kicker addition

[ALX357]

'Am considering adding a lower HP motor to the starboard side of the transom, outside of the rudder. A quiet 4-stroke motor, somewhere in the 6 to 9.9 hp range, not a built-in gas tank, for economical and quiet long term motor cruising at low speeds. As a back-up auxiliary, it is not going to replace the 50 hp center-mount.

My MacX carries only the load of motor(s), 24 ga. fuel, 10 gal HOH, 3 gal. portapot, 3 batteries, some tools, and 4 persons. - IOW, a medium lload.
( NO 30 gallon water tank, no air conditioner, no microwave, no plumbed head, no teak interior furniture, no motorized capstan, no pressure water system, no sub-woofered Fosgate amped stereo, no steel roll bar arch, no dragged or decked dingy. )

How much motor power is necessary to get the boat fast enough to empty ballast ? ( Speed vs. HP )
How much motor power is needed to manage the boat in a cross breeze ?
Any suggestions from experience involving specific brand and model of MOUNT / motor and location on a MacX ?

Your personal experience and knowledge would be greatly appreciated.

[bubba]

I have seen folks with a 2.5 hp for there dingy and kicker. This discussion has been kicked around and the 70hp 4 cycle I have or the 50hp 4 cycle at below 2000 rpm's get about the same fuel use as a smaller moter pushing at the same boat speed. So keep the rpm's down and you will get better fuel use.

[Trouts Dream]

As a back-up auxiliary, it is not going to replace the 50 hp center-mount.

How much motor power is necessary to get the boat fast enough to empty ballast ? ( Speed vs. HP )
How much motor power is needed to manage the boat in a cross breeze ?

I assume you will be keeping the 50HP available for these 2 tasks anyway.

[LOUIS B HOLUB]

'Am considering adding a lower HP motor to the starboard side of the transom, outside of the rudder. A quiet 4-stroke motor, somewhere in the 6 to 9.9 hp range, not a built-in gas tank. As a back-up auxiliary, it is not going to replace the 50 hp center-mount.

Im toying with the same idea, but less auxiliary HP than 9.9 or 6. Something just to move the boat in an emergency, and light enough to easily remove from the boat to storage, or to the dingy. I would like having 4HP, maybe 5HP and with the ease of a manageable weight.

My former Mac, a classic, had an 8 HP 2cyc. Merc. It moved the boat well, full ballast. The size & weight of that 8HP prevented removing or installing it when the boat was afloat.

[Love MACs]

How much motor power is necessary to get the boat fast enough to empty ballast ? ( Speed vs. HP )
Your personal experience and knowledge would be greatly appreciated.

Still begs the question? I am curious about that answer also.


Allan

[Trouts Dream]

I have a 97X (loaded for overnight cruising) and I need a minimum of 7.5 knots to drain and usually target for 8kts.

[ALX357]

Horsepower cf. speed on the MacX ?? anyone ? I know the speed required to empty ballast already, of course.

The idea was to have back-up/alternate for the main motor, and would sure like to have the kicker alone able to empty ballast.

How much motor power is necessary to get the boat fast enough to empty ballast ? ( Speed vs. HP )

Any suggestions from experience involving SPECIFIC brand and model of MOUNT / motor and location on a MacX ?

Your personal experience and knowledge would be greatly appreciated.

The suggestion that for a given speed, any size horsepower engine will get the same gas mileage is suspect at best.
I am thinking at 5 knots, an 8 hp motor will get better economy than a throttled back 50 hp.

[Frank C]

. . . The suggestion that for a given speed, any size horsepower engine will get the same gas mileage is suspect at best.
I am thinking at 5 knots, an 8 hp motor will get better economy than a throttled back 50 hp.

Actually Alex, I believe your basic assumption is incorrect, as long as you're measuring strictly four-stroke engines of various sizes (cubic displacement). I did some extensive research using Google. But I never saved any of those results, so all the following is strictly from memory.

• It takes a certain fixed horsepower to move identical boats at a given speed, and;
• Four-stroke internal combustion engines burn nearly identical fuel to produce a given horsepower.

Stated otherwise, producing a specific horsepower requires a defined amount of fuel energy, regardless of the size of engine. The essence of this claim is that one horsepower (550 ft.lbs/sec) requires a given no. of calories or BTUs. The physics are well-defined and most modern IC 4-stroke engines are very efficient ... especially with computer controlled EFI.

Granted, when comparing 4-stroke engines, a 50hp vs. 10hp, the larger engine might have slightly greater friction losses in producing an identical 10 horsepower. But those losses are nearly insignificant to the overall work being done. If you jump to a much larger engine size, you might find the friction losses become more significant?? But Billy's 140hp Suzuki has generally reported the same fuel consumption (or even better) than the 50s in his flotilla.

We must, of course, assume that the loads are constant, so a boat with heavier engine must carry less cargo. A useful search here might be using Billy as a target. Curiously, I'd also be guessing that Billy's single 140hp Suzuki weighs less than a pair of 4-strokes, the 50hp plus the 10hp.

Reading through these archives, you'll find many times that users report about 10 to 12 mpg at "hull speed" and that companion boats traveling at the same speed burn about the same amount of fuel ... again, regardless of their engine sizes (caveat, 4-stoke engines). If you're finding excessive fuel consumption, I'd suggest your best solution would be changing to an EFI outboard. You can find "demo" outboards on the iBoats.com website, usually late model engines with very little usage, at very attractive prices. Just a SWAG, but I'd bet that last year's Suzuki 50 EFI isn't terribly much more expensive than buying a new 10-horse outboard.

Finally, of course, your other goal is having a backup engine. I'd prefer a very lightweight 2.5 hp instead, foregoing some of the backup functions that are now reliably performed by the modern EFI primary motor. However, judging from my experience with Suzuki's EFI-60, I feel no need for a backup engine. Besides that, I doubt that a 10hp would be adequate to empty the ballast.

[atzserv]

Frank,

You mention 10-12 mpg at "hull speed". What is the hull speed on the Mac? I have an M but I would guess it is close to the same for the X.

I haven't done enough distance to get a milage figure but would be nice to know for lots of other things to watch for as far as engine performance.

Gary

[ALX357]

last years Yamaha 50 hp, $ 4,399.00 is a good deal, but nowhere comparable in price to the typical thirteen-hundred dollar low power 4 stroke.


I believe that 4-stroke internal combustion engines have a relatively low efficiency, as far as output compared to fuel potential energy. Somewhere below 50%. If a motor is running at its best energy efficiency at WOT, any lower RPM is going to be even less efficient. So .... a throttled back 50 hp motor running at 3000 RPM , compared to an 8 hp running wide open, - to yield a sustained speed of, say 8 knots, ?? So the energy "wasted" by a larger engine is going to be a greater amount that that of a smaller one, percentage of waste being the same. This greater waste of energy equates to more gasoline used. ( i think )

Now, if you are comparing a 150 hp motor to a 50 hp, that is a factor of three times the power, whereas the comparison of a 50 hp to an 8 hp, has a factor of over SIX times. 'Going on the assumption that the power is somewhat related to the displacement of the cylinders, etc. If they both get the same mileage, then they would both idle as long on a gallon of gas also ?

I freely admit I have no direct experience with these comparisons in the real boating world, only have the one motor, and that also I am not an engineer, -- but larger engines in a given vehicle, don't they usually eat more gas, for a given speed ??