A “Bruce number” is used by multihull designers to compare the power to weight ratio of multihulls.
Go to:
http://www.multihullboatbuilder.com/bruceno.html
to find the Bruce number for your multihull.
Comparisons:
Ok multihullers time to publish your numbers-besides it will be a good database for all of us!
Doug Lord
–High Technology Sailing/Racing
My Multione cat= 2.079 (top rigs, couting all that roach)
Bigger no. is better?
Luff 'em & leave 'em.
Bigger number is better but the number doesn’t take into account ,as best as I can tell, what the POWER TO CARRY SAIL is. Its fine to have a high ratio but can the boat carry the sail area? In what wind strength? It would be good if a number could be derived that would be dimensionless yet reflect power to carry sail. But since that factor is generated by so many different means on different boats I don’t know how it could be done…
Your BN is excellent!
Doug Lord
–High Technology Sailing/Racing
Parlier’s cat (not scaled)=2.284 [:o]
Luff 'em & leave 'em.
Just ran across a post by Peter Birch(page 4, General Discussion, Jan.24th,2004) describing the weights of competitive Australian multihulls as being between: 3-4kg(6.6-8.8lb.)
That results in Bruce numbers between 1.659 for the lightest to 1.507 for the heaviest…
Doug Lord
–High Technology Sailing/Racing
how about your F48 class multi??..oops sorry, there is no such multi from microsail (yet), and how about your X3…dang…same,no X3 (yet)…better have a Bruce number, than nothing at all! [;)]
Wis
if it isn’t broken, don’t fix it!
This is an attempt to build a database of important comparative information regarding various multihull desgns. If you have nothing to contribute please don’t post here.
Doug Lord
–High Technology Sailing/Racing
I guess you should do same…
how can you post some Bruce Number without an X3 having hit the water?? maybe I am just wrong…but I still didnt see an X3!..I’d better stop here [;)]
Wis
if it isn’t broken, don’t fix it!
The Bruce Number is a DESIGN comparison and the BN of the X3 is listed in the first post.
Doug Lord
–High Technology Sailing/Racing
More:
A) D4Z Trapeze Power Ballast System Formula 48 Catamaran:
Doug Lord
–High Technology Sailing/Racing
Doug, 4kg for an F48? never actually realised that, thats a bit on the heavy side. I would think 2.5kg would be the absolute max you would want, my basa mini40 came out around that. Making multi’s heavy is just a no go, as the heavyer the boat the less its going to accelerate in gusts, so the power from the rig either drives the lee bow down or the windward hull up (depending on the volume distribution, use of T foils etc).
Luff 'em & leave 'em.
Some who sail heavy multis say that they perform better in light air because they don’t stop in the lulls; Chris Jackson said that if the boat (mini 40) isn’t 2kg or less then it is too heavy.
The heavy boats will have substantial more power to carry sail as the wind picks up. Boats like the D4Z where 25%+ of displacement is in removable movable ballast have high power to carry sail coupled with lite weight in lite air(ballast removed).
But the foiler can be built lite and has unlimited power to carry sail so there are some limitations on using the Bruce Number to estimate a designs’ performance or to compare it with another multi.
Without some number that can factor in power to carry sail the comparisons using BN’s may only be valid-to whatever extent-in lite air and on full size boats with no movable ballast or foils. Further, BN’s don’t reflect the intangible’s that show up on a race course: 1)susceptability to pitchpole/capsize,2) ability to tack.
BN’s are calulated using the max sail area either upwind or upwind + downwind but every model multihull will need to reduce sail as the wind picks up. The BN at that sail area will be much lower than with the maximum. Model multihull’s, under both the mini 40 rule and the Formula 48 rule, are allowed to vary displacement as well as sail area further clouding the BN picture.
So for models I’m not sure that the BN represents a good comparative tool if used alone.
Doug Lord
–High Technology Sailing/Racing
Well, I am certainly happy we cleared “THAT” up !
You want to build a database of numbers that don’t mean anything because of difference in wind strengths. I would also think the “number” of hulls would play a significant role. If one changes a rig size, that too changes the numbers by a significant amount. Not sure what good your number are - or how important they are to the average multihull sailor. The race results - and your one quote pretty much sums it all up.
