Discussion in 'Boat Design' started by Magnus W, Jan 12, 2019 at 4:06 AM.

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### Magnus WJunior Member

I've lately been putting my grey matter to use contemplating net economy for the boat I'm going to build.

In my business I mostly get paid by the hour when transporting goods and/or passengers a certain distance. So in general it's in my interest to go slow and in my customers interest that I go fast. This is something for me to balance but my question doesn't include this aspect of the total equation, just the boat.

It's most likely going to be a single water jet installation and when looking at the calculated performance data the fuel per distance is pretty much constant between 18 and 27 knots which translates to 50 to 75 percent powertrain load. Not taking into account the most efficient engine rpm but I'm well above it for all loads anyway so it may not matter much for this calculation.

Since fuel cost per distance is irrelevant for the normal speed range, how should I calculate the best load (ie speed) in order to reduce costs? (And lets also assume that the extra man hours required from going slower are free.)

In my mind 50 percent is enough to secure that the diesel (442 kW max) is running at a high enough load to clear its throat. Also, 75 percent is well below the maximum continuous rated 90 by a comfortable margin.

On the good side of going slower is a 250 engine rpm drop as well as lower thermal and pressure loads. Overall lower strain on the hull, transmission and jet is also on the plus side.

On the other hand running hours (at speed) will increase by 50 percent if I reduce from 27 knots to 18.

All relevant powertrain maintenance is in relation to running hours and while it's easy enough to split the cost per the different hours required to get a buck per mile figure it's rather the opposite to accurately measure the potential gains by slowing down.

While I think it's beneficial to slow down I can't know until I have empirical data. But perhaps there's a way that the industry looks at this. Or at least a rule of thumb.

It would be very interesting to see how you calculate the worth of lower wear and tear vs increased scheduled maintenance costs.

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### JSLSenior Member

do you have any more specs on the boat- length, weight etc..

3. Joined: Nov 2017
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### Magnus WJunior Member

13 meter lwl and 3,2 m beam at chine.
Jet performance calculated at 10000 kg, guesstimated dry weight circa 7500 kg.

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### BlueBellAhhhhh...

Gains can be measured by GPS.
Engine load/speed optimization is an important variable to consider too.

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### TANSLSenior Member

Just an idea : your business will be profitable, not on the basis of saving fuel, but on the basis of transporting as much more good merchandise, minimizing the variable expenses per kg transported. I mean you should look for the cheapest speed.

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### Magnus WJunior Member

This is true and as I stated there aren't any fuel savings to be had from going slower – same gas mileage between 18 and 27 knots (give or take). So for now it comes down to costs from wear vs scheduled maintenance.

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### jehardimanSenior Member

Unless you are contracted for a fixed schedule or in the 'small package trade", I doubt you will find any economic advantage to going slower at lower prime mover efficiency vice going bigger at higher prime mover efficiency, it runs against the economic rules of transport capacity and the modern Gabrielli–von Kármán diagram.

Are small ships a thing of the past? https://www.boatdesign.net/threads/are-small-ships-a-thing-of-the-past.48500/page-2#post-672016

https://trainsnboatsnplanes.files.wordpress.com/2010/01/gkupdated.jpg?w=500&h=626

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Way way too many unknowns to make such an assertion of what you would like or want.

You can only do so wirth facts, not supposition.

Thus "design" the boat.....then establish its resistance over the speed range that interests you. Then you'll know what power is required....then you can start to begin to make some assumptions based upon facts and then come to a conclusion. Otherwise all you'll do is go around in circles.

It's called comprise.
All designs, successful designs, are a good compromise of all the competing elements.

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### Magnus WJunior Member

I understand that there are too many unknowns to be certain of anything. I was just wondering it there's a way of determine or estimate the value of reducing load in terms of lower unscheduled maintenance/longevity of the drivetrain.

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### rxcompositeSenior Member

Since you have the data, you can plot the fuel cost for different speed/Load. Speed will cost more. Engines go by ratings and you can use the data provided by the manufacturer. That is the engine hours or the Time between Overhaul (TBO), the number of running hours allowable before you overhaul (include the drivetrain). You can now predict more or less when the boat is going to overhauled. Tally that with hull maintenance cost and other related equipments.

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### Magnus WJunior Member

Fuel cost per unit of distance vs speed is constant for loads between 50 and 75 percent. Engine and jet maintenance are related to running hours only (well, some annual items but only for low running hours so not applicable in my case). Type of duty, like constant, cyclic, average load etc is reflected in the engine rating which in turn dictates the scheduled maintenance so no official maintenance gains by reducing load.

I have tbo for the jet but so far nothing on the main engine option. And certainly nothing that suggest what may be gained, or lost, by altering the load.

