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Old 05-09-2013, 07:53   #1
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Cruise speeds and terminal hull speeds?

It's odd that this information seems a bit hard to find. I mean something simple like:

What is the cruise speed of a Lagoon 440 while running under power?

Sure it depends a bit on wind, weight, and engine size, but still. There really is very little info out there regarding these speeds.

It feels like every catamaran I've chartered, from a Lagoon 380 to an Orana 44 ends up maxing out at 7 knots and that just feels plain slow when you're motoring. It's fine when you're sailing because for me, I already assume that I'm not doing this to make the best time. But when I'm motoring it's often because there isn't any wind, or I need to make the best possible time and sail 100% on course.

I thought catamarans suffered less from terminal hull speed? EG., that simple formula of 1.34 x sqrt(waterline).

It hasn't felt like that to me. I definitely sense a speed hump at or around 7-8 knots. IE., a point where I need an every increasing amount of power to go the same amount faster.
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Old 05-09-2013, 08:09   #2
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Re: Cruise speeds and terminal hull speeds?

Dont forget Prop sizing and type - most boats are propped for economical cruising and of course folding feathering comes into it.

I can get 8.2 knots on flat water with a clean bottom - thats with outboards.
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Old 05-09-2013, 08:31   #3
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pirate Re: Cruise speeds and terminal hull speeds?

Its built for charter... smallest engine one can get away with... cheapest prop... the same with the mono's... lucky to get 5kts in flat water on my ex-charter 331..
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Old 05-09-2013, 08:45   #4
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Re: Cruise speeds and terminal hull speeds?

Seriously, I was half tempted to rig up the dinghy's outboard to the stern for another 0.25 knots. It's a shame that these boats which look so elegant and quick ... get passed up easily by huge floating blobs burning 10x more fuel.

Aren't power cats quicker than motor yachts of equal length?
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Old 05-09-2013, 14:12   #5
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Re: Cruise speeds and terminal hull speeds?

You can safely assume that around hull speed, the propulsive power is proportionnal to the speed at the 5th power. Then, to increase speed from 7 to 7.25kts, you have to multiply the engine power by (7.25/7)^5=1.19, assuming that the propeller efficiency remains the same (neither ventilation nor cavitation).

This increase in power isn't always available.

Assuming that the engine specific consumption (in gal/hp/hr) remains the same, the increase in fuel consumption for a given distance is then proportional to the speed at the 4th power, resulting in this case in 15% more fuel needed.

Alain
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Old 05-09-2013, 15:06   #6
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Re: Cruise speeds and terminal hull speeds?

you might want to learn about the difference between displacement and planing hulls
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Old 05-09-2013, 16:06   #7
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Re: Cruise speeds and terminal hull speeds?

Engines in sailing catamarans are auxillaries, not primary propulsion. Your sails are your primary propulsion. The engines are designed and spec'ed to get you in and out of the harbor or marina. They are not meant to cruise at maximum speed. The engines need to be light and not require much fuel so that you get good sailing performance, not great motoring performance. Many Cat companies offer power versions of some of their Cat models. The power versions have much bigger engines, much bigger fuel tanks to support those engines and much faster motoring cruise speeds. My boat cruises at 6 knots on one engine, 7 knots on two, and tops out at about 8.7, all on twin 40 hp Yanmar saildrives. The power version of my cat has 200+ hp engines, cruises at 14 knots and tops out at 16. My boat holds 110 gallons of Diesel, and the Power version holds about 500. My boat is much faster sailing than under power. If I had 500 gallons of fuel on board and huge engines my boat would probably have trouble moving under sail. I think the same is probably true for Lagoons.
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Old 05-09-2013, 16:42   #8
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Re: Cruise speeds and terminal hull speeds?

Quote:
Originally Posted by tamicatana View Post
you might want to learn about the difference between displacement and planing hulls
And you might want to learn about High Speed Displacement Hulls

Heres a good article from the Late Malcolm Tennant
Catamaran Comparisons by Malcolm Tennant

This catamaran from Chamberlin did a trip from Brisbane to Hobart without refueling (1109nm) cruising between 13-14knots using 1.3L/nm
PassageMaker - September 2008
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Old 05-09-2013, 17:18   #9
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Re: Cruise speeds and terminal hull speeds?

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you might want to learn about the difference between displacement and planing hulls
High speed displacement hulls

Multihulls
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Old 07-09-2013, 08:22   #10
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Re: Cruise speeds and terminal hull speeds?

