Replacing diesel with outboard

bdmitchell · #1 11-26-2002

My husband Peter and I own a 28 ft. Morgan Tiger cub. We decided to replace our 18hp Yanmar diesel with a 4-stroke outboard with a long shaft. We are unsure whether or not a 15 hp will be adequate. Should we consider something larger? Any thoughts on this would be appreciated. We will be cruising the East Coast - NH to FLorida and then crossing to the Bahamas.

Jeff_H · #2 11-26-2002

The Tiger Cub is a really neat boat. I have not thought about them for probably 20 years. As to your plan to replace the inboard diesel with an outboard, with all due respect, given your proposed use of the boat, this is a really spectacularly bad idea. Get the diesel rebuilt. Most diesel rebuilds are really cheap compared to buying a big enough outboard to do what you are proposing.

The reason that I say that going to an outboard is a bad idea is that you will potentially be doing a huge amount of motoring and a lot of it will be in a very short chop. Outboards are next to useless in those conditions.

Beyond that, outboards require more fuel for a given speed and distance, so you will need to add bigger tanks, offsetting any space savings from removing the diesel.

As to size, you can probably get by with as little as 10 hp, but 15hp should be way more than adequate. The key will be to find an outboard with proper gearing and a ''power prop'' or ''high thrust prop'' rather than the normal standard ''speed props''.

Jeff

pirateofcapeann · #3 11-26-2002

I''ve been pushing my 28 foot wooden Herreshoff Solitaire with an extended deep shaft 8hp Mariner for 5 years now. Switching from an inboard Volvo MD7A to an outboard was the best decision I''ve made for the boat. As you well know, on the smaller cruising boat, space down below is at a premium. Not having to share it with machinery and fuel is a big boon indeed. I’d only report two downfalls. A: The battery charging rate of an outboard leaves much to be desired and B: I wish I had opted for a 15 hp model as the added kick when needed would be appreciated.

In rough seas, until my boat gets moving and gets her stern wake built up, she likes to hobby-horse. This caused me some initial concern, as she’d alternately hang the prop out of the water then submerge the power head. In response to my query, the manufacturer assured me that these motors are tested for such extremes and it wouldn’t present a problem. They must have been right because my outboard is still just as dependable as it was when it was brand new!

NH to Florida? Gosh, you’re missing out on the best cruising grounds on the east coast, and arguably, in the world: Maine!

flyfisher · #4 12-02-2002

But, isn''t gasoline much more likely to cause an explosion than diesel? I''ve avoided outboard equipped boats for sale because of this safety factor. Thoughts?

paulk · #5 12-02-2002

IMHO that somebody out there like Honda makes a diesel outboard in the 15hp range. Might be somewhat heavy, of course...

Jeff_H · #6 12-02-2002

While gasolene is more prone to explosion than a diesel, the whole engineering of a marine gas engine installation is intended to minimize the chance of explosion. In my lifetime virtually every sailboat that I have known of that exploded (with one exception) had a diesel engine and a propane stove. It seems amazing to me that the very same folks that would not have a gas engine onboard think nothing of carrying propane, and carrying propane without the bilge blowers and explosion proof electrical systems found on a gas engine boat.

Jeff

jklewissf · #7 12-02-2002

I sailed for years in florida and had a boat about the same size with an outboard. It was useless in anything but calm seas as the motion of the boat was always pulling the prop out of the water. Im sure that was hard on the motor.

It was a long shaft ob with a very low motor mount.

The other concern I would have is that the range on a gallon of fuel is much less on a gasoline engine.

Maybe a 20 hp yanmar would be cheaper and lighter than what you are thinking about doing. Even if the weight were greater, it would be in a better place on the boat.

pirateofcapeann · #8 12-03-2002

Gasoline, in its liquid state, is not flammable. It is the vapor that is explosive. For this reason, I keep my gas tanks outside of the hull. For local cruising, say within a 50 mile radius, I carry a 6-gallon gas tank of mixed fuel connected to the motor, a 5-gallon can of unmixed which can do double-duty both in the generator or mixed for extra motor fuel and a 1 gallon can of mixed for use in the skiff motor. These have homes in the foot-well of my cockpit and are not in the way. In fact, they make for rather nice foot-rests. On cruises of greater length, I lash two more 6-gallon tanks up on the foredeck.

My 8hp burns ¾ gallon per hour at cruising throttle which gives me a steady 5 knots. That equals out to a 160 mile “steaming” radius with a full load of fuel. Incidentally, my old inboard MD7A diesel had the same ¾ gallon pre hour consumption rate. With a 20-gallon tank and an actual useable amount of around 15 gallons before the internal baffles were useless and the fuel turned to foam, I still had to carry extra fuel!

Unlike some sailors, I live on a budget. I don’t mind laying to my own hook or eating Stop & Shop brand foods. It makes me feel good when I can enjoy all of the same ambiences of a place that the big boys do, without paying the big bucks at the marina. One of the ways I save money is, I carry a foldable two-wheeler cart. With this, and some spacers to go over the tank handles, I can in one trip, port all of my tanks to a gas station ashore and avoid the outrageous fuel prices found at the marinas. I found, on the average, fuel prices ashore run around 1/3 less.

I suppose I could write a book on the benefits of an outboard over an inboard. I could write about the superior maneuverability with an outboard, or the lack of excessive vibration from a diesel motor bouncing around in the bilge. How about getting rid of the excessive heat in the cabin and the noise and the odor. Cutlass bearings, shaft alignments, intakes for this and more holes for that! How about the hours of maintenance and enough spare parts to just about build a second motor, or the pile of wiring that would make an Italian spaghetti cook proud! Sometimes it felt as though my boat was merely a support platform for the friggin’ motor! No more!

I don’t sound too opinionated, do I?

In the summer, my outboard quietly rests, always ready, atop it’s bracket. It’s connected to the hull by its two clamp bolts and locked with a piece of chain, period! All of its systems, parts and gear are completely integrated in its one case. A few spare parts are below, tucked away in a small plastic toolbox. In the winter, I bring it ashore, put it in the cellar on a motor stand and take care of the yearly maintenance. There she’ll sit until spring when she’ll most likely fire on the first pull.

Last I heard, my old MD7A was still running the hydraulic power at a logging operation up in New Hampshire! Good place for it!

bikersailor · #9 12-03-2002

I agree with Jeff H. The OB might be a good option to simply go in and out of a slip or if the boat was less than 25 ft., but you most likely will be disappointed in it for extended cruising. With due respect to pirateofcapeann, IMO an outboard is much less efficient, does not have an adequate alternator for house battery charging, and can be very disconcerting in any kind of seaway. I went from an OB to diesel auxillary and would never (willingly) go back. Your money is better spent rebuilding the diesel.