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Old 04-29-2019, 08:49 PM   #41
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Stop, stop stop

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Originally Posted by HeadedToTexas View Post
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?

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 doesnt 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
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Old 04-29-2019, 09:49 PM   #42
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If you are keeping the boat 'forever' and that is exactly what you want then you are missing nothing.

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.

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Old 04-29-2019, 11:09 PM   #43
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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.
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Old 04-29-2019, 11:26 PM   #44
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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.
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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Old 04-30-2019, 03:23 AM   #45
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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?
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.
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Old 04-30-2019, 06:55 AM   #46
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And your

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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.
Manufacturer?
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Old 04-30-2019, 06:56 AM   #47
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Also

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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.
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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Old 04-30-2019, 07:02 AM   #48
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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.

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Old 04-30-2019, 07:45 AM   #49
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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.
1500 rpm (not rpms) is very slow IMO and Im 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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Old 04-30-2019, 07:55 AM   #50
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1500 rpm (not rpms) is very slow IMO and Im 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?
1500 rpm is in the range for many diesels that run for generating power, pumping and hydraulics at extensive hourly usage.
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Old 04-30-2019, 07:56 AM   #51
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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
Never really had problems traveling at 7 knots for most trips except for those times when we were in 4-5 knot currents.
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Old 04-30-2019, 07:58 AM   #52
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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.
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Old 04-30-2019, 08:02 AM   #53
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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.
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Old 04-30-2019, 02:12 PM   #54
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You are WAY over thinking this.
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.
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Old 05-01-2019, 06:35 AM   #55
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"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.
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Old 05-01-2019, 07:10 AM   #56
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Hehe, fair enough. Thanks all for the input. Looking forward to the channel ahead.
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Old 05-01-2019, 08:53 AM   #57
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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.
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Old 05-01-2019, 09:32 AM   #58
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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.
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Old 05-01-2019, 03:18 PM   #59
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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 youre doing may be because you like a stiffer boat or the extra space usually found in SD boats.
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.
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Old 05-01-2019, 05:05 PM   #60
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Very. 2 x 54hp NA yanni's pushing 50,000lbs. 6kn cruise at 1800rpm, ~2gal/hr combined.

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