VI in boat engines, rethink.....

[Schmoe]

Been scanning the older posts and come across a great deal of debate on using the st. weights vs. multi visc. Some posts even declared that the Quicksilver 15W-40 is a straight weight since there are no VI improvers. Why would they call it a 15W-40 then?? But, where I need guidance is the diesel engine oil for a gas boat engine. Thought that was the way to go, but read some more that maybe it isn't because it could leave deposits within the engines. Are we comfortable that the Quicksilver has enough corrosion inhibitors that a diesel oil can't touch, even if it's CI/4 rated? I do believe that VI would get the living be-jesus squished out of them in a boat motor and maybe that's why they are not good, which would lead to other deposits.
I, just this year, went with the Pennz. straight wt. HD 40 for my 18 foot 3.0LX Merc. First weekend out, oil pressure held steady around 40 psi. This weekend, noticed it would drop to around 20 at idle after a run, but jump back up to 40-45 when running. Last year I used Valvoline's 20W-50, oil pressure would jump up to 60 psi and drop to 20 at idle. That made me a little nervous. BUT the Valvoline obviously had VI's in it, but pressure fluctuated by 30 to 40 psi. Wouldn't think that would happen to that kind of oil. What about the st. weight Pennz. 40, no VI's in it, it fluctuated too, granted, not as much? The little 3.0 works a lot harder than their bigger cousins, but oil pressure differences puzzle me. School of thought was the VI's would "adjust". But both kinds seem to indicate to me, that after some use, they ALL thin out, regardless of VI or not so don't waste your time and money trying to determine a correct multi visc. oil for a boat engine. I think even Bob uses straight weights on his boats, or knows lots that do. (all conditions were the same, speed, temp, amount of oil in engine, etc. etc.)

Also, switched to a 18 pitch 4 blade comprop. Unless you want quick planning and low top end, don't waste your money. But it will make your boat jump, I mean JUMP, out of the water. Would be my pick of props for pulling skiers and tubers.

[ June 10, 2003, 04:23 PM: Message edited by: Schmoe ]

 

[userfriendly]

shcmoe,
I thought that Quick Silver (not the horse) was a 25W40. Shell used to offer that marine grade, but I think they dropped it. Is that boat engine a BB or SB Chev? If so, the best way to increase oil pressure at all RPMs is to install a bigger oil pump that also has a high pressure by-pass spring. I think that going up in viscosity to gain pressure is a band-aid approach as all you may be achieving is by-passing more oil around the engine instead of pumping the oil through it.

 

[Drstressor]

Quicksilver is 25W-40 and is not supposed to contain VI modifiers. It is CH-4 rated, so it is very similar to HD diesel oils except that it cost a lot more. If you look at the viscosity specs for some straight 40 weights you would think they were 25W-40s. A good synthetic diesel oil is probably your best bet for a hard working 3.0 L used during the summer. There is really no need to obsess about this. Your engine will more likely die from a leaky manifold than from wear as long as you change the oil before winter storage.

BTW, I just switched to a 4 blade prop myself. I just fish, never ski. But I have plenty of LAF (lard *ssed friends). I took a 15% hit at top end, but the boat sure does plane better.

 

[JonS]

I heard Quicksilver 25w-40 can be called a multi-grade with VI improvers is because it meets the cold flow specs for a 25w (not 15w-40)

 

[Schmoe]

drstressor,
Did you go up in pitch, or remain the same? I went from a 19, since they had no 19's, I went to a 18 thinking quicker take off's and less strain on the motor, to a 18. Called Overtons. They said send it back and they will swap it for a 20. I'll try that. Seemed like the engine was over reving. I was watching the tach, but I know the "feel" of the boat. Top end disappeared. One thing I did notice: the comps are around 13.5 to 13.7 blade width while the aluminums are 14. Should have done my homework better and calculated the differences between the cup, blade width and pitches and go with the 20 in the beginnning. The more I think about it, wonder if I should just get the larger 22. Wonder if that would really affect my wide open throttle rpm, 4200???

[ June 12, 2003, 11:14 AM: Message edited by: Schmoe ]

 

[Drstressor]

For skiing, you want to use a prop that lets the engine rev to the middle of the recommended rpm range WHEN UNDER LOAD. This usually requires a higher pitch prop than you get with a stock boat. Increasing the diameter of the prop, within reason, generally gives you a better hole shot as well. For general use it’s usually best to prop a boat so that it runs toward the high end of the recommended rpm range when lightly loaded.

I went from a 3 blade 17" pitch to a 4 blade with the same pitch and lost 15% of my top end speed. But my max rpms empty went from 5200 to 5500 (90 hp Honda OB). Much better hole shot and better performance with a heavy load with the 4 blade. I think the extra drag from the 4 blade takes away the top end speed.

Composite props are interesting. They flex a bit (reducing the pitch) when under load, which is like having a lower gear during acceleration. I had one as a spare on a bigger boat 15 years ago. You were not supposed to run them full throttle in those days. But I understand that the newer composite materials allow normal operation. I would imagine this would be a good thing for a ski boat.

 

[friendly_jacek]

I use a 4-blade comprop in my 18' Bayliner. I picked it up for $50 on ebay and it is more scratch and dent resistant than the stock aluminium it replaced.

 

[Drstressor]

"This usually requires a higher pitch prop than you get with a stock boat."

Sorry, I meant to type lower pitch prop. I should profread my posts.