5 hp Outboard on 30’ sloop (8600 lbs) vs 30 hp Atomic 4 on ICW

[boatman61]

Fitted an 8hp L/S Suzuki to a 37ft Waquiez for a delivery, the diesel engine was shot and needed a rebuild.
It performed well on the downriver run from Seville to the ocean and the 90nm we had to motor when the wind died just past Lisbon.
Just make sure the props as low as possible and if there's wind.. Sail.

[Svsumurun]

On our old P30, the PO did exactly this - the A4 had died , and they put a mount and 9.9 on the port side stern ( the shape of the stern plus the rudder I think was the issue besides the swim ladder).
They brought it from Ny city to Lake Erie and I remember him telling me against tide and current in the Hudson it was 2 knots at full throttle .
No idea how that would compare, as going up the Detroit river years later against bad current that year it was all the A4 would do to keep moving forward!
In the right conditions, sure , but not sure why? Is the A4 out of commission? It would definitely be easier I would think , and better on fuel ( that boat was 1 gallon an hour under most conditions)

[Adelie]

Originally Posted by Jammer

I'm in complete agreement with Jim, actually.

The OP asked whether the 5 hp would be sufficient "in calm water or moving along with the current." So I answered the question.

I agree that to windward or in chop it would not provide satisfactory power.


It would be worthwhile, if proceeding with the 5hp, to fit the shallowest pitch prop that is available for it.

Yep, get the flattest pitch prop you can, and if you can get a bigger diameter, better still.

[Adelie]

Originally Posted by Jammer

....

I would be surprised if there would turn out to be a meaningful difference in fuel economy.

Having just spent some considerable time researching this:

4-stroke outboards in the 2-15hp range deliver 9.4-11.4hp-hr/gal
4-stroke inboards deliver 11-12hp-hr/gal.
That's about a 10% advantage. The larger, slower prop will improve on that by some amount I have not researched. Let's call it somewhere in the 15-25% range of total advantage.

2-stroke outboards get 6.8-8.1 hp-hr/gal.
Diesels inboards 16.5-18.5 hp-hr/gal.

For diesel tractors this is a very serious economic issue that is closely tracked and the best tractors currently barely hit 20hp-hr/gal, most are similar to diesel inboards.

[thomm225]

A small 4 stroke outboard would work great in the ICW getting much better gas mileage than an Atomic 4 also.

Plus you can get a new one for about $1650.

I can cross about 20 miles of "open water" on a good day with about 1 and 1/2 gallons of gas.

Recently I had sailed 50 miles North and needed to get back but the wind was on the nose.

I had a bit over two gallons of gas.

I still motor sailed for close to 6 hours of the 10 hour return trip. Luckily I did have the tide

https://www.youtube.com/watch?v=cpZhq35etcw

[rslifkin]

Originally Posted by Adelie

Having just spent some considerable time researching this:

4-stroke outboards in the 2-15hp range deliver 9.4-11.4hp-hr/gal
4-stroke inboards deliver 11-12hp-hr/gal.
That's about a 10% advantage. The larger, slower prop will improve on that by some amount I have not researched. Let's call it somewhere in the 15-25% range of total advantage.

2-stroke outboards get 6.8-8.1 hp-hr/gal.
Diesels inboards 16.5-18.5 hp-hr/gal.

For diesel tractors this is a very serious economic issue that is closely tracked and the best tractors currently barely hit 20hp-hr/gal, most are similar to diesel inboards.

That's only at the peak of the efficiency curve though. Under light loads, the efficiency of a gas engine goes to crap. Combine that with the (much bigger) A4 being very old tech (low compression, primitive carb and head, etc.) and I wouldn't be surprised if it burns at least as much fuel.



That said, I'd rather have a working A4 than an undersized outboard, especially somewhere like the ICW where motoring is pretty much the only option. The A4 would push the boat at higher speeds and be able to motor in worse conditions than the outboard can (even better if the A4 has a reduction gearbox and can turn a bigger, slower prop).

