Wide open throttle (and Hurth ZF-25 gearbox)

David Vogel

Greetings all,

I am, amongst other things, in the process of writing up some findings arising from the earlier failure of the transmission cooler for the Hurth ZF gearbox, as happened on approach to Tahiti last year.

However, in the interim, the following may be of interest, and help someone avoid some costly mistakes.

The ZF 25 (250 HSW) hydraulic box is rated for continuous operation at 3,200RPM. The under load WOT (Wide Open Throttle) Max revs for the (105HP, intercooled turbocharged) Yanmar 4JH-HTE is 3,750RPM (or thereabouts, depending upon your reference). I regularly run up to WOT in order to assess the state of the turbo-charger, and prop cleanliness, during which I expect to see >3,500RPM. With the recently newly cleaned and serviced prop (coated also with PropSpeed), I momentarily exceeded 3,800RPM and still accelerating before retarding the throttle. Note, this RPM is whilst underway. Whilst stationary (e.g. tied up at the dock), the achievable revs with the Bruntons H6 Autoprop can be higher. Be warned.

One further interesting thing to note, with respect to our transmission and WOT, relates to the power output curves for the YANMAR 4JH3-HTE, when compared to the acceptable power INPUT to the ZF gearbox under Pleasure / Light / Medium usage conditions (as defined by ZF). In addition to the limiting max continuous 3,200RPM applicable to the Hurth Transmission ZF25-2.80-1 (#ZF25-2.80) gearbox, the maximum input power for continuous operation is 55kW (73HP). The 4JH3-HTE power output curves indicate that 55kW is achieved at ~2,400RPM.

THEREFORE, the Maximum Continuous Power for this engine-gearbox combination is limited to 2,400RPM.

Having recently suffered severe power loss due to carbon build-up in the turbocharger exhaust plenum chamber (which itself was due to extended periods of low-power motoring), I am now paying much more attention to avoiding carbon build-up in the turbo. We went for a SM with a larger engine for the express purpose of being able to ‘muscle’ through rough conditions, and specifically to minimise our time-of-exposure when running the passes of the island-atolls of French Polynesia. Being able to achieve only 2,200RPM at the precise time of needing to leave an atoll through the pass (due to impending inclement weather conditions), against opposing wind and current, is an uncomfortable position to be in. Anyway …

My operating procedure now is: at the end of a passage (typically, entering a bay for anchoring), I gradually step-wise increase the throttle to fully forward (2,000 -> 2,400 -> 2,800 -> 3,200 -> WOT), noting the engine coolant temp, exhaust and STW/SOG achieved at each step. In relatively calm conditions, I expect to achieve >3,500RPM, with no puffs of black smoke as I advance the throttle. I hold WOT for no more than 5 seconds, before retarding the throttle to set 3,200RPM (roughly equivalent to 80%, which is also the max continuous RPM for the gearbox, although exceeding the Max Continuous Power), which I then hold for no more than 3 minutes, during which I expect the temp to increase, but no more than 5ºC (roughly 78 to 83 ºC). I then retard the throttle to the gearbox-limited Max Continuous Power of 2,400RPM for a further 2-3 minutes to maintain the cooling water flow, and then reduce power as required for anchoring manoeuvres. If we are straight onto the anchor, I then maintain the engine at idle for at least 5 minutes after the “back-down” procedure is complete, which I normally do at 1,500 RPM, but sometimes 2,000RPM if there is any doubt, or if we’re close to underwater obstructions. During the cool-down before shutting down, there is no revving the engine as recommended by some (reportedly to clean the injectors). I believe that the cool-down period is imperative to keep the oil flowing freely through the turbo for a time as it spins-down and cools from incandescent temperatures. As I understand it, to go from normal under-load operations immediately into shut-down will promote heat-soak of the oil remaining in the oil-chambers of the turbo bearings, ‘cooking’ the oil to a congealed state, which then inhibits oil flow on the subsequent start-up, leading to earlier bearing wear in the turbo.

I now do this run-up procedure every 6-8 hours of continuous motoring, the frequency of run-up depending on the power setting underway (low power = more frequently); previously, it was every 12 hours, but this proved to be not often enough. Note: if you have a dirty prop (or fouled turbo), then achieving WOT (or even sometimes even above 2,2200RPM) is not possible, and any un-burnt carbon in the exhaust resulting from excess fuelling (as will be the case in a mechanically-metered engine such as the 4JH3-HTE) will exacerbate any potential or pre-existing problems with the turbo.

As an aside, due to the gearbox apparently being the limiting factor for max continuous power, I am considering to install a temperature gauge on the gearbox. The background to this is, that I had previously read somewhere that the Hurth Gearbox could accept up to 50kW (or thereabouts) without a cooler. Meaning about 2,200RPM for the 4JH3-HTE. When I needed to run the engine whilst having by-passed the transmission cooler for the Hurth gearbox, fortunately I was scoping the gearbox temperature from the helm using a laser-pyrometer, and observed that the gearbox temperature started to rise above 80ºC as soon as 1,500RPM was exceeded (this was in optimal conditions of no-wind and calm-water); 1,800RPM was unsustainable for more than a 30-60 seconds, before the temp started to climb rapidly – reducing to idle-in-gear was the necessary remedy. Continuous operations at 1,200RPM was more comfortable than 1,500RPM. I feel certain that I would have quickly and catastrophically damaged the gearbox if I had been operating (without the transmission cooler) at the planned 2,200RPM, and therefore would recommend that 1,200RPM be considered as the max ‘normal operations’ operating RPM if one has to bypass the transmission cooler, leaving 1,500RPM in reserve for temporary contingencies.

I would also like to install a pressure gauge on the gearbox (in order to detect an early loss of gearbox pressure due to a failed heat-exchanger) but, on our installation, the pressure take-off is already used for the shaft brake.

I hope that this information and general background is of some assistance, and I would be very interested to hear any comments from the group on the evolved ‘run-up’ and shut-down procedures, and on power settings in general.

Thanks, and with kind regards,

SM#396, Perigee

From: <main@AmelYachtOwners.groups.io> on behalf of David Odell <david.wanderer@...>
Reply-To: <main@AmelYachtOwners.groups.io>
Date: Sunday, 16 January 2022 at 1:30 am
To: <main@AmelYachtOwners.groups.io>
Subject: [AmelYachtOwners] Wide open throttle

This is my first post as my wife Susan and I just bought A54 #153, originally named Aora, now Dragonfly. Excited to be a part of this community, and what great boats these Amels!
My question pertains to running the Volvo Penta at wide open throttle. I previously had a trawler with a 170 hp Lugger diesel. The Lugger technical support manager advised the fleet to run the engine at wide open throttle after 6-8 hours of engine usage. We would run wide open throttle for 5-10 minutes to burn off all of the carbon build up and to heat up the gaskets and seals. Engine temperature would go from 180 degrees F to about 195 degrees F. We would never let the engine temperature exceed 205 degrees F. 
A few questions:
1.  Do you run your Volvo Penta at wide open throttle and for how long?
2. What is your normal engine running temperature, and what does the temperature reach at wide open throttle?
3. Under normal engine operating conditions, is there an RPM “sweet spot” where the engine is happy and the fuel burn is efficient?
Thanks in advance. 
David Odell
A54 #153 Dragonfly
Southport, North Carolina
Currently lying Le Marin, Martinique