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### rxcompositeSenior Member

You can check the manufacturer for the engine performance curves. They publish the fuel consumption, torque and BHP curve, ect. If you are running the rpm is in the band where the torque flattens out, then you are in the sweet spot (assuming your jet is matched to the load curve). There is nothing much you can do.

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### BarrySenior Member

Max horsepower curves with fuel consumption usage are probably not the source you need to design a power package.
Most graphs show only the maximum hp that is developed at a particular rpm and the corresponding fuel consumption.
Generally, these Max HP Fuel Usage numbers (at the rpm specified) are not the most efficient fuel efficient rpm to run an engine. (fuel efficient in this sentence meaning the amount of fuel
consumed to produce a particular max hp value at a specific rpm)

Say an engine produces a max of 100 hp, uses 8 gph, at 3000 rpm, as illustrated on a HP -Fuel consumption curve. On a dyno, you have provided the absolute maximum fuel flow and held the rpm at the 3000 rpm. There will be an excess of unburned fuel, high EGT's.

So you have a boat that you need 100 hp to the shaft, look at the graph of a motor, and design around this available max hp graph. (assume a straight shaft)
You will more than likely find that your fuel efficiency, in this case defined as the amount of fuel burned per mile, km etc is not very good

So instead you change the gearing, instead of 1:1 as referenced above, kick the rpm OF THE ENGINE up to 3400 rpm. While the engine can produce say 120 hp at this rpm, you are only taking 100 hp of the available max output, EGT's drop, less unburned fuel exist, etc and you get better fuel efficiency.

Remember the horsepower production of an engine is determined by the amount of hp that the jet, prop, requires at that speed.
We have seen many instances that we might be taking 100 prox, due to the jet hp input curves provided by the manufacturer, at rpms where the engine could produce upwards of 250 hp,
and found high fuel efficiencies.

Of course, other factors impact finding the perfect rpm for a particular set up. The inlet speed, ie boat speed, will change prop thrust characteristics and hp requirements AND of course

I would suggest that you do not rely on just the HP-Fuel Consumption curves provided by the engine manufacturers as design points but check out with other people who have
built boats within the scope of what you are trying to do and see what they are getting for their configuration of engine, gear ratio ( ok with a jet normally 1:1 but not always) speed etc

To illustrate how weight can affect performance
A couple of boats ago, I ran a 30,000 pound 40 foot aluminum boat with twin 350 hp diesel IO's. The display gave me fuel consumption, NMPG. Over time, I found that
3050 rpm gave me the best fuel efficiency when fully loaded, 400 gallons of diesel, 120 gallons of water, food supplies for 4 people for a week. On the way out, we would use get about 1.1
NMPG and about 21 knots and on the way back, same rpm, 1. 25 and 23 knots. So same rpm, loaded and unloaded weight, but a different outcome on performance figures.

Ie different prop face loading, and a different inlet speed to the props

In a summary, if I may, if you need 100 horsepower for a particular jet, boat, speed, weight combination, you will likely find that the rpm that you want to run,you find to be most fuel efficient (miles per gallons burned) will be at an rpm much different than the one that is shown on HP-Fuel Consumption tables.

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Yes there is:

That'll give you all the data you need. And then you will be able to match the engine and prop to suit your speed v fuel consumption at a given speed.
Can't do any number crunching of "savings" etc without this data.

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### Magnus WJunior Member

This has been done and as per the data there are no fuel gains to be had between 28 and 18 knots – the fuel per distance curve is flat in this region.

So again, here I'm not looking for ways to save fuel. I'm trying to determine other benefits of slowing down.

Example:
Say the hull has been designed to have a lifespan of 30000 hours under a certain set of circumstances. At a certain displacement, speed and applied power (thrust) we have a certain speed that will give me the 30000 hours. In my case 27 knots at 10000 kg with 75 percent power (for one of the powertrain design options). If I reduce the power to 60 percent the speed drops to say 23 knots which in turn reduces the stress on the hull. How many more hours can I get from the hull?

And I'm not really expecting a way to determine the exact figure, although I'm sure it can be calculated with sufficient data. I assume that total cost of ownership analysis for large vessels include these types of considerations. But a ballpark figure or rule of thumb would be nice.

Where I come from, aviation, the rule of thumb for a turbine engine is that a 10 percent reduction from max continuous ITT results in a 50 percent longer time between hot section overhaul (or 50 percent lower costs for an overhaul in the case of hours being the only TBO factor to consider). Of course the gains may vary greatly between different engine types and even individuals but it gives the operator an indication as to what general gains can be had from reducing power. For larger turbine engines the manufacturer will supply this data, for smaller ones it's down to experience and said rule of thumb.

Back to my boat. Neither the engine manufacturer nor the jet manufacturer can provide any hard data, they refer only to the rating and how that relates to life expectancy. Nor can the boat yard building the boat give me anything more than the "guaranteed" number of hours and while I assume it can be calculated with accuracy the effort to do so would come with a hefty price tag.

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