Is it ok to run boat for long periods of time at a steady RPM?

04cobra23

04cobra23

Member
Aug 21, 2014
344
Boca Raton Florida
Boat Info
340DA and 215LR
Engines
Twin 8.1's and 4.3 Merc
I made the run on the Okeechobee waterway a few months back from Delray beach to Sarasota. It's time to bring her back to the east coast. Took me almost 2 days to cross (had some engine issues on second leg of trip so couldn't run above 2500rpm) and total run time of about 19 hours. First day 10 and second day 9. If the boat was running correctly I think it could be done in about 15-16 hours. Anyway, back to my question.. Is it "ok" for the boat to run for that long at such a steady rpm? My boat is a 03 34 with 8.1's:smt038:smt038
 
Generators run for days at a constant RPM.
No idea about big block gas at high RPM. We run 1400 RPM for 12 hours at a time a few times a year. Once we ran at 2000RPM for 16 hours.
 
It is my experience that many commercial (opposed to military or aviation) designs don't always deal well with long period, high stress loading and vibration.

Is it "OK" ... well the components are generally designed to run continuously, but maintenance practices after leaving the production floor can degrade the overall robustness of the boat as a "system"! I do a lot of my own maintenance work and I will add things like "backup clamps" and "safety wire" to critical or high stress components to beef up the original design. (and I use the word "design" loosely)

As for making your trip ... I would go over the running gear carefully to check leaks for loose fittings and check the operation and calibration of engine instruments and move ahead. Any time I'm running at more than 40% to 50% of max, I kinda go on "Hi sens" for changes in sounds and vibration. Once at a comfortable cruise RPM I tend to leave it there - there are stresses that come with changing rpm - so I tend to leave it alone as long as she is running smoothly.
 
The short answer is YES, providing you're not exceeding manufacturer suggested cruising RPMs. Don't forget important fact that by running at planing cruise speed, you'll never run for 16 hrs non-stop. Majority of recreational planing hulls are designed to run upto 10hrs. You'll have no choise but to stop for fuel. Any fuel stop is at least 30min to 1hr. That's plenty of time for enngines to take a break.
 
Alex F said:
The short answer is YES, providing you're not exceeding manufacturer suggested cruising RPMs. Don't forget important fact that by running at planing cruise speed, you'll never run for 16 hrs non-stop. Majority of recreational planing hulls are designed to run upto 10hrs. You'll have no choise but to stop for fuel. Any fuel stop is at least 30min to 1hr. That's plenty of time for enngines to take a break.
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Alex, How did you determine that a planing hull is designed to run for up to 10 hours? What happens at hour 11? Interested in seeing where this data came from.
 
ImpulseIII said:
Alex, How did you determine that a planing hull is designed to run for up to 10 hours? What happens at hour 11? Interested in seeing where this data came from.
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He's got a spreadsheet mapping out all variables, hull shape, number of chines, engine size, fuel consumption ratios, coefficient of drag efficiencies, etc. He then concluded that an average cruising burn rate of 40 gallons/hour would require a minimum of 400 gallons of fuel to burn for 10 hours.

But yeah, what happens in hour 11?
 
In the 11th hour, you run out of fuel... I believe is what he is saying.
 
JimG said:
In the 11th hour, you run out of fuel... I believe is what he is saying.
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No, that is not what he is saying. I am just looking to understand the engineering and architecture that limits plaining to 10 hours. I can run on plane for more than 10 hours, but now I want to understand why I should not do that.

Something like inquiring minds want to know.
 
Cruise as long as you want - your engines will be just fine.

Regarding what Alex said, I took it to mean that he was talking, very generally, about the relationship between fuel tank size and range - especially for a gas-engined 30'-ish boat of this style. I would agree that Alex is pretty much spot on. But again, this is just a very general statement as some will go a little further on one tank and some will go less. Obviously conditions can change this, but if my assumption about what Alex is saying is correct (regarding the OP's boat), then he is pretty much on target... 10 hours-ish is the approximate time limit for a boat like the OP's while at cruise before the go-juice runs out (which of course means you would run less time to have some gas in reserve).
 
You know, several, you know, dozens of boats cruising in their 11th hour spontaneously combust each year. It's just not really widely reported.
 
LTD.330 said:
You know, several, you know, dozens of boats cruising in their 11th hour spontaneously combust each year. It's just not really widely reported.
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he read it on the internet:smt043
 
We routinely run our boat for extended hours (8-10) at a time, cruising 10-20 miles off shore. The Crusader 8.1s use no oil. Our manual states that engines can be cruised at 75-80 percent of WOT (4200). We run them around 3400 with no problems. And, we do all of the recommended maintenance spelled out in the manual.
 
