380: Re Power with Bigger Yanmar

[kauai_420]

What is the biggest SD yanmar that will fit in the lagoon 380?
I would like to get 10 -12 knots or more motoring.

[careka]

The boat can take up to 60 HP total, and the hull speed is, ca 8,5 knots.
Dont even think of trying to get more out of it with motors running. waste og time and money.
with sails you will easy get more speed.

[westwinds]

Originally Posted by careka

The boat can take up to 60 HP total, and the hull speed is, ca 8,5 knots.

Originally Posted by careka

Don’t even think of trying to get more out of it with motors running, waste good time and money.
with sails you will easy get more speed.

Quite true, the only reason to have more horsepower than needed for hull speed is if you are trying to move the boat from a lee shore against strong winds. The marine architect most likely did supply engines that are more than needed for hull speed for this reason. Even at hull speed, you are going to use an awful lot of fuel. What's interesting is to make a graph of miles per gallon fuel. Start by making a table of RPM vs. speed. Get a stop watch and GPS on a calm day and get your data. You will then need graphs supplied by the engine manufacturer to get horsepower vs. RPM. There should also be a graph of horsepower vs. fuel consumption. From this you can calculate miles per gallon. There is going to be a huge variation in miles per gallon depending on speed. I did this for a 30 foot sailboat and got 6 miles per gallon at 3400 RPM and 5.6 knots, AND 14 miles per gallon for 2000 RPM and 3.3 knots. Your mileage may vary.

[westwinds]

Here are the graphs for the GM Yanmar diesels:
http://www.sailingboatefaki.gr/yanmar%20gm%20series.pdf

[djmarchand]

A good rule of thumb for a displacement hull is that it takes 15 hp per 10,000 lbs displacement to push the boat to hull speed (the conventional 1.34 definition which may not apply to cats with their long, narrow hulls). With a Lagoon 380s cruising weight of 20,000 lbs and a lwl of 36, then that is 30 hp to push it to 8 kts.

30 out of 56/58 (the maximum output of two 3gms or 3YMs) is about as much as you want to pull out of those engines on a continuous basis.

To push it to 10-12 kts is going to take a lot more horsepower, probably more than the hull can stand.

David