Gardner
In the limit a multihull operates as a single hull. For the single hull in the water, length and slenderness are the main factors affecting hydrodynamic drag - in the limit longer gives faster and narrower gives faster.
The sail carrying ability gets down to where the stabilising mass is located and how much there is. In this regard a proa is superior to either catamaran or trimaran.
With a cat and a tri the centre of mass is on the centreline of the boat because they are symmetrical side-to-side (unless pumped ballast is used). The modern tris are superior in the limit to cats because they have long slender amas able to carry the entire displacement. The larger, beamier central hull for accommodation on a tri will be flying when on the limit but has lots of windage.
With a proa it is possible to have the centre of mass much closer to the windward hull as symmetry side-to-side is not important. The windward hull can be heavy but squat to minimise windage as it is really flying ballast in the limit and does not need any hydrodynamic merits. The lee hull can be long, light, low and slender to maximise hydrodynamic performance while minimising windage.
So finding a single formula for rating these different configurations will not be meaningful. The proa has the fundamentals on its side if not the level of development seen with the other configurations.
Rick W
On 26/05/2010, at 12:33 PM, Gardner Pomper wrote:
Hi,
I have been occupying myself with what seems to be my favorite hobby; designing harryproas that I never get around to building. One of the things that occurred to me, is that I don't seem to have a good way to predict performance. I have been using the typical SA/D and Bruce number calculations, but I can't get these to provide me with number I can believe, based upon my personal experience and the speeds shown by Rare Bird on the Youtube videos.Here is a table I compiled:Boat Type Length Disp Sail Area SA/D Bruce #ME Cat 30 catamaran 30' 6000 500 24 1.2ME cat w/screacher catamaran 30' 6000 720 35 1.5Elan 7.7 trimaran 25' 2000 310 31 1.4Rare Bird harryproa 50' 10000 774 26 1.3Harry harryproa 40' 3500 452 31 1.4Harrigami harryproa 35' 2500 333 29 1.3Camper (mine) harryproa 30' 2000 392 39 1.6Contrarry (mine) harryproa 38' 3500 728 50 1.8It is a little hard to read, but basically it says that Rare Bird should be slower than my Maine Cat with the screacher up, or my Elan trimaran at any time. The Maine Cat never got over 12 kts, and that was surfing down a wave. With the screacher in 15 kts of wind, it would top out at about 10 kts. The Elan may have hit 15kts at one point, but that is when it was blowing 20+ kts and I was terrified of a capsize.So, given that the longer, sleeker, harryproa hulls are the deciding factor, what metric should I use to compare a harry design with "normal" multihulls?The reason I ask is that I may be overreaching for sail area on my designs, based on these numbers.Thanks,- Gardner
Rick Willoughby
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