Crossover water supply to Tides shaft seals

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Ortolan
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Crossover water supply to Tides shaft seals

Postby Ortolan » Sun May 14, 2017 5:34 pm

Do any PDQs with the Tides or other dripless shaft seals have a crossover supply line for operating with one engine? The water injection to the seal provides cooling & lubrication. Their instructions clearly state this is essential, as the "off" engine still has the shaft rotating - hence requires water injection. A crossover hose provides water flow to both shafts when only one engine is running.

While this might be an uncommon situation, getting home on one engine could cause damage to the other seal. Having your boat towed is also a similar problem ... Being towed, with the crossover in place, one engine should be running at a fast idle - without the crossover, both engines should be running.

On the other hand, according to a Steve D'Antonio article, a crossover hose could, under certain circumstances, cause engine water ingestion problems...

I'm aware of the many, many seal issues with the PDQs, but haven't read anything about a crossover hose & ours doesn't.
Russ
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Re: Crossover water supply to Tides shaft seals

Postby SecondWind » Mon May 15, 2017 3:26 pm

Put the non running engine in gear to keep the shaft from turning.
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Re: Crossover water supply to Tides shaft seals

Postby Ortolan » Tue May 16, 2017 8:44 am

That's what I used to do with my sailing cat - with folding props. However, the drag & torque on a large, fixed, 4-bladed prop locked in place by the transmission at 8 knots would be tremendous. I checked the ZF transmission manual & it states you can (safely) leave the transmission in neutral when on 1 engine or being towed, but doesn't say you must.

I recall a couple PDQs mentioning running on 1 engine at times to save fuel at low speed, but that would certainly be negated with the drag of 1 locked prop.
Russ
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Re: Crossover water supply to Tides shaft seals

Postby deising » Wed May 17, 2017 7:08 am

My ZF transmission manual states that the tranny uses hydraulic pressure from the engine output shaft to perform its functions. If the engine is off, it cannot be locked into gear and will freewheel regardless of the position of the selector.

We have the Tides seals on our prop shafts with water injection, but no cross-over. I have had to run for several long days on one engine due to an initial problem after purchase, and have chosen to run on one engine numerous times when I don't need to go more than about 6-7 kts. I don't think I am saving much in fuel (really impossible to tell without expensive fuel monitoring instruments), but it keeps the engine hours down somewhat.

I ran my sailboat inboard for over 3,000 hours without any water cooling on the dripless shaft seal and never had a problem, but I realize these are different. I have not seen any leaks on this boat, but maybe I will cease running on one engine unless I have an emergency.
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Re: Crossover water supply to Tides shaft seals

Postby SecondWind » Wed May 17, 2017 3:06 pm

I would consider locking the shaft with vice grips rather than chance expensive damage. We're only talking about a temporary situation here, in limp home mode.
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Re: Crossover water supply to Tides shaft seals

Postby deising » Thu May 18, 2017 8:31 pm

Not being argumentative here, but what expensive damage are we talking about? The transmission manual states that freewheeling the shaft is not a problem whatsoever. so that is not an issue.

If letting one shaft freewheel without some positive flow of cooling water to the dripless shaft seal is the only issue, then we need to know the details. At what speed is the dead shaft rotating when the boat is moving at, say, 6-7 kts on one engine? I suspect it is nowhere near what the operating engine's RPM is. Is that really an issue and how much?
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Re: Crossover water supply to Tides shaft seals

Postby duetto » Fri May 19, 2017 6:45 am

i believe that it depends on the particular shaft seal. we've had both tides and pss so i'm somewhat familiar we them. as i recall tides is pretty emphatic about needing water flow and will wear out without it. pss says you don't need water at displacement speed.
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Re: Crossover water supply to Tides shaft seals

Postby deising » Fri May 19, 2017 7:14 am

Well, guys, I guess I am guilty of not fully understanding exactly what the limitations were for our Tides shaft seals. Taking Tides warnings at face value, and not having a crossover water connection, I will no longer plan to run on one engine deliberately.

It is interesting to note that with at least 30 hours of single-engine running (typically at 2000 RPM, but occasionally at 3000) on each engine, I don't even have a drip of water past the seals. Maybe I lost some life, but at least they are not leaking.

Thanks to all for raising the issue and discussing the details.
Duane Ising
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2006 PDQ MV 34 - hull 91, 75HP, 3-blade

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Re: Crossover water supply to Tides shaft seals

Postby Ortolan » Wed May 24, 2017 11:17 am

I've decided not install a crossover hose, but will refrain from running on one engine - didn't often anyway. Henry on SnoDog had evaluated efficiencies & found that you didn't really save much fuel unless running under 6 knots.

The only change I'm planning is to replace the cheapie hardware store type reinforced hose to each Tides seal with type B fuel hose as recommended in the Steve D'Antonio article. The first half of the hose runs through engine compartment (lots of heat) & the second half is below the waterline (a flooding danger if comprised). The hose should be free & slack the last 2' so that it's not pulling on the seal at all. One of mine had been too tight & even pulled the clamp out of the fiberglass. New high-end hose clamps are called for as well - one of the two hose clamps on one engine had rusted thru & was just sitting there. Periodically, you should remove the hose from the seal & check for full water flow with the engine idling (you could get a partial blockage at the "T").
Russ
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Re: Crossover water supply to Tides shaft seals

Postby DickTuschick » Wed Aug 09, 2017 4:34 pm

Hi Folks,
We now have about 20000 miles on our boat and a significant portion of those miles are on one engine. On one engine we typically run at about 6.5 to 7 knots.
We have never had a problem with either the transmission or the Tides shaft seal. When running on one engine the speed is so low that there does not appear to be sufficient friction heat build up to cause any problems. The shaft seals are always wet with a reasonable amount of water in the shaft tube to disburse any heat that does build up.
Hope this info helps.

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Re: Crossover water supply to Tides shaft seals

Postby deising » Wed Aug 09, 2017 5:05 pm

Yes, Dick, it helps and I feel much better now about my previous one-engine runs.
Duane Ising
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2006 PDQ MV 34 - hull 91, 75HP, 3-blade

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Re: Crossover water supply to Tides shaft seals

Postby Ortolan » Fri Aug 11, 2017 5:37 pm

Thanks Dick! I feel better now - This is what I get for reading the instructions!
Russ
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