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

I have been contemplating the many issues surrounding choosing a trawler and cruising the Loop and other destinations. After lots of thinking time, I believe we are completely committed to cruising at 7 knots.

When I put all of our priorities into the grinder, I keep coming up with a Mainship 390 as it checks the most boxes of anything I've seen. The only real issue is the big diesel under the deck. I get the designer's objective, but I don't want to run at 15 knots, even for 20 minutes every few hours. Pushing a 390 through the water at 7 knots should only take 60 horsepower or so. Why not repower with a new, efficient Hyundai or even a classic old Lehman 120?

Buying a boat needing repowered means there's a good chance it will need air conditioners and gen set and electronic work. Seems to me like the right plan (for us) is to find one with a good hull and solid decks and take a year or so to refit what needs refitting with the right power to go slow. What am I missing?

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

Texas: I like the Mainship 390 a lot (the 400 trawler even more). I toured a 390 over the winter and was very impressed with it. I eventually moved on because I'm not a fisherman and saw the huge cockpit as a wasted space for me.

About your repower question: You'd really have to crunch the numbers on it. After spending $20k+ on parts/labor would it really be worth it? Why not just run the Mainships current "big" motor at hull speed? That's what many folks do. Also keep in mind that boats are designed with certain power plants in mind based on their hulls. I don't know if putting in a motor that's 1/4th the size of the factory motor would be the best call. But it's definitely doable. Anything is on the table if you've got the bucks to do it. Maybe consult with Mainship first and talk to a naval architect as well.

That's pretty much what I did.

The fuel consumption numbers from my motor swap are below. If you find a boat with a bad motor, it makes sense. Otherwise, most won't cruise the boat enough to recoup the investment.

Fuel consumption numbers

The thread about my refit project.

My short haul refit

Ted

Sometime you may be fighting a current and 7 knots isn't enough or trying to make a bar crossing before the CG closes it. If you run a higher hp engine slower it uses much less fuel. Maybe more fuel than a much smaller engine, but I bet never enough to cover the cost of engine replacement.

O C Diver said:

That's pretty much what I did.

Ted

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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.

Lepke said:

Sometime you may be fighting a current and 7 knots isn't enough or trying to make a bar crossing before the CG closes it. If you run a higher hp engine slower it uses much less fuel. Maybe more fuel than a much smaller engine, but I bet never enough to cover the cost of engine replacement.

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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?

I like the idea of repowering but it dosn’t usually result in a cost effective move.

There’s lots of ducks that could line up though.
The new engine can be a piece of cake for some and a nightmare for others. Exhaust can be easy, basically the same as the old engine or on the wrong side and no end of problems related.
On our repower I moved the engine mounts for better balance. I put the aft mounts back on the BW gear instead on on the bell housing. Worked out well.

A significant downside is the possible reduction in value at selling time. I wouldn’t worry about that because I believe there are buyers that would welcome smaller engines. Also taking old heavy engines (like FL’s) out and replacing w small lightweight engines much to huge weight reductions can be had. The engines would be more efficient just because they are new but the large reduction in overall boat weight will probably lower fuel burn more than anything else.

There could be unforeseen problems. An unlikely example is ....The boat could have had excessive engine vibration during R&D and been modified to cope w the vibration. But big old engines usually run at lower speeds and newer engines would most likely be running at higher engine speeds. A problem here would be hard to predict if the boat would like the higher rpm. This is perhaps a bad example as it’s not very likely to come to pass. But an example of an unforeseen potential problem.

As I said I like repowering as it gives the boat owner the option to make her what he/she really wants from a boat. And w so many trawlers out there that are overpowered w old heavy engines they are a natural for smaller engines and a repower.

I do not know about that specific hull but some semi displacement hulls are squirelly with a following sea and improve with a little more speed. It may bw worth checking with 350 and 390 owners in the mainship section.

You'd be way ahead to buy a boat powered the way you want it and avoid having to repower. I have been down this road with twins, and you shouldn't be surprised if the project cost is much higher than your highest estimate, both in the labor required and hardware cost. Lots of things pop up and the meter is running. The only case to be made for buying a particular boat for repower is if it's a very nice example and you're getting it real cheap.

Gdavid said:

I do not know about that specific hull but some semi displacement hulls are squirelly with a following sea and improve with a little more speed. It may bw worth checking with 350 and 390 owners in the mainship section.

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Right. I run my old Mainship 34 at displacement speeds in flat water. But in any sort of waves the ride and handling are much improved by more throttle. There are lots of times speed is your friend.

