Running boat at 8 knots for long periods of time?

Dani-Lu

Dani-Lu

Well-Known Member
Apr 20, 2007
1,352
Delray Beach, Fl
Boat Info
2001 410DA
Engines
3126 Cats
We are getting closer to retirement and our plan to buy a displacement or semi-displacement trawler for cruising 4 to 5 months or more per year in retirement. I am not sure we will like cruising at 8 kts, with a maximum speed of 10 kts, which is why we are also looking at semi-displacement trawlers. I also like knowing I have reserve power when needed, like string currents or rough inlets. I love docking my 2 engine boat, but many trawlers are built with one engine, with the shaft and prop protected by the keel. The single engine trawlers usually have bow and stern thrusters, but I don’t know if I will like or feel comfortable docking on one engine, even with thrusters.

Will I do harm to my Cats running at 8 kts all day or even 24 hours straight with the low gph burn rate? Will I need to run the engines up to cruising RPMs, during the run or at the end of the run to clean out whatever builds up when running at low rpms? The reason for this question is, I would like to use our current boat for the first year of cruising the coast/keys/bahamas, before making the move to a trawler. I would want to run our boat at trawler speed, to get a feel for this type of boating. Will I do harm to my engines or turbos running them at 8 kts?

Lastly, should I move this thread to the diesel section?

Thanks,

Jeff
 
Simple answer - it depends....

In general, running a big diesel slowly for long periods tends to make them last forever...... But, if the load on the engine is TOO small, it can result in 2 things, 1 - glazed bores/rings, resulting in either excess oil consumption, or fuel contaminated oil.... And 2- a condition I know (others may call it something else) as "wet stacking", combustion temperatures too low resulting in very incomplete combustion, resulting in unborn the fuel collecting in the exhaust or if you have one, the turbo housing - it's not super detrimental, but it does result in a very thick smokescreen when you crank up the throttle (it's unlikeley on a boat propulsion unit, it's far more common on diesel gensets that are grossly under loaded and left running for long periods)

I doubt you will have any issues, but if you want to be sure, just power up to 50% or more, for 20 minutes or so once a day....

Some fisherfolk have outboards that become troublesome from trolling at idle for hours on end... The fix is usually to run it flat out for ten minutes or so on the way back to the ramp.
 
Diesels do this all day everyday in 100’s of applications. Look At trawlers, tugs, military equipment, apu units, generators...the list goes on.

I would simply say on a boat with turbos you would want to bring up the RPM’s now and then so they get used.

I have trawled in our 460 for 5 days solid. I do think you should make sure your exhaust is set up correctly and that following seas wont be an issue.
 
Jeff,

Your engines have a sweet spot at about 1450-1550 rpm. The turbos are just beginning to work and the engines are under load but the fuel burn is down at about 6 gph. On my 450, that rpm setting is enough to begin to lift the bow but is well off plane and is about 12 kts but our hulls are different so I don't know what speed that would be for you.

I happen to run that speed a lot since we have a lot of local traffic and a long no wake zone to get to the Gulf of Mexico and open water. My oil samples come back perfect every year and my engines show no adverse effects from running at this level of output.
 
To echo what Frank says:
My wife likes to cruise at slower speeds and I often do this at about 1500 RPM on days when it is quieter (less boat traffic) in the Bay. While I generally like to cruise faster, I have come to appreciate the ability to interact more with passengers at the slower speeds. My 410 seems to really like that speed and although I don’t have flow meters on this boat, the burn rate seems to be really low just judging by my fuel usage.
Unfortunately during busier times in the Bay you take your life in your hands trying to cruise the narrow channels at the slower speeds.
As a side note: Years ago I was told that the speed limit in the State Boat Channel is 12 MPH. Yet anything under 25-30 MPH will get you swamped on the weeekends.
I did have flow meters on my old boat, and although that one had twin 5.7 gassers, the lower cruising speed in that boat resulted in a really low burn rate.
While it’s certainly tempting to go with a slower single engine Trawler style boat, I think I’d really miss the availability of power and the maneuvering of the twin engines, especially when negotiating a sloppy inlet.
 
