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Old 26-08-2017, 08:26   #1
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How fast do sailing catamarans motor?

My question relates to sailing catamarans or trimarans: Is there a reliable way to determine approximately how fast they motor? I know that most catamarans sailing speed ranges between 7-15knts but I was wondering how they perform under power. I understand that a larger engine will cause them to go faster, but do they have a max hull speed like a sailboat, or will they perform more like a large pontoon boat or a "power catamaran" under power?

For example, lets say their is a 30ft sailing catamaran with a 60hp outboard attached to it. If the beam is 14.5ft and the displacement is 7000 lbs. Is there an equation that I can use to determine approximate cruising speed or can anyone reference and examples that may be similar to this case so I can work off a baseline?

Thanks a ton.
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Old 26-08-2017, 08:54   #2
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Re: How fast do sailing catamarans motor?

You should be able to sail in light winds with a multihull.

There is not much reason to consider motoring speeds.
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Old 26-08-2017, 09:47   #3
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Re: How fast do sailing catamarans motor?

The calculations for a catamaran are more complicated. The formula for catamaran hull speed is 1.34*(wetted length)1/2; however, this drag formula is generally not the limiting factor for catamaran hull speed. This is because boats with waterline length to beam ratios greater than 8:1 are not limited by hydrodynamic drag factors, whereas smaller boats need to plane to do so (planing requires enormous amounts of power for displacement hulls). A more important factor to consider is the prismatic coefficient, Cp. Cp = V/(LBP*Am), where V is the volume of water displaced by the hull, LBP is the length between perpendiculars, and Am is the area at midship.

I felt really smart for a few seconds....but I copied all that from internet...have no idea what it means.
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Old 26-08-2017, 10:16   #4
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Re: How fast do sailing catamarans motor?

I forgot to mention the following in my previous post.
Very fast boats actually require a high prismatic coefficient, which in turn requires a less-narrow boat. However, narrower hulls can get away with a lower prismatic coefficient. The ideal range of Cp for a catamaran is between 0.61 and 0.65. There are a few ways of increasing the prismatic coefficients: sailors can use bulb bows, a wide planing aft segment, or a flat hull rocker in conjunction with a bustle aft. Though high prismatic coefficients increase drag at low speed, at high speeds they can reduce drag by as much as ten percent. I think what all this means is that if you put a 60hp engine in a 30' cat, it gonna go like hell. That's all I got to say about it; and I'm sticking to it.
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Old 26-08-2017, 11:37   #5
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Re: How fast do sailing catamarans motor?

Most sailing catamarans can sustain 6-8 knots under power but not much faster. I don't know why.
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Old 26-08-2017, 13:25   #6
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Re: How fast do sailing catamarans motor?

My Prout Snowgoose will do 6knts in calm flat water at a good cruising RPM with a 30hp Yanmar. More often that not I get around 5-6knts. The same Cat with a 40hp outboard will do close to 8knts.

I've been on larger catamarans that will easily do 10knts using just one engine, but perhaps something in the 7knt range is more common for Lagoons and the like.
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Old 26-08-2017, 13:57   #7
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Re: How fast do sailing catamarans motor?

Lagoon 450 cruises on one 54 HP engine at 2200rpm around 6knots at consumption of around 3.8 litres per hr.

With Both engines at 2400 revs 8.3 kn with 9.4 litres per hour.
Both engines flat out 9.1 knots 3000 rpm fuel consumption is 20.5 Litres per hour
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Old 26-08-2017, 14:11   #8
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Re: How fast do sailing catamarans motor?

For a typical 38ft cruising cat you will motor at about 8-9 knots top speed. Going 8 knots will use about 50% more fuel than 7 knots so you motor at 6-7 instead.

Drag is related to the square if the boat speed so going fast needs huge amounts of power and fuel. I added a second motor to my boat - and got an extra 1 knot top speed with almost double fuel consumption - 9.9 knots now.

Sailing cats and power cats have very different shapes. A power cat has straight aft sections and a sailing cat has upsweep in the keel line. The two are very different and you can't make a power cat shape work well when sailing in lighter winds and upwind and you can't make a sailing shape work well at high speeds under power.

Just chill out and go at 7 knots. Even though our boat has hit 19.9 knots, we average about 7.5 to 8.5 most of the time. You don't go that fast.
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Old 26-08-2017, 14:29   #9
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Re: How fast do sailing catamarans motor?

Isn't it about power versus drag? if you fit a big enough engine and open the throttles wide you get on the plane and can start accelerating till you get to the next limiting factor.

That could possibly be the motors breaking away from their mountings and setting off on their own as you exceed the structural limits of the boat........
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Old 26-08-2017, 14:34   #10
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Re: How fast do sailing catamarans motor?

Quote:
Originally Posted by twohapence View Post
Isn't it about power versus drag? if you fit a big enough engine and open the throttles wide you get on the plane and can start accelerating till you get to the next limiting factor.

That could possibly be the motors breaking away from their mountings and setting off on their own as you exceed the structural limits of the boat........
Funny you should say that............
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Old 26-08-2017, 14:57   #11
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Re: How fast do sailing catamarans motor?

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Funny you should say that............
Do go on.......
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Old 26-08-2017, 15:00   #12
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Re: How fast do sailing catamarans motor?

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Originally Posted by twohapence View Post
Do go on.......
I had a Dory rated for 40 knots max.. fishing boat...
Strapped an 85hp on the back... soon was sat in the front with me along with the transom...

Delightful sunset as my mates pulled us to shore.......
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Old 26-08-2017, 15:20   #13
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Re: How fast do sailing catamarans motor?

It has been my understanding that a displacement hull, one that moves through the water by displacing it rather than planing above it, is limited in speed by its length at waterline. Push it beyond a certain point with progressively larger engines, and there is a correspondingly large bow wave poised to form to oppose any further increases in hull speed.
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Old 26-08-2017, 16:44   #14
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Re: How fast do sailing catamarans motor?

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Originally Posted by catsketcher View Post
For a typical 38ft cruising cat you will motor at about 8-9 knots top speed. Going 8 knots will use about 50% more fuel than 7 knots so you motor at 6-7 instead.



Drag is related to the square if the boat speed so going fast needs huge amounts of power and fuel. I added a second motor to my boat - and got an extra 1 knot top speed with almost double fuel consumption - 9.9 knots now.



Sailing cats and power cats have very different shapes. A power cat has straight aft sections and a sailing cat has upsweep in the keel line. The two are very different and you can't make a power cat shape work well when sailing in lighter winds and upwind and you can't make a sailing shape work well at high speeds under power.



Just chill out and go at 7 knots. Even though our boat has hit 19.9 knots, we average about 7.5 to 8.5 most of the time. You don't go that fast.

I'm guessing for most sailors a 7.5-8.5 kt. Average would be their most memorable sail of the year!


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Old 26-08-2017, 17:28   #15
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Re: How fast do sailing catamarans motor?

Searunner 40 trimaran, 30 hp Yanmar with folding 2 blade 18x12 prop = 8 knots at 2300 rpm, 20 knots under sail, unless I'm really nutso.
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