I bought my specific Bayliner 4788 because it needed a repower.

The OEM engines were 330 Cummins. I could have easily put 220 HP Non turbo 200 HP cummins engines in the boat and save a few bucks. Probably save some fuel as well.

Glad I went with the factory reman 330’s, having 8 years under the keel.

There are times when it makes sense to trade fuel dollars for speed. Not every journey, but they happen, and I’m glad to have the reserve horsepower.

I have repowered and retanked and new stern drive. My boat is semi displacement and was powered with an old 50 years old Merc 270 hp stern drive. Turns out the 270 hp is only 240 hp as in those days hp was measured at the engine, now it is measured at the prop and you lose about 10 % of hp in the conversion. The engine was horribly over propped by the previous owner but even so it achieved 11.2 knots. My fuel tanks (2) were a total of approximately 104 gallons and I went up to 160 gallons.

For the area I cruise, most of the time I'm happy at around 7 - 8 knots but I like to go quickly at the beginning and end of the cruise. Crossing the Strait of Georgia from Comox to Powell River is a bit boring, especially after you have done it a number of times so I like to move quickly until I reach Powell River, then slow down. I also like the extra power so I have moved up to a Merc duoprop 3 with 350 hp measured at the prop. This will get me up on plane and get me out of bad weather quicker, take me through bad weather better, through strong currents the like of which we have around here.

The extra fuel tanks will not be filled most of the time, I don't like a lot of fuel sitting around getting old, even with stabilizer added. But I have the option to fill up for long distant jumps or to move quickly to a location while I burn boating dollars in fuel.

If your intent is low V/L cruising, long distance, ocean or long coastal passages, occasionally rough water, and owning for a long time, then perhaps skip the semi-displacement Mainship and look for a full displacement vessel, or one with better buttock lines and a smooth transition astern.

BTW, is that Mainship a single diesel?

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.

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Most engines in these ranges have a standard size bell housing (I believe a SAE #3). For this housing, the transmission has a standard adapter plate that allows it to bolt up to any SAE #3 engine. Having an industry standard makes it easy. In reality I would have been better off with a different transmission ratio. Mine is approximately 2.5:1 and ideally a 4:1 would be perfect. You have a lot less HP, so you want to make the engine run at its normal operating RPM for your 7 knot cruise. Unfortunately, my boat needs a down angle transmission and 2.5:1 is about the tallest gearing I could get. While I did get a flatter pitched prop, the engine still cruises about 300 RPM below optimal.

The engine I removed was quite a bit larger, very desirable in my area, had only 1,000 hours on it, and looked like it had been sitting on the showroom floor. Getting $15,000 was pretty much its market value. Doubt you would get much for the one out of the Mainship.

Ted

HeadedToTexas said:

When I put all of our priorities into the grinder, I keep coming up with a Mainship 390 as it checks the most boxes of anything I've seen. The only real issue is the big diesel under the deck. I get the designer's objective, but I don't want to run at 15 knots, even for 20 minutes every few hours. Pushing a 390 through the water at 7 knots should only take 60 horsepower or so. Why not repower with a new, efficient Hyundai or even a classic old Lehman 120?

What am I missing?

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You'd likely never recoup the cost of repowering by using only slightly less fuel than if you just run the boat at 7 kts.

HeadedToTexas said:

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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If you look at engine fuel curves, you'll see fuel consumption at low RPMs is also low. Even with twin 450s, our curves suggest about 4 gallons/hour (total), or about 1.75 NMPG, at 7 kts. So if you repower, you might get down to 2 GPH? (Really?) At what cost? How long to amortize? Is an extra 2 GPH (really?) a big deal -- if the other features of the 390 floats your boat?

I don't see much consequence of running at low RPMs, and usually only bother to run at higher RPMs when more speed is also useful for whatever reason. Usually sea states.

Jeff F said:

Right. I run my old Mainship 34 at displacement speeds in flat water. But in any sort of waves the ride and handling are much improved by more throttle. There are lots of times speed is your friend.

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Ditto. It's not like the ole 34 could get up and fly, but there were certainly times when rocking and rolling at 7 kts just wasn't comfortable and a bit more speed, maybe coupled with some tacking, could make it better.

ksanders said:

There are times when it makes sense to trade fuel dollars for speed. Not every journey, but they happen, and I’m glad to have the reserve horsepower.

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Yep. For example, 7 kts on the Chesapeake from Annapolis to Norfolk... is a long slog. Running at 18 kts means I can at least get that over with within a given 1-day weather window if necessary... instead of hopping down one leg at a time and maybe with lay days in between for weather.