We have twin 454s. We cruise at 12 to 1400 RPM for hours (Put on 200 +hours a year). Once every few days we go real fast for 10 minutes at 25 gallon a side real fast is expensive. At 1400 RPM we burn 3 to 4 gallons per side an hour and go 6 to 7 knots. Over the year we average 1 US gallon per knot. Being retire we are not in a hurry unless there is a storm or no place to anchor that night. Trawlers we have seen tend to run same speed we do at 1,000 RPM or less. Most small sail boats run at very slow RPM, go 4 knots and they do not seem to have engine problems. I have a nephew who has a 32 foot sail boat his wife referred their cruising as snarling.
 
I took my last Sundancer, and 1997 400DA with 3116 Cats all the way across France using the canal system. 5 mph really is the only speed you can travel at, we covered 1,500 miles like that, over 6 weeks in total, and when we got out to the Mediterranean, opened the throttles, no smoke, up on the plane and it ran perfectly.

Just my 2 cents worth.


Graham
 
Jeff, to answer your question, when I bought our boat with Cat 3406C's I was told by the Cat mechanic that the engines liked to be run at 89% throttle 20% of the time. That keeps things cleaned out and makes the Cats purr.

Now, to address the part where you asked about one engine vs two, displacement vs semi displacement, I'd suggest you charter a few different types of boats to see what you really like about them.

Also, buy your second boat first. By that I mean, many people buy a boat they like the looks of, only to discover it really doesn't meet their needs/wants. So they sell it after a year and buy what they should have bought first. That can get VERY expensive. So take your time, do your homework and walk lots of docks.

Then, buy your second boat first without making a mistake on buying the wrong boat first.
 
fwebster said:
Jeff,

Your engines have a sweet spot at about 1450-1550 rpm. The turbos are just beginning to work and the engines are under load but the fuel burn is down at about 6 gph. On my 450, that rpm setting is enough to begin to lift the bow but is well off plane and is about 12 kts but our hulls are different so I don't know what speed that would be for you.

I happen to run that speed a lot since we have a lot of local traffic and a long no wake zone to get to the Gulf of Mexico and open water. My oil samples come back perfect every year and my engines show no adverse effects from running at this level of output.
Click to expand...


Frank, I will have to check if it is the same sweet spot for my boat. Thanks, jeff
 
JVM225 said:
To echo what Frank says:
My wife likes to cruise at slower speeds and I often do this at about 1500 RPM on days when it is quieter (less boat traffic) in the Bay. While I generally like to cruise faster, I have come to appreciate the ability to interact more with passengers at the slower speeds. My 410 seems to really like that speed and although I don’t have flow meters on this boat, the burn rate seems to be really low just judging by my fuel usage.
Unfortunately during busier times in the Bay you take your life in your hands trying to cruise the narrow channels at the slower speeds.
As a side note: Years ago I was told that the speed limit in the State Boat Channel is 12 MPH. Yet anything under 25-30 MPH will get you swamped on the weeekends.
I did have flow meters on my old boat, and although that one had twin 5.7 gassers, the lower cruising speed in that boat resulted in a really low burn rate.
While it’s certainly tempting to go with a slower single engine Trawler style boat, I think I’d really miss the availability of power and the maneuvering of the twin engines, especially when negotiating a sloppy inlet.
Click to expand...


John, When I run at night I usually run at 8 kts. I feel it is a nice slow speed allowing for the reaction time you need at night. But not many boats waking me at night. But I agree, I have gotten rocked big time when idling in the state channel.
 
Gofirstclass said:
Jeff, to answer your question, when I bought our boat with Cat 3406C's I was told by the Cat mechanic that the engines liked to be run at 89% throttle 20% of the time. That keeps things cleaned out and makes the Cats purr.

Now, to address the part where you asked about one engine vs two, displacement vs semi displacement, I'd suggest you charter a few different types of boats to see what you really like about them.

Also, buy your second boat first. By that I mean, many people buy a boat they like the looks of, only to discover it really doesn't meet their needs/wants. So they sell it after a year and buy what they should have bought first. That can get VERY expensive. So take your time, do your homework and walk lots of docks.

Then, buy your second boat first without making a mistake on buying the wrong boat first.
Click to expand...


Great advice, on buying second boat first. My 410 dancer, was almost a 360 dancer. I realized the 410 was a better boat for our family and bought it. Had I bought the 360, the 410 would have been the 2nd boat. :)

I agree, renting a few different trawlers is a great idea. I think we are going to wind up with a twin engine, semi-displacement, so I have the power if and when needed, as well as for docking and less draft. I think we will most likel be coastal boaters, keys and bahamas, so a blue water single engine displacement trawler (Krogen, Nordhavn) is over kill and very expensive! I have a lot to learn, but we are getting there. I know I want a pilot house and a stand up ER, to live on the boat for 4 to 6 months a year. The question remains, where will our land based home be?
 
