Run Allison Transmission oil pump with engine off

[sailorryan]

Hi,

This is my first post but I frequently read items on here and you guys are a great help in thinking from all the angles on Hatteras usage/equipment/maintenance. My wife and I picked up a very well maintained and modified 50' 1967 MY awhile back. This is our first powerboat (we have 2 sailboats pre marriage that we are in the process of consolidating, and I grew up on boats so its not our first boat). I am in my mid 30s so have not been around the block mechanically speaking and like to read and see what people say, hence I am here.

We have twin DD 8v71N and for reasons beyond the depth of this post, we need to run on 1 motor occasionally, free spinning the other prop. We have Allison transmissions, and I know you cannot free spin these as the oil pump is on the engine and it causes tons of wear. My question is as follows, disregarding why we need to run on 1 motor, and eliminating the option of blocking the shaft or removing a prop, what is the best way to allow us to free spin a prop with the motor off?

My thoughts were to rig power to the transmission oil pump bypassing the engine so we could cycle oil when the motor is off and if heat is needed run the webasto/everhot through the motor to keep it warm. (Is this needed?) I already have it setup to heat the motors) Getting a new tranny that allows free spinning is an option, but I would rather not do that right now.

I am not sure if its a good idea to cycle transmission oil when the engine is off (of if it would actually get to where its needed). Any ideas? Outside the box thoughts are more than welcome!

Thanks!

[Boatsb]

Why not run on one engine with the other at idle?

Your need is not easily met without a major change to the gear.

I'd look to raise the bridge not lower the river.

[sailorryan]

Why not run on one engine with the other at idle?

Predominately because its bad it run at idle for extended period of time without load for various reasons I have read here (wet stacking etc).

I have read that when you build a new setup, you pump oil through the block before cranking it over, just curious if there is anything preventing doing this on an ongoing basis (for the tranny, not the block) or there are better ideas.

[Scott & Malina]

Sailorryan,
Assuming you have an Allison "Twindisk", according to Twindisk and I quote, "most current Twin Disk production transmissions can be backdriven so long as the speed does not exceed normal vessel speed."
Now having said that, you need to determine if you have a "current" production transmission. So, for the sake of argument and disagreement, I would recommend you consult with a Twindisk dealer/service center and discuss your particular transmission options with them, as there are a few regarding the ability to backdrive your transmission.

Scott & Malina

[sailorryan]

Sailorryan,
"most current Twin Disk production transmissions can be backdriven so long as the speed does not exceed normal vessel speed."


Scott & Malina

Thanks Scott, I skipped over that. I am aware that the twin discs are good to go re: free spinning the prop. I was under the impression that I had a m20 (previous owner scribbles-was anal so I put some weight in it). Upon further digging, I found a spec sheet with mh20, might just be a general DD printout. No markings indicate 1 vs other. I have serial num etc but the plate doesn't say a model, just GM and based on pics I think its not a twin disc--- unlucky.

If anyone knows a way to definitively check, I am all ears. I have loads of manuals, even the original 50 year old sales booklet. I have read through most, but so far no luck.

The Allison serial num lookup said mine did not exist. =)

[sailorryan]

As a heads up, I was PMd a suggestion. Add 2 more pumps, one to each engine, in parallel, so either engine (or both) can supply both transmissions. I am checking into this as well and will post info I find.