Sustained speed question in 185

H

hemi2nv

New Member
Jun 2, 2010
59
dallas
Boat Info
2006 185 Sport
Engines
4.3l
How long is it ok to run across the lake at 4500 rpms? Is it out to run from one side or the lake to the other for 8 to 10 minutes at 4500 rpms? I have a 14 3/8 x 18 prop and it gets out of the hole very well but top speed is about 45 mph at 4800 rpms. Is it ok to turn 4300 to 4500 for 5 to 10 minutes?
 
Yes, but it would be better if you didn't make a habit out of it.
 
Hmmm. How much weight and where is it? I have your boat (exactly) and I can easily get 45 mph at a lower RPM (about 4200 or so). The boat will go 55 flat out (with only me and a dog and no gear, plus --I think-- about half a tank of gas). I don't do that very often. Play w. the trim; the speed on ours is quite sensitive to the exact trim angle. At any rate, I agree w. the previous answer; it cannot be a good idea to run for a long time near the top end of the boat's range.
 
They wouldn't make the throttle open all the way if you weren't supposed to run that way. Run it like you stole it!
 
I'm in Texas at about 500' and there was chop this weekend about 1' tall. I had 3/4 tank of gas, gear, and 5 people on board. I know the 14 3/8 x 18" prop cuts down on the top speed but it will pull a 170 lb wakeboarder out of the water and plane out in 3 seconds with 4 people on board. We use the boat to wakeboard and tube but there are those times when we want to run across the lake and I don't want to damage the boat.
 
quailrunner said:
Hmmm. How much weight and where is it? I have your boat (exactly) and I can easily get 45 mph at a lower RPM (about 4200 or so). The boat will go 55 flat out (with only me and a dog and no gear, plus --I think-- about half a tank of gas). I don't do that very often. Play w. the trim; the speed on ours is quite sensitive to the exact trim angle. At any rate, I agree w. the previous answer; it cannot be a good idea to run for a long time near the top end of the boat's range.
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do you have the mpi or carb? what pitch prop?
 
fc3 said:
They wouldn't make the throttle open all the way if you weren't supposed to run that way. Run it like you stole it!
Click to expand...


Funny is funny.

If you want the engine to last, the rule of thumb is 75% of WOT. So to say if you are turning a maximum of 4800RPM you can run 3600 (or 4500 close enough :smt101)

10 minutes on or about at full throttle is excessive (IMO).
 
Your owners manual states that marine application motors were designed to run at wot. For extended periods of time..... Now take that into acct when thinking about mant. Etc. I have no issue in running mine wot for 20-30 mins at a time running up and down the lake for the poker runs we enter etc. My boat is a 96. Looks like I bought it last year. We take excellent care of our boat and we never skimp on maint. Yes we rebuilt our motor this year with over 800 hrs on it. Was it because I had to? Nope, just knew it was getting a little tired and needed a freshen up. So take into acct how well you are gonna take care of it and keep up with it. If you are gonna skimp on yearly maint. Then no don't run it wot
 
hemi2nv said:
How long is it ok to run across the lake at 4500 rpms? Is it out to run from one side or the lake to the other for 8 to 10 minutes at 4500 rpms? I have a 14 3/8 x 18 prop and it gets out of the hole very well but top speed is about 45 mph at 4800 rpms. Is it ok to turn 4300 to 4500 for 5 to 10 minutes?
Click to expand...

WOT on your engine is in a range of 4400 to 4800. Depending on where you are in that range, multiply that number by .75. So, if you turn a WOT of 4600, keep your running speed at 3450 or lower to avoid unusual wear on your engine.
 
Firerooster said:
Your owners manual states that marine application motors were designed to run at wot. For extended periods of time..... Now take that into acct when thinking about mant. Etc.
Click to expand...

Do you have a page number and source for that statement. I've never seen that running a WOT was a good practice, but I've only read the manuals for 350s, 454s and 8.1s. I'd like to read all about it.
 
Its in the mercruiser owners manual, dont remember the page. I am sure its in the operating section. I will look when I get home. Mine is the 454 with BIII. But doubt that would matter in statement like that too. When I read that few years ago I thought thatitwas weird too. But I asked around and I got the same answer I gave....maintain it and no problem
 
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Well it sound like it should be ok to run 4300 to 4500 for 5 or 6 minutes here and there. I do plan on servicing the boat regularly and i won't be running it like that all the time. I even considered moving up to a 19p prop but I think I would miss the torque out of the hole since watersports is about 75% of the boating I do.
 
Your engine will last longer if you avoid doing the wot thing on a regular basis. Most of the manuals I've read advise against doing it often. Just look at some of the posts on boating boards and all of the questions related to mechainical failures. Many of those questons come from people who overload their engines. Factory reps from Crusader recommend running at WOT at the start of the season to make sure the engine reaches spec and then once a month or so to the end of the season to make sure nothing has changed. Checking in this manner requires only a minute or two and then the advice I've received from the Crusader rep is to limit rpms to 75% for carburated engines and 80% for EFI engines. Personally I keep my cruise speed to 75% of WOT and routinely make a 10 hour run from Harbor Springs, Michigan to Grand Haven. I've been doing that trip for many years and have never had a mechanical problem (knock on wood). I do maintain the engines by the book.
 
Mercruiser owners manual 7.4l alpha, bravo I,II, and III. Page 14. In bold states that from the factory the rev limiter is set at safe operating rpm's. If changed or with loading and prop changes can cause varying speeds and should be considered. Operating at near wot or at rev limiter is safe however will increase service intervals
 
What rpms do you turn when the rev limiter kicks in? 75% of wot?
 
Now this is where getting the correct information about your own boat is vital. (for you carb guys it may not be as easy) My mechanic hooks up his computer to my boat everytime it comes in or service etc. The factory has my rev limiter set at 5125. According to mercruiser for my motor it should be between 4800 and 5300 depending on set up etc. with mine being set at 5125 we just left it alone there. In all the history of my boat the computer only shows me hitting the rev limiter 4 times. ( I am the original owner of the boat. it is a 1996, with 843 hours on the motor end of last season, rebuild was done this spring and currently 43 hours on motor snice rebuild) At WOT my rpm's are 5054-5103 an the computer shows throttle position at 98.4% even though I physically cannot push the throttle any farther. So given this thead I sat down and did the math. with my physical WOT RPM's at 5054 that is 98.6% of my set rev limiter, and 95% of the maximum allowable rev limter per factory specs. So in all I am well within allowable factors. Now I have changed my bravo III props from the stock 3 and 3 blade 26P to 4 and 3 blade 26P in doing this change with the computer testing my RPM's have had less then a 4 RPM change throughout the range.

So what I am saying is, if you are really that worried about the numbers and how close to WOT you can run your particular boat. You need to invest in a computer program or pay your mechanic to go out with you and give you the full details on your boat.

in answer to your question I am roughly about 3900 RPM's at 75% throttle. which puts me about 46 MPH on fairly smooth water.
 
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It's not just RPM, it's also load factor. You can only measure load with an engine dynometer. Lacking that one can infer load from RPM, throttle position, manifold pressure, etc. Generally speaking if the engine is at or near WOT and within the max RPM range, it's loaded to 100%.

Some ECMs compute load factor and can display it. Ford's do. GM's MEFI does not. I don't know if Smartcrap does.

Best regards,
Frank
 
A rebuild at 843 hours pretty much sums it up. Properly operated, these engines will go 3000 hours.
 
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