I'm under propped. What should I move up to?

[rslifkin]

It might be worth checking fuel consumption at 8 kts on one engine vs 2. You might find that lower speeds (5 - 6 kts) is more efficient on one engine, but to do 8 kts, you might do better with both running (as you might be able to stay below the point where the props start to slip / cavitate like crazy).

[a64pilot]

Originally Posted by rslifkin

It might be worth checking fuel consumption at 8 kts on one engine vs 2. You might find that lower speeds (5 - 6 kts) is more efficient on one engine, but to do 8 kts, you might do better with both running (as you might be able to stay below the point where the props start to slip / cavitate like crazy).

That and it’s real common for engines to go very rich at high power to protect themselves from overheat and detonation.

This chart is for my little Suzuki and shows air to fuel ratio, you’ll see it only goes to max lean between 3,000 and 5,000 RPM, lean at low RPM runs rough at too high a power and it overheats so look at how rich it gets at high RPM/ power
This is a sort of typical AF ratio even for carbureted engines, just they don’t control it so exactly as there is no computer of course.

[kmacdonald]

It takes x amount of power to move boat y at speed z. The amount of fuel needed to create power x is a constant. The prop will have little influence in that. Prop the engine to get full recommended rpm @ wot. They may be under propped because someone wanted to cruise with one engine and achieve full rpm @ wot. Under propping would allow that. The way to save fuel is to slow down and maybe use one engine. Experiment a little on your trip. If the props are way off you are wasting power churning water. Cavitation.

[MicHughV]

Besides sailboats, I also run a twin engined fishing boat. I've had a lot of time to experiment with one engine...two engines...etc.....my engine instruments are all digitized so I know the exact fuel consumption at a glance vs. rpm, etc.....to .10 of a gallon/hour.

There's no question, that the speed/fuel consumption performance of the boat is directly related to whether the two engines are synched or not. Even a difference of only 100 rpm, can make one engine run harder than the other, jacking up the fuel consumption considerably and making a marked difference in speed and boat handling.

I'm talking 4,300 rpm vs. 4,200 rpm. and across the range, but most noticeable when on a plane at speed. At that rpm, one would think, there is not much difference between an engine running at 4,300 rpm and it's neighbor running at 4,200 rpm, but I can tell you that the difference is easily detected in speed, performance and fuel consumption.

Not only that, the one engine that is taking the brunt of the pushing power is also laboring, which can be clearly noted by the sound of the engine.

By adjusting the engine throttles so that both engines are running at the same rpm is instantly noticable in sound, engine harmony, and reduced fuel consumption.

Whether this is applicable to a multihull sailboat propulsion or not, I cannot say, but I suspect the same reasoning could likely apply. ie, better to have both engines running at the same rpm, rather than one engine doing all the work.

[Chotu]

Interesting . Maybe as said a little earlier, 2 engines running at moderate rpm might produce better thrust and fuel efficiency than one at WOT? I know consumption goes way, way down at lesser rpms but was always taught that one engine is less fuel no matter how hard you drive it.

However, in this case, if 1 engine has too narrow a propeller, doubling the active surface area, but running at much lower RPMs could theoretically save fuel. The extra output of one engine beyond what the prop can transfer to the water wouldn’t be wasted in a jet of water doing nothing.

I wish I saw this dynamic before. I’d like to try this out first.

Do you have a use case for running one engine at full speed vs both at the same speed for fuel consumption?

Originally Posted by MicHughV

Besides sailboats, I also run a twin engined fishing boat. I've had a lot of time to experiment with one engine...two engines...etc.....my engine instruments are all digitized so I know the exact fuel consumption at a glance vs. rpm, etc.....to .10 of a gallon/hour.

There's no question, that the speed/fuel consumption performance of the boat is directly related to whether the two engines are synched or not. Even a difference of only 100 rpm, can make one engine run harder than the other, jacking up the fuel consumption considerably and making a marked difference in speed and boat handling.

I'm talking 4,300 rpm vs. 4,200 rpm. and across the range, but most noticeable when on a plane at speed. At that rpm, one would think, there is not much difference between an engine running at 4,300 rpm and it's neighbor running at 4,200 rpm, but I can tell you that the difference is easily detected in speed, performance and fuel consumption.

Not only that, the one engine that is taking the brunt of the pushing power is also laboring, which can be clearly noted by the sound of the engine.

By adjusting the engine throttles so that both engines are running at the same rpm is instantly noticable in sound, engine harmony, and reduced fuel consumption.