Another example, the Cape Fear River down by Southport was a bit snotty when we got to it. OK, not to worry, we just got up on plane for about an hour, fixed.

-Chris

When I repowered my "ex" old 34 Mainship model 1 it made it a whole new boat.
Went from original 160 Perkins to 270 Cummins.
Speed AND economy improved greatly. So did handling in most sloppy seas.
Best I could get with the Perk was about 2.3 NMPG at 7 knots.
With the new power, I could get 3.2 NMPG at 7 knots and 2.3 NMPG at 13 knots.
Original engine fastest cruise speed was about 9 knots. New engine fastest cruise seed was 15.5 knots.
In typical following/quartering seas at the old cruise speeds the boat would "wallow" in each trough. With the new speed it could stay straight.
No comparison. Go for power.

Great perspectives and feedback, folks. Thank you.

This Mainship 390 needing repowered is just an idea or a search objective. I've got years to go before buying, but I see them from time to time priced $60,000 or more below the normal market value for this model. Seems at that point, one could come out about even once the cost of repowering was added back in. As many of you have pointed out, repowering to the same or even more power may be smart.

And there is certainly a broad array of options out there. When all the compromises of those models are considered, the 390 keeps arriving at the top of my list, hence the focus on that model. The KK39 is up there too, but will probably remain too far above budget when buying time arrives.

HeadedToTexas said:

I have been contemplating the many issues surrounding choosing a trawler and cruising the Loop and other destinations. After lots of thinking time, I believe we are completely committed to cruising at 7 knots.

When I put all of our priorities into the grinder, I keep coming up with a Mainship 390 as it checks the most boxes of anything I've seen. The only real issue is the big diesel under the deck. I get the designer's objective, but I don't want to run at 15 knots, even for 20 minutes every few hours. Pushing a 390 through the water at 7 knots should only take 60 horsepower or so. Why not repower with a new, efficient Hyundai or even a classic old Lehman 120?

Buying a boat needing repowered means there's a good chance it will need air conditioners and gen set and electronic work. Seems to me like the right plan (for us) is to find one with a good hull and solid decks and take a year or so to refit what needs refitting with the right power to go slow. What am I missing?

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Headed,

You really don't NEED a high speed option, but some folks WANT speed and that's fine. There sure are a few times that speed comes in handy, and I can think of a few on the Loop, but can be done just fine at 7 knots (I did it that way).

I could argue strongly that downsizing doesn't make a lot of sense. I'd bet that you won't get much better MPG and the cost could be significant, with a lot of hidden extras.

Basically, it take xxx hp to move the boat xxx speed. And if you use a small engine or a large engine to get that xxx, there really isn't enough difference in fuel to be significant for coastal cruising and the loop. Sure, there may be some efficiencies in the smaller engine, but you have to factor in the prop, transmission, hull design and probably a lot more.

When I did the Loop in the Mainship 400 (very similar to the 390) I got 2.5 smpg over the whole trip, including going fast, slow, no wake zones, etc. After some observation and comparing notes, gut feeling the Cummins is slightly better on fuel than the Yanmar which I have, and the 390 and the 350 get slightly better mileage. However, not close to scientific, just a gut feeling.

But FWIW, I did have an acquaintance that ran his single engine Mainship 400 side by side with a twin of the same model, and over several hundred miles, the twin burned less fuel. Go figure.

As to the requirement to run at high speed.... there is no requirement, at least not in the Yanmar maintenance manual. Assuming thats to burn out the carbon from operating at low speed. The manual says to "race" the engine in neutral, going from high rpm to low rpm about 5 cycles. You can run at high speed and most likely do the same, but no requirement.

SeeVee,
The rule I frequently use for fuel efficiency thinking is how much power per ton required. If you take out weight the boat gets easier to push .. less power required. You can loose way over a ton removing just one FL. But removing two probably make a whole new feel to the boat and would most likely handle much better.

"But FWIW, I did have an acquaintance that ran his single engine Mainship 400 side by side with a twin of the same model, and over several hundred miles, the twin burned less fuel. Go figure."

Heard that before but there are many variables. Just the cruising speed of one skipper over another could produce the statement you made above. But you say "go figure" meaning it shouldn't happen I'm guessing.

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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"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

smitty477 said:

"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

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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

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.

ranger42c said:

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

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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

Shrew said:

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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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.

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 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?

HeadedToTexas said:

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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"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.

rsn48 said:

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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"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.

rsn48 said:

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.

Click to expand...

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.

smitty477 said:

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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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.