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Old 12-11-2012, 17:06   #1
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Passage Speed

Recently I've been looking at various boats, with a view to a future purchase, that is hopefully going to happen sometime next year, but will be a 'training boat' rather than a dream boat :-)

However, having considered all options, I wondered something...

In the top speed pecking order, things seem fairly clearly defined:

1st: Tri
2nd: Cat
3rd: Mono

However, over a long passage (giant French boats excluded ;-)

Which would be quicker?

I ask this question as I saw a review for the SunFast 3200, talking about an 11 knot average, which seemed pretty quick for a mono. I wondered what the performance of similarly priced tri's/cat's would be over a longer period (I appreciate its not quite like for like, but something like a DragonFly 920 / 28 and maybe a Gemini 105).

Its just for interest really, but it got me thinking, so what does everyone think?
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Old 12-11-2012, 17:41   #2
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Re: Passage Speed

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I ask this question as I saw a review for the SunFast 3200, talking about an 11 knot average,


I dont think so. In fact if it will average 11 knots (i.e. 260 mile days) then I will buy one.

On the issue of passages, check in with folk who have travelled together on rallys etc, eg 44CruisingCat on this forum did a run with a number of other boats of varying types to Vanuatu recently. Also the Cairns Louisiades rally had a range of boats with good cats consistently faster, and the longer the faster.
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Old 12-11-2012, 17:49   #3
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Re: Passage Speed

In a displacement hull (ie mono) maximum speed is a function of waterline length.

The formula, as I recall, is 1.5 times the square root of the waterline length.

It is possible to exceed this speed while surfing, but maintaining surfing depends on conditions that would not be consistent over a long passage.

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Old 12-11-2012, 17:55   #4
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Re: Passage Speed

Sorry, I should have posted this initially, here is the review I'm refering to:

Jeanneau Sunfast 3200

Its a plaining hull, so should be capable of exceeding hull speed, given the right conditions.

But that aside, is 11knots average speed considered a fairly acceptable speed for a multi over 24 hours?
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Old 12-11-2012, 18:05   #5
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Re: Passage Speed

As far as I know, most monohull owners will budget on average passage speed of 5 knots. For catamarans / trimarans, you might budget a little more... maybe 6 knots.

For sure there are boats / sailors that can and do achieve higher average speeds than that, but to be honest, if you think that a 32 foot monohull can average 11 knots for more than an hour or two, especially in cruising trim (as opposed to racing trim with a full racing crew and every scrap of sail up, downwing in big seas), you ain't gonna be one of those, so work with 5-6 knots average on passages and you'll be not far off the mark.
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Old 12-11-2012, 18:09   #6
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Re: Passage Speed

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Originally Posted by westsail374 View Post
In a displacement hull (ie mono) maximum speed is a function of waterline length.

The formula, as I recall, is 1.5 times the square root of the waterline length.

It is possible to exceed this speed while surfing, but maintaining surfing depends on conditions that would not be consistent over a long passage.

Sail on, sailor!

Bill
From memory, the formula is more like hull speed = 1.34 x sq root waterline length. But these days modern monohulls get up and plane a lot easier than of yore. But, having said that, geting a monohull to plane for long periods of time, even for a light weight, modern hull shape requires a lot of sail area and a lot of skill and concentration.
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Old 12-11-2012, 18:12   #7
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Re: Passage Speed

Ah right, thats essentially what I'm trying to work out. So in cruising trim, would it be fair to say that say a Dragonfly 920 would cover similar ground to a modern, plaining hull 32 foot mono (very roughly)?

As for racing trim, does the same logic apply?

i.e. Over a 24 hour race, for arguements sake, although a tri/cat may perform better if conditions permit, assuming conditions are 'mixed' lets say not all downwind, moderate sea state etc. Could I expect a similar performance too, or if in racing trim (with the appropriate crew), would the multi's outperform (again I'm talking 30footish, not the large ocean crossing type)?
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Old 12-11-2012, 18:27   #8
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Re: Passage Speed

My understanding is that as a general rule, multihulls have higher speed potential than monohulls. This potential is generally highest downwind and least hard on the breeze. However, multihulls do have a more significant loss of performance if they are loaded much above optimum trim and loading above optimum is definitely a common occurence for long-term cruisers so that, as a very general and broad-brush rule, while multihulls should be significantly faster than monohulls, as live-aboard cruising platforms, in reality they often aren't as much faster than monohulls of similar length as their potential might indicate.

