How fast do sailing catamarans motor?

[Scotty Kiwi]

Originally Posted by Kenomac

Fountaine Pajot Cumberland 47 powercat video. Change the hull shape a bit, bin the costly sails and rig, then.... off you go!

What a video! The boat, the man, and the two girlfriends, buzzing around like a blowfly in a bottle.

Of well, that's powerpoking opps powerboating

[nuri kongur]

21 ft sailing catamaran with 20 hp suziki outboard 2 litres per hour 4 knots and 4 litres when the speed is 5 knots. I can motor up to 6,5 knots, but I don't know how much fuel the engine consumes. I can sail up to 7 knots per hour

[mikedefieslife]

Originally Posted by rbrentp

Just to pile on. I have a Prout Snowgoose with twin outboards. One is the XLong shaft Honda 15 and the other is a Yamaha 25 Long.

The honda pushes the boat at about 6 knts at 3700 rpm and a half gallon per hour. The Yamaha pushes at 6 knts at 3400 rpm at 3/4 gal per hour.

The honda is four blade and five inches deeper in the water. The Yama is 3 blade. Together they push to 8. The Yama might do better with an extension kit and four blades as it cavitates at about 3600 or any wave action.

Eventually I will sell the Yama for a second honda. For similar results I will burn less fuel.

What model Snowgoose do you have?

Can you share an image of how you've set-up your engines on the rear? Most with outboards use a single in the place of the drive leg.

[rbrentp]

DISCLAIMER: The work was done by a previous owner and I do not take credit or abuse for it. Picture on the hard was original with twin Yamaha 25s. Cavitation was terrible. The second in the lovely algae laden water of Galveston, shows the Honda on the left. At the top of the picture you can just see the engine cowlings above the rear deck. There are "boxtops that serve as covers, sound baffles and places to sit that cover the holes.

The transom is too low (obviously) and the bottom paint line indicates how over loaded she was when we bought her. Even so we hit 10 knots under sail.

Next bottom job this will be modified extensively to get more flow but it gives the idea. Being able to burst one forward and the other reverse allows her to turn on a dime.....very handy in tight marinas in the wind that never blows from the right direction.

As an aside: the hydraulic cylinder shown with the Honda was bought on ebay from wholesaledirect_01 as new. The mess is from a "new" cylinder that was on a shelf for 11 years. All cleaned up now but avoid that guy like the plague.

[smj]

For those using outboards on your multi I would recommend this product.
http://www.powerthruster.com
We have used these on 3 catamarans and it's $89 well spent.

[YARGESOL]

When I'm dreaming about my next boat,my dreams are concentrated arround this cat: Motorsailer Cordoba,Malcolm Tennant design:
"CORDOVA:-
18.3 m [60'] motor/sailer catamaran

At the time of its construction, this boat was the largest vinylester structure ever constructed in the Southern Hemisphere. What made it unique was that it was the first of our motor sailer designs to use our evolving CS high speed displacement power boat hull form. The 'Cordova' was the first of our designs to apply our unusual design philosophy to the design of motor sailers. What we did was to fit sails to what was a power boat hull form. It was the first motor sailer in the world to really embody the concept of the "best of all possible worlds". It sailed much faster than any comparable monohulled vessel ever could [24 knots under sail was recorded]. It also motored faster than most power boats at 18 to 20 knots. Along with this it incorporated the accommodation space of a fairly large sized house. The generous amount of space available was further enhanced by the interior fitout which utilized normal domestic appliances and commercial air conditioning. Another first in the marine area."
She's got 2x150 hp motors -not that much,for her size.
My current cat is also Tennant designed Timeliner 46. When surveyor at Ft.Lauderdale advised me to put levers on full throttle,she'd run 9 kts. I still feel the motors are small for her size/weight (2x27hp 3GM30).Good for a moderate sea and wind. I'm planning to unload a lot of stuff kept on board by previous owners and brought myself. Than she will do better. Max was surfing 3m swell at 15.8 with a 70 sqm ATN asimmetric spinnaker...blew up aspy (repaired)was skearing...