<font size=“2”><blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>BN’s don’t reflect the intangible’s that show up on a race course<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”></font id=“size2”>
<center>[:-indifferent]</center>
My point in saying what I did was to caution people not to draw too many conclusions, at this point, regarding the application of Bruce numbers to rc multis.To collect the information now is important since perhaps some correlation between BN and performance will be shown as time goes by. I do think that there is a degree of correlation NOW in light air based on European race results.
I also think that by thinking about these numbers, their derivation and what they DON"T show in rc models someone may be able to come up with a number that more closely tells the rc multihull performance story by reflecting righting moment more accurately.
How to relect tacking ability may be the biggest problem.“Cats don’t tack as well as tri’s” is not good enough(and not always true) but how to gather the data to include in a performance estimating number I don’t know.
Doug Lord
–High Technology Sailing/Racing
Hey Dick, just out of interest, put in the numbers for your Mutione Tri, and even those for your fullsize cat…
I know the number doesent tell you alot, but its kinda like comparing SA/displacement Ratios for mono’s, it just gives you an idea of the potential performance compared to other boats. Yes i know most mutis make full use of mutiple rigs and whatever, but then mono’s do that also…
Luff 'em & leave 'em.
Doug -
until you made the first topic post, it would be my opinion that 95% (or more) of the readers of the forum had NO idea what a Bruce Number was, let alone having them draw conclusions or correlations from the calculated numbers. Kind of like comparing horsepower to weight of a car, or sail area to hull length - not really useful information.
Matthew -
here are the numbers you requested:
<u>18 Square Meter catamaran </u>
194 sq. feet of sail area and 330 lbs.
BN = 2.016
<u>MultiONE trimaran</u>
7.64 sq. feet of sail area and 3.5 lbs.
BN = 1.819
<u>F-48 Trimaran</u>
9.72 sq. feet of sail area and <font color=“red”>(*est) </font id=“red”>5 lbs.
BN = 1.823
<font size=“1”><font color=“red”>(*Estimated final weight - under construction)</font id=“red”></font id=“size1”>
In order for my F-48 to meet the same BN as the 18 Square, I have to get the weight down to 3.7 lbs. which would be quite a challenge.
Missing in the above number, is ANYTHING to do with actual sailing weight, and actual moving ballast. Also how far outboard the ballast is placed, wind strengths, water/wave conditions, and rig tuning. That is why I’m saying the numbers have little if any use for r/c mutlihulls. In the end, it will be my boat against your boat around a closed course. If I can tack, gybe, pick the favored side of the course, get a clear air start, hit the wind shifts, and have a bit more boat speed than yours, you must admit that Bruce Numbers have virtually no impact how we would finish. In fact, based on a different post about slightly heavier boats having a possible advantage to “coast” thru tacks or light air lulls, a lower number might be “more” important.
Finally, whether you use one of Doug’s patented reefable rigs, or you grab the sail area off a 36/600 monohull for heavier wind - suddenly the BN numbers change again - and my question remains - “So What” and “Which One Is Important?” I would submit that my lower sail area and lower BN number would be “more” competitive in heavy wind than your full sail area boat in the same wind strength.
All we have successfully done is to create more confusion with “numbers” for the beginning sailor. Based on the posted numbers, it “appears” that Doug’s X3 (whenever it happens to appear) should be faster than my 18 Sq. Cat, and if I could drop 30 lbs. <font size=“1”>(easy if I replace aluminum mast with carbon)</font id=“size1”> on my 18 Square - getting the BN down to 2.08 I should be able to beat a 32 foot Warren tri - or a scaled UP Open 60 ??? Come on - No correlation whatsoever! If it is, I certainly would be happy to race my 18 Square against Doug’s X-3 for the <font color=“pink”>“pink ownership slip”</font id=“pink”>! [:-eyebrows]
In the end, if the owner can keep the multi on it’s feet, sail around the course and finish, it matters little what the Bruce Number is. His concern is how many boats were ahead of him as he crosses the line.
Regardless - those are the numbers for what they are worth. Readers can draw their own conclusions as to how “important” the Bruce Numbers are. To me, if I was a buyer, I would be more interested in the actual racing “pedigree” and finishing history of a boat in various wind strengths and regattas.