[canvasman]

I also have a Bristol 27 I acquired in May. After working on it pretty much daily it was finally splashed last week. It has a Tohatsu 6hp Sail Pro with the 25 inch shaft mounted in a well that has been modified (by a previous owner) to allow the motor to kick up. When lowered, the prop sits not much higher than an inboard prop would. We motored out into Narragansett Bay at about half throttle, wind on the nose of about ten to twelve knots, waves less than two feet and against the current. The GPS read four knots and the knot log read six. The Bristol 27s are around 6600 lbs. Granted, these were not rough conditions but we were impressed by our progress. All my previous boats had diesels along with their benefits and drawbacks. As I know the outboards will have. As an aside, a friend with a wooden sailboat still has the original Atomic 4 and it runs and sounds great.

[thomm225]

Originally Posted by rslifkin

That's only at the peak of the efficiency curve though. Under light loads, the efficiency of a gas engine goes to crap. Combine that with the (much bigger) A4 being very old tech (low compression, primitive carb and head, etc.) and I wouldn't be surprised if it burns at least as much fuel.



That said, I'd rather have a working A4 than an undersized outboard, especially somewhere like the ICW where motoring is pretty much the only option. The A4 would push the boat at higher speeds and be able to motor in worse conditions than the outboard can (even better if the A4 has a reduction gearbox and can turn a bigger, slower prop).

Small 4 stroke gas outboards are very efficient on fuel right there with small diesels.

In the ICW, a small outboard would be especially good due to it's new technology and fuel economy

They do pretty well in open water also .....

https://www.youtube.com/watch?v=ri0c5GnZ1I8

https://www.youtube.com/watch?v=Jz9x5oRthfk

[rslifkin]

Originally Posted by thomm225

Small 4 stroke gas outboards are very efficient on fuel right there with small diesels.

No, they're not, although they should a bit better than an Atomic 4 (ignoring propulsion efficiency differences due to gearing, prop size, etc.). My 6hp dinghy outboard (modern 4 stroke) burns a little over 1/2 gallon/hr at WOT. So it's making 10 - 11 hp-hr / gallon. Modern bigger gas engines will manage 12 - 13 hp-hr / gallon (partly due to it being hard to make small engines as efficient, plus outboards have to run colder than ideal due to the raw water cooling system).

A typical modern-ish small sailboat diesel will manage around 18 hp-hr / gallon (and the efficiency will be much more consistent from idle up to full power than a gas engine). And some of the bigger marine turbodiesels are pushing into the 20 - 21 hp-hr / gallon range now.

[thomm225]

Originally Posted by rslifkin

No, they're not, although they should a bit better than an Atomic 4 (ignoring propulsion efficiency differences due to gearing, prop size, etc.). My 6hp dinghy outboard (modern 4 stroke) burns a little over 1/2 gallon/hr at WOT. So it's making 10 - 11 hp-hr / gallon. Modern bigger gas engines will manage 12 - 13 hp-hr / gallon (partly due to it being hard to make small engines as efficient, plus outboards have to run colder than ideal due to the raw water cooling system).

A typical modern-ish small sailboat diesel will manage around 18 hp-hr / gallon (and the efficiency will be much more consistent from idle up to full power than a gas engine). And some of the bigger marine turbodiesels are pushing into the 20 - 21 hp-hr / gallon range now.

I can cross 20 miles of open bay with about 1.5 gallons of gas.

3 weeks ago I made it back from 50 miles out motorsailing. I had about 2 gallons onboard.

Not sure when you used an outboard last to motor 20 miles. 1960 - 1970 maybe?

On top of all that, the the A4 is mounted INSIDE THE BOAT!

Unless it's new or Moyer's Rebuilt, it most likely will stink up your boat with leaking fuel if the boat doesn't already smell of gas

[rslifkin]

Originally Posted by thomm225

I can cross 20 miles of open bay with about 1.5 gallons of gas.

3 weeks ago I made it back from 50 miles out motorsailing. I had about 2 gallons onboard.

Not sure when you used an outboard last to motor 20 miles. 1960 - 1970 maybe?

On top of all that, the the A4 is mounted INSIDE THE BOAT!

Unless it's new or Moyer's Rebuilt, it most likely will stink up your boat with leaking fuel if the boat doesn't already smell of gas

If an A4 is stinking up the boat with fuel, there's something wrong with it and it needs an immediate repair.

