**Sailor Thom Davis shared the following with me last week as pertaining to guessing the speeds of trimaran designs.** I’d love to hear from others as to whether or not they feel this little formula reflects a general “accuracy” or not.

*Thanks for sharing this with us Thom … perhaps this post will inspire input from others that have some thoughts on this topic.*

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Just in case smalltrimaran readers are interested in guesstimating the speed potential for different trimaran designs, here’s a formula that works reasonably well for a good guess.

1) Take the Length in Meters raised to the 0.3 power.

2) Take the Sail Area (main, jib, spinnaker all added together) in Square Meters raised to the 0.4 power.

3) Multiply those together.

4) Divide by the boat weight plus crew weight in KG raised to the 0.325 power.

… Numbers above 1 are fast boats.

… Numbers below 0.8 are slow boats.

I didn’t arrive at this, those Aussies developed it. It can’t be really accurate since you can’t tell how a boat will point (or fail to point) and hull shape will definitely slow some boats down more than others…but your readers might want to know if the boat they are considering is fast or slow.

The Aussies use it as the start for a rating formula. Basically, just take the elapsed time and multiply by “the number”. Lowest result wins. In friendly competition, numbers should be adjusted so the same person/boat doesn’t win all the time.

Respectfully,

Thom Davis

I went ahead and calculated “the number” for several designs that I have sailed or owned using published specs and using 170 kg for crew weight assuming 2 on board. The number reflects reality (relative speeds) between them…

Triak 0.71 (slow boat)

Windrider 17 0.89 (with a 13 meter spinnaker)

Hobie Getaway (catamaran) 0.98 (with a 13 meter spinnaker)

Weta 1.00 (Singlehanded-with 2 on board it is slightly faster than windrider 17)

Corsair F242 1.02

Boats I haven’t sailed but I was interested in

Corsair Pulse 600 1.03

SeaRail 19 1.08

Let me see . . . Hull length 4.50 meter, sailarea 7.00 m2, weight boat 93 kg, weight of me 91 kg, . . . Something like . . . 0.6?? Very slow. Ok, I used a gps and clocked 10 km/ hour in 4 beaufort wind . . still is not very fast for a trimaran of 4.5 meter length, but certainly not slow in my experience.

Nepau, my trimaran, 5 metres , 11.5 square metres sails (white sails no gennaker) 180 kgs crew, 110 kgs hull,

Caluclations give a 9.975 figure … There must be something wrong. My tri is surely not as fast as a Weta on just white sails…

Operation # 4 is ambiguous… divide by kgs of crew and boat, and THEN raise to 0,325 ( final result is 2.7)

OR, divide by (boat weight+crew weight) raised 0,325 ??

The results is then close to 20…

some examples would be appreciated…

Sorry if it is ambiguous, Stephano. I get 0.68 for your boat plus crew. It is, indeed, slower than a weta. (Boat Length to 0.3)*(Total Sail Area to 0.4)/((Boat Weight plus Crew Weight) to 0.325)

I apologize if the characterization of fast or slow boats is an irritation to anyone. To me, a fast boat is one that has a top speed in excess of 16 kts. To me, a slow boat is one that goes to weather at less than 3 kt VMG (in the direction of the wind). Again, I think 3 kt VMG is a slow boat because I routinely sail against a 3 kt current so a slow boat won’t get me where I want to go.

This is all in good fun and helps inspire a lot of folks to think about these things.

OK… now I know where to set correctly the parenthesis… Nice VMG predictor for future tri !

Thumbs up !!

Stefano

Future trimaran, 6.8 metres, 420 kgs, 250 payload , 23 sq metres sails + 21 spinnaker = 44.

Calculations provide a promising 0.974 … Glad to read this in comparison with Hobie getaway …

It is an interesting challenge.

I wonder how the length-beam ratio would step in here… Narrower the best ( 1:10) , or semi-planing (1:8.2) ? Anybody can step in with some numbers or sound experience ?

Most of the newer designed tris seem to be going toward the 1:10 with reverse bows and larger volumes forward. Gotta assume Nigel Irens knows what he’s about. Then again, don’t design anything by what I say…I’ve never built anything from scratch.

Mr Thom Davis, with all respect, 16 kts boat speed would made my 4.50 meter long trimaran more of a rocket instead of a boat while for a container cargoship of 300 meter length 16 kts make it a sloth.

