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Old 09-24-2022, 12:46 PM   #1
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Fast vs Slow

Haven't been on here for a long while but I'm having a relapse of the boat disease. Trying to decide if I want to buy another boat and do the Loop. Been looking at single engine slow boats to twin aft cabin boats. The two boats that I keep coming back to are the Heritage East 36 and the Sea Ray 420 aft cabin. Obviously these are two very different boats. Can anyone tell me what kind of fuel burn I would be looking at if I ran the Sea Ray at 10 - 12 knots? What about maintenance on two engines vs one, etc.? Previous boats were a Catalina 22, a Pearson 34-2, a CHB 34.
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Old 09-24-2022, 01:07 PM   #2
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Sea Ray at 10-12 knots probably about as bad as if on plane.

I'd guess a 420 has a waterline length of maybe around 36-38' (although you could find the real number) and if so that'd be a least expensive speed of about 6-8.25 kts.

Two engines, twice the maintenance. OTOH, not necessarily horrible... and with a built-in "get home" (or at least get somewhere) capability.

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Old 09-24-2022, 01:42 PM   #3
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The Sea Ray hull is designed as a planning hull. That means the hull will be plowing water until it can get up on plane. in doing that its fuel efficiency will be very poor at 8-10 knots and as the engines pick up speed to get the hull at 14-15 knots you would be burning a lot of fuel getting it up there.

I tested a Sea Ray 420 several years ago. The bottom line is that compared to true trawler hulls it burned a lot of fuel at any speed.
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Old 09-24-2022, 01:45 PM   #4
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Planing hulls can run efficiently at low speed although some won't handle well when going too slowly. For many planing hulls you'll start to plow and waste fuel once you get within a knot of hull speed. We usually slow cruise around 6.5 kts in our 38 footer for this reason.
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Old 09-24-2022, 01:53 PM   #5
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Any issues with engine problems due to running at slow RPM for extended periods?
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Old 09-24-2022, 01:55 PM   #6
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We run our 41PC mostly around 10 mph. We are getting about 1 mpg. We have twin Cummins 6CTAs that are 450hp. That is fast enough so that the steering has firmed up. What engines does the Sea Ray have in it? Is it cored below the waterline? If so be very careful in checking it out.
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Old 09-24-2022, 02:19 PM   #7
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Any issues with engine problems due to running at slow RPM for extended periods?
Not particularly, if you have it running at correct operating temps during all that.

Pays to check for each engine/manufacturer, but generally once the coolant and oil temps are in the correct operating range everything's good.

Some recommend a period of higher speeds/RPM at the end of a long slow run... but even that may vary depending on whether inter/aftercooled with turbo -- or not -- and so forth.

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Old 09-24-2022, 02:21 PM   #8
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We run our 41PC mostly around 10 mph. We are getting about 1 mpg. We have twin Cummins 6CTAs that are 450hp.

That's surprisingly low. We could get closer to 2 NMPG -- with the same 450Cs in our previous 42 -- running at ~8-ish kts.

That's from the fuel curve on the Cummins data sheet though (compared to actual speeds through the water at various RPM), not from a Floscan or similar.

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Old 09-24-2022, 02:26 PM   #9
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Engines run the gamut from the 300 horse 3126 Cats, 430HP Volvos to the 430 horse Cummins. We haven't narrowed down a particulay boat yet. I believe that hull is cored. How do you like the Cummins engines? My experience with engines is Yanmar on the sailboat and the old Ford Lehman on the trawler.
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Old 09-24-2022, 02:31 PM   #10
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Makes sense. I would think you would want to blow it out a little after a long low speed run.
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Old 09-24-2022, 02:49 PM   #11
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That's surprisingly low. We could get closer to 2 NMPG -- with the same 450Cs in our previous 42 -- running at ~8-ish kts.

That's from the fuel curve on the Cummins data sheet though (compared to actual speeds through the water at various RPM), not from a Floscan or similar.

-Chris
It is my best guess, never really measured it accurately.
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Old 09-24-2022, 02:52 PM   #12
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Engines run the gamut ... to the 430 horse Cummins. We haven't narrowed down a particulay boat yet. I believe that hull is cored. How do you like the Cummins engines?

