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 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:

• A fundamental transmission ratio error. Instead of copying readily available, existing designs Diport AGs "solution" over-torqued our engine. Our transmission ratio was 1.85:1 instead of 2.78:1. It's not surprising that with such a fundamental mismatch the maximum boat speed was just about as fast on one prop @ 4000 RPM as on two at 3100 RPM (the max. the engine could attain under dual propellers)... The fastest we ever went was 6.5 knots, not 8.
• The hoses in the system "sang" because all of them were too small, causing high flow rates and excessive pressure drop. All hydraulic hoses in our boat had to replaced with the next size up! Even more damning was our discovery in the hydraulic oil tank where hoses two sizes too small were feeding the suction side of the system.
• The bypass valve settings were set too high - instead of stalling w/o killing the diesel, a lobster pot wrapped around a prop causes the pump control cables to jump from their mountings. The whole system comes to a catastrophic halt.
• Least annoying is the cacophony caused by the mounting method for the motors: Motors and sail-drives are bolted directly to the hull, enhancing noise transmission tremendously.

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:

Lessons learned along the way for our propulsion system:

• Use a competent, local hydraulic design firm. Most US firms I have dealt with are more forthcoming about fixing their mistakes... The current head of Diport AG, Mr. Bechler, was nice enough to review the system his company had designed, came to the same conclusion regarding the inadequacy of the design, yet did not offer to fix it. We had to buy the replacement motors, hoses, machine the necessary parts, etc. ourselves. He did provide a nice diagram though of a installation cross section showing just how different the next motor up would be.
• Disallow the Diport AG mounting method. While Volvo installation requirements call for the isolation of the sail-drive from the hull (using a rubber diaphragm and mounting it to a suspended engine via the transmission), our sail-drive was attached directly to a ring glassed into the hull. The effect: a lot of noise transmission from the drives and motors to the hull, which are louder than the engine (!!!).
  
  Even if the saildrive was tghe only option, Diport AG should have reviewed existing installations and chosen 23cc motors instead. As shown above, Diport AG chose not to mimic an existing product. Furthermore, the saildrive should have been installed such that it would not touch the ground before the rest of the hull did. This is an oversight on the part of Prout Catamarans...
• Instead of using a saildrive, we could have installed a simple stern tube and a 54cc hydraulic motor into the rearmost bilges. The benefits would have included a more robust design (higher torque no gearing), fewer transmission losses (no fragile saildrive), better protected propellers (not hanging lower than the keel), a standard interface for the Spurrs cutters (i.e. actually cut lines, not just spin ineffectively), etc. Furthermore, the slower turning motor would have produced less noise and virtually no vibration since a stern tube setup produces much less drag than a saildrive.
• Install adequate hydraulic hoses. Given the flow requirements, even a cursory look at the Gates or other hydraulic hose catalogs indicates that our A&B lines should be ¾" ID, the S-lines 1"ID. This was another fundamental, basic error on the part of Diport AG. By one estimate, we are losing 6 psi of pressure for every foot of undersized A&B hose ~ 120 psi per side. The suction lines are even more sensitive.
• Install a proper Hydraulic Oil Reservoir, not one where ¾" suction lines are interfaced with ½" hose inside the reservoir while ¼" return lines are hooked up to ¾" hoses. This was perhaps a installation error. Suction lines cannot tolerate such pressure drop anywhere along the way to the pumps.

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:

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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

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Total	42 hours