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Old 04-29-2019, 10:18 AM   #21
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"Those are two important victories, but getting 3.5 NMPG is huge."
We have owned 2 - 45' Bayliners and one 47 Bayliner with each one of them getting 3.5 nmpg at low speeds. Each one of them had twin engines and each one could easily run on plane for long distances. Hp for each was between 220 and 310 per engine dependent upon the exact year and model (1986 - 1995) - YMMV

My earlier "1.75 NMPG" comment was just from a quick re-glance at our performance curves... but yours made me look back at our actual records.

These are twin engine totals... based on averages of two-way observed speeds at given RPMs, and Cummins fuel curve predictions at those RPMs... so not metered values as from a FloScan, but...

At 5.8 kts, we'd be seeing around 1.2GPH/4.83 NMPG. That's a tad slow for my tastes. At 6.45 kts, it'd be 2.2 GPH/2.93 NMPG... and at 7.05 kts it'd be 3 GPH/2.35 NMPG. The calcs are of course not as precise as the two decimal places infer...

Our waterline length (nominal 42' boat) is either 35' 10" from our own owner's manual, or 43' 10.5" from the 2008 version of the owner's manual for the same model. Either way, I think our maximum theoretical hull speed is approx 8 kts... and at 8.20 kts, we'd be seeing 4.8 GPH/1.71 NMPG.

Anyway, it's hard to imagine an MS390 repower that would improve all that significantly enough to pay for itself.

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Old 04-29-2019, 10:30 AM   #22
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7 knots at Max cruise would be horrible. We ran from Nantucket to Woods hole this summer against a I had to run a mac cruising rpm, just to hold 5-6 knots. I should have been doing 11kts at that rpm.

Because we were much later than anticipated arriving at Woods Hole, Instead of being at slack tide, I was opposing the full tide. I crawled through at around 3.5 knots at max cruise rpm. A 7 knot boat would not have made it though Woods Hole.

I run our mainship 350 (same hull and layout as the 390) at 8 kts.
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Old 04-29-2019, 10:34 AM   #23
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My earlier "1.75 NMPG" comment was just from a quick re-glance at our performance curves... but yours made me look back at our actual records.

These are twin engine totals... based on averages of two-way observed speeds at given RPMs, and Cummins fuel curve predictions at those RPMs... so not metered values as from a FloScan, but...

At 5.8 kts, we'd be seeing around 1.2GPH/4.83 NMPG. That's a tad slow for my tastes. At 6.45 kts, it'd be 2.2 GPH/2.93 NMPG... and at 7.05 kts it'd be 3 GPH/2.35 NMPG. The calcs are of course not as precise as the two decimal places infer...

Our waterline length (nominal 42' boat) is either 35' 10" from our own owner's manual, or 43' 10.5" from the 2008 version of the owner's manual for the same model. Either way, I think our maximum theoretical hull speed is approx 8 kts... and at 8.20 kts, we'd be seeing 4.8 GPH/1.71 NMPG.

Anyway, it's hard to imagine an MS390 repower that would improve all that significantly enough to pay for itself.

-Chris
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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Old 04-29-2019, 10:37 AM   #24
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7 knots at Max cruise would be horrible. We ran from Nantucket to Woods hole this summer against a I had to run a mac cruising rpm, just to hold 5-6 knots. I should have been doing 11kts at that rpm.

Because we were much later than anticipated arriving at Woods Hole, Instead of being at slack tide, I was opposing the full tide. I crawled through at around 3.5 knots at max cruise rpm. A 7 knot boat would not have made it though Woods Hole.

I run our mainship 350 (same hull and layout as the 390) at 8 kts.

Yes - agreed. There are times when we go thru NYC and up the Hudson where traveling faster actually 'saves' fuel to some extent. The other option is to only run at favorable or neutral tides which would take a number of extra days and is not possible for the entire trip.
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Old 04-29-2019, 11:44 AM   #25
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When I was repowering looking at a 6 Volvo Penta 240 hp, Volvo put me through the ringer sending out a questionnaire that would have made a marine engineer proud. The issue wasn't whether the horse power would move my boat, it would. The problem was would the engine be too stressed for the boat size/weight/hull configuration.

Below is an article on gas versus diesel in boats but it also covers smaller engines doing bigger jobs, you might find it helpful:

https://www.yachtsurvey.com/GasDiesel.htm

Also, having more horsepower as you will see in the article can result in reduced fuel used, seems counter intuitive doesn't it? Take my boat with the 270 hp and now the 350 hp engine. To achieve 7 knots, guess which one has the higher rpm, yes - the 270 hp engine. So using a smaller engine to do the same job as a larger engine, stress it more in terms of heat and wear but also increased fuel needs. So that is why some here say "I increased the hp and my fuel burned went down." And this is why Volvo was giving me a hard time about putting in a smaller engine with the same output as the old engine, it would be put under more stress.
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Old 04-29-2019, 11:55 AM   #26
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Excellent stuff and a great example of the value this forum delivers. Thanks all.

