Real cruising multihull speeds

[BigCat]

Originally Posted by Therapy

HUH? ..........

Huh, what? It seems pretty clear to me. Skinny hulls are faster than fat hulls. Push your hand through the water, finger tips forward--easy. Pushing your hand through the water sideways--harder.

[drewke07]

I think this thread is concerned with the average daily speeds that one can expect with a cruising multi. And in my view it is pretty simple. A longer boat will be faster, a narrower hull will be faster, and a boat that is properly loaded will sail well. But the most important part of the equasion is the desire of the captain and crew for a comfortable passage.

Sure, a serious sailor could push out 25% more miles most days, but who wants to scare the kids? In any event every boat owner will find his groove with his boat and be able to travel the world at what he believes to be a reasonable speed. That becomes his "real cruising multihull speed"

[Troutbridge]

Originally Posted by BigCat

The Broadblue 385 is one of those boats where the builders give you a displacement without telling you what its basis is-loaded, empty, or half-tanks. Looking at it, one can see that there is no trampoline, and that the bridge deck takes up a very high percentage of the boat's overall length-well above average. Clearly, the design goal was a maximum of accommodations for its length, and other considerations were secondary.

Using the figures given by the builders, it has a DL ratio of 156, a SA/D ratio of 18.75 with the gennoa, and the waterline square root is 5.97. Looking at drawings of the interior, I guesstimate the hull waterline beam at 5', which would make the hull L/B about 7.2. So, one wouldn't expect a lot of speed from this design. If, as I suspect, the displacement is unloaded, one would have to correct the DL and SA/D figures, and speed expectations would be even lower. As the saying goes, 'built for comfort, not for speed.'

(Further research tells me that the displacement given is the 'light' displacement. Add a hundred gallons of fuel and the same of water, at least two people and their gear, and you have added a good 3000+ pounds. That lowers the SA/D to 16.8, and the D/L increases to 186. This combined with beamy hulls will give those who understand these figures the expectation that the Broadblue's performance would indeed be similar to an equivalent monohull.)

Not quite. Obviously I talked to the builders, their figures are basically with half tanks, so +50 (imperial) gallons of fuel and the same for water.
The design criteria was for 'Blue water cruising' for a couple & occasional guests. The ability to carry a decent load featured high in the priorities but work was done on the underwater profile to 'speed the boat up' a bit. People with more experience on the boat than me get better average speeds, but then as I said I sail very conservatively. The hull form is a compromise between speed and load carrying, they weren't quite so concerned about maximising living space as the boat was not designed with the charter market in mind.