Good info. Thanks for the links. I find hull design fascinating.

I think part of my issue also has to do with where I've been sailing. Here on the Chesapeake (departing from Annapolis) I've been planning trips that involve about 30nm from destination to destination (+/- 5-10 nm). So I'm normally looking at 4-5hrs of cruise time at around 7 knots. That can end up being quite a lot unless you plan your days efficiently (eg,. early rise, quick breakfast, X amount of time ashore, etc...). That often defeats the whole point of a nice relaxing sailing trip.

I have yet to sail with enough wind to push a catamaran more than 8knots. So the problem remains. I've got a bit too much distance with not enough power. It gets even more frustrating when there is no wind at all and you have to motor ... because you're back down to that snails pace.

It seems as though things are quite different in other parts of the world where, as one might suspect, there are much more active sailing (catamaran) industries like BVI, etc... Those strong steady winds make all the difference. Plus, I would guess you're there for at least one week at an absolute minimum. So the entire notion of a rigid time-base schedule can get tossed overboard and you can just enjoy the ride, or not and just enjoy the lovely clear water.

I also recall reading about the Lagoon 440 which was for a time built with 75hp engines, but the added weight and fuel consumption were simply not worth the ever-so-slight increase in speed. So they stopped even making them with the 75hp engines.

Why don't they offer more turbo charged power plants? They save weight and give you all that extra power if you want it. I guess they don't want to give people the option to burn so much fuel and then run out of gas, which would likely happen much more often resulting in complaints about design flaws.

Anyhow, thanks for helping me flesh out this particular little inlet of indulgence for my need for speed.

I need to get out of the Chesapeake.
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Old 07-09-2013, 09:46   #11
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Re: Cruise speeds and terminal hull speeds?

1989, 40 foot trimaran YANMAR ENDEAVOR, designed by Gino Morelli, powered by twin Yanmar 25 HP diesel outboards, a crew of three, with about 400 gallons of fuel, motored from San Francisco to Honolulu in just over eight days, averaging about 15 knots. No sails.
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Old 07-09-2013, 13:41   #12
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Re: Cruise speeds and terminal hull speeds?

A couple of things:

The 1.34 in the formula is actually another variable that takes into account the length to beam ratio. To really get a big bump, you need crazy high ratios to make a big difference. Going from a mono 3:1 to 8:1 on a cruising mono, it might get you a 1/4 knot. The high performance boats can be upwards of 20:1 but they sacrafice carrying capacity and other good cruising features.

The other thing I've noticed is hull shapes on sailing cats seem to be optimized for sailing. On a nice broad reach, the bows are pushed down and the stern comes up a little due to the sails pushing from above. When motoring, the prop tends to pull the stern down.
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Old 07-09-2013, 15:06   #13
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Re: Cruise speeds and terminal hull speeds?

Quote:
Originally Posted by Captain Bill View Post
Engines in sailing catamarans are auxillaries, not primary propulsion. Your sails are your primary propulsion. The engines are designed and spec'ed to get you in and out of the harbor or marina. They are not meant to cruise at maximum speed. The engines need to be light and not require much fuel so that you get good sailing performance, not great motoring performance. Many Cat companies offer power versions of some of their Cat models. The power versions have much bigger engines, much bigger fuel tanks to support those engines and much faster motoring cruise speeds. My boat cruises at 6 knots on one engine, 7 knots on two, and tops out at about 8.7, all on twin 40 hp Yanmar saildrives. The power version of my cat has 200+ hp engines, cruises at 14 knots and tops out at 16. My boat holds 110 gallons of Diesel, and the Power version holds about 500. My boat is much faster sailing than under power. If I had 500 gallons of fuel on board and huge engines my boat would probably have trouble moving under sail. I think the same is probably true for Lagoons.
Yep, this Is the answer.
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Old 07-09-2013, 15:10   #14
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Re: Cruise speeds and terminal hull speeds?

Quote:
Originally Posted by valhalla360 View Post
A couple of things:

The 1.34 in the formula is actually another variable that takes into account the length to beam ratio. To really get a big bump, you need crazy high ratios to make a big difference. Going from a mono 3:1 to 8:1 on a cruising mono, it might get you a 1/4 knot. The high performance boats can be upwards of 20:1 but they sacrafice carrying capacity and other good cruising features.
According to tank testing done by Edmund Bruce, at LWL:BWL ratios around 8:1, bow wave propagation is reduced to the extent that the hull speed "limit" no longer applies.

Most cats are above 8:1, very few monos are.
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