Among the things that drew me to trawlers and cruising in general are the patience and advanced planning they demand. Previous facets of my life have relied on tides and weather and that has left me keenly aware of the benefits of waiting for the right time. I expect to go slow now only while under way, but also overall. We are definitely no hurry folks.

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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Old 04-29-2019, 12:09 PM   #27
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Excellent stuff and a great example of the value this forum delivers. Thanks all.

Among the things that drew me to trawlers and cruising in general are the patience and advanced planning they demand. Previous facets of my life have relied on tides and weather and that has left me keenly aware of the benefits of waiting for the right time. I expect to go slow now only while under way, but also overall. We are definitely no hurry folks.

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?
"Easy for me to say that today, but is it practical to cruise for years and wait out every front or tide?"

Depends entirely on where you are and where you are going.
In our case the answer would be No - but that certainly may not apply to anyone else.
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Old 04-29-2019, 12:11 PM   #28
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When I was repowering looking at a 6 Volvo Penta 240 hp, Volvo put me through the ringer sending out a questionnaire that would have made a marine engineer proud. The issue wasn't whether the horse power would move my boat, it would. The problem was would the engine be too stressed for the boat size/weight/hull configuration.

Below is an article on gas versus diesel in boats but it also covers smaller engines doing bigger jobs, you might find it helpful:

https://www.yachtsurvey.com/GasDiesel.htm

Also, having more horsepower as you will see in the article can result in reduced fuel used, seems counter intuitive doesn't it? Take my boat with the 270 hp and now the 350 hp engine. To achieve 7 knots, guess which one has the higher rpm, yes - the 270 hp engine. So using a smaller engine to do the same job as a larger engine, stress it more in terms of heat and wear but also increased fuel needs. So that is why some here say "I increased the hp and my fuel burned went down." And this is why Volvo was giving me a hard time about putting in a smaller engine with the same output as the old engine, it would be put under more stress.
"So using a smaller engine to do the same job as a larger engine, stress it more in terms of heat and wear but also increased fuel needs."
Diesel fuel loads should be about the same for the same boat speed - stress may be another thing altogether dependent upon the exact engine.
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Old 04-29-2019, 12:21 PM   #29
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When I was repowering looking at a 6 Volvo Penta 240 hp, Volvo put me through the ringer sending out a questionnaire that would have made a marine engineer proud. The issue wasn't whether the horse power would move my boat, it would. The problem was would the engine be too stressed for the boat size/weight/hull configuration.

Below is an article on gas versus diesel in boats but it also covers smaller engines doing bigger jobs, you might find it helpful:

https://www.yachtsurvey.com/GasDiesel.htm

Also, having more horsepower as you will see in the article can result in reduced fuel used, seems counter intuitive doesn't it? Take my boat with the 270 hp and now the 350 hp engine. To achieve 7 knots, guess which one has the higher rpm, yes - the 270 hp engine. So using a smaller engine to do the same job as a larger engine, stress it more in terms of heat and wear but also increased fuel needs. So that is why some here say "I increased the hp and my fuel burned went down." And this is why Volvo was giving me a hard time about putting in a smaller engine with the same output as the old engine, it would be put under more stress.
Excellent article and it makes perfect sense, especially in terms of life expectancy and reliability. In terms of age too. Makes me wonder where the balance lies between the benefits of sheer mass from old engines like the Lehman 120 and efficiency of new common rail engines.
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Old 04-29-2019, 12:26 PM   #30
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Depends entirely on where you are and where you are going.
In our case the answer would be No - but that certainly may not apply to anyone else.
Our plan is to take long enough to get to know the boat, then poke our way around the Loop taking as much time as we feel like. That might be a year and a half, or it might be several years. By my way of thinking, and knowing our travel habits on foot and by other modes, that means we will stay in port during marginal weather and plan our routes by tides and other current and traffic variables. Being in no hurry at all would seem to make it easier to avoid the avoidable hazards.
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Old 04-29-2019, 12:35 PM   #31
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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.
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Old 04-29-2019, 12:46 PM   #32
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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'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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Old 04-29-2019, 12:52 PM   #33
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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.
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Old 04-29-2019, 01:13 PM   #34
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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?
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.
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Old 04-29-2019, 01:34 PM   #35
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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

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.

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Old 04-29-2019, 01:59 PM   #36
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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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Old 04-29-2019, 02:23 PM   #37
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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.
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.

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

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Find the right boat for you, and then spend time with her and listen. She'll tell you what she needs...
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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Old 04-29-2019, 03:01 PM   #38
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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.


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 itís systems were probably neglected as well.
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Old 04-29-2019, 03:09 PM   #39
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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.

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/s...tml#post761468

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Old 04-29-2019, 07:59 PM   #40
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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?
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.
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