Repower? How committed are you to life at 7 knots?

I may be the only one on TF that doesn't track fuel mileage. When we bought our boat I knew it wasn't going to be a fuel skimping boat but I also knew that fuel is probably one of the smallest costs of owning a boat.


Our boat has twin Cats, turbo'd at 580hp each. My hull speed is just about 9kts. AT idle I'm running about 7kts and if I want to get below that in a no wake zone I run with one engine in neutral.


My cruising speed is between 17-22kts, depending on how much of a hurry I am in. The boat does well at both speeds, rides nice, handles, well, etc. WOT on my boat is 30.3kts and we verified this during the sea trial when we bought her 9 years ago.


Since we boat on a river that usually has a 2.5kt current, being able to run at 7kts was not something I wanted to do. When we hit the Snake River in the spring the current can easily be 4.5-7kts. Not something I'd want to do with a 7kt boat.


About 90% of our cruising is done at just above idle unless we're heading upstream then I'll kick it up to about 1000 rpm's and that puts us about 10kts.


I guess my point is that the speed is there if I want/need to use it, but the higher fuel economy is there if we're just cruising to enjoy a day on the water.

GFC said:

About 90% of our cruising is done at just above idle unless we're heading upstream then I'll kick it up to about 1000 rpm's and that puts us about 10kts.

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I've run small center consoles all day at 30-40 knots, but running that big thing at 30 knots would be something to feel. Quite impressive. So if 90% of your time is at idle or just above, how often do you rev up to address carbon build up concerns?

Or maybe that is just one of the diesel myths I need to dismiss?

My first question is the boat in question NEEDING a repower??? If that is the case then it is a totally different story. But if you are taking out a perfectly good engine then I would not do that.

Also you say you are in no hurry on the loop. The weather/seasons push you around the loop. If you take more than one season then you will be leaving your boat somewhere over the winter...maybe that is what you were referring to.

HeadedToTexas said:

I've run small center consoles all day at 30-40 knots, but running that big thing at 30 knots would be something to feel. Quite impressive. So if 90% of your time is at idle or just above, how often do you rev up to address carbon build up concerns?

Or maybe that is just one of the diesel myths I need to dismiss?

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Not so sure that is an issue with all diesels. It is with my 12v71’s, but they were designed in the 30’s and are an all mechanical injection system. Any engine with a computer should be able to run at idle indefinitely, although the manufacturers handbook is the ultimate guide.

I have been told by several Detroit experts that the real reason to run on plane every few days is to put enough heat into the engine to evaporate any condensation in the engine oil since they run so cool at 1050rpm. I’ve also been told that Detroit’s will never really warm up at idle in neutral and need a load to work against to come up to full operating temperature.

smitty477 said:

That makes sense as the speed is the key in most all of these calculations.
taking some liberties for rounding to allow easy recall our typical numbers were like this...
4 nmpg at 6 knots
2 nmpg at 8 knots
1 nmpg at 10 knots
0.90 nmpg at 16 knots

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Yep, close enough. And down to 35 GPH/23 kts at 2400 RPMs, and 48 GPH/26-ish kts @ WOT. Clean hull and gear, light load, etc.

-Chris

Find the right boat for you, and then spend time with her and listen. She'll tell you what she needs...

Baker said:

My first question is the boat in question NEEDING a repower??? If that is the case then it is a totally different story. But if you are taking out a perfectly good engine then I would not do that.

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I have seen several boats for sale far below market value because they need serious engine work and my interpretation of those is that they are candidates for repower projects, though this thread is helping me understand why repowering to a lower power engine may not be the right approach.

I am looking forward to a project boat as I have a number of quirky desires that don't seem to be part of the standard equipment packages on 20-30 year old boats. It had occurred to me that the engine may be one components that could benefit from a modern replacement. Maybe not though.

Baker said:

Also you say you are in no hurry on the loop. The weather/seasons push you around the loop. If you take more than one season then you will be leaving your boat somewhere over the winter...maybe that is what you were referring to.

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I understand the endless summer aspect of Loop timing and progress. A multi-year Loop would certainly involve over winter storage.

bridaus said:

Find the right boat for you, and then spend time with her and listen. She'll tell you what she needs...

