Inboard vs. Outboard for Cruising

[newhaul]

Originally Posted by brianlara 3

Not so Thomm. Firstly I motor at only 60% throttle load.
Second. My boat weighs abt 6,500 lbs normal weight.
38 litre freezer
480 ah agm batt
170 litres fuel
150 water.
A 35lb manson on bow, 25lb on stern.
Bow chain = 250 ft @ 5/16" stern chain 26ft 3/8"
Loads of power tools, 60 amp batt charger, inverter, 2kv generator, Springfield Ladderback helm chain on a transverse slide and 360 deg swivel, bread maker, 60 days of good food half of it frozen, 20lb and 10lb propane bottles..and on &on.
Believe me now?

Actually I do believe he was referring to me in my lil islander lead sled.

[brianlara 3]

Originally Posted by newhaul

Actually I do believe he was referring to me in my lil islander lead sled.

I told you I was slow. Second stuff-up today.....sigh.......

[valhalla360]

Originally Posted by Adelie

The diesel inboard's 50-75% fuel efficiency advantage I mentioned earlier was a mistake. That adavantage is for a diesel inboard over a gas/petrol inboard. For a 4-stroke outboard it would be about twice the range for the same amount of fuel and for a 2-stroke 2.5 or so better range.

Odd, our prior boat came in outboard or diesel. The diesel owners reported only around 20-30% fuel efficiency improvements.

[Adelie]

Originally Posted by Cadence

Take all the low HP things with a grain of salt. I doubt some people know how to figure there flat water speed with no current or wind?JMHO

That would be "their" not "there"

I would be one of those people that can. By several methods too.

1. Borne & Witte's nomograph is published in one of Ian Nicolson's books

2. Gerr's method from "the Propellor Book"

3. Skene's has a graph relating Speed-LWL ratio to thrust required per long ton. This one works best for calculating speed from a trolling motor. Knowing that outboards produce about 25 lb thrust per HP you can get a good ballpark for them too.

4. Psychosnail has a pretty good boat speed calculator which seems to be an implementation of Gerr's formulas.

For my Cal20 I figure I can hit almost 6kt with a 4hp. The motor is in a well so in heavy weather I should be able to do ok

I should be able to do 3kt using 10-12amps for a trolling motor and 4-4.5kt running at 30amps.

[SailingFan]

Well, to chime in on this one, I have an inboard Yanmar (8 hp) and an outboard Evinrude (8 HP also, but with slightly smaller prop, just barely). Both props have three blades. The sailboat prop is non-feathering.

Now, the Yanmar was not running when I bought the boat, I assumed it was dead and that I would have to remedy the situation at some date in the future by either fixing it or removing it for the additional storage space that removal would provide. The outboard is exceptional for a 2-stroke and as a long shaft design its prop is about precisely the same depth as that of the inboards blades. I used the outboard exclusively on my trip up the ICW from Daytona, and it did not fail me, not once. However...

There were multiple occasions when there was cavitation when I was dealing with wakes of other vessels. I can imagine that in open water with chop the outboard would quickly become rather cumbersome, but I also can honestly state the boat will turn in her length, pivoting on the keel, in a tight space, and that is by far an excellent situation, given how tight the St. Augustine marina was for us to navigate (in high winds, with contrary currents with the tidal flow as we wound through a maze of mega-dollar yachts that cost more than any house I have ever set foot in, to include some pretty schmantzy houses).

The outboard was quite valuable and effective, though it did go through some fuel. I cannot state how the inboard compares in effective horsepower though, because my three inch fouled hull still hit 5 plus knots with the outboard, and I am unsure if the inboard would have given more considering the state of hull fouling we had for the trip. I will repost later when that is evaluated, after she is back in the water again. My theoretical hull speed is under 7 knots, so I doubt the inboard would have given me much more than the outboard did in terms of speed.

The outboard was noisy. The inboard would likely be noisy as well, but I don't know how noisy it would be. Again, it was not running, and I only recently have gotten the cause of it's silence addressed, and am only now ready to bleed the system and attempt a start. I may do it Tuesday this week, in fact. We shall see.

I can also state that the outboard performed excellent second duty as a launch engine, but the long shaft made it a little tricky on ramps because of the increased depth, and also made it a bit difficult to steer the launch because of the geometry of force that long shaft offered in turns on a flat bottomed launch, and the horsepower was not terribly high, so the launch was almost planed but not quite, and would side-slip in turns easily. Still, beats swimmin' in gator infested waters!

I think I agree with the one that mentioned that the outboard was a good backup for the inboard which was itself a good backup for the sails on a sailboat. That seems to be a reasonable evaluation and represents my own experience in this process. If I had an oar or two on board, I would be even happier, but I do have a galvanized ten foot long pole for poling off of docks and the like, so I suppose that will have to do for now. I don't have tons of room anyway, but the thought of pulling the inboard and selling it off so I can make better use of the space it currently occupies keeps entering my mind, until I consider how much "fun" would be experienced in choppy water along a stone jetty with an outboard that was cavitating in the lumpy soup.

