WOT rpms

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Gregory S

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Anybody ever heard that we get 10% less wot rpm said in salt water as opposed to fresh? Somebody told me that today. Higher specific gravity?
 
Dunno, but as a WAG it doesn't seem likely as average SW has only a 2.5% greater SG than FW. ( 1.025 vs 1.00.)

Maybe in the Dead Sea (1.17) or Great Salt Lake ( close to Dead Sea) ??
 
I never heard it until today. My brother put his boat in salt water for first time today. 454 Crusaders maxed out at 4100 rpm. Mechanic told him that he would get 10% less rims. Thought he was full of it.
 
I would think it would be just the opposite. Salt water has more bouancy than fresh, so theoretically, the boat will ride slightly higher in the water, meaning less surface friction which would make the engines to not work so hard...more RPM's.
 
I was thinking the same thing.

Greg, was the boat loaded the same way? Fuel, water, etc.? Bottom clean? Props tuned recently?
 
Clean bottom , 1/2 tank fuel, no fresh water. The mechanic/Captain is no rocket scientist. They take the boat out for a shakedown, several minutes in, they get an engine alarm. Mechanic says oil pressure and temp are good, so disables alarm. STBD engine over revs, come back to dock. STBD transmission very hot with no ATF reading on dipstick. he didn't think about transmission temp alarm obviously. I told my brother to fill the transmission with the appropriate fluid and see how the transmission acts. I'm not optimistic though.
 
IMO the boat sits lower in fresh water and would push more volume, probably more friction because of more wetted surface.. Sme weight however. Both engines the same RPM despite the problems?
 
yep, both engines the same until transmission started to act up.
 
Hopefully it didn't fry. Vdrives, i/o's or straight inboards?
 
If there was no fluid in the gears hopefully they looked into where it went. It's either all over the bilge or I guess it went out the oil cooler. I would have thought there would be some water in the gear oil tho.
 
If a leaking trans. cooler is the culprit, there should likely be some surface sheen evidence of ATF behind the boat at rest.
 
That's the discussion we just had. Where did it go? None in the bilge so must be a leak in the trans/oil cooler. He says he didn't see a sheen in the water, but had to go somewhere.
 
The bottom line is the mechanic should be off the boat if he disabled an alarm and let it run until it blew up.
 
Yep, not letting him near my boat. Disabled an alarm????? WTF?
 
Well, I know people that did it. For example; Detroit diesels are notorious for having no oil pressure at low/idle RPMs. On our boat after a long hard run in the Summer we'd be docking and when shifting (at idle RPM of course) due to normal engine RPM dropping with the shifts, the oil pressure alarm would briefly sound. I was always concerned but knew that they all had issues of this sort because they were "meant to spin!".
But one day I was on a friends boat with similar Detroits and noticed that one of the fuses at the dash board was backed out. We had been in discussion about what happened on our boat and he said "this is how I took care of the idle alarm". Ignoring an occasional blip is one thing depending on the beast, but crippling the alarm upon initial discovery or permanently is over the top.
 
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