Cheapskate that I am, I always look for a good used boat that has been through most of the depreciation curve. Ours was 17 years old when we bought her. Condition is a much better indicator than age.
 
Based on the people we have met that go long distance a single engine will use less fuel. A diesel will use less than gas. The smaller the engine the less fuel. We once met a person who repowered a gas wood boat that was the same length and width as ours with a single small 4 cylinder Issu diesel. He could go 8 knots top speed and burnt 3 liters an hour. We burn 6 US gallons an hour at 6 to7 knots. If you are going to go far and live on the boat permanently: big, slow and stable are your requirements. There is a reason the Krogen, Nordhavn cost more. Once we met a person with a custom made boat. He called it his Alaska boat. It had two of every thing that was important to ensure he could keep going including two complete generators. Where you are going to boat is important. If you need AC 24 hours a day an efficient generator is required. If it is going to be cool and wet efficient heating system is required. We have seen a lot of fishing boat conversions. Some good others not so good. 40 foot fishing boats go slow, burn little fuel, can handle most weather and take a great deal of abuse. It boils down to how much you want to spend and how long you can be together with your partner before things deteriorate. When anchored it is difficult to go for a walk. When we decided to go further we went north for 6 weeks and anchored most of the time. Did not kill each other so we figured we could go longer.
 
Dani-Lu said:
John, When I run at night I usually run at 8 kts. I feel it is a nice slow speed allowing for the reaction time you need at night. But not many boats waking me at night. But I agree, I have gotten rocked big time when idling in the state channel.
Click to expand...
I do the same at night. Occasionally encounter small boats with no lights on out there and the extra reaction time is critical. My night vision isn’t as good as it used to be either. More easily blinded by ambient light than I used to be so I make sure to take it slow. These past few seasons I’ve encountered more boats running with their “headlights” on at night. Unbelievable! This season, out of frustration with their stupidity, I’ve lowered myself to aiming my spotlight right back at them a few times to give them a clue as to why boats don’t come with “headlights”.
Still nothing like a nice moonlight cruise on the calm bay though.
 
we run quite a bit at 1000-1050rpm. I will always run it up to 1750-1850 for about 10-20 minutes before the trip is over just to clean everything out. I also like to make sure that the turbos are spooling up and running clean.
 
Dani-Lu said:
Great advice, on buying second boat first. My 410 dancer, was almost a 360 dancer. I realized the 410 was a better boat for our family and bought it. Had I bought the 360, the 410 would have been the 2nd boat. :)

I agree, renting a few different trawlers is a great idea. I think we are going to wind up with a twin engine, semi-displacement, so I have the power if and when needed, as well as for docking and less draft. I think we will most likel be coastal boaters, keys and bahamas, so a blue water single engine displacement trawler (Krogen, Nordhavn) is over kill and very expensive! I have a lot to learn, but we are getting there. I know I want a pilot house and a stand up ER, to live on the boat for 4 to 6 months a year. The question remains, where will our land based home be?
Click to expand...

Our long range plan is the same, sell the house and live aboard full time. Renting is a great way to check out a few different types of boats to see what works and what does not for you. We have chartered a few from a company in Cape Coral, Florida. They have a good selection of new and old trawlers (and a 54 Sundancer to boot). They are a pleasure to deal with and reasonable cost. Might be worth a look. Southwest Florida Yachts - www.swfyachts.com. Also, we love boating in that area! Nice destinations and beautiful waters.
 
We have seen a lot of live a boards. Some have been full time residence at their yacht club. Yacht clubs once you are a member cost less than getting a slip at a private marina in Canada. One yacht club we have visited found the number of live a boards had increased to the point the club was become "Trailer Park for boats" The club implemented new rule "Your boat must leave the club under its own power for 48 hours twice a year" This resulted in several boats being removed from the club. We track what we spend on fuel, moorage away, food and drink. On our trip north we spent 134 a night and our trip in Washington we spent 128. Just saying living on a boat costs us more per year than having a house. We were on the boat a total of 3 months this year. Cost per day do not include maintenance or permanent moorage.
 
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