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Old 10-25-2016, 07:29 PM   #81
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Hi Eric,

Glad to pop in once in a while an try an offer a different way to look at things...

As to the term "magic", just a term that I use that describes how easily each particular hull can move forward under the vessel & weather conditions at that time . It can vary even with the same boat...Really, all that term means is just:

"Magic" for that boat based on the the particular hull and conditions at that time, the owners wallet as to MPG going from Point A to Point B, his need for getting someplace (time value), and of course the comfort of the journey itself, and what the passangers wants to enjoy along the way...........May or may not be "hull speed" based on an equasion, but certainly 7-10 kts is close enough for 90+% of boats talked about here fall into that range when being on "plane" is really not an option.........................


As to all the rest------------All is good.


Tony
....always have tried to convey the almost infinite amount of variables in this topic.

Sure, rough rules of thumb.... but the variables in hull design that NAs have confirmed and variables in the propulsion chain that engineers have confirmed seem lost on the 1.34 crowd.
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Old 10-25-2016, 10:19 PM   #82
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The slower you go, the better your NMPG numbers will be.
Pretty much true for displacement boats in still water, but not if there's opposing current.
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Old 10-26-2016, 06:25 AM   #83
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cafesport - great graph, it looks similar to the 3 graphs in post #1 which started all this by saying "fuel used per mile".


GPH - I have always used gph as an indicator of engine load. Knowing your engine load at any speed matters to make sure you are not overloading the engine - gph , EGT and boost are methods to measure loads and keep you safely within proper engine parameters. Having gph (or boost or EGT) historical benchmark values and comparing those to current cruising conditions is an easy way to help ensure a long engine life.
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Old 10-26-2016, 07:16 AM   #84
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Wifey B: Answering for him.

You were a very bad boy to say that.

That's pretty funny actually.

I would love to help you BandB but as it was discussed, my math must be bad, so I'll give some tips to those who may want to use a calculator rather than their fingers and toes like I have evidently done.

It is in fact extrapolation and not interpolation. For those less enthused about math, that means that you are trying to find an ESTIMATED value that is beyond the range of the data that is already presented, not data points between a set.

Unfortunately, the CAT fuel graphs, or in your Nordhavn, (we do all agree that it is a reasonable estimation right?) are curved and so you cannot apply a linear formula to it where y(x)=y1+x-x1/y-y1(y2-y1).

You have to apply a polynomial formula (which quite frankly is a pain in the butt) called the process of finite difference. I had to break out my handy TI-85 calculator to do that math, but I'm sure anybody who wants to spend the time could do the same by hand.

Or the easiest way would be to get your 6th grader to do a science fair project.
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Old 10-26-2016, 09:21 AM   #85
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Since I was told there are too many variables to answer my questions in post #51, I'll ask a slightly different question.
How do you calculate engine load?
I have twin Perkins engines and want to shut one engine down without overloading the other engine while trying to maintain the previous speed with two engines under the same conditions.
I can actually measure the fuel burn with my Floscan on the one running engine so I really don't need to calculate it. Just thought a formula would be nice so I could compare it to the actual measured fuel burn.
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Old 10-26-2016, 09:44 AM   #86
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If you use 17.5 bhp /per gallon per hour, you'll be close enough for a NA Perkins.. If turboed, use 18 BHP per gallon per hour.

So, if you are using 4 GPH running on one engine, then its 70 to 75 BHP.. The rest is easy....

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Old 10-26-2016, 09:57 AM   #87
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FOG,
Additionally,
Percent of max load is derived by knowing the fuel burn at WOT and at what speed or rpm you want to know the percentage of max load. Say you find (usually from the engine manufacturer) that your engine burns 6gph at WOT and you burn 3gph at your cruise speed you cruise at a 50% load.
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Old 10-26-2016, 11:41 AM   #88
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Quote:
Originally Posted by FOG View Post
Since I was told there are too many variables to answer my questions in post #51, I'll ask a slightly different question.
How do you calculate engine load?
I have twin Perkins engines and want to shut one engine down without overloading the other engine while trying to maintain the previous speed with two engines under the same conditions.
I can actually measure the fuel burn with my Floscan on the one running engine so I really don't need to calculate it. Just thought a formula would be nice so I could compare it to the actual measured fuel burn.
Assuming you don't have a gauge telling you load, the best approximation of load is fuel usage. You simply take gph at the current speed and divide it by gph at WOT. So, if 2300 RPM uses 72 gph at 42 knot and 2000 RPM uses 56 gph at 37 knots, then 2000 RPM is 56/72 or 78% load. That particular boat recommends 80% as cruising load so that would be approximate cruising speed.