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I have doubt of that and look forward to listening, but the opportunity to buy a boat who, at least temporarily, can't talk and teaching her to talk is pretty attractive. Guess what I mean is buying a boat whose engine needs rebuilt or replaced, air conditioners need replaced, refrigerator, batteries, genset, electronics, anchor all need replaced, would give me the opportunity to choose what I want in all those roles rather than learning to live with what someone else wanted 20-30 years ago. Or at least that is the strategy I am exploring.

HeadedToTexas said:

I have seen several boats for sale far below market value because they need serious engine work and my interpretation of those is that they are candidates for repower projects, though this thread is helping me understand why repowering to a lower power engine may not be the right approach.

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Chances are better than not that a boat well below market value that needs to be repowered also needs a whole lot more than new engines. If the engines were neglected, the remainder of the boat and it’s systems were probably neglected as well.

HeadedToTexas said:

I have doubt of that and look forward to listening, but the opportunity to buy a boat who, at least temporarily, can't talk and teaching her to talk is pretty attractive. Guess what I mean is buying a boat whose engine needs rebuilt or replaced, air conditioners need replaced, refrigerator, batteries, genset, electronics, anchor all need replaced, would give me the opportunity to choose what I want in all those roles rather than learning to live with what someone else wanted 20-30 years ago. Or at least that is the strategy I am exploring.

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Could be an OK strategy -- depending on the starting condition of whatever you buy -- if you can do (some of? most of? all of?) the work yourself and if you enjoy doing the work, etc.

But it can be a boatload of work, with a leaking wallet, and maybe with no actual "boating" involved for the first several (weeks? months? year? years?)...

If you haven't already, see here for related discussion:
http://www.trawlerforum.com/forums/s30/budget-advice-buying-old-vs-newer-44033.html#post761468

-Chris

HeadedToTexas said:

I've run small center consoles all day at 30-40 knots, but running that big thing at 30 knots would be something to feel. Quite impressive. So if 90% of your time is at idle or just above, how often do you rev up to address carbon build up concerns?

Or maybe that is just one of the diesel myths I need to dismiss?

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I don't know if it's a myth or not. But the other 10% of the time we're moving is in a high rpm range and high speed. My WOT rpm is 2150 and the cruise rpm is around 1700. I get the ol' girl up on plane and run her for awhile from time to time, but don't run anywhere near 2150. At WOT she's gulping 65 gph. That's about .5mpg. YIKES!

A few years ago we were headed up the Snake River with some other boats from our club. We were last in line because the others had passed us. I "snuck" up behind the boat ahead of me, coming up at about 25kts (against about a 4kt current), about 3kts faster than the other boats. When I got about 50 yards behind the one ahead of me I hit the horn for a second or two. He didn't know I was there and told me later it scared the bejesus out of him.



BTW, here's a wake shot taken on the sea trial while running at 30kts. Too bad you can't see me in this photo or you'd seen the HUGE smile on my ugly mug.

Stop, stop stop

HeadedToTexas said:

Wow, great outcome. That's exactly what I had in mind. Is it unusual to find engines that mate up to transmissions with no modifications? And is it unusual to sell an old diesel for $13,000? Those are two important victories, but getting 3.5 NMPG is huge. Very nice.



Fighting currents with a set up that maxes out at hull speed is one of those things I was missing. Good point.

Until this repower thing cropped up, I had intended to simply run the huge diesel at 7 knot speed and just grin and bear the gulping fuel every few hours at high revs. Is that really such a big deal? How does one run at higher revs for 20 minutes every few hours on the Dismal Canal or the Erie Canal? What are the consequences of not running at high revs but once a month or so?

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You are WAY over thinking this. Your boat will burn only a little more producing 60 hp than a smaller motor. Period. It takes a given amount of energy to push your boat through the water. It doesn’t much matter if you have one engine or four. Diesel produces the energy.

Just drive your boat slow. And no, you DO NOT have to run your engines at 80 percent or 60 percent. You WILL NOT hurt your engine by running slow. If you want to run it fast periodically, go ahead. But you are not helping or hurting the engine by doing so.

So you can spend thousands of dollars and hope that in your lifetime you could recoup the cost and aggravation of swapping engines, or just go out and enjoy your boat.

Most of our engines will go 10000 to 20000 hours or more if properly maintained. I am guessing my 2003 boat will go to the scrap heap with working Cummins 330s. I now have 1850 hours on the boat running at 8 -8.5 knots. And I put on about 300 hours per year.

We just finished a lengthy thread discussing this very thing a few weeks ago. But what we are dealing with is physics, I.e. the amount of energy required to move a given weight through water.