I really need to get that Yanmar running....

[brianlara 3]

Sailing Fan. A cavitating outboard is never fun. Even on the ICW you cavitated...not good.
Get that Yanmar going!
To hell with the space it takes up, get it going.

[thomm225]

Originally Posted by newhaul

Actually I do believe he was referring to me in my lil islander lead sled.

Yes, and I was mainly talking fuel mileage.

These newer model 4 strokes are pretty good on gas............even better with a bit of wind especially the smaller ones. (mine's a 2011, 5hp w/internal tank and external attachment)

I was motoring home across the bay a while back after the wind dropped to about 4 knots or so when a cloudline passed overhead bringing the wind back up to about 15-16.

I was so near my creek that I left the engine on and just rolled out the jib. The autopilot is doing the driving. (I just wish the video was more clear)

https://www.youtube.com/watch?v=yEw7pYeO0Oc

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

[hamburking]

Originally Posted by ET12

Wouldn't it be easier to work on an Engine outside the boat? Or in a standing position.

If you drop a part or tool while working on your inboard diesel engine, you pick it up. The same thing while working on your ouboard....splash...

[kmacdonald]

A diesel is about 30 to 35% more fuel efficient than a four stroke outboard. A four stroke outboard is about 15 or 20% more efficient than a two stroke outboard at cruise speed. At idle the four stroke outboard is waaaaaaaay more efficient than the two stroke. Differences in prop efficiency can also make a difference.

[valhalla360]

Originally Posted by hamburking

If you drop a part or tool while working on your inboard diesel engine, you pick it up. The same thing while working on your ouboard....splash...

This is highly dependent on the design.
- With many (but not all) inboards, if you drop the tool it is lost forever inaccessible down in the bilge.
- If you are doing major work, you pull the outboard up and into the cockpit where a dropped tool is easily reached. Also, there are easy options: I used to slide the dingy under the engine and drop it onto the floor of the dingy (still on it's bracket) to do oil changes and lower unit fluid changes. If I dropped the wrench, it fell into the dingy. Similarly I knew another guy with the same boat who simply strung a tarp under the motor to catch anything that might drop.

I would still far prefer to be out in the open air if I have to work on the engine.

[hamburking]

Originally Posted by kmacdonald

A diesel is about 30 to 35% more fuel efficient than a four stroke outboard. A four stroke outboard is about 15 or 20% more efficient than a two stroke outboard at cruise speed. At idle the four stroke outboard is waaaaaaaay more efficient than the two stroke. Differences in prop efficiency can also make a difference.

AGREE!!!

In addition, outboards usually have small portable gas tanks, and inboards have larger, integrated tanks.

My experience:

C&C25 with an 8hp 4stroke long shaft. Pushed the boat fine, but noisy. The prop would come out of the water in big seas. 5 gallon tank would run for 10 hours, giving a range of about 50 miles.

C&C27 with yanmar diesel. Built in 75 litre tank. Pushed the boat well, the prop was well under the boat, no trouble at all ever. The yanmar drank less than 1 litre per hour. So I could run for OVER 3 DAYS continuously, giving a range of about 400 miles.

BIG difference.

[valhalla360]

Originally Posted by hamburking

AGREE!!!

In addition, outboards usually have small portable gas tanks, and inboards have larger, integrated tanks.

Tank size and engine type are unrelated design considerations. On our prior boat, it came with diesel or outboard and had the same tank configuration.

[newhaul]

Originally Posted by hamburking

AGREE!!!

In addition, outboards usually have small portable gas tanks, and inboards have larger, integrated tanks.

My experience:

C&C25 with an 8hp 4stroke long shaft. Pushed the boat fine, but noisy. The prop would come out of the water in big seas. 5 gallon tank would run for 10 hours, giving a range of about 50 miles.

C&C27 with yanmar diesel. Built in 75 litre tank. Pushed the boat well, the prop was well under the boat, no trouble at all ever. The yanmar drank less than 1 litre per hour. So I could run for OVER 3 DAYS continuously, giving a range of about 400 miles.

BIG difference.

OK here is the rest of the story
75 ltrs is equal to about 20 gallons
So let's bring things to same same
Outboard 200 miles on 20 gallons
Inboard 400 miles on 20 gallons.
I personally think you are a bit low on diesel consumption more likely 1.5 Ltrs per hour. Which would give you about 300 miles
Still a significant improvement but that's the difference btwn diesel and gas.

[newhaul]

One thing I have not seen listed. My 8 HP outboard is also the motor for my tender so I don't have extra weight of an additional engine .

[kmacdonald]

My Capri 22 got 17 mpg with a 3.5 HP two stroke at 4.5 kts. It gets 20 mpg with a 4 HP Tohatsu four stroke at 4.5 kts. These are real measured numbers, not guesses.
The 4 HP will push it into strong wind and waves but the 3.5 HP would not. High thrust prop on the 4 HP makes a big difference.