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Bounty Boats continued and developed the idea used by the F. B. Wilds Caribbean / Bermuda range of utilising hydraulic drive and the significant advantages this offers in boats.

As there is no direct mechanical link between the engine and propellor, the use of hydraulic drive allows the engine to be positioned almost anywhere within the boat rather than being in a fixed position in relation to the propeller and the precise alignment this usually requires to reduce vibration to a minimum. Bounty chose to put the engine, transversely mounted, under the aft deck as in the Caribbean design allowing easy access to most of the main components for servicing and maintenance without disturbing any of the accommodation. All of the drive components being in the stern also greatly reduces the amount of engine noise in the forward 'living' area when the boat is under way. Most Bounty's used the quieter BMC 1.5 power unit (although some did have alternative engines fitted) as original, rather than the Perkins 4.107/8 favoured by F. B. Wilds, which along with a better designed exhaust system reduced noise still further. It will be found that some of the Bounty Buccaneer designs in use do not utilise hydraulic drive in an effort to keep initial build costs down, but these were invariably purchased as mouldings and thus are not genuine Bounty built versions. In this case the engine is mounted under an engine cover within the galley and drives via a conventional marine gearbox and shaft.


In simple terms, the hydraulic drive itself consists of an hydraulic pump mounted to the bell-housing of the engine where a gearbox would normally be. The pump is in constant mesh with the engine from start- up. This is fed with a supply of oil from a reservoir through a strainer and in idling mode, oil is circulated via the control valve, which provides the function of a conventional gearbox, back to the reservoir. When forward or reverse gear is selected via a conventional lever at the helm and Morse type cable, the control valve forces fluid in the appropriate direction through the hydraulic motor which is connected to the propeller before returning that fluid to the reservoir. In early Bounty's the hydraulic motor was mounted to a conventional shaft and stern tube as in normal inboard installations but later the hydraulic motor was mounted directly in the keel of the boat just in front of the propellor, reducing requirements for shafts and potentials for vibration. In this case there is usually a vessel mounted above the final drive, connected to it via a pipe which lubricates the final drive on a "constant loss" basis. This needs to be kept topped up with hydraulic oil. A heat exchanger type oil cooler is fitted into the system to cool the hydraulic oil.

Three different types of hydraulic components are available, gear, vane and piston type. The original Bounty system was usually the lower pressure but higher flow type consisting of a gear type pump and vane type motor, gear and vane types being compatible in the same system. Piston type hydraulic systems are more efficient, having more of the characteristics of conventional shaft drive without the perceived "slip" of hydraulic systems, but work on less flow at a much higher pressure and are not interchangeable with gear and vane components. The piston type system has something of a a reputation for having a slightly louder characteristic hydraulic drive "whine" but this is counteracted with lower revs for a given speed than gear and vane types.


Well looked after all of these systems are very reliable and should give many years of trouble free use, providing that oil levels both in the hydraulic system and final drive lubrication are maintained and that the oil cooler is kept clear of obstruction. In any case, there should never be any loss of hydraulic fluid, as any loss is indicative of a leak on the system, requiring immediate attention.







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Great article  Jill. Personally, the quietness obtained by having the engine aft, enabled by this type of drive, is a big plus when choosing a hire boat. Soundproofing engine boxes has never been a strong point on Broads hire craft.



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Interesting,we have this on our Bounty and find it fine although ours whines a bit.Hope to hear more regarding servicing etc ,which was covered in previous posts with a guide to settings.Great to see this part of the site picking up with the work being done to bring old posts back by Captain Joshie.

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Surely hydraulic drives are inherently far less efficient than a prop-shaft?


Maybe not "far less", but yes, they are less efficient than a direct shaft drive.


The conversion of engine rotation to hydraulic fluid movement and back again to movement in the short prop shaft creates heat through the additional friction.


At low horsepower ratings it's not a big reduction in efficiency though, and as people have said, the removal of the engine from the cabin space makes a huge difference in noise levels at the helm, and stops the engine taking valuable space in the centre of the boat.

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 I think the other thing over shaft drive is that a long crooked shaft that's been there for 30 years isn't particularly efficient either.


But you can then intuitively estimate the engine revs by the tingling sensation from the floor..... ;)

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  • 1 month later...

Hi all, I think the post that has been referred to elsewhere regarding hydraulics was a rather long thread, where a few of us had discussed our respective issues and fixes.


I have a 82 Bounty 34 centre cockpit, ex hire, located in France.  It has a Perkins 4.108 motor and a gear pump and vane hydraulic motor.


My saga (and expansion of my knowledge on the subject) has continued unabated.


Last year I think I posted a series of numbers that I had produced by comparing engine revs to driveshaft revs at various rpm points, and showed we had an effective reduction in gearing presumably through wear and a slight leak in the valve system.

We replaced the valve unit over winter and had a noticeable improvement in performance - roughly a 100rpm less for any given speed.    

After LOTS of thought, I am about to replace the entire drive train with new, I am thinking about changing the geared pump for a far more efficient (and more expensive) vane pump as well if I can work out how to get the adaptor plate modified.


The hydraulic train is incredibly inefficient, particularly when worn, and a more sensible person would just get another boat, but something keeps making me perservere to see if I can get hull speed out of the old dear one day!


I will post updates after winter, when I have something to say!





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