Whether this is applicable to a multihull sailboat propulsion or not, I cannot say, but I suspect the same reasoning could likely apply. ie, better to have both engines running at the same rpm, rather than one engine doing all the work.

[MicHughV]

think of it this way...

2 guys are pushing a car down the road.

it requires 100 lb of force to keep the car going at 10 mph.....ie, overcome weight, air resistance, etc.

guy # 1 has both hands on the car and his shoulder leaning into the car....
guy # 2 has one hand on the car and just loping along.

because guy #1 is basically doing all the work, the force he provides on the car enables to car to move at 10 mph.
because guy 2, is basically just loping along with one hand on the car, his force alone is insufficient to propel the car at more than.... say 2 mph.
in other words, guy #1 is moving the car faster than than the guy # 2 is, in effect, guy # 2 is doing nothing, because the car is moving away from him faster than the speed he can provide.
even though guy # 2 has his hand on the car, his efforts amount to nothing, and guy #1 has to do ALL the work.

hope this analogy makes sense to you..

[Chotu]

Yes, I definitely understood all of this from your very first post. Made sense. Your first post was quite clear.


What I was asking in addition is if you have any Additional information for the case where it is just one guy pushing the entire car And the other one staying home. Versus two guys pushing it each equally. In the case where both push equally, they both are putting out about half the power. In the case where one of them does the whole job, they are putting out 100% of the power. But, we have to operate a brain, we have friction from shoes, things like that.

So one guy can do the whole thing while the other one stays home, or, they can both divide the work equally.

Which case will the guys need less food and calories?

I was always taught that one guy doing all of the work would need less food. Because you don’t have to run two brains, two sets of shoes losing energy to friction, etc.

Originally Posted by MicHughV

think of it this way...

2 guys are pushing a car down the road.

it requires 100 lb of force to keep the car going at 10 mph.....ie, overcome weight, air resistance, etc.

guy # 1 has both hands on the car and his shoulder leaning into the car....
guy # 2 has one hand on the car and just loping along.

because guy #1 is basically doing all the work, the force he provides on the car enables to car to move at 10 mph.
because guy 2, is basically just loping along with one hand on the car, his force alone is insufficient to propel the car at more than.... say 2 mph.
in other words, guy #1 is moving the car faster than than the guy # 2 is, in effect, guy # 2 is doing nothing, because the car is moving away from him faster than the speed he can provide.
even though guy # 2 has his hand on the car, his efforts amount to nothing, and guy #1 has to do ALL the work.

hope this analogy makes sense to you..

[rslifkin]

I know as an automotive example, when pulling a trailer up a hill with a gas powered vehicle, sometimes downshifting a gear beyond the highest it'll pull the hill with produces the same fuel burn, but with more reserve power available. The difference between the engine lugging along at almost WOT (with RPM constrained by load) and running rich in power enrichment mode versus turning an extra 1000 RPM, but not in power enrichment sometimes leads to equal fuel burn.

Gas engines don't typically have as flat an efficiency curve as a diesel. Usually best efficiency is just short of where it has to start going rich for safety and it drops a bit from there to WOT. And efficiency falls off badly at light load, which is why shutting one down at low speed saves fuel (while it makes a much smaller difference with diesels).

So sometimes a counter intuitive change puts you in a much better part of the efficiency curve and you come out ahead. But the behavior varies between engines, so you pretty much have to try it to find out.

[MicHughV]

chotu, I'm an engineer (now retired) by profession...so I understand (for the most part) the science behind various and sundry suppositions...

so to answer your question in the simplest way I can think of..the classic definition of work is:

"Work transfers energy from one place to another, or from one form to another..

so the work of the pusher (guy # 1) transfers his force (energy) to the car to overcome the force (energy) of the the car's weight, friction, etc.

so we get into static and dynamic forces, which can be defined as:

Statics deals with stationary situations mostly. It works on rigid bodies with or without an applied force. ... Whereas dynamics deals with situations where the rigid body is in motion. This motion adds a couple of more variables to the situation as speed, acceleration, friction, and so on, requires "external energy" to overcome...I repeat "external energy"..ie, the pushers....

..also don't forget the car could roll down a hill on it's own without the help of any pushers at all....but I won't go there right now...

..in simpler terms, energy does come not from nowhere....it requires, in the car example, muscle power (energy) from the pushers, which in turn requires food (fuel)) to provide the muscle power.