When considering load to run an engine or engines, please use the recommendations from the manufacturer. I've seen a lot of things presented here on engine load, of both propulsion engines and generators, that is very contradictory to the manufacturer's recommendations. It doesn't just vary by basic engine but varies by the configuration as often there are multiple versions of an engine, designed for different uses.
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Old 10-26-2016, 11:43 AM   #89
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Fog, With your flowscan you can easily construct your vessels prop curve and compare that to the manufacturers flywheel curve to get the percentage load. Read Tony's article and make sure you can achieve wot before you start this, otherwise you may just need someone to watch your exhaust while you run.

Bandb your unlikely to find any 900 rpm data for that particular engine since it's idle spec is 900rpm.

Fwiw just about all of the data you see manufacturers throw around in advertising their boats ranges use data from the engine manufacturers. The boat test of the sea ray cited in post 69 does so as well. The data is somewhat useful but not as useful as a true propeller power curve which can better quantify the numerous real world variables all boats deal with while underway.

Here is an interesting link for the non math challenged;
http://continuouswave.com/whaler/ref...owerCurve.html
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Old 10-26-2016, 01:01 PM   #90
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Runnin on one engine IMO presents different conditions. Overloading is easy to do. I found that if I throttled up to the point where rpms increased and speed did not as much then backed off a bit the engine seemed not to rise in temp or sound loaded for what that's worth. Furthermore I backed off the point where steering became difficult. On my44' MY that was about 6 kts.
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Old 10-26-2016, 01:01 PM   #91
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Originally Posted by cafesport View Post

Fwiw just about all of the data you see manufacturers throw around in advertising their boats ranges use data from the engine manufacturers. The boat test of the sea ray cited in post 69 does so as well. The data is somewhat useful but not as useful as a true propeller power curve which can better quantify the numerous real world variables all boats deal with while underway.
In my experience, very little of the data is based on what engine manufacturers provided. The post in 69 is based on a real test of the boat. The test information I cited earlier was all from actual tests. All the information I share in this regard is based on actual real world data. Yes, there are parameters. The tests indicate those such as "Acme 4 blade 23x26.5p, 3 persons, 30% fuel, 50% water, 50 lbs of gear, 78 degrees, 67% humidity, wind 15-20 mph, seas choppy."

Many of the boat and engine combinations have been around for a long time. Boats are regularly tested by boat manufacturers and by third parties. Now, for the information builders use in advertising, they do tend toward being conservative.
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Old 10-26-2016, 05:44 PM   #92
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Good information. I now know how to achieve what I wanted.

Thanks to all for your feedback.
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Old 10-28-2016, 09:37 AM   #93
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Well jst did a quick check of miles covered and what I've ran through the floscans, and my 120hp 30 year old lehmans are doing .801gall per mile (both). In a previous post I mentioned some problems with high engine room temps and efficiency. We installed high cpacity exhaust fans changing the e.r. air every min and blowers pushing outside cooler air directly at the engine intakes. Ball aprk we are running at 20% less fuel at the same RPM and 3/4 knot faster. We've dropped the E.R temp by a good 25 degrees F. All the rest of the Engine room componenets, batteries, a/c pumps, water pumps are performong better. When I started this project to increase my efficency I found very little info generally. However I did find some helpful sites. Which showed some possible gains, wereas the engines temps have remained stabil the ability of the engines to breathe cooler denser air and get more bang for your buck has been very positive.

Previously when I got the boat E.R Temps could hit 130F now we cruise at 1800 (7.3kts) with E.R. temp around 103F..
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Old 10-28-2016, 09:43 AM   #94
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Hello Martin J


"Ball aprk we are running at 20% less fuel at the same RPM and 3/4 knot faster."


