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Old 06-10-2016, 09:02 AM   #1
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Re-prop for more efficient cruise?

Still don't quite understand over/under propping and probably never will.
But is there any advantage going to a different prop and or gears on a boat that will run at 15-20 knots to get better fuel usage?
I'm still considering motor yachts with larger gas or diesels that can cruise at 15-20k but will probably cruise at 7-8 knots and have no need for a prolonged cruise at higher speeds.
Mostly concerned with extending the range as much as possible.
So will it help to re-prop to get the most efficient engine speed down to 7-8 knots?
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Old 06-10-2016, 09:32 AM   #2
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repropping wont change fuel use if the engine is propped correctly. If you want lower fuel use slow down and or get a lighter boat. Don't believe the makers weight as it probably does not represent the model and equipment you have.


A lot of people will talk about peak torque but that data is not available from makers at other than full load curves. Partial load data is usually not available.


In any event consider that it will always take the same power to move a boat in the same conditions regardless of the engine. Some engines are more efficient with the modern electronic controlled diesels at the top and old two stroke at the bottom.


Do a little research on brake specific fuel consumption (BSFC) to understand that fuel used doesn't vary much across similar engine classes.


FYI my 38,000 # ACMY got 2-3Nmpg at below 8 knots and .5 or so at 18K. I ran slow in the waterways and fast if I had to cover a large open stretch.


In 15 years of cruising fuel was not the biggest cost by far.
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Old 06-10-2016, 10:16 AM   #3
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In theory re proping a fast boat down to be more efficient at displacement speeds would work just fine,

BUT,, the lower more efficient fuel burn will only be on the order of 15%-20% , at slow speeds the fuel burn is light so a pair of props would take the loop to pay off. 20% of 3GPH isnt much.

Most boat engines are small enough that underloading and slobbering seldom come into play.

An overpowered displacement boat re prop makes the most sense as the advantages exist , tho the biggest is operating at far lower RPM at cruise , nice and quiet.

And the next owner wont have you put the speed boat props back on.
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Old 06-10-2016, 10:38 AM   #4
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You can't have it both ways.

You can prop for best efficiency at one speed. That's it. With a propperly propped boat WOT (rated rpm) is the only engine speed where ideal engine max load is achieved. And you'll need to "overprop" some to get the best efficiency at your chosen speed (excluding WOT) and the overpropping produces negatives (one could say side effects) that aren't worth the very small increase in fuel economy.

But if you need to save 5% of your fuel burn overpropping will do it. But only at displacement speeds or very slightly above ... definitely not at 15 to 20 knots. Only on overpowered boats can overpropping be effective enough to be worth doing. And that's only if you never ever run the engine or engines at more than about a 70% load.

The above is only an opinion and it's my opinion that overpropping should almost never be done and if I knew the details of your application I'd almost certianly say not in your application specifically. I have a favorite saying about this ........ too much to loose and too little to gain. Also no engine manufacturer recomends it.

That being said if you run your boat at 25 to 30% load almost all the time and occasionally at 50% load for short periods overpropping may be beneficial. Of course that depends on how much overpropping. An EGT can tell you w at least a fair degree of accuracy if you've increased the throttle too much and are overloading the engine but the location of the sensor is critical as very small variations in location can produce inaccurate readings.

As I see it the only worthwhile motivation for overpropping is noise reduction and/or vibration. And re vibration and noise I think these problems should be addressed specifically rather than resort to overpropping.

But unless you've got a old lifeboat w a Full displacement hull that needs only 15hp and some PO installed a 6cyl 100hp diesel engine and you can't afford to repower I'd say forget overpropping.

However re your statement,
"is there any advantage going to a different prop and or gears on a boat that will run at 15-20 knots to get better fuel usage?"

The answer is yes .. if it's not the best gear ratio and prop for a specific boat at a specific speed. Ideally one would choose the right gear ratio (for propeller shaft speed) and the best prop for a specific speed. One can optimize for speed on any boat under any given circumstances considering all the many variables. But if you optimize for any one thing you will pay the price at other speeds and/or circumstances. But any given prop will only be ideal at one speed .. no other speed. Unless of course you have a variable pitch propeller.
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Old 06-10-2016, 11:09 AM   #5
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I have a boat that can go faster, as well as cruise slower. Probably similar circumstances to the boat you mentioned.