...seeing as guy # 1 is doing all the work, it stands to reason, he will require the most nutrients (fuel) to push the car.
...guy # 2, is also using energy (therefore consuming fuel) with his one hand on the car as he still has to walk alongside the car, and hence also using nutrients (fuel), but his efforts do not provide any meaningful power as the car is in effect being pushed away from him.

so guy # 2 is in essence is consuming fuel (energy) but not providing any meaningful force to push the car.

so guy # 1 has to burn extra fuel to provide the additional thrust that guy # 2 is not providing, while guy # 2 is still consuming fuel, but not providing any meaningful thrust, so yeah, guy # 2 might as well stay home and not burn any fuel at all.

guy #1 has adequate strength to provide his share ( 50 lbs thrust) to push the car, but is now being asked to provide 100 lb thrust, which is twice as much as he can reasonably be expected to do, requiring him to burn twice the energy, plus a bit more to overcome various other forces (ie, the drag from the non-rotating prop)

Pretty soon guy # 1 is going to be overcome by pushing twice his required load....his muscles will tire, his back will ache and his heart is being overworked....while guy # 2 is busy admiring the flowers..

I may have rambled on here a bit, this requires a bit of hand waving and perhaps some sketches to fully explain but I hope you get the gist of it.

My opinion, in simple terms, would be to run both engines at the same rpm for best efficiency and fuel consumption.

[a64pilot]

As a general rule an internal combustion engine’s efficiency peak is around 65% to 75% of max RPM.
Bu that’s the engines efficiency peak, meaning at that specific RPM and loading, it’s producing power at the least fuel burn.
However it’s not really that large a difference and is a almost never the max MPG speed, even in an automobile, reason is there are other factors that that way over come the slight efficiency increase at a set RPM/ load. Meaning the drag increase at speed.

I would assume those engines have a Can Bus as most modern engines do, and if they do it’s pretty sure that they can supply quite accurate fuel burn, that data fed into most chart plotters will give you things like fuel range if tank capacity is known and instantaneous MPG read out, just like an automobile.
Now buying that stuff isn’t ever going to pay for itself as far as fuel mileage is concerned, but I have graphed over the years three different boats, and the fuel mileage vs speed increase was rather dramatic.

Two engines producing the same HP as one is most often inefficient, the reason is there is twice the friction of the engines and twice the losses from running a water pump etc. But one of the major reasons to run one at a time is to cut the maintenance and maintenance expense in half and double the life of the engines, fuel savings is secondary for most people.


However it’s quite likely that as your engines can’t push the boat to a speed that won’t result in excessive prop slip and excessive cavitation, you may have a case that running both at a moderate RPM, and by moderate I mean lower than you want to and save fuel.

Maybe someone has a speed vs drag curve for a displacement hull.
https://images.app.goo.gl/sNQnUkT7Lw3eERU7A

this will work, this I think is for a rowing shell, which is of course pretty much the ultimate narrow beam hull, a regular “fat” displacement hull the curve would be steeper, but your hulls aren’t as “good” as a rowing shell either, your somewhere in between.

Anyway if you look at the graph roughly to go twice as fast requires four times the energy, now that’s not a set rule, the faster you go the steeper the curve gets, meaning that pretty soon to double the speed you require ten times the power and more.
Now think of that power increase as a fuel burn increase, and you get the idea that speed costs fuel, and the faster you go, the more fuel you burn, there is no point where an increase in speed results in a decrease in fuel consumption.

I can promise you that if fuel cost is the primary consideration that going slow will save you the most fuel, along with downloading a current app so that your not motoring into current but will wait until you have a current behind you to go. A 1 kt current is very common in the ICW, and 1 kt doesn’t sound like much until you consider that it’s the difference between 5 and 7 kts over the ground for the same fuel burn, if your trying to save fuel, it matters a lot.
Wind is also a killer, motoring into a 20 kt wind slows me down by a kt and a half, where motoring with the tailwind adds at least a kt. So watching winds matters too, there should be days that you don’t travel, and days where you leave before first light and days you don’t leave until lunch due to currents from tides.

Go slow and play the tides / currents and watch winds and you will burn half the fuel or less.

On edit, you can’t compare a planing hull at planing speeds to a displacement hull, especially running one engine for several reasons, first they can’t plane on one, and secondly the drag of a prop not producing thrust at planing speeds is a truly huge amount, I mean an enormous amount.
You can tilt a motor completely out of the water can’t you?
Absolute best mileage will be one one engine and it running slowly, I mean fast idle slowly and playing the currents / wind. But most would go insane doing that, so we motor at a more moderate speed.