I don't really want to get too specific with numbers because frankly I'm not qualified to comment on actual crusing multihull performance - the multihull miles I have done were on a high performance racing cat, while racing (it was super-fast, scary fast even) but that performance isn't at all transferable to cruising performance on cruising multihull boats.

I will suggest, however, if you are giving more than a cursory glance at the potential speed performance of a boat that you intend to cruise, you probably need to readdress your list of criteria. Cruising is, for most people, a short handed (1 or 2 person) game, where speed is not of the essence and the journey is more imprtant than the destination. How comfortably you get there and how comfortable you will be when you get there are bigger considerations than the speed at which you could, potentially, get there...
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Old 12-11-2012, 18:32   #9
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Re: Passage Speed

Yep, that makes sense, its more about curiosity at the moment. I'm not going to be buying anything racey for the significant future. I'm just curious really, I like the idea of fast cruising, but of course like you say, the reality may not be the same as the ideal, due to provisions/baggage etc and of course conditions!
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Old 12-11-2012, 19:19   #10
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Re: Passage Speed

I believe on paper the Drago would win followed by the Sunfast and then the Gemini would come last.

However, it is all up to how you set up the track and how you gear the boats - you know some are more prone to lose speed if overloaded and it is easier to overload some of them. Etc.

BTW as cruising boats, I think the Garmin would win (most space and best stability) followed by the Sunfast and then the Drago would come last - very little space and can pitchpole too).

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Old 12-11-2012, 19:25   #11
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Re: Passage Speed

Just to correct a misconception. There is no such animal as hull speed. All the rule of thumb formulas are really just for describing a very narrow band of boats. Specifically those with very traditional length to width ratios. Any boat that falls outside of these parameters, these formulas just do not apply.

The reason this is important is that very narrow long hulls (like trimarans and catamarans) have length to beam ratios closer to the 8:1 ratio as compared to monohulls which are more in the 4:1 range. This means that the multihulls remain in displacement mode at speeds well in excess of a monohull of the same length. This is also why the original ULDB's like the Olson 30 were so fast, they are very skinny relative to other boats their size.

In practice, until you get beyond 60' it is unreasonable to expect even 200 mile days (8.33 kn avg) in a monohull. And even there you would expect those boats to be fully crewed raceing boats.

For a crusing boat to routinely hit 8.33kn average for days at a time you would need to be closer to 80'. This can be moderated somewhat by going to a very easily driven boat like the Dashew's but as mentioned above, thier boats are very long and narrow for the length, thus allowing them to have higher peak speeds, driving up the average speeds.

A catamaran on the other hand has much narrower hulls than the mono for the same length, and thus can maintain higher displacement speeds in the same conditions. Once they can plane them all bets are off, but planing on a cruising boat is frankly unrealistic, the amount of weight in gear needed to cruise comfortably just isn't going to allow planing speeds.

See http://www.bethandevans.com/pdf/200mile.pdf for more information.
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Old 12-11-2012, 19:51   #12
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Re: Passage Speed

Agree with this, except that 8:1 is a pretty fat cat. Most would be closer to 10:1, we are 12.5:1. And we live aboard, carry all the neccessities, watermaker, washing machine, etc.

We usually plan to average 8 knots, and usually do except in very light or very heavy conditions.
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Old 12-11-2012, 20:09   #13
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Re: Passage Speed

The best predictor of CRUISING speed for monohulls that I have found is the Leonard/Starzinger formula.

L/S formula is Daily Average Mileage =24*(2.62 + 0.066*SA/D+ 0.051*LWL ) obtained from their website and based on reviewing the logs of a bunch of boats they met while cruising and running the data thru a mathematical regression to determine what the important variables are.

If your passage is short enough that you can expend a racing level of effort to keep the boat going or you have a larger than normal crew I expect that this formula can be bettered by a significant amount.
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Old 12-11-2012, 20:28   #14
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Re: Passage Speed

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Originally Posted by Adelie View Post
The best predictor of CRUISING speed for monohulls that I have found is the Leonard/Starzinger formula.

L/S formula is Daily Average Mileage =24*(2.62 + 0.066*SA/D+ 0.051*LWL )
Please specify the units. I assume
SA = sail area, but it is square metres, square feet, acres, etc? Likewise
D = displacement, but is that kilograms, pounds or tons (imperial long, imperial short or metric)
LWL = waterline length, but metres, feet, rods, perches or furlongs
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Old 12-11-2012, 21:18   #15
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Re: Passage Speed

Just use the most obvious units. It works.
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