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Discussion in 'Announcements & FAQs' started by Forever Mates, Aug 5, 2014.
I would be intersted in finding out more about the tabs also!
I can tell you my 270/454 9ft beam will struggle to stay on plane at 3200rpms with three adults fully loaded. I only use my tabs to level the boat, tuck the bow a bit or get on plane. I would think running them down at a lower rpm would still use more fuel. 3600-3800rpms for me seems ideal. My 4bbl is probably open a bit but really kicks in at 4200rpms. Not sure when the larger 750QJ opens. My CV-23 with 5.7L opens at 3300rpms with a 650QJ. Ideal cruise with it is 2900rpms for 4.5mpg (2mpg with 4bbl open). On the 270 with a load of six adults I have to stay at 4000rpms to keep the hull responsive on plane. My WOT is 4600rpms with a 4 blade 17P 14.25" Merc Stainless on my Bravo 1. I like to cruise 25-30mph gps. Lie O Meter reads about 5mph faster. I get about 1.3mpg at 3800rpms, Mike.
You are absolutely correct. That's why I don't look at GPH, I look at MPG. You've essentially confirmed the MPG, but by doing a different calculation.
I asked the question earlier if running at higher RPM is detrimental over the lifespan of the engine. No one really offered an answer, but, of course I have my own ideas. In a boat with 20+ yo engines, my thinking was that if there's no benefit to running at the higher RPM (other than to avoid abuse dished out by friends), I'm not going to push them.
That remains my philosophy with this boat as well.
I would think once you free the hull up from the water at speed there would be less stress on the engines. I trim my drive up a bit once on plane and it gives me about 200rpms so I drop back 200rpms to cruise. I doubt any V8 hurts its self at 3500rpms. I do know 454 big blocks don't last as long as 350's when run above 4000rpms constantly, Mike.
From the mid '80's until the late '90's, Thermo Electron (builder of Crusader Marine engines) was one of my accounts for fabrication work. I had a 300 Weekender with a pair of 350's, and I asked the director of engine development about prolonged high rpm usage. He said that the engine could run forever at up to 80% of the maximum rpm's it was designed for, which in that case was 4500 rpm. So, 3600 was no more harmful than 3000. I've used that as a guide since, and don't exceed those numbers for any more than a necessary burst of speed.
I've heard the "3/4 throttle all day long" argument. Sounds like what the Crusader guy told you. Is that 80% of throttle/RPM or 80% load? Being nit-picky here but dlesel guys tell me that 60% of RPM is not necessarily the same as 60% load. Might not even be relevant to gasoline engines.
I tend to run the same theory as yourself with this. My beliefs are mostly driven as a mechanic, and feeling for engines, I have never liked running engines at high RPM for extended period.
A lot of people on here particularly with diesels refer to the high engine hours that the motors achieve in trucks and machinery etc, in these cases they are generally not running at extended high revs.
In a vehicle, the engine revs to get you going, depending on the motor, and let's stick with gas, automatic, generally around 3-3500 they up change to drop you back into the torque curve around 2000.
Depending on the motor and gearing etc, highway cruising is generally somewhere between 1800-2600.
Now I know we are not in cars.
So coming back to boats.
In my waterway, the furthest we can go in one stretch opened up,would be 1nm. So what revs I am going to run it up to, really doesn't concern me as it's not going to be there for long.
But I do several long open sea runs a year. The run a couple of weeks back, would have been about 90mile, of fast running, 10mile of trolling.
I sat most of that at 20-21kn, with around 3' of swell running, which in mine is around 29-3000RPM, I am comfortable at that. Normally outside I run around 2600 17kn, which I find more pleasant
I know I have done calcs and lifting the speed up a bit more, does improve the fuel rate per nm. But I'm not comfortable with the engine at those revs for a long period, and generally with the sea conditions I am in......the ride
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Without all the technical analysis, IMHO….
With the bestest of care, pm, and use, your engines only have so many rpms in them…components have to/will wear out. Get up on plane and go…that’s what we bought them for.
How do we get on plane and go without those rpms…we can’t. We gotta use some of them up.
The propulsion on our boats is not custom designed for it. In ‘04 one 200 Select bowrider option was 260hp, 5.0mpi, 2.20 B3, 24P prop. That same combination was used as twins on the 280DA. SeaRay uses a handful of std engines, drives, drive ratios, and props. They match them up with each boat model as best they can. Some combinations compliment themselves better than others. They try to give us some reasonable mix of performance, longevity, and value.
So what’s the best rpm? I look at fuel usage in the format MPG, you can look at it anyway you want. When my boat is giving me the best MPG there are reasons…the list could be long but would include all the PM I’ve done, the props that don’t bounce off of every sandbar, and the clean hull I have because I sometimes jump in 60F water to scrub it. This best MPG suggests it’s a point were the engine is working easier than other planing speeds/rpm below it. I’ll bet you a picture of my boat trimmed out properly at 3,500rpm will show less hull in the water than at 3,250rpm...what might that benefit?
I cruise my boat between 3,400-3,600rpm. I doubt my engine life will be any shorter than the other 280DA guy that never exceeds 3,400. We’re probably using up about the same number of rpms going from point A to B. So why do it…I’m getting there faster, I’m burning less fuel, and I feel it’s easier on the engines as illustrated by the lower fuel consumption.