I'm not sure what you'll gain by intentionally overpropping as far as fuel used. I say that because I have compared our fuel burn at displacement speeds, with several twin engine go slow only boats in the same size range and found that we have similar fuel burn at displacement speeds.

If you really want to save fuel, go smaller, or get a single engine full displacement boat.

Otherwise enjoy the choices that a SD boat provides.
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Old 06-10-2016, 12:31 PM   #6
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There are things that you can do to help keep your engine/s in the highest BSFC range both at cruise and high speeds.
Autoprop, a self pitching prop, has the ability to vary it's pitch automatically so that at lower speeds your engine is nearer its higher BSFC ; ie: under more load.
Gori has a 2 speed propeller, which is "shifted" by use of your existing shift control. It could provide the same advantage but is not automatic so could overload your engine if not operated and understood correctly.

Both these devices are aimed at motorsailors so good luck getting any information about their use on a powerboat although two articles have been published on the Autoprop.

A third option would be a 2 speed transmission. The 2 speeds were designed for fishing trawlers towing & running.

None of these options will EVER pay for itself as a retrofit.
Seriously overpropping to save at cruise speed will damage your engine at higher speeds. Thus the 2 speed option or controllable pitch option are necessary.
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Old 06-10-2016, 12:34 PM   #7
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Nomad:

Quote:
That's it. With a propperly propped boat WOT (rated rpm) is the only engine speed where ideal engine max load is achieved.

What makes WOT an ideal load?
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Old 06-10-2016, 12:38 PM   #8
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Engine manufacturers are strongly against over propping. For Cummins and Yanmar, failure to prop a new installation to reach rated WOT, will cancel the warranty. That is a pretty strong message from the engine manufacturer. As a couple of others have said, over propping will not save you very much fuel, and fuel is a relatively small portion of the overall cost of owning and operating a boat. So far this year alone we have burned 1155 gallons. Within 50 miles of here the diesel price runs between $2.02 and $2.37. Shopping fuel price can save you more than over propping and does not break the warranty. If this information does not convince you, talk to a reputable prop shop. in my experience they also recommend against over propping.
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Old 06-10-2016, 01:08 PM   #9
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The slobbering 375-425 HP turbo twins in the mid-40 foot semi-planning hulls might be good candidates. They'll get over the hump and semi-plane in the 12 knot range with 350 HP, or so. Maybe reach 16-17 with 500 HP. The 300 hp excess is for a few knots more. These boats are also great candidates for a twin 250 repower. An issue to consider with reprop is minimum speed and low speed maneuverability around the dock...too much thrust at idle.
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Old 06-10-2016, 01:21 PM   #10
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When I do sea trials for a survey we always run to full throttle. The engine mechanics want to see if the engine will get to the deign max RPM, if not they want to know why. I have a listing now on a 53 Defever POC ( Performance Offshore Cruiser) which has twin CAT 3208 rated at 375 HP. The boat does not need that much power it cruises at 8 to 9 knots. At some time many years ago the props were changed to get better efficiency at low speeds but it will not reach rated RPM or ever get to 15 knots. That is fine for that design boat.
I had a listing a few years ago on a 70 Hatteras motor yacht and it would not even come close to rated RPM. The owner neglected to tell me that the props had been changed before he bought the boat. When we did a sea trial the buyer said to call him back only when the boat would reach rated RPM.
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Old 06-10-2016, 01:28 PM   #11
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Ok, so what if we just talk about gas engines, specifically 454s. Would it be possible to re-prop to get better mpg at 7-10 mph?
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Old 06-10-2016, 01:42 PM   #12
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folivier,
Yes ... but to little advantage.

Fuel is cheap now. What will you what to do when it gets expensive again?
And when fuel goes up your boat will be worth far less. Perhaps a boat more suitable to your needs is in your future .. the near future.