[mrybas]

Cowther 150 2x Nani 21hp
Cruising weight in flat water:
One engine cruising rpm: 5.5 knots
Both engines wot: 8.5 knots

I recently emptied the boat of most junk and would do 9.5 knots

[admiralslater]

Here is a list of boats that I know about . I will try to year list ,size ,design ,hp max speed motoring and sailing .
1 1973 SL 30 (Dads first design) 15 hp out board,6.5 knots and 14 knots
2 1980 45 foot custom with very skinny hulls . 20 hp outboard 12 knots and 21 knots .
3 1985 catalack 27 20 hp out board 6 knots and n/a . It squatted at 5.5 knots and no hp was going to make it faster .
4 1975 Brown Searunner 40 ,20 hp gas inboard 7.5 knots and 23 (surfing in a storm on Lake Ontario during an overnight race . I #%^*+ my pants .
5 1986 PDQ 27 9.9 Yamaha 4 stroke 7.5 knots and 18.3 (chute )
6 2003 PDQ 36 Twin 9.9 Yamahas 7.6 mast up 8.3 mast down . 18.5 (chute)
7 2003 Outremer 45 2x28 hp diesel with sail drives , 7.75 and 16 .
These are max speeds that I experienced.The Brown 40 was raced hard and had a huge chute we saw over 16 often . The Pdq 27 was home built by my brother and I as a poor mans stiletto. The Pdq 36 has dagger boards a taller rig and weighs 4 tons with out rig or water .

[valhalla360]

No there is not a simple formula like for traditional monohulls.

The reason is multihulls take many forms and are designed for different purposes.

Your run of the mill cruising multi will likely gain you 10-20% over a similar length monohull but it will vary with loading. Heavily loaded it will be about the same as the mono but kept light, you likely have a good turn of speed over a similar mono.

If you get a performance oriented multi and keep it light, you can blow away traditional hull speed sometimes by a factor of 2 or more without jacking up the HP to crazy levels.

Assuming you reinforce it, you can do similar increases by adding lots of HP. Gemini has a 15kt boat they are selling where they mostly just added lots of HP and reinforced the transom. Original boat tops out around 7-8kts with standard 25-30hp engine.

[smj]

With a single Honda 15 on our 38' catamaran that's 3 ton light displacement we get low 6's at cruising rpm and low to mid 7's full throttle.

[Catmandu]

What's interesting about cats is how fast they will motor on just one engine and how little difference the second engine makes. Adding a second engine only seems to increase the speed by 15% or so. For example when running one engine at 2,500 rpm you will not see the second engine start to make a difference until it is at at least 2,300 rpms. We experienced this on every cat we sailed.

As stated earlier, there are many variables: hull shape, hp, waterline length, cleanliness of hull and running gear and WIEGHT.

FWIW, Our 55' cat with 54hp Yanmar motors at 8.5 kts on one engine and 10.3 kts with both WOT. At cruising rpms (1,800-2,200) she motors at approx 6.9-7.3 kts on one and 7.8-8.6kts on two. Wind and waves can affect these numbers significantly.

YMMV.

[ZULU40]

Aside of displacement theory, planing theory and all else.

Power is a cube root law.
It takes 8 times the power to go twice as fast. (2x2x2=8)
One other contributing unit of power would add 1/8th or 12.5%, so you are pretty close
.

[sb1]

Originally Posted by Roy M

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.

Simpson 34' cat, 2 x 18 hp Yanmars with fixed blade props = 6 knots at 2300 rpm (if we are lucky), so far managed 13 knots under sail (once) but usually 7-8. But hey, we love it

[Mr B]

Originally Posted by Kita

How about a Gemini 105mc catamaran motor in flat water about 6.5 knots at a little less than a gal per hr I think. That seems to be hull speed unless you have current to help or slow you down.

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,

[buzzyearlight]

The responses from this forum have been tremendous, Thanks everyone for giving me a better understanding of catamaran motoring speeds!