Here is some SeaRay info on fuel usage, note this is not boat model specific. It’s interesting that they’re measuring fuel usage at a ‘Cruise’ rpm of 3,500. It indicates engine torque, and hp specs have been purposeful. But…is the 3,500rpm a result of design for best fuel usage or engine longevity…I think both…now pick a boat, a drive, a prop, throw them together and lets see what we really get. Carbed, MPI, IO and V-drives are represented… look at the rpm for the 8.1Horizon. Read the comment above the chart…this is likely accurate info in itself but it’s ballpark when applied to actual boats. In my case, my boat betters those numbers.
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Getting to the heated heart of the topic…there is no across the board, single exactly right, most efficient rpm for gas powered boats. Was ‘3,500rpm’ ‘generally speaking’ a good answer? I knew what he meant…but maybe the OP didn’t…so, yes we need to be careful what we say…especially ‘technical experts‘.
I’m really diggin’ this paragraph thing…hope I don’t run out of space bar.
I concur. Very good. However, sentence structure, though better than last time, still needs to be taken up a couple of levels. Especially the copious use of three dots. That has to go.
Ms Wendt? :smt043 I was writing as if I spoke...honest.
Keep going off topic and you guys are going to get called to the principals office again.
In order to stay compliant with a technical reply in a non-tiki bar thread I submit the following in response to what "engine load" means:
Engine Load, as commonly viewed on diesel ECM's, is calculated on the amount of fuel used by the engine vs. the maximum amount it could use at a given rpm. Generally speaking of course...
1996 270 sundancer 7.4lt single. What rpm and speed to be most fuel efficient 2 people on board, full tank 100 gallons. Should I get her on plain and then what rpm ? I,m learning that,s why i,m asking.
Send a PM to nickysc with your question. i believe he has a 270SE with a fuel injected 7.4. If your 270 is fuel injected, he should be able to help.
its 7.4lt carb. it seems to run nice at 3000rpm at about 22-25 mph Its trimmed down on stern drive, Just play with stabilizers to drop rpm,s and keep speed the same?
Thank you very much, I will try this tommorrow. We are really enjoying our 270 and I am impressed with the help that this forum has given me. Unfortunately its Aug 7th and not May 7th hope we have a nice fall. Thanks Again and I will return with more questions no doubt. lol
First of all using "fuel economy" and boat is the same sentence doesn't make sense. with a boat there is no such thing as fuel economy, just fuel consumption. :wow:
The one answer to what is the best RPM to run the boat at is "it depends"
I have a fuel flow meter on my GPS so I can see the instantaneous readout of litres per Km and for my boat the best I can get is between 1.2 to 1.4 This is dependent on the amount of weight in the boat, on sea conditions and the trim position of the drive. The optimal setting in terms of RPM varies as these vary.
Since I cruise long distances I try to optimize my fuel burn per km.
What I found is that to get the optimum I don't set it by RPM but rather by fuel flow rate. A rate of 55 to 65 litres per hour is my optimum point. This will be dependent on the load in the boat.
As Scott said trimming to just below the point that the boat starts to porpoise is the bet way to get to the optimum point. Once I get on plane I set my flow rate to about 61 then trim up the drive. Doing so get me a couple hundred RPM more and 5 to 6 km per hour more. You can almost feel like the boat has shifted gears at this point.
One other thing that I noticed is that I use less liters per km when there is a very slight chop on the water than when it is dead calm.
Oh after having said all that once I optimize my settings for throttle and drive trim I'm usually tuning over at 3400 to 3600 RPM.
All this is with a 2001 260 DA with a 7.4 MPI engine.
I just sold my 1991 270 Sundancer(also known as a 20 in later years) in June of 2014. I owned it for 10 years and got to know it pretty well. With a 7.4 carb. and a Bravo II it seems to run well at 3200rpm with average speed of 22 mph. I burned 11 to 13 GPH. I burned more fuel (closer to the 13GPH with the tank full) Great boat you'll love it.
Frank is absolutely right as I have two of those lovely engines in my boat. At 3200 RPM on a long run I wonder if there is a leak as the engines drink in the fuel!!!! But at 3000 RPM all is well and they sip it. So I enjoy the cruise at 3000 RPM at about 20-21 knots. But at 3200 RPM I do get a 24 knot cruise, but that consumption reduces my miles per gallon.
To the original poster you have to look at the ratio of miles traveled over gallons consumed to get mile per gallon. That is your best indicator of a good cruise for your boat.
If any of you are going to enlarge your trim tabs, consider adding a hull extension to it. I did with mine and don't even need to use the tabs to get on plane.
This last one both engines are running, just that the fuel sensor bounce from .4-.6 gph because there out of there range. But at no wake speed your lucking to see over 2 mpg.
Over all I usually figure about 1.55 mph to be safe. Last two years I have seen about 1.6 to 1.7 cruising. Two years ago I had it in a slip (I think the boat got heavy with all the crap we found out we just had to have plus the growth on the hull. Last year I went out three time's with it on major trip, we were just heavy with people and gear. (Also run 21 Vengance props last year, Don't think there going to work as good as the 19 Mairage props.)