As a parting shot you may want to search "running a twin single" or variations of that and find the lengthly thread about that. Leave your slip as a twin, cruise as a single and make your landing as a twin when you finish for the day. All the time w the boat listed. Just a thought.
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Old 06-10-2016, 02:13 PM   #13
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Quote:
Originally Posted by folivier View Post
Ok, so what if we just talk about gas engines, specifically 454s. Would it be possible to re-prop to get better mpg at 7-10 mph?
I think not, at least not in any easy way.
Gasoline engines have a min BSFC too but it is usually at max volumetric efficiency which, depending on camshaft and MANY other things, can be at more RPM not less as in diesels. Plus you would never be able to obtain a fuel map to guide you. Not that diesel manufacturers publish them anymore either.

These are fuel maps for diesel engines One showing the effect of using a 2 speed transmission (or prop) to keep load in BSFC "sweet spot" resulting in a 12% fuel savings at cruise
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Old 06-10-2016, 02:54 PM   #14
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This thread is just like one several months ago that went on for 100 posts or so before petering out. Let me summarize:


You can pick up some mpg improvement by over propping, but you need to make a big change- 4" of pitch or more to make any real difference. Since that is too much of a pitch change to make on your existing props then it takes new props to do it. You will never recoup the cost of the props in fuel savings unless you cruise for many thousand of hours.


The mpg improvement is maybe 10-15% by over propping 4+ inches. It is hard to tell exactly how much unless you have the detailed fuel map for the engine and that isn't available to the public.


And as our resident broker notes above, you will probably have to go back to the original props if you ever want to sell it. Who else (but you) wants a motor yacht that can't go fast when needed because it is over propped.


Any other way of doing it like an adjustable pitch prop or a two speed transmission just isn't going to happen for a recreational boat and the cost will be much more than new props.


In other words, fuggitaboutit.


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Old 06-10-2016, 05:53 PM   #15
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Another option;

Autoprop: Propellors

"With other feathering and fixed pitch propellers performance under engine is a compromise. No such compromise exists with Autoprop, her blades admittedly rather an odd shape, are the result of a huge investment in design time and testing. The Autoprop blades, unlike those of conventional propellers, maximise the thrust delivered whatever rpm the yachts engine is running at. Conventional propellers are designed to provide maximum thrust at the engines maximum output; at any other point up to maximum revs the propeller will be operating at less than its potential. Autoprop however, subtly varies its pitch to optimise thrust whatever the engine's revolutions."
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Old 06-10-2016, 06:06 PM   #16
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Thanks for the education. A couple of the boats in my budget that we really like the layout are only available with gas engines. And my research looks like the best mileage is 1mpg at 7-8 knots.
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Old 06-10-2016, 06:10 PM   #17
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My hat goes off to whoever pre-visualized how the auto prop works!

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Old 06-10-2016, 11:48 PM   #18
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Quote:
Originally Posted by folivier View Post
Thanks for the education. A couple of the boats in my budget that we really like the layout are only available with gas engines. And my research looks like the best mileage is 1mpg at 7-8 knots.
Gas engines aren't so bad. Quiet, smooth and not too heavy. Running at or close to WOT almost as efficient as a diesel. Too bad they can't be run that way.
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Old 06-11-2016, 06:53 AM   #19
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What makes WOT an ideal load?

It is the ideal load for a mfg to specify , as it is seldom used.

Look at any "prop graph" and you will see hoe drasticaly the props ability to absorb power drops , even with a few RPM pull back.


The is a delight for the eng mfg as the lower the load on the engine , the easier it is working so the best chance to pass the warentee period at no expense.

A look at the fuel map (post #13) will show that an engine propped to full RPM will always be out of the best fuel burn zone.

The Mfg could care less , your fuel use is not on his tab.

Displacement boats with stoopid sized engines can usually reprop with good results.

A 2 speed tranny or a CPP is a better solution , but too costly for the first owner of a coastal boat.

For a low load on a gas engine some folks will re adjust the carb so the cruise RPM is on the small primaries , and not open the fuel gobbling secondaries.

This cant be done to all carbs , but worth a try if most cruising is just above idle.
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