Reduction In Performance - Thick Oil?

[DockHoliday]

This season my boat is just a tad sluggish. With a full load recently, I had a heck of a time getting on plane. Can't recall having this problem before. My WOT rpms use to be 4,700, but now are only 4,200. Props are in good shape, hull is reasonably clean, engines run fine, etc.. One thing that is different is that the motors (VP 5.0, EFI) are filled with straigh 40W dino. I had previously run synthetic 5W30.

Could the thicker oil make a noticable difference in available power?

 

[TooSlick]

Yes, my neighbor picked up about 300 rpms going to the Amsoil 15w-40 in his twin Mercruiser Inboards (37' Searay). Top end went from 4400 rpms to 4700 rpms on that boat. So I'd expect the opposite to also be true....

TS

 

[androbot2084]

A lot of marine mechanics say that you have to use a straight SAE 40 grade oil in a marine engine and they recommend against synthetic 15w40 multigrades. What the mechanics don't understand is that the 15w40 synthetic marine oil is the better oil.

 

[Schmoe]

I've heard of that happening down here also. But when oil is up to temperature and that size of an engine, I'd think that it wouldn't really make that big of a difference. Could be your density altitude. High heat and humidity make for some difficult breathing.

 

[Cicero17]

Where do you get a full sythetic marine oil? Mercury says to use 25w-40 oil and Quicksilver comes in dino or synthetic blend but no full synthetic.

I just did my first, 20 hours, oil change and didn't notice any changes. I used the Quicksilver synthetic blend.

Searay 180 with I/O, love the boat and wonder how we ever got by without it.

 

[friendly_jacek]

I bet the 5W30 was loaded with friction modyfiers.
Next time try Delo 400 15W40 CI-4+ as it also is friction modyfied (Moly).

 

[DockHoliday]

The 5W30 was Amsoil S3K. The owner of my marina had a fit with he learned I was running 5W30, and changed it with 40 Wt. I posted this on here last year (post name old school).

I have several jugs of Delo 400 15W40 waiting for the fall oil change. This is intended for the rinse phase of the ARX (also posted in this forum).

 

[Steve S]

doubt it. 500 rpm is quit a bit of drop.

 

[alreadygone]

Have you changed fuel filters? Has marina changed suppliers? Higher alcohol blend? throttle cables out of adjustment?

Bob

 

[TooSlick]

Doc,

The owner of your marina is living in the dark ages when it comes to lubrication.

I'd go back to using a synthetic - the Amsoil 10w-40, Four Stoke Marine oil if you insist on a 40wt - and I think you'll find most of your missing rpms.

TS

 

[DockHoliday]

quote:

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Originally posted by alreadygone:
Have you changed fuel filters? Has marina changed suppliers? Higher alcohol blend? throttle cables out of adjustment?

Bob

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Bad fuel is a definite possibility. I don't fill up very often, and it costs a to do so, so this variable is hard to solve for. However, I do use FP. The fuel filters are changed each fall as part of the winterization process.

 

[DockHoliday]

quote:

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Originally posted by TooSlick:
Doc,

The owner of your marina is living in the dark ages when it comes to lubrication.

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Agreed, but this is a common view point, and is even common here in this forum on BITOG.

quote:

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Originally posted by TooSlick:


I'd go back to using a synthetic - the Amsoil 10w-40, Four Stoke Marine oil if you insist on a 40wt - and I think you'll find most of your missing rpms.

TS

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The Amsoil 10W30 and 10W40 marine oils were not out, or I was not aware of them, when I made the switch to series S3K. I will probably switch to the 10W30 once the Delo is used up.

 

[Geoff]

Since I could feel the difference going from 15w-50 to 5w-40 I would say that yes, viscosity has a lot to do with it. Straight 40 weight is thick stuff.

However, the affect will diminish significantly on hot engines. Try again after running it for a while near WOT. Then again if you are running in cold water (around 75 deg), the oil coolers may keep the oil cool enough that it never really gets close to the 212 F point where viscosities are getting closer together. Also, do you have 140 F or 160 F thermostats?

Keep in mind Volvo specs straight 30W synthetic for its GM based engines. I would not be surprised if most 30W synthetics also meet 5w- or 10w-30 ratings.

The argument for 40 grades: UOAs I have seen on the GenII/III GM blocks tend to be better with 40 weights. Also, fuel dilution is a problem in marine engines, since they are tuned to be conservative and idle a lot. (Lean mixtures on heavily loaded engines can burn a hole through the pistons very quickly). Fuel dilutes oil and will reduce the viscosity.

Also, xw-40 grades probably run off slower, helping to control corrosion or rust.

Other than all that, you are slightly over propped at this point. More gear? New bimini top? Bilge have water (8 lbs/gal)?

Dropping an inch in pitch will give you 200 rpm. Reducing the cupping on the blades can do same. Most prop rebuilders can change the cupping on aluminum props very easily. Get the revs in the right recommended WOT range. This will balance the mechanical load between the engine and the drive properly.