thickness (not diameter) of the pulley caused the belt to run at slightly off from 90 degrees. This lead to a failure of the pump after 1 further year. It turns out the Sherwood water pump fitted by Westerbeke has one bushing and one bearing. The slight sideways tension on the belt caused the shaft to “eat” its way through the bushing. We rebuilt the pump.
  • After another 2 seasons the same thing happened again. We are now planning to remove the Sherwood pump and replace it with  a Jabsco pump – at least Jabsco has worldwide parts availability which is more then Sherwood has. The mechanic replacing the Sherwood pump is of the opinion that the pump was not designed for belt drive (as it had a bushing and bearing instead of 2 bearings) and should not have been fitted in that manner by Westerbeke.
  • After 9 years the starter motor failed and had to be replaced.
  • After 10 years the hour counter on the engine display panel stopped working. Of course we had ordered the enhanced control panel. Little good it did us.
  • After 11 years the electrical supply to the fuel lift pump came apart. This cost quite a bit of money to set right as the engine was considered “rare”.
  • There has been quite a lot of upheaval in the parts network in Europe for Westerbeke. Some dealers have dropped the product line; some dealers will only stock parts for certain engine models. As customers of this system we find it quite ad hoc and unprofessional. We would not really recommend a Westerbeke to anyone in Europe, there is no advantage to always being told you have a “rare” engine when you need service. And then the service parts are not easily available.

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