If you're going slow enough, the outboard will be more efficient than the A4 just because it's a smaller, more modern engine. But if you're trying to motor close to hull speed, the difference is likely to be pretty small (partly because a 5hp outboard likely won't get a decent weight 30 footer to that speed and a bigger outboard will burn more fuel).

I wasn't even born in the 70s, so my outboard experience is more recent. The small modern 4 strokes are nice and efficient (I've recently put about 25nm on my dinghy with a 6hp while testing some stuff and have burned maybe a gallon of gas). But they're not some magical propulsion device that can push anything on minimal fuel worlds better than any other propulsion choice. And on a sailboat with any amount of overhang in the stern, outboards do have their limitations outside of calm weather simply due to the mounting location.

[thomm225]

Originally Posted by rslifkin

If an A4 is stinking up the boat with fuel, there's something wrong with it and it needs an immediate repair.

If you're going slow enough, the outboard will be more efficient than the A4 just because it's a smaller, more modern engine. But if you're trying to motor close to hull speed, the difference is likely to be pretty small (partly because a 5hp outboard likely won't get a decent weight 30 footer to that speed and a bigger outboard will burn more fuel).

I wasn't even born in the 70s, so my outboard experience is more recent. The small modern 4 strokes are nice and efficient (I've recently put about 25nm on my dinghy with a 6hp while testing some stuff and have burned maybe a gallon of gas). But they're not some magical propulsion device that can push anything on minimal fuel worlds better than any other propulsion choice. And on a sailboat with any amount of overhang in the stern, outboards do have their limitations outside of calm weather simply due to the mounting location.

Repair?! Are you kidding.

That would be like taking your 1970's car in and saying the engine leaks and smells please fix it for me this afternoon!

Those engines are ancient leaky and smelly plus they not only leak gas and oil but do so inside the boat.

This is why when James Baldwin does a refit on a Good Old Full Keel Cruising Boat he always goes the outboard route in boats up to 32'

9,000 lb Alberg 30 here: (Tohatsu 6 HP 4 stroke Sail Pro Outboard; removed A4)

https://www.youtube.com/watch?v=mDRrJ_udl7Y

[rslifkin]

Originally Posted by thomm225

Repair?! Are you kidding.

That would be like taking your 1970's car in and saying the engine leaks and smells please fix it for me this afternoon!

Those engines are ancient leaky and smelly plus they not only leak gas and oil but do so inside the boat.

Ancient design, sure. And they may leak oil. But if they're leaking gas, something is broken. They don't do that by design. FWIW, my mid 80s vintage gas inboards don't leak a drop of fuel or stink up the boat. And they're still old school carbs, no modern fuel injection or anything. Same story with the generator, and that's pretty much a 1940s design engine with a 1970s design carb on it (so not much more modern than an A4).

[thomm225]

Originally Posted by rslifkin

Ancient design, sure. And they may leak oil. But if they're leaking gas, something is broken. They don't do that by design. FWIW, my mid 80s vintage gas inboards don't leak a drop of fuel or stink up the boat. And they're still old school carbs, no modern fuel injection or anything. Same story with the generator, and that's pretty much a 1940s design engine with a 1970s design carb on it (so not much more modern than an A4).

Totally different deal on a power boat.

First off you aren't down there with the engine(s) like you are on a sailboat.

An old A4 and it's associated old fuel tank and old fuel lines can saturate a sailboat with the smell of gasoline........it can be quite awful to deal with.

Much better to have 3-4 gallons of gas onboard for your outboard with maybe extra in plastic jugs on the bow if motoring long distance on the ICW

[rslifkin]

Originally Posted by thomm225

Totally different deal on a power boat.

First off you aren't down there with the engine(s) like you are on a sailboat.

An old A4 and it's associated old fuel tank and old fuel lines can saturate a sailboat with the smell of gasoline........it can be quite awful to deal with.

I'm sure it's common, but it's not normal. If you're smelling fuel from old stuff, then it's time to rebuild the carb, replace the lines, confirm the tank isn't leaking, etc. And then the fuel smell should be gone.