With the 8.1L Mercruiser Gas V8's, we have run 3200-3500RPM (maintaining 19-20 knots) for 5.5 hours. Which got us from Emeryville, CA to Monterey, CA. There was 4-5 foot sea, and wind was 15-20 knots. We burned close to 200 gallons of fuel. Un-flat conditions cost us 3-4 GPH. Leading me to think there are heck of a lot of variables in running "long periods at a steady RPM". In the land of cars, we all know that excessive idling in your driveway, and constantly driving slow speeds around town, can cause fouled plugs, carbon build-up, and even corrosion in engine components that never get to full operating temperature, or designed loads. Boats do not like to idle about for hours-on-end either. If you do like the occasional Cocktail Cruise, or sightseeing adventure, it is good if you can run the boat up to speed for a few minutes on your way back to the barn. Also, you should run the blowers at low RPM. Especially on Carbureted boats. During an engines 'Break in period' (usually around 10 hours per the Manufacturer) it is very important to vary the RPM's. Mostly this is to help suck oil up into the rings, but it also helps all the moving parts wear into a stable relationship they will share fort the rest of their working life. Once the engines are broken-in, the time you can run at cruise is dependent on weather conditions and proper mechanical maintenance. Given flat water, no wind, reasonable temperatures, and normal loads, the run time should just be dependent on fuel capacity. And whether your wallet explodes and sinks the boat.
 
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Lazy Daze said:
Cruise as long as you want - your engines will be just fine.Regarding what Alex said, I took it to mean that he was talking, very generally, about the relationship between fuel tank size and range - especially for a gas-engined 30'-ish boat of this style. I would agree that Alex is pretty much spot on. But again, this is just a very general statement as some will go a little further on one tank and some will go less. Obviously conditions can change this, but if my assumption about what Alex is saying is correct (regarding the OP's boat), then he is pretty much on target... 10 hours-ish is the approximate time limit for a boat like the OP's while at cruise before the go-juice runs out (which of course means you would run less time to have some gas in reserve).
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Thank you, Dennis. Your interpretation of my statement is exactly what I meant.When I said "...upto 10hrs", that exactly what it means, from zero upto ten hours.

For sorcastic minds, according to my "spreadsheet" at 10hrs and 1st minute, you're SOL.The statement was very general to help the OP get an idea, but in reality it applies to number of hulls. The math is very simple. Let's take a look at few:

1. My POed 320DA, 200g tank, basic cruising fuel burn was around 25GPH. 200g/25gph=8hrs, pull the throttles back a little until SC shows you're burning 20GHP, then you're at the targeted 10hrs mark. 340DA can easily fall into the same basic math and acheave the same 10hrs cruise.

2. 40-45'er DA with 400g tank, at regular cruise would burn around 30-35gph, which gives you more optimistic 11-13hrs. Those with CAT 3116 will have even more optimistic numbers.

3. 45'er DB with 350g tank, at regular cruise burns around 35gph, which gives you targeted 10hrs of cruising.

4. 480/500/520DB with 500g tank, at regular cruising speed will burn in the neighborhood of 45gph, which gives you around 11hrs.

So, as you guys can see, my very basic math applies to many planing SR hulls ranging from 35-50+ footers, gas and diesel.

You can argue all day long, but in general terms of planing hulls, we have 10hrs cruise boats. Obviously, the extend of your cruise will depend on number of variables, with primary one is how far do you push your throttles.

Having said that, my primary point to the OP was that he doesn't have to worry about putting his engines at risk of cruising for "too long", simply because he will have no fuel left to present a "danger" to the engines.
 
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Great information and explanation Alex F!

Thanks for the confirmation!
 
I guess t his will screw up Alex's example, which it appears is based on rated cruise rpms. But, I've got a 450DA with 350hp Cat 3116's. The tanks hold 400 gal X 85% for reserve= 340gal / 16.5 gph (75% power -18 kts which is comfortable on plane for me) = ±21 hours ..........I'm just glad these boat have head systems on them!
 
fwebster said:
I guess t his will screw up Alex's example, which it appears is based on rated cruise rpms. But, I've got a 450DA with 350hp Cat 3116's. The tanks hold 400 gal X 85% for reserve= 340gal / 16.5 gph (75% power -18 kts which is comfortable on plane for me) = ±21 hours ..........I'm just glad these boat have head systems on them!
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A decent Head seems to matter more as I get older ;0) Wait... I am pretty sure here is a joke there somewhere.
Seeing as Alex is working with a pretty wide brush, there will be examples on each side of the curve. Our 380DA has 275 Gallons and runs 33GPH at cruise. Assuming we stayed at 3400RPM for the whole trip, I would be calling Vessel Assist well before we made 10 Hours. Sill this is a good exercise to work through on CSR, rather than 3-Miles out from Santa Cruz at 7PM.
 
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