Bigger Outboard on Sailboat

[michaelmrc]

just a question im curious as to why you dont see bigger outboards on sailboats.say 20-30hp O/B on a 30-33 ft sailboat. im would imagine that it would be far cheaper to replace a 20-30hp diesel with a same rated outboard. can someone give me the pros and cons of both... thanks

[Ex-Calif]

Weight aft of the transom
Strength & design of the transom structure

The Macgregors use some big engines but the structure and boat is designed for it.

[michaelmrc]

could this not be reinforced or moved forward. i see many boats with the ob in the aft lazerette that has been cut out and reinforced

[David M]

Once a full displacement hull reaches hull speed, it takes a tremendous more amount of power to increase the speed slightly. Its pretty much a cubed relationship.

For larger sailboats, the amount of pitch makes it impracticable to put an outboard on the transom. It would take a ridiculously long shaft (the vertical element of the outboard) to make it work.

[michaelmrc]

i understand but the costs savings to go to an outboard must be quite significant. price difference of say a 25 hp westerbeke vs say a 25 hp mercury 4stroke is quite substantial i would imagine.

[David M]

Yes, inboards are more expensive on a per horsepower basis, especially diesel inboards. But after a certain length range, you have no choice but to go with an inboard for a sailboat.

For sailboats, that LOA is typically on the mid twenties...although there are a few extreme examples outside of what is typical.

Some sailboats have a well where they can put an outboard but these in my opinion are too much of a PITA than its worth. I don't think there are any newer sailboats that have an outboard well.

Also, you really don't want a gasoline inboard engine inside a boat for safety reasons. That can be argued but I know I prefer not.

[perchance]

They did. They call them saildrives.

[David M]

Hah ....forgot about those. They still don't have the advantage of regular outboard in being able to easily remove them to take them in for servicing. Also the older ones cannot be fully retracted out of the water although there are some newer ones that can. I don't know the cost of sail drives, but I cannot imagine them to be as inexpensive as an outboard on a per horsepower cost.

[Ex-Calif]

Many of the Maxi's here have been converted to outboard power. The owner looks at the cost of replacing/overhauling the Volvo and decides to pull it and hang an outboard. So for cost reasons it is being done.

Once you put an "outboard" motor in a well in the hull doesn't it start to become an inboard?

Fundamentally, the cost of inboard diesels are disproportionate to their HP, utility and technology. These things should be as cheap as chips. They are not because they are specialized, marinized and the market for them is very small.

I venture that mercury sells a lot more 25-50hp outboards than Volvo or yanmar sells diesels.

[beau]

In designing my 40ft motor/ sailor,
I found it very hard to justify buying a diesel.
They are heavy, cost the earth, and do not have the longevity of diesels in trucks or tractors.
The reason that diesels have a high failure rate in cruising boats, is that diesels run best under a load. A lot of cruisers run their motor to recharge batteries or only use them for getting in/of the harbour.
The cylinders glaze over and life of the motor is significantly reduced.

The boat should be designed for Outboards.
( I would not recommend an Inboard petrol motor on a yacht)

It is also not a good idea to just hang them out the back.
Get the lower geared/ high trust/larger propellor model with a long or extra long shaft.
The only down side, in my mind, is you will need larger fuel capacity for petrol motors.
The four strokes give better fuel economy over a wider range of RPM than a two stroke.
Two strokes are lighter, much easier to fix and much much cheaper.(half price?)
If your new model two stroke outboard motor is run at 3500-4500 rpm, fuel consumption will not be that much different from a four stroke.

I am in the process of putting 2 X 90 two stroke outboards on my 40ft aluminium fast displacement cruising trimaran.

One motor at 4,000rpm will give me a cruising speed of 10 knots.
The other motor is a spare. Or by running both motors together I will get 16 knots plus if I need it.?

We have sails, when there is wind, for sailing downwind or on a reach.
We can't Tack.
But hey, the sails, mast and rigging cost a total of $2,000.

I know lots of cruisers (particually mono's) will swear that Diesels are the only way? However modern outboards are much better than they used to be, they are light and cheap.
More and more Multihulls are now going with two outboards, properly mounted. I would recommended buying larger HP than generally recommended and run them at no more than 1/2 to 3/4 throttle giving more economical fuel consumption.