Read this article. It’s pretty good
http://www.oceannavigator.com/March-...l-consumption/

[MicHughV]

a64pilot.....interestingly, if you go to Youtube and search " Robalo twin vs. single" there is a video documentary showing a test lake with two similar boats, one equipped with twin 150 hp Yamaha engines and one with a single 300 Yamaha hp engine, the boats are run side by side, etc and the end of the video depicts charts which compare speed, fuel burn, mpg, etc..

however, I have the same identical boat, equipped with the twin 150's and I can tell you without a shred of hesitancy, that unless both engines are run at identical rpm's, speed, handling and boat performance and most importantly fuel consumption are very much impacted. This is quick to notice by reducing the rpm on one engine by 100 rpm and watching the speed dial and fuel consumption be affected.

still, it's hard to compare and evaluate this situation without also delving into the different gear ratio's of the 150 vs. the 300.

in the horsepower wars, a 150 Yamaha is a 2.7 liter engine, whereas a 200 Yamaha is a 2.8 liter engine. Only 100 cc difference, yet providing 50 more hp, ha, but one must take into account the different final drive ratio's and also the max. operating rpm of both engines.

there is " sales and marketing" at play with engines that sometimes have little do with engine performance.

regardless, I remain steadfast in my opinion, that a twin engined boat is best run with both engines engaged and running at the same rpm.

[MicHughV]

and finally....yes, the non running engine will require less maintenance, but the engine that is doing the twice the labor will eventually require a lot of maintenance or even replacement....
more efficient ???

[Chotu]

Great comments, guys. Thank you!

Plenty to think about and experiment with. I wish I knew all of this BEFORE I bought the engines and built the outboard wells and slides and everything. I saw so many awesome performance cats with outboards and wanted to keep the sailing performance great (my drive units retract out of the water for sailing performance), that I missed the crucial prop diameter issue. I just assumed 30hp is 30hp. A 20hp diesel is specified in the plans.

Assuming these are going to be auxiliary engines for a sailboat and not as heavily relied upon as the case where they are the main propulsion system, another important way to save fuel is to get the damn mast on already!! Ha ha ha.

Thanks again for these comments. I’m finally understanding.

[MicHughV]

"horse power" is not to be confused with " torque"
"torque" is the actual work being done..."horse power" is the effort to keep it going

...torque can be measured on a dyno....horse power is basically a mathematical equation.

like riding a bicycle...your legs ( torque ) provide the pedal power, but your lungs ( horse power) provide the effort to keep your legs going...

The horse power wars is basically all marketing as few people understand the intricacies of the two together...

[a64pilot]

Originally Posted by MicHughV

a64pilot.....interestingly, if you go to Youtube and search " Robalo twin vs. single" there is a video documentary showing a test lake with two similar boats, one equipped with twin 150 hp Yamaha engines and one with a single 300 Yamaha hp engine, the boats are run side by side, etc and the end of the video depicts charts which compare speed, fuel burn, mpg, etc..

however, I have the same identical boat, equipped with the twin 150's and I can tell you without a shred of hesitancy, that unless both engines are run at identical rpm's, speed, handling and boat performance and most importantly fuel consumption are very much impacted. This is quick to notice by reducing the rpm on one engine by 100 rpm and watching the speed dial and fuel consumption be affected.



still, it's hard to compare and evaluate this situation without also delving into the different gear ratio's of the 150 vs. the 300.

in the horsepower wars, a 150 Yamaha is a 2.7 liter engine, whereas a 200 Yamaha is a 2.8 liter engine. Only 100 cc difference, yet providing 50 more hp, ha, but one must take into account the different final drive ratio's and also the max. operating rpm of both engines.

there is " sales and marketing" at play with engines that sometimes have little do with engine performance.

regardless, I remain steadfast in my opinion, that a twin engined boat is best run with both engines engaged and running at the same rpm.

You can’t compare a planing boat under plane, to a displacement vessel.
Your 300 Hp? What size and weight of boat? I’m 41.5” and 24,000 lbs, and 40 HP, there is a rather large difference.

But engine wise, the Suzuki 9.9 to 20 HP are the same engine. The Mercury Verado 225 to I believe now 400 HP are the same engine.
Using one engine for a broad stead of power is common and makes sense from a manufacturing and parts standpoint.

Your boat will burn much less fuel at its displacement speeds with one engine running and the other trimmed out of the water, try it to see. Now displacement speed are speeds at which you are making no or only a very small wave, which due to your waterline length is probably much less than you think it is. It’s a speed before your bow begins to lift even a little bit