Theoretical Hull Speed of Multihulls

penultimate

New member
Joined
12 Sep 2004
Messages
345
Location
Cargreen, Cornwall
Visit site
Bladerunner's post re Theoretical Hull Speed set me thinking about Multihulls. I seem to remember something called the Herreschoff Coefficient which is a length / beam ratio above which a multihull is not limited by wave making resistance and hence has no theoretical upper limiting hull speed. The figure of 15:1 comes to mind. Is this figure correct? Also, reason would suggest that there should be a gradual reduction in wave making resistance rather than a cut off point at a particular length / breadth ratio?
Sorry to be a bore, but I'm waiting for the fog to lift.
 

boatmike

Well-known member
Joined
30 Jun 2002
Messages
7,044
Location
Solent
Visit site
You need a hydrodynamicist to answer this one fully but the simple answer in practical terms is that you are correct in assuming that the resistance of the hull increases as the beam of each hull increases relative to its OAL. It never reaches zero but becomes much less significant at your value of 15-1. The rule you speak of is like many others. A simplification of the relationship of beam to OAL in median sea conditions that modifies the well known but equally oversimplified rule regarding the speed relative to the square root of waterline length. It is also necessary to consider wetted area resistance which will increase with depth of hull/keel, but this is less of a limiting factor than beam. Thus racing hulls on cats have great length and little beam or depth being very light displacement. Unfortunately this does not help the cruising yachtsman because this type of hull will not carry much load. For the cruising yachtsman it is necessary to increase the beam of the hulls to carry load which reduces speed. There is no such thing as a perfect boat...... The fog remains...... /forums/images/graemlins/confused.gif
 

snowleopard

Active member
Joined
16 May 2001
Messages
33,645
Location
Oxford
Visit site
above about 8:1 the wave-making resistance reduces and above 10:1 you can keep getting faster by applying more power.

there are exceptions where hull form generates waves, bill o'brian and tom lack's designs for example but round-bilge forms without excessive rocker generally have no hull speed effect.
 

boatmike

Well-known member
Joined
30 Jun 2002
Messages
7,044
Location
Solent
Visit site
Er... Keep going faster by adding more power sounds like there is no limit to how fast you can go....... Wanna think about that?
 

snowleopard

Active member
Joined
16 May 2001
Messages
33,645
Location
Oxford
Visit site
i've thought about it a fair bit!

in a multi, the drag increases with speed so while 50hp extra may get you from 5 knots to 10, another 50hp might only get you 2 more and the next 50hp only 1 more. it's certainly not a linear progression. obviously if you start pumping in huge amounts of power something will eventually break. the workable upper limits however are pretty high, you've only to watch a fast cat ferry going by to see that.

the big difference between the multi and mono scenarios is that with a wave-making hull form there is a sharp increase in drag once you reach hull speed whereas with a multi the increase in drag is progressive.

take 2 boats of 35 ft lwl

speeds with various powers are something like-

25hp 50hp 100hp
non-planing mono 7kn 7.9kn 8.1kn
multi 7kn 10kn 14kn

(i get 11 knots from 56hp on that lwl)
 
Top