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Introduction to the Hydraulic System

Since Prout Catamarans had no expertise whatsoever designing hydraulic propulsion systems, my father turned to Diport AG, a.k.a. Swiss-Hydro, a specialized design bureau that he found on a boat show. The representatives of the company reassured my father that these sorts of propulsion systems were their bread-and-butter business and that we could have a standard working system in no time.

Among other things, Diport AG chose our engine for us, since the M60 Perkins model has bolt-on compatibility with a Rexroth line of hydraulic pumps. The Diport AG representatives also claimed that our boat would be able to travel at 8 knots, and that lobster pots would merely block the affected hydraulic motor without stopping the rest of the system. This is the system that Diport AG had us install:

Hydraulic System Diagram as Installed on our Prout Escale catamaran
Hydraulic Layout: Engine (at left) drives two pumps in series. Each pump is attached to a separate motor.

Further on, we discovered that the claims by Diport AG to be dubious at best. Perhaps by now Diport AG has managed to gain enough experience at the expense of customers like us to improve the quality of their designs. After all, the number of fundamental mistakes in their design made Prout Catamarans look professional! After Diport AG supplied all the parts, Prout Catamarans managed to install them properly for the most part. Yet, our system had some maddening flaws:

The engine was oversized to 60 Hp so that we would derive about 40 hp of prop power after hydraulic and sail-drive losses (85% and 95% efficiency, respectively). Forty horses at the propellers were supposed to accelerate our Prout Escale to about 8 knots of speed.

The sail-drives are the 120S model from Volvo with an interface plate to accommodate Rexroth hydraulic motors instead of a Volvo MS2 transmission. A review of existing Volvo sail-drive combos such as the MD2020 (a three cylinder 20 HP motor paired with the 120S sail-drive) shows a 2.5:1 transmission ratio. However, the name of the game is to ensure that propeller size, pitch, and rotational speeds are equivalent, not just the overall transmission ratio.

Thus, if a Volvo MD2020 engines have a max. RPM of 3,600 and a overall 2.5:1 transmission ratio, the fastest their props will turn in water is 1440 RPM (16" diameter 3-blade folding prop, 11 degree pitch). Our Perkins runs up to 4,000 RPM and the consequent transmission ratio (assuming we want to achieve the same propeller speed) has to be around 4000/3600*2.5=2.78.

Since the sail-drive has a transmission ratio of 2.2:1, we can now calculate the transmission ratio required for the rest of the system. As the variable displacement pumps put out 19cc oil for every engine RPM and since we use fixed motors, the calculation becomes: 2.78 = (x/19)*2.2 where x is the cc size of the motor.

Assuming we want to achieve exactly 1440 RPM at the propeller, we'd need a 24cc motor. Rexroth still manufactures the A2FM fixed motor line and the closest displacement is the 23cc motor. Another, cheaper option would be to reduce the size of the propellers yet again. As designed, our boat carried 17" diameter, 3-bladed MaxProps. Nanni Diesel uses 20 hp engines and a 2.2:1 transmission ratio from 3,600 RPM on 14" 2 bladed propellers. However, upwind performance of the system would have been compromised.

At the end of 2000 we were looking at the following options:

Measure Cost Pro Con
Replace current drive system with twin engines $25,000
  • Standard System
  • Ensures proper performance
  • Saildrive taken care of
  • Cost (ouch! ouch! ouch!)
  • Engines in living quarters
  • Cramped access
Replace 2,600 RPM, 65 hp Perkins for current engine $9,000
  • Slow RPMs of engine would allow perfect propeller speed in the water
  • Hoses may be adequate
  • Cost (ouch! ouch!)
  • Saildrive still a problem
  • Bolt pattern on engine is different and would require a new plate
Replace current 16cc hydraulic motors for 23cc models $4,500
  • Better torque
  • Engine/Prop match
  • Could fix saildrive concurrently
  • Could replace hydraulic hoses concurrently
  • Cost
  • Installation (need to re machine mounting plate)
Cut down propellers to 14" $400
  • Easy
  • 10% loss of performance going upwind
  • Saildrive still a problem
Install Z-drive, relocate engine further back $6000
  • Prout "standard" installation
  • Potentially much simpler to maintain than hydraulic system - certainly easier to "de-lobster pot".
  • Potentially less noise in hull due to relocation of hull penetration
  • You can lift Z-drive out of water during sailing to reduce drag, chance of catching lobster pot
  • Much better transmission efficiency, lower fuel consumption for given boat speed
  • Boat would have better front-to-back trim due to engine being further back
  • Cost (ouch!)
  • I have yet to find a Z drive long, strong enough for a 60 hp engine
  • Maneuverability isn't as good with Z-drive as with twin propellers
  • Z-drive can come out of water during storms and is thus much less desirable than propellers that are deeper in the water..
  • The long bending moment makes this quite a fragile rig.

Lessons learned along the way for our propulsion system:

Actually, the best option would have been to stay away from hydraulic drives and go with dual 25hp engines and a huge diesel tank in the center nacelle. Besides greater redundancy, a dual engine set-up is standard across the world and would have made servicing the system far simpler. The larger fuel tank would have given us better range. However, installing engines into living spaces would have also required some careful venting (consider that proper venting is not a Prout Catamarans specialty) to prevent smells and carbon monoxide from entering the cabin.

In the end, we decided to repair the hydraulic system rather than replace it because it seemed like the less expensive option. A more critical review of our propulsion system after the flooding should have prodded us to bite the bullet and install twin drives instead. To Prout Catamarans credit, they tried in vain to dissuade us from the hydraulic system and use one of their proven set-ups instead...

Best Estimate for Time Required:
Diagnose over torqued engine 2 hours
Confer with Diport AG re: lack of performance 10 hours
Review existing literature, search internet, consult colleagues, arrive at solution 30 hours

Total 42 hours