How fast do sailing catamarans motor?

[Taichungman]

St. Francis 50 .....2-57hp Yanmar common-rail fuel injection.
Motored for 4 days between equator and St. Lucia last year.
1 engine 2000rpm's 7kts...of course potential to go faster but best use of fuel at that speed. If you have a pocket full of money, you can go faster.

[tomfl]

Originally Posted by danielamartindm

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.

As was posted earlier this is true of what I will call traditional designed monohulls; but if a boat has a length to beam ratio of greater than say 10:1 (think the earlier poster said 8.5:1) then the bow wave no longer is a factor. The 10:1 ratio is often mentioned as the dividing line between condomarans (which are not limited by bow waves) and performance catamarans which will normally blast past a condomaran with it's fatter hulls.

Another consideration for cats is the beam of the boat. While the 10:1 ratio is not limited by the bow wave there is a bow wave; in fact there are four bow waves, two from each hull. While the outboard bow waves are not a real issue the two bow waves between the hulls can cause problems. This is why modern cats have their hulls farther apart than most older cats.

I had an interesting online discussion with Ian Farrier about how the faster multihulls have sugar scoops that often times have their aft most section slightly above the water to prevent the stern of the boat squatting in the water and causing additional drag.

Just as an aside the trimaran used in "Water World" really seemed to move along in the movie. In part this was because there were two boats. The fast one was powered by a 220HP Volvo engine. There are also several very fast, and in my opinion over powered, tris and cats that travel as very high speeds.

As with most things related to boats if you are willing to pay the big bucks you could get a NA to design a tri that could easily cruise at 20+ knots and most likey sail at that speed as well.

But in the real world it is unlikely you could buy a multihull that would motor at even 10 knots, and something like 6-7 knots is realistic. Unless you are a Russian mafia billionaire.

[Smooth Cruiser]

As others have said you will pretty much find that 6-8 knots is normal for a multihull motoring and 10 is about the most you could achieve even with lots of extra horse power.

I know a few cats that are designed for sailing and have tried to get motoring faster (putting 75hp engines in each hull in a 50ft boat) and all that happens is they bog down in the sterns and start pushing s**t uphill. They still max out at about 10knots.

Similarly there are a few sailing cats that have been designed to motor fast (15+ knots) and they could only achieve this by adding lots of bouyancy in the sterns to keep the sterns up and a proper trim. This design then is no longer optimised for sailing, and looks more like the hulls of a power cat.

So if motoring at speed is important to you it can be done, but you will compromise sailing performance. In this case you should buy a power cat.

[Kenomac]

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

https://m.youtube.com/watch?v=WtRysaFr_2U

[twohapence]

Originally Posted by danielamartindm

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.

I am not an expert so this is just my limited understanding of a complex subject.

You have theoretical maximum hull speed, or displacement speed, which is when the boat is effectively sitting in a hole between the bow wave and the stern wave. It takes power to get out of that hole and bring the bow wave a lot further back so you can ride the wave. If you have enough power then displacement speed is not a limitation.

Lighter boats, e.g. catamarans, dinghies, speedboats and so forth get on the plane relatively easily. Different conditions apply to a hull on the plane. Heavier boats, such as monohulls with loads of ballast to keep them upright, supertankers, etc are less likely to be able to do this.

Getting a big motor boat onto the plane takes a lot of power which is why they usually create massive amounts of wash. All that power has to go somewhere. If they moved at hull speed they'd be more fuel efficient and cause less wash.

I do wonder when the green lobby will look at power boats and how fuel hungry they are when moving fast.

[catsketcher]

Under power even my 10-1 hulls generate a large wake at high speeds. You can't get around the physics that resistance goes up by the square - its an immutable law - even for planing or narrow hulls and reasonably light boats.

The attempts to combine the two different hull types - a la Macregor 26 - results in a poor sailing boat.

[catsketcher]

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.

20 knots in a Searunner - surely you are joking. 14 maybe but I can't see the good old girls hitting 20s without you going totally nutso. My performance cruising cat has never hit 20 - got to 19.9 on a wave once - just missed out.

[mikedefieslife]

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 terrible video in terms of marketing. Thy really shouldn't have let Youtube stabilise it.

Anyway, about the boat. I imagine the cost of sails and rigging is a less than a 2 years fuel. 40 litres per hour is not uncommon for a motorboat.

[Jimske]

Well that makes me feel better period on my 31 searunner I use a 9.9 outboard and in Flat Water I can get about five knots with 4000 4500 RPMs. I have to make my fuel consumption on that motor about 2 hours of Motoring per gallon. But that is definitely Flat Water. A little Kern against me even I'm going to drop down to 3.8 knots. My friend put a 20 horsepower on his he does much better. I'd like to have a 20 horsepower but I couldn't find one with an extra long shaft

[twohapence]

Originally Posted by Jimske

Well that makes me feel better period on my 31 searunner I use a 9.9 outboard and in Flat Water I can get about five knots with 4000 4500 RPMs. I have to make my fuel consumption on that motor about 2 hours of Motoring per gallon. But that is definitely Flat Water. A little Kern against me even I'm going to drop down to 3.8 knots. My friend put a 20 horsepower on his he does much better. I'd like to have a 20 horsepower but I couldn't find one with an extra long shaft

Try a Yamaha 9.9 Extra Long Shaft high thrust or the similar Mariner. They'll give you more drive with a bigger prop turning more slowly.

[smj]

Originally Posted by Jimske

Well that makes me feel better period on my 31 searunner I use a 9.9 outboard and in Flat Water I can get about five knots with 4000 4500 RPMs. I have to make my fuel consumption on that motor about 2 hours of Motoring per gallon. But that is definitely Flat Water. A little Kern against me even I'm going to drop down to 3.8 knots. My friend put a 20 horsepower on his he does much better. I'd like to have a 20 horsepower but I couldn't find one with an extra long shaft

We have had good luck with the Honda hi-thrust 15 with xtra long shaft. Seems to have a little more thrust and speed compared to the Yamaha hi-thrust 9.9

[gilligansail]

6-8 knots on my Outremer 49
6 with one engine
7.5 with 2 running

[Cheechako]

My 42 cat, heavily loaded motored at 8-9 knots, flat water, no problem. two Yanmar 3GM30 diesels/ max props. The real stuff is motorsailing though. They do superb at that.

[Cadence]

All I can offer tri info.. My 31SR would attain 7.5 Kn as hull speed with a 15nHonda 4 stroke at about 2/3 throttle. I could squeeze 8 wide open. I only tried that once for curiosities sake.

[snokid]

ok I'm a total novice when it comes to displacement boats. What I can tell you is I have a 25' boat with 340hp and it will around 60 mph. I have had bigger offshore boats in the past up to 42' with 1200hp 130mph boats.

Now I was happy to be hitting 7knts yesterday!!!!

but what happens if I take that 50hp diesel out and stuff a supercharged monster down there?

I know it costs more for fuel...

bob