How fast do sailing catamarans motor?

[Cadence]

Originally Posted by Mr B

Gemini 105mc does 7 knots, constant average at 2500 RPM on the Westerbeke 27 HP diesel at 3/4 gallons an hour, USA Gallons,

Thats loaded to the hilt and the plimsol line 2 inches below the waterline,

8 knots at 3000 RPM flat stick, Against an out going tide, GPS, It was doing 1.5 knots,
It takes 3 knots to make the Gemini sail with only a Genoa up.

Its about 3 knots under wind speed till it gets to about 12 knots, Then I keep it under that, 6 to 9 knots is comfortable,

Over 12 knots, The Gemini gets quite hairy, As in, Its moving very fast thru the water,
Single handed and in a very big ocean, I keep the knots down,
Very safe boat in very big waves,

A great write up. TY

[Kita]

Originally Posted by Mr B

Gemini 105mc does 7 knots, constant average at 2500 RPM on the Westerbeke 27 HP diesel at 3/4 gallons an hour, USA Gallons,

Thats loaded to the hilt and the plimsol line 2 inches below the waterline,

8 knots at 3000 RPM flat stick, Against an out going tide, GPS, It was doing 1.5 knots,
It takes 3 knots to make the Gemini sail with only a Genoa up.

Its about 3 knots under wind speed till it gets to about 12 knots, Then I keep it under that, 6 to 9 knots is comfortable,

Over 12 knots, The Gemini gets quite hairy, As in, Its moving very fast thru the water,
Single handed and in a very big ocean, I keep the knots down,
Very safe boat in very big waves,

I think I need to clean up my hull,it has some green slime still there,& I will get my prop checked, it has a little deformed spot..maybe a problem. .I just got mine & it has been sitting a while & I will clean up hull when I get home.. it seems as though mine is a bit more sluggish than what yours is... it won't do 8 knots by motor, I guess funk on the hull & a ding in the prop could account for the difference. It might get almost 7 & the fuel is about right..thanks, I was wondering

[Mr B]

Originally Posted by Mr B

Gemini 105mc does 7 knots, constant average at 2500 RPM on the Westerbeke 27 HP diesel at 3/4 gallons an hour, USA Gallons,

Thats loaded to the hilt and the plimsol line 2 inches below the waterline,

8 knots at 3000 RPM flat stick, Against an out going tide, GPS, It was doing 1.5 knots,
It takes 3 knots to make the Gemini sail with only a Genoa up.

Its about 3 knots under wind speed till it gets to about 12 knots, Then I keep it under that, 6 to 9 knots is comfortable,

Over 12 knots, The Gemini gets quite hairy, As in, Its moving very fast thru the water,
Single handed and in a very big ocean, I keep the knots down,
Very safe boat in very big waves,

It had clean Hulls as you can see,,
The first piccy i was sitting on the slipway for a week with a broken drive shaft, Bundberg Qld,
The second piccy was waiting for the tide to change at Maroochy River,
Clean hulls make all the difference to speed,
Even if its overloaded to the hilt,

[SailFastTri]

39 foot Dragonfly 1200 tri with Volvo Penta MD 2200 59 hp: 6-7 knots when saving fuel and keeping the noise down 2500-3000 rpm, 7-8 knots when pushing 3000-3500, 8-9 knots at WOT 4000 RPM MAX.
Dragonfly also made the boat with options for a 105 hp engine, or with a 150 hp engine. A friend who had the 150 hp model would cruise above 10 knots with a top speed of 14 knots under power. The rear end would squat under that much power. It's a sailboat.

[CatNewBee]

Potentially a cat can motor as fast as it can sail, the limiting factor is the hull-speed as with other vessels. at a certain point you can increase power, but the speed will not go up. The speed depends on hull length and width.

But sailing catamarans are not built to race on motor, so the engines are under-powered, they are only there to fight a light tide or river current, maneuver in harbors or help during light winds.

So 6kn seems quite reasonable for them under motor.

[valhalla360]

Originally Posted by Kita

I think I need to clean up my hull,it has some green slime still there,& I will get my prop checked, it has a little deformed spot..maybe a problem. .I just got mine & it has been sitting a while & I will clean up hull when I get home.. it seems as though mine is a bit more sluggish than what yours is... it won't do 8 knots by motor, I guess funk on the hull & a ding in the prop could account for the difference. It might get almost 7 & the fuel is about right..thanks, I was wondering

You would be amazed how much difference a little growth makes.

Had a 3400 with 40hp outboard and it would do 8kts full throttle and an easy 6-6.5kt cruise at half throttle, when we first got her (fresh bottom paint).

