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Old 08-10-2020, 11:48 AM   #1
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Sea Trial of Gas Big Block Engines

I have a sea trial coming up on the 18th of August. I have been advised to be sure to run the engines at WOT for 10-15 minutes to see how they handle the prolonged load and heat dispersion. I usually only run gassers at WOT for 3-5 minutes to see if they reach rated RPM and show any signs of overheat or missing. The engines are the original 1987 Crusdader 350 hp closed cooled with 600 original hours (no rebuilds). Is 10-15 minutes excessive or reasonable? I suppose if a bad storm were coming in I might run those motors at 3800 to get in quicker (4000-4400 is rated max) so why not run them at max for ten minutes?

The boat is 34,000 pounds and is a planning hull. I intend to find her sweet spot for mileage and run it there (Garmin 5015 with flow meters). I typically never run my big blocks over 3200 for a cruise RPM. Bill
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Old 08-10-2020, 11:56 AM   #2
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I'd consider running WOT for more than a couple of minutes to be excessive on any engine that's not meant to run at or close to WOT continuously. I'd open them up long enough to see what RPM you get and see if the temps continue to climb or if they stabilize (I'd consider anything under 200 fine).

On the 350hp Crusader 454s, I'd want to see 4200 or better at WOT ideally. 3400 is considered max continuous on those. I typically run at 3300 for a planing cruise with my similar 340hp Mercruiser 454s (mine are only 2 bolt main blocks though). A short run around 3500 - 3600 if a sprint is needed would be fine, but I wouldn't run above 3400 for more than a few minutes. If they turn closer to 4400 at WOT, I'd be more ok with pushing to 3500 for a little longer.

On a 34k lb boat, you'll likely find you're running at max continuous any time you're on plane unless it's a very efficient hull. I'm only 26k lbs (and a bit short on prop blade area I think) and while the boat will top out in the 25 - 27 kt range, I can barely hit 18 kts at 3400. There's a bit of room for improvement in my prop selection which should help, but I certainly wouldn't expect your boat to be any faster at max continuous.
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Old 08-10-2020, 12:00 PM   #3
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I have a new Merc engine and the break in procedure is over 20 hours and you are not suppose to go to WOT until hour ten. And then at WOT for only a minute or two every so often.
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Old 08-10-2020, 12:01 PM   #4
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Great opinions, thank you. The boat is a CC Catalina 480. That Uniflite hull is regarded as unusually efficient and "easily driven". I will be looking to the Garmin for fuel calculations and determining what gives me the best MPG.........Errrrrrr.........GPM. Sorry. Bill
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Old 08-10-2020, 12:05 PM   #5
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For years I was unaware of the differences between Mercruiser and Crusader 454's. I assumed they were the same. Boy was I wrong. Regardless, I try to baby my engines which is really what led to this post. Hopefully, I am going to own this boat so why would I want to beat on it. On the one hand to make sure it is up to the task. On the other hand I would not because it is not intended to be run in that manner. If it were a diesel I would assume it would be far more common to sea trial in this manner. Bill
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Old 08-10-2020, 12:08 PM   #6
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My boat doesn't go in the water for another two weeks and the place that installed the new Merc engine wants to do sea trials. I won't bore you with why but the only way I have figured out what prop to use on the boat is using a prop selector software program out there, found on the Merc site, but other sites as well. I know my engine installer guy (and new leg) will want to take the boat up to WOT and try and convince me it will be okay for a couple of minutes. I have no intentions of letting him on the boat until hour ten.
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Old 08-10-2020, 12:10 PM   #7
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RSN48, I think you are ok with short wide open burst to seat the rings fully up in the cylinder. Doing it for any length of time is probably not advised at that young age (bearing and surface break in). Bill
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Old 08-10-2020, 12:13 PM   #8
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That hull shouldn't be too bad to push, but it's still a lot of boat for those big blocks. The biggest issue with them is just that max continuous output is annoying low relative to WOT. I can't find a WOT speed mentioned for that boat with gas engines, but some quick searching indicates they're good for 24 - 25 kts at WOT with 375hp Cat 3208s. The gassers are about 1400 lbs lighter for the pair, so I'd figure you might see 24 kts at WOT with the gassers. That'll probably leave you in the 17 - 18 kt range at 3400 (max continuous). Not bad for that much boat, and you'll probably find best planing mpg somewhere in the 3200 - 3400 range I'd guess. Figure probably 0.5 nmpg on plane, maybe a hair better if you're lucky.

Low speed MPG (7 - 7.5 kts for that boat) will be about double or a little more what you get on plane, I'd expect (probably between 1 and 1.2 nmpg). Gassers lose a lot of efficiency at light load, so unfortunately you won't see the triple or better at low speed that diesels often get compared to running fast.

Durability wise, as long as you don't run them above 3400 regularly and are good about maintenance (especially manifolds, risers, and making sure the cooling system is healthy), those engines will live many hours. One of my 454s got a long block a couple years ago due to an oil line failure (before my ownership), but the other one is showing 1550 hours. No perceptible difference between the 2 other than the older one going through slightly more oil. It's spent many hours turning 3300 - 3400 and still checks out as quite healthy.
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Old 08-10-2020, 01:02 PM   #9
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I think you are ok with short wide open burst to seat the rings fully up in the cylinder. Doing it for any length of time is probably not advised at that young age (bearing and surface break in). Bill

Tell that to Mercury.... lol.