BTW, I just returned to Virginia from a winter on the Bahamas. My 50000 pound boat! Using engines rated for a total of 660 hp used only about 4.5 gallons an hour. If I had had one engine I might have burned 4.45 gallons an hour, but sometimes it is nice to have reserve power.

Gordon

smitty477 said:

If you are keeping the boat 'forever' and that is exactly what you want then you are missing nothing.

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I keep going back to the above comment.

I think if you repower a SD with an engine much smaller than the original engine you will have a difficult time selling the boat if you ever decide to sell.

I think that most folks who NEVER wanted to cruise faster than 7 knots, would simply buy a full displacement boat.

Jim

JLD,
Yes but there’s few FD boats to choose from and typically the SD boats have more space on board and are slightly cheaper. But I think the price range is a function of perceived quality of a specific boat in SD boats.

Gordon J said:

Just drive your boat slow. And no, you DO NOT have to run your engines at 80 percent or 60 percent. You WILL NOT hurt your engine by running slow. If you want to run it fast periodically, go ahead. But you are not helping or hurting the engine by doing so.

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My engine manufacturer understands their engines pretty well (!) and definitely state not running them "slow", and also revving them every so often (they do define both better than that and I'm paraphrasing).

Of course your engines and manufacturer may not so state. But it's not fair to others to make your blanket statement.

HeadedToTexas said:

To that end, I hope to avoid issues like entering passes or rivers against tides or crossing open water in unfavorable forecasts. Easy for me to say that today, but is it practical to cruise for years and wait out every front or tide?

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This brings up not only the question of what speed you may want, but the question of how much reserve power do you personally need?

For example, personally I never had need for much. The thousands of miles of ocean travel was always at one speed. Heavy tidal currents... waited them out. But I didn't live in Bay of Fundy, Anchorage or other challenging areas like some here. My boat was powered at 6hp per long ton. If I repowered I would install half that (and will on my new build).

Regarding to "crossing open water in unfavorable" weather. When the seas build past 12ft, hitting 20ft or more, are you really going to want tons of reserve power to battle against the angry Gods? No way. You'll power back to 4 knots and focus on surviving.

Mr. HeadtoTexas raises very practical comments and questions here, not theoretical.

And your

mcarthur said:

My engine manufacturer understands their engines pretty well (!) and definitely state not running them "slow", and also revving them every so often (they do define both better than that and I'm paraphrasing).

Of course your engines and manufacturer may not so state. But it's not fair to others to make your blanket statement.

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Manufacturer?

Also

mcarthur said:

My engine manufacturer understands their engines pretty well (!) and definitely state not running them "slow", and also revving them every so often (they do define both better than that and I'm paraphrasing).

Of course your engines and manufacturer may not so state. But it's not fair to others to make your blanket statement.

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Please define slow. I run my engines at 1500 RPMs. Not really slow for these engines which do 2800 at WOT.

While traveling at 7 knots clearly isn't for everyone, there are a tremendous amount of monohull sailboats and displacement hull trawlers doing it. I have 14,000 miles on my boat at 7 knots or less without issue, but then I'm a patient person.

Ted

Gordon J said:

Please define slow. I run my engines at 1500 RPMs. Not really slow for these engines which do 2800 at WOT.

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1500 rpm (not rpm’s) is very slow IMO and I’m sure the manufacturer mentioned above would agree. My Mitsubishi S4L2 is rated at 3000rpm. Only 200rpm less than yours. 1400rpm is the engine speed that I come out of the marina fairways and held for 5-10 minutes for warm up. I basically always run at 2300rpm for cruise and on rare occasions 2000. Would never run at 1500rpm all day.
What is your engine load at 1500rpm?

Nomad Willy said:

1500 rpm (not rpm’s) is very slow IMO and I’m sure the manufacturer mentioned above would agree. My Mitsubishi S4L2 is rated at 3000rpm. Only 200rpm less than yours. 1400rpm is the engine speed that I come out of the marina fairways and held for 5-10 minutes for warm up. I basically always run at 2300rpm for cruise and on rare occasions 2000. Would never run at 1500rpm all day.
What is your engine load at 1500rpm?

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1500 rpm is in the range for many diesels that run for generating power, pumping and hydraulics at extensive hourly usage.