If one sits in a kayak and does go 3 kts windward it is experienced as a good speed, often.

Now of course the experience is not much of a science, but I think that speed would be related to the size of a boat in the first place. Second to the type of boat (multihulls are considered as being fast boats in general) and may be also the purpose of the boat (cruising vs match sailing for example).

But, apart from these, the proposal to use a formula to such as boat speed in consideration with fast or slow is a good matter and in such the approach is very welcome, so I am glad with this formula anyway. (I consider myself as being to lazy to calculate much as I want to build and sail and then find out if my idea is slow or fast. It provides one with many boats though . . . ;) Calculating more would certainly provide better boats in my case!)

So my thanks for your efford and my respect for your aproach mr Davis.

I appreciate the thoughts Otosj. I have a Triak which is a sailing kayak and I know exactly what you mean by 3 kts seeming fast when you are 6″ above the water. The Triak will actually reach about 10 kts downwind with spinnaker up, so that’s a hoot! But I also sailed with a friend on his 4.4 meter Weta which occasionally hit 20 kts…sailing enjoyment isn’t about speed for everyone.

Well said Thom!

Hi all, i own (since may 2017) a Magnum 21S: 6,3 m length, 350kg + 85kg weight (singlehanded), 16,5 sqm main + 6,5 sqm jib + 16 sqm gennaker.

That makes 1,73 * 4,32 / 7,20 = 1,038. Looks as if this boat is fast. But til now i never was faster than 6.5 knots! (But i´m not an experienced sailor! Looks as if i make something wrong.)

The Magnum S trimaran is designed to be a high-performance boat – http://www.ahoy-boats.com/magnum-21S-trimarans-specification.htm

There are a lot of factors relating to potential speed, however, that don’t have anything to do with the boat’s design. Learn how to sail that boat well in strong, steady wind my friend :-)

All I know is my windsurfmaran is going to be going mach 1 after I get the new amas together :)

This looks like a promising tool, thanks for that.

I ran my intended parameters through it, and it is:

L 4.1m, SA 12.3sqm,Displ 190kg, Result 0.757

So not quite a fast boat, but upper end of slow, if that makes sense.

Now should I try to increase the sail area?

Hi,

I’ve been chewing on some of these numbers, and the previously quoted result for the WETA is different from my result. I used the numbers from the website, and got 0.78 (main and jib) and 0.9 (main and screecher) but this was with a hull weight of 72kg as per 2017 spec. If I use the sail area as total of main,jib and screecher (19.5 sqm) the result is 0.966

Can you use the total sail area like this?

Yes, you are supposed to use the main, jib plus screecher all together for the overall result. I may have used a smaller crew weight when I did the calculation which probably explains the variance between my result (1.00) and yours (0.966).

OBTW, IF you omit any large downwind sails in the calculation; that is, just use jib and main for a sloop…then the resulting fraction “should” come pretty close to the percent of windspeed your boat will go upwind. It seems pretty close for the boats I’ve owned or sailed. You may not have noticed, but your boat usually sails a pretty steady average percent of windspeed over the range of winds above 6 kts and until you are approaching the max speeds of the boat.

Dries, you would be better off trying to make it lighter if you can.

AJ, I used length, 4.4; SA 19.5; Weight 72+85 or 157 and get .989 for Weta. I probably rounded up since I consider the Weta to be a fast boat.

Trimaran (and catamaran) calculations are automatically done for you at multihulldynamics.com. Inputs are length overall, waterline length, beam overall, beam between the ama centerlines (if you know it), sail area, displacement, bridgedeck clearance for cats (if you know it), and beam of main hull (if you know it). Units can be metric or English. The site then will calculate sail area/displacement, Bruce number, average boat speed over 24 hrs, Kelsall Sailing Performance number, Texel rating, stability speed (vaka begins to lift) and Kelsall Stability Index (wind speed to capsize). A bunch of multis already are done. I think they all are bigger than most we look at here.

I ran this formula for Frank Smoot’s Slingshot 19 with folding amas and 234 square feet of sail (more than he shows in his plans). This formula gave me 1.03 with myself alone (about 200 pounds) and 0.87 with by 300 pound brother along. So according to the comment above, it should match wind speed upwind with me alone. The multihull dynamics site gives a Kelsall performance number of 15.75, which I think means max speed 15.75 knots in a 10 knot wind.