BREAK-IN PROCEDURES

“Break-In” Procedures for 4-cycle MerCruiser Engines

IMPORTANT: The first 20-hours of operation will have a dramatic affect relative to overall engine performance and expected engine life. Proper break-in practices reduce the potential for excessive oil consumption. It also helps to achieve better engine performance levels for a longer period of time.



FIRST 20-HOURS:

1. Do NOT operate the engine below 1500 rpm for extended periods of time. Shift into gear as soon as possible after starting the engine. Advance the throttle above 1500 rpm if conditions permit safe operation.

2. Do NOT operate at one continuous speed for extended periods of time. Vary the throttle position every ten to fifteen minutes.

3. Do NOT exceed throttle during the first 10 hours of operation. During the next 10 hours you may occasionally operate at full throttle for no more than 5-minutes at a time.

4. Do NOT operate at full throttle until the engine reaches normal operating temperatures.

5. Avoid full throttle acceleration from an IDLE speed.

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Old 08-10-2020, 01:45 PM   #10
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I won't argue with the experts, but number 3 is the only one I would have any issue with. I would hit WOT for a very short time, and I would not run WOT for any length of time beyond 5-10 seconds. Again, please take my advice and throw it in the trash. Mercruiser has a reason for saying this.

One of the reasons I was willing to sacrifice diesel for this purchase is that I felt the mileage could be borderline acceptable at hull speed, which is what I would have had with a FD trawler speed and slightly more speed with SD, but if you push a wake with an SD you get pretty poor mileage. The Gasser gives me the option of going fast if I need to although I purposely have been wanting to slow my life down and thought a single diesel at hull speed was just the ticket for that. With gas fuel is cheaper, maintenance is cheaper, repair or replacement is WAY cheaper. I wanted diesel. I settled for gas. Hopefully the trade offs will result in a positive experience.

My Bertram 35 was 23000 lbs and 19 degree deadrise. It could hold plane down to 2300 rpms at about 14 mph. 2800-2900 got me about 18 mph and 21.5 GPH combined. 3200 would push it to 20 mph but the economy dropped to 28-30 gph. Top speed was around 25-26 mph at 4000 (have a tanker follow you).
The dead rise of 19 degrees hurt speed/economy, but the ride was sublime. My Chris 35 was 10-13 degrees and was much faster, but the ride was inferior when compared to the Bertram. This new boat is much heavier and longer than the 35 Chris SF I owned, but with a similar dead rise I think. Should be interesting (teeth rattling). There is always hull speed and at the longer waterline the speed will be more. Slightly.

The new boat has a 6 foot cockpit added to the popular 426 Catalina model (same hull as Corinthian 480 and Uniflite 48 yacht fisherman, and I think possibly even the Camargue 48). The props and shafts are located well forward of the transom in the same location as the 42 foot hulls (no change in molds for shaft location). I wonder what effect having the props so far forward has on the efficiency? Bill
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Old 08-10-2020, 01:59 PM   #11
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With the props and rudders far forward, it should be fine efficiency wise, but in a following sea you'll need to dump on the power and run with or ahead of the seas, efficiency be damned. Letting the waves overtake you is likely to be an ugly ride, as you'll get shoved around a lot with the rudders so far forward.

The basic hull form doesn't appear too different from my 381 Catalina (which has slightly less deadrise at 10 degrees), so here's what I'd expect: It looks to have very little keel, if any (can't find a great picture of one on the hard), so directional stability in flat water may not be the greatest, but less keel will mean more speed and less handling quirks at high speeds (which would need more power to really matter).

In a head sea (especially if it's steep, short period chop) you'll want to slow down a little (15 - 16 kts) and use the big trim tabs to keep the bow down. Too far down and it can drop too fast between waves and slam into the next one, so you'll have to experiment to find the sweet spot. It won't ride like a deep V running upwind, it'll pound if you push it too hard or let the bow get too high, although the longer waterline length will help this somewhat. Expect it to be a wet ride if you're at any angle to the wind in a head sea, as you'll be punching a good bit of water out of the way and any crosswind will blow a lot of that onto the boat.

With a following sea, the low deadrise hull should surf easily and can be quite a bit of fun provided you're going fast enough for good handling and not getting pushed around too much. The forward positioned rudders may hurt you here.
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Old 08-10-2020, 02:39 PM   #12
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Thank you again for the generous sharing of opinions. As for the sea trial, the captain/broker, surveyor, and my father in law (expired license surveyor) all plan to pin it and see what she does. In the end it is my call and I will not let it go on too long. Whatever too long is My gut feeling I suppose. Bill
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Old 08-10-2020, 04:50 PM   #13
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I cant say I would run them at wot for more then a few (2-3) min max. Anything more is just abuse in my book. Most often I feel if there is a cooling problem it will show as soon as your on-step for any amount of time. And as far as getting recommended rpm....mine would not reach the factory max of 4500 as its over propped. But dam its nice to have them bigger props at my 2800 rpm 3 hour runs !
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