<center>(in my opinion of course)</center>
BN’s have the same value in rc models as they do in full size boats in light air but when the ways that righting moment is generated are considered, as the wind picks up, the value may change as already mentioned.
Based on what Chris Jackson told me a while back the higher BN boats have the best race record in all conditions in European racing. So the value or lack thereof of BN numbers in rc multihulls will be come more clear as the multiONE, mini 40 and F48 classes grow.
This is directly related to higher BN numbers and the effect of stability: an AMYA official told me the other day that the biggest reason for the lack of interest in multihulls ,in his opinion,is the general perception that they will capsize/pitchpole in the blink of an eye.
In many respects higher BN number multi’s may be harder to keep upright than lower BN number multi’s but that depends entirely on HOW RIGHTING MOMENT IS GENERATED which is probably the single most important question a beginner can ask when considering a multi.That question should include athwhartship stability(capsize) as well as ,most importantly, pitch stability…
Doug Lord
–High Technology Sailing/Racing
Personally, I think you are placing the emphasis (and voodoo math to some) in the wrong area.
48 inch beam is 48 inch beam, but 1400 square inches of sail is different from 1100 or 800 or even 600. Beam is irrelevant if a sailor switches down in rig size depending on wind strength. Between lowering the center of effort, and reducing the available power, I liken a multihull “under-canvassed” if you will, as to driving a sports car with a governor on it. Keeps you from getting into trouble.
You keep skipping over the part that has been admitted for some time in the European multihull community, and that is a “fact” that most now feel the Mini40 (or F-48) has too much sail area, and have and are considering a rules change to reduce the sail area.
I would be pleased to correspond with “said AMYA official” if you would be so kind as to give me his name? My guess is that instead of any kind of intelligent conversation, your efforts at education were again directed toward moving ballast and foils - and the asscoiated high costs of only those two D.L. alternatives. Also interesting is that the “AMYA official” wouldn’t have been directed to the class for comment and views. Kind of like me saying that your boats are known for poor performance and extremely high cost.
Who was the offical and under what circumstances did a conversation about multihulls lead to that view?
By the way - are we racing for “pink ownership slips” ? My 18 Square against your X3 with spinaker? If I win (which I think I can almost guarantee) I will donate the X-3 (with spinnaker) to the AMYA for their next public auction. I propose the early part of September at a site, perhaps half way between us. I would assume that given your posted comments about molds and prototype testing you will be available? The other opition is I can probably hook up with an existing 18 Square owner near you and simply fly in and charter his boat, with an agreed upon delivery of my boat to you should I lose. After all, your published BN indicates a much faster boat than mine - and also it seems to be illegal in the Multione or F-48 class for class racing - but you can provide the specs. Wold more interested in how you came up with your posted numbers. Not sure I can beat Matt with his big-roached MultiONE, however. His BN are too high
I must point out that a higher BN does NOT necessarily mean that a small boat will beat a large boat-large being 4 plus times the length, two to three times the beam and 10 plus times the sail area! You can create BN’s for any size boat but their major value is in comparing similar boats. By no means should the BN be misinterpreted to indicate that a multiONE could beat Cheyenne! Even if the multiONE’S BN was higher! BN’s compare the power to weight ratio of individual boats and ,again, only have value in comparing more or less similar boats.
The “too much sail area” thing about mini 40’s is only (maybe)true if you don’t consider a mini 40 designed with a movable ballast system or automatically stabilized foils both of which are legal in the mini 40 and Formula 48 classes and both of which make the legal sail area “carryable”
in a much wider wind range than is possible with just a lightweight hull-and neither of which has been explored by anybody in Europe very much though Pierre Gonnet(I think) won a major mini40 regatta using movable ballast.
In this country the only two rc multihull builders are both experimenting with movable ballast and one is building foil equipped boats.
Which brings us back to BN’s being possibly accurate comparisons in light air but probably not when different types of righting moment are brought into the equation: foilers can be very light yet have virtually unlimited righting moment and movable ballast boats may show a decreased BN while having a doubled righting moment…
Doug Lord
–High Technology Sailing/Racing