Our experience with the 6CTAs (450Cs; 450hp/430bhp) was good, we'd have 'em again if we found a boat -- appropriate for that power range -- that suited us.

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Old 09-24-2022, 03:04 PM   #13
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Makes sense. I would think you would want to blow it out a little after a long low speed run.
It's not a bad idea. How much it's needed or how frequently will depend on the engines in question and just how slow you're running them. Some tolerate light loads well, others load up more and need to be burned clean periodically.
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Old 09-24-2022, 03:49 PM   #14
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Engines run the gamut from the 300 horse 3126 Cats, 430HP Volvos to the 430 horse Cummins. We haven't narrowed down a particulay boat yet. I believe that hull is cored. How do you like the Cummins engines? My experience with engines is Yanmar on the sailboat and the old Ford Lehman on the trawler.
I don’t like cored hulls, fairly big chance of water getting in. We love the 6CTAs. I don’t care for Volvos, poor part support. We had Lehmans in our last boat and loved them. Great support from American Diesel.
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Old 09-30-2022, 03:01 PM   #15
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Everyone has an opinion, so here's mine. You shouldn't be focusing on fuel burn to drive your purchase decision. If you are planning to do the loop and have time that you don't need to rush, speed is probably not that important. Define the mission for the boat and what things you want and don't want. Worry about fuel burn later.
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Old 10-01-2022, 03:47 AM   #16
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Slow could save you lots of money on fuel, but unless you want to travel an unreasonable number of hours each day, the number of marina nights on your itinerary will increase. This could more than offset the fuel savings of slow (unless you're very much into anchoring). On the plus side, slow gives you a quieter ride and the opportunity to see more places, resulting in a more enjoyable trip overall, in my opinion.
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Old 10-01-2022, 05:17 AM   #17
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Slow could save you lots of money on fuel, but unless you want to travel an unreasonable number of hours each day, the number of marina nights on your itinerary will increase. This could more than offset the fuel savings of slow (unless your very much into anchoring).
The Loop is typically done in around a year. Part of this has to do with the desirability of being North in the summer and South during the winter. An average trip is probably around 6,000 miles. If you travel every 3rd day, that's about 120 cruising days averaging about 50 miles a day. Pretty reasonable to do slow or fast.

For most Loopers, the trip is sightseeing with a commitment to being in certain regions at certain seasons of the year. Reducing the number of days kind of defeats the purpose of the trip.

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Old 10-01-2022, 08:14 AM   #18
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Been brought up on non turbo naturally aspirated engines. The mantra was 15 minutes every 4 hours. Then turbo the same. Now turbocharged, aftercooled and common rail and I’m told by half the mechanics it doesn’t matter and by half it still does. So generally do at least 10 minutes of WOT every 4 hours if situation permits. If nothing else think it makes my ablative paint work better.
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Old 10-01-2022, 08:23 AM   #19
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Been brought up on non turbo naturally aspirated engines. The mantra was 15 minutes every 4 hours. Then turbo the same. Now turbocharged, aftercooled and common rail and I’m told by half the mechanics it doesn’t matter and by half it still does. So generally do at least 10 minutes of WOT every 4 hours if situation permits. If nothing else think it makes my ablative paint work better.

It's hard to put a solid rule on that one. How much an engine will gunk up and need to be burned clean varies both on the engine in question (some handle light loads quite well, others don't) and just how gently it's being run at slow cruise. It also depends on whether the oil gets up to full temp at slow cruise or not.

On the oil temp one, I keep debating adding thermostats to the oil coolers on my engines. The oil definitely runs too cold at slow cruise, although it's not a major issue. No noticeable moisture issues from it, but the oil does pick up a little bit of fuel (not enough to smell it in the oil, but enough that if I run them hard for a bit the oil level does drop a little and then level off).

Realistically, I don't run them up as often as many people suggest to (but gassers aren't as sensitive to it other than the oil temp issue). Typically they get a short run on plane (a little below max continuous, not at WOT) every 5 - 10 hours of slow running, but depending on where we're cruising, etc. they've gone as long as 25 hours without being run up.


One of the sailors in our marina did learn about diesels loading up after idling too long the other day. He left his idling for about an hour and was surprised when he throttled it in gear to see some significant black smoke for a few seconds (which he said he's never seen from that engine).
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