If you are running 3/4 knot faster at the same rpm it has nothing to do with engine room temps.
Taking that 3/4 knot increase at the same rpm observation further you will see less fuel burned per distance if you have increased speed at the same engine rpm.
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Old 10-28-2016, 09:53 AM   #95
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Quote:
Originally Posted by bayview View Post
Runnin on one engine IMO presents different conditions. Overloading is easy to do. I found that if I throttled up to the point where rpms increased and speed did not as much then backed off a bit the engine seemed not to rise in temp or sound loaded for what that's worth. Furthermore I backed off the point where steering became difficult. On my44' MY that was about 6 kts.
Bayview,
Sounds like running both engines would be better.
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Old 10-28-2016, 12:13 PM   #96
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I agree with smitty, if the boat is going faster it's because it's likely lighter, more slippery or the props are cleaner. Have a look at the article Tony cited in this thread called propellers move boats then take a look at this PDF from David Gerr about power curves: http://www.gerrmarine.com/Articles/E...owerCurves.pdf


I'm curious as to what temps your getting once everything gets heat soaked, say running 50 or so hours continuous. It's one thing to bring down the charge air temps. Cooling fuel and big chunks of metal are more problematic. Our er seems to run the same temps in Maine as the Bahamas once everything has been running for a few days. It's great for drying laundry underway.

Bayview if you are propped right there is plenty of reserve power available to run on just one engine. See the Gerr PDF in the link above. You can run it like you would run a controllable pitch prop and just watch the exhaust gas temperature to stay away from the edge of the curve.
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Old 10-28-2016, 02:49 PM   #97
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I posted that once and got a bunch of crap about it. One thing for certain though, in a displacement or semi displacement boat, your fuel economy goes down very quickly above hull speed.
My fuel consumption more than doubles going from one knot below hull speed compared to hull speed. The boat can't exceed hull speed. So, normal speed is hull speed minus one.
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Old 10-28-2016, 02:56 PM   #98
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Sounds to me like you found your "Magic Speed"..


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Old 10-28-2016, 03:38 PM   #99
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Hi Smitty
During my research I came across a engine research facilty, very comprehensive, thats were I came across the most easily understood research info spreasheets etc. You could plug in air temp, speed, altitude, air pressure a huge load of varibles. from this I was able to increase my HP at a given RPM to obtain better combustion , by the engines ingesting colder and denser air. Previously when in the open sea I would notice that the tachs would dip slightly as seas passed underneath the boat etc. from this we were aware we were not getting constant BHP (it was dipping and rising as the props came under load) due mostly by imgesting hot air at 130F. The calcs proved that the BHP would increase to near original Lehman test figures if we could get the temps down to under 100F. Previously the engines were ingesting hot thin air and making up for it by increasing the fuel and we were using higher RPM to make the speed. Basically running very inefficiently. The engine room was 15ft 6in x 9ft 6inx 6ft 4" not including Bilge areas The previous owner had just one extractor fan a small PAR which extracted som 350 cu ft of the total 950cuft every min. This coupled with the natural aspiration of the engines would not control the heat to a reasonable level. Currently I have some 1000cuft extraction and some 700cuft intakes aimed at the engine intakes, plus the vented exterior engine room deck door. Plus the engines themselves ingesting cooler air. This has provided better combustion and more BHP for any given RPM, with leaner fuel consumption etc. Constantly smmother RPM giving better speeds in a seaway etc.
If you look on line there is a very comprehensive manual by caterpiller concerning the need for cooler efficient engine rooms vs performance.
Some of this may sound wrong however in practice, the boats performance is up!
Unfortunately I cannot give you the links (they are in my desktop), but will add them to this forum when we return from cruising in January.
I am sure when I publish the links, many other members will be reviewing there engine room temps. Part of our log routine is to record E.R. Temps

Safe sailing!!
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Old 10-28-2016, 03:49 PM   #100
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Smitty

Heres the link to the Cat manual http://s7d2.scene7.com/is/content/Ca...13-53120-44971

And a good article BySteve D'Antonio Venting the Engineroom - Professional BoatBuilder Magazine
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