A year or so later in the Carolinas with some growth, cruise was down to 4.5kts and top end was 7kts. Jumped in cleaned the bottom at an anchorage and speeds were back up to normal.

Later we swapped for a 25hp outboard and top speed went down to 7.5kt but cruise speed stayed about the same.

[valhalla360]

Originally Posted by CatNewBee

Potentially a cat can motor as fast as it can sail, the limiting factor is the hull-speed as with other vessels. at a certain point you can increase power, but the speed will not go up. The speed depends on hull length and width.

But sailing catamarans are not built to race on motor, so the engines are under-powered, they are only there to fight a light tide or river current, maneuver in harbors or help during light winds.

So 6kn seems quite reasonable for them under motor.

This may or may not be true. 6kt in particular depends greatly on the hull design. For a 30' fat hulled cat, 6kt is a nice cruise speed but not hull speed. For a 45' performance cat, 6kt would be a slow cruise speed.

Also, most cats can outperform motor under sail. Part of it is raw power but also, you have to consider where the forces are applied.
- Under motor, the force is applied just below the water line and it tends to dig a hole resulting in the boat squatting and constantly climbing a hill. This is most pronounced as you try to push to hull speed.
- Under sail, the center of force is 15-20' above the waterline and tends to push the bows down keeping her more level partially negating the hill climbing effect.

[Kita]

Originally Posted by Mr B

It had clean Hulls as you can see,,
The first piccy i was sitting on the slipway for a week with a broken drive shaft, Bundberg Qld,
The second piccy was waiting for the tide to change at Maroochy River,
Clean hulls make all the difference to speed,
Even if its overloaded to the hilt,

What a coincidence I am sitting here with a broken I guess you could call it a drive shaft. The manufacturer called it an isolator, but it is the part between the drive leg and the engine. I guess the bolts come loose just a little bit and shook it to death. But I made 700 miles out of a 800 Mi trip

[Mr B]

Originally Posted by Kita

What a coincidence I am sitting here with a broken I guess you could call it a drive shaft. The manufacturer called it an isolator, but it is the part between the drive leg and the engine. I guess the bolts come loose just a little bit and shook it to death. But I made 700 miles out of a 800 Mi trip

Sorry for the thread Hijack,

You mean this little Beasty, I welded up the holes and retapped them,
I was told it came out of a Morris. Mini, which it didnt, So I wasted a week waiting for the wrecker to come back from holidays,

It went clatter clatter, Then nothing, Floating down the river, amongst all the anchored boats,
I dropped the anchor, That was great till the tide changed, A Danforth, It rolled over and got a rock stuck in the cleavis pin, Im floating down the river again, Dragging the anchor,
I caught a pile and tied off, I got the work boat from the slipway to come and get me,
This river is full of Bull Sharks, Im not going into it,

[Kita]

Originally Posted by Mr B

Sorry for the thread Hijack,

You mean this little Beasty, I welded up the holes and retapped them,
I was told it came out of a Morris. Mini, which it didnt, So I wasted a week waiting for the wrecker to come back from holidays,

It went clatter clatter, Then nothing, Floating down the river, amongst all the anchored boats,
I dropped the anchor, That was great till the tide changed, A Danforth, It rolled over and got a rock stuck in the cleavis pin, Im floating down the river again, Dragging the anchor,
I caught a pile and tied off, I got the work boat from the slipway to come and get me,
This river is full of Bull Sharks, Im not going into it,

Yep that's it!

[44'cruisingcat]

Originally Posted by CatNewBee

Potentially a cat can motor as fast as it can sail, the limiting factor is the hull-speed as with other vessels. at a certain point you can increase power, but the speed will not go up. The speed depends on hull length and width

Many cats are not constrained by hull speed.

[weavis]

Originally Posted by 44'cruisingcat

Many cats are not constrained by hull speed.

This is a question I've wanted to ask for a long time. I have been told this..

How does a Cat overcome the hull effect?

[44'cruisingcat]

Hull speed is basically the point where you don't have enough power to climb further up your bow wave. Boats with fine hulls like rowing sculls etc, simply don't generate much in the way of bow waves. Most cats hulls are fine enough.

[weavis]

Originally Posted by 44'cruisingcat

Hull speed is basically the point where you don't have enough power to climb further up your bow wave. Boats with fine hulls like rowing sculls etc, simply don't generate much in the way of bow waves. Most cats hulls are fine enough.

AAh.
That explains the very blade like look of modern cruising Cat hulls.

Thanks for that.

[44'cruisingcat]

Check out the bow wave. Up to 30 knots, theoretical hull speed less than 15



https://youtu.be/yrCh8NKMXyo