O C Diver said:

While traveling at 7 knots clearly isn't for everyone, there are a tremendous amount of monohull sailboats and displacement hull trawlers doing it. I have 14,000 miles on my boat at 7 knots or less without issue, but then I'm a patient person.

Ted

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Never really had problems traveling at 7 knots for most trips except for those times when we were in 4-5 knot currents.

I have no idea what the engine load is. And, I guess I am not concerned about it’s loading. 1500 RPM puts me at just about Hull speed.

Past boats cruised at 20 K but I often cruised at hull speed. Didn’t see a big need to go fast and burn a bunch of fuel on a lot of levels, including environmentally. I know travel at 6 to 7 K all of the time and enjoying the journey.

Gordon J said:

You are WAY over thinking this.

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I am definitely prone to way overthinking things and this is likely no exception. The reality that pushing a 20,000 pound boat through the water at 7 knots takes 60 HP whether that 60 HP is developed by a 330 HP diesel or a 75 HP diesel now makes complete sense. Not sure I completely bought or understood that a couple days ago when starting this thread.

My thinking on this stems from the timing of our life's boat chapter. I plan to retire a few years before my wife and I see that as time to find and outfit the right boat. The term "outfit" started as updating electronics, but grew in scope as I learned more and more about systems, power, electricity, plumbing, etc. Again, likely overthinking.

Still, I find the idea of outfitting a boat with everything I want quite attractive. Most everything I own is customized to some degree (I got that from my Grandfather), so I'm predisposed to seeing things like boats or homes or cabins more as a blank slate than a finished product.

"so I'm predisposed to seeing things like boats or homes or cabins more as a blank slate than a finished product"

Thats a great concept after a couple of years of cruising teaches you what you like and dislike.

"'I am definitely prone to way overthinking things and this is likely no exception. The reality that pushing a 20,000 pound boat through the water at 7 knots takes 60 HP whether that 60 HP is developed by a 330 HP diesel or a 75 HP diesel now makes complete sense. Not sure I completely bought or understood that a couple days ago when starting this thread."

There will be many many more surprises ,RELAX.

Hehe, fair enough. Thanks all for the input. Looking forward to the channel ahead.

I like seeing non-turbo engines at cruising speed to be making about half of their max power. So a Ford Lehman that could make 120hp it is nice to cruise at 60hp and 3gph. Engine is loaded enough there to keep clean, and noise is not too bad. Probably in the 1500-1800rpm range, depending on the boat and propping.

With regards to high output engines (turbo, aftercooled, etc), there is nothing wrong with running them at 60hp either. Sure they can make say 300hp, but running at 60hp engine makes plenty of heat to be happy. The turbo and aftercooler are there, but basically doing nothing to help nor hinder the engine.

Most of the trawler engines are in the 6-10 liter range, and whether they are turbo or not, the burn rate for making 60hp is going to be very similar, probably all within 10-15% between the best and the worst. Not worth fussing over.

Many thumbs up for the above post by Ski in NC!
purrfect

Smitty wrote;
“1500 rpm is in the range for many diesels that run for generating power, pumping and hydraulics at extensive hourly usage.”

Yes very common but most all experience fairly high loading often in-between the hours of low loading. And of the diesel engines that fail due to underloading are mostly generators that run too long w/o being loaded.

Fletcher500 wrote;
“I know travel at 6 to 7 K all of the time and enjoying the journey.”
Then IMO you should have a big Kady Krogen or Willard.
But doing what you’re doing may be because you like a stiffer boat or the extra space usually found in SD boats.

Nomad Willy said:

Fletcher500 wrote:
“I travel at 6 to 7 K all of the time and enjoying the journey.”

Then IMO you should have a big Kady Krogen or Willard.
But doing what you’re doing may be because you like a stiffer boat or the extra space usually found in SD boats.

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Some thread drift, but the OP appears to have his answer.

We chose the boat based mostly on the layout internally, as well as externally in regards to fishing and diving.

The Hull is actually closer to a FD, then SD and likes hull speed best. Throttling up over about 1,500 RPM just digs the stern in, creates a big wake, and excessive fuel use. IE, the boat will do 8-10K if needed, but its happiest place is 6 - 7.5K IMO.

I knew this going in, and it fits our cruising style. I have been in a rush for the past 30 years with the job, family, etc. Slowing things down has been a good thing.

Very. 2 x 54hp NA yanni's pushing 50,000lbs. 6kn cruise at 1800rpm, ~2gal/hr combined.