Optimizing a Tri for Light air

One of my frustrations with my corsair F31 is the radical difference between its relative speed in light air, vs its speed in heavy air. I like to compare my boat to the Melges 32, which beats us in light air, and which we beat in heavy air. The boats are pretty close in many ways (except they kill us with that giant mast head spin):

M32 Corsair 31R
Weight 3775 3750 lbs
LWL 31’ 10” 30'
Mail 445 254
Jib 254 218
Spin 1302 996

The weights are a bit of a guess-- my boat was weighed on a scale with everthing ready to sail --sails, motor, gas. Not sure what the Melges figure (from the Melges web site includes).

We satart to beat the Melges upwind at maybe 12-15 knots of breeze.

Anyways I have been thinking about how one might design a tri which with the same sail area, and payload capacity as the F31, but which could hold its own off the line (in speed and height) with the M32 in 5 – 10 knots of breeze.

Isn’t the M32’s advantage in light air a simple result of a superior ratio of wetted area to sail area? My thinking is that with two hulls in the water, the tri must have a greater wetted surface for the same displacement—and that wetted surface is the biggest source of drag in light air. In heavier air, I think that our greater windage becomes an issue, but by then our righting moment starts to kick in.

Am I right is believing that wetted surface is the key?

If I am, the secret to matching the M32 would be to reduce weight— I think 3000 LBS ready to sail should be achievable in a boat that is still as cruisable as an F31. I would make it a bit longer, a bit shallower, use a vertical gybing daggerboard, a high aspect under hull rudder, less rocker and less depth in the main hull, and floats that are moderate—not too flat, reasonable prismatic, etc. Obviously, I would have to build in carbon, with minimal accomodations and systems-- but I think it is doable in backyard tech.

Are these the right things to concentrate on?

 

I think you are focussing in the right area's. It seems the toughest part of your brief is to keep the same level of amenity and payload as the C31 but if you extend your waterline length to get the displacement and improve the fineness ratio you should be on track albeit with a narrower and shallower package. A drum style rudder system like the melges would be great for your boat as well and keep your easy beaching ability intact. I also agree that you should be able to build the boat as a homebuild.

It's a tough comparison because the boats are quite different by nature even if they are similar in length I imagine your corsair would make a much more pleasant cruising boat than their stripped out melges32.

 

One of the features that makes the Buccaneer 24 good in light air is the amount of dihedral in the float /hull arrangement, the boat is able to just about keep both floats out of the water !
RR

 

You can't win

Monos are good boats for a reason. Why do multis beat them usually? Because monos have higher FORM drag (high speeds) and lower stability (higher winds). In light winds the highest drag is from induced drag (making your boards work too hard) and wetted surface area. WSA is the real killer for multis.

Have a look at the little dinghies they race in lakes and ponds in the UK. Light weather and not long distances and they end up looking like little round tubs. A sphere is the perfect light weather boat (as long as the wind doesn't blow much over half a knot or so) A half sphere is the ideal shape for getting max volume in minimum surface area. Your Farrier is not at all like this and so you get killed by the WSA. I am sure the Melges crew get forward to get the broad bum out of the water and get the boat floating on the round forward sections to reduce WSA- this is light wind mono 101.

Monos kill multis even more when they can heel over to leeward to get the sails to draw earlier. As they heel the reduce wetted surface even more. Then on top of that a mono is so much more responsive to sail in light winds. You can feel the boat groan when the crew walks heavily on deck. Tacking is much easier (you may even roll a sport boat) and so you lose far less speed every tack. You also can tack on windshifts that a tri (but especially a cruising cat) would lose heaps of momentum in. Tactically a mono will love light winds in shifty breezes and a course straight upwind and then straight down.

Depending on the mono you will be in power up gear (I call it first gear) until the boat starts sighing. Then you start going for speed (2nd gear). Its not really until the mono reaches max stability that the multi will really make huge strides - monos are pointing now. When you start to fly a hull (just reaching max stability) the monos are depowering all they are worth.

So unless you make a tri like a mono - fat, you are going to find yourself getting beaten in the very light stuff by a similar high tech mono. Try and be nice about it. I am sure you get your own back most of the time.

cheers

Phil

 

So now to the question

Vincent I think your new tri should be the opposite way around. In fact I have often thought of a rule beater tri with much the same idea as Wave rider. That crafty Davidson from the late 70s (I think - be kind Gary)

You go for a deep and semicircular main hull that is quite short and has a canoe stern underwater. For a 30ft tri make this section about 23ft. On the end of it you put an absolutely flat stern extension (Your cockpit will go over it) 7ft long. The stern extension is about 40mm above the water with the crew forward. In light winds you get your crew forward and the whole extension lifts out. Of course your floats are high set but long, high volume and low rocker. The idea is that as soon as the wind comes up the floats take displacement and the main hull sits on the dead flat bum. If the rule measures main hull waterline you even get a bonus in the measurement.

Now I haven't even drawn this on paper, let alone done the calcs but I would like to see some new ideas on how to make tris even better in light winds. Its a good project.

Makes me remember trying to catch a big fat Harris 42 tri in my slim (highWSA/volume) Twiggy. I couldn't in the 6 knot downwind breeze. It made me change my idea of a cruising boat - get it to 8 knots quickly was what I told my designer (it gets to 9-10 pretty easily too)

cheers

Phil

 

Garys Groucho Marx (Im sorry for the spelling) Seams to be realy quick even in light winds with no amas in the water, just a foil or two.
I agree with Catsketcher that for light winds a half sphere would be the best, but it would ofcourse be worthless in moderate winds and more. Therefor, if you want to increase lightwind performance, get both amas out of the water (either by bc24 balance them or by puting "light wind" foils on them) and keep leangth on the aka down as much as possible. (Some of this is already covered by Catsketcher, and as usual i must add that i do not sail a multihul, and have not done, but i will.)

Kind regards
/Brorsan

 

Hi Vincent,

the main problem with your performence difference is that you are compairing apples with pears. The Melges 32 is a flat out racer with nearly no interior and your f 31 R is a pimped performance oriented cruiser. If you are thinking about a new boat to beat the melges you should upgrade the class of the boat to have a Cruiser-Racer, not the details.

You are right in saying that the wheight and the wetted surface is the main problem. For a multihull the F31 has a really fat mainhull with cwl/bwl of 8:1. It should be 10:1 for a good one to go faster than hull speed early in light winds.

There is no need to build a carbon boat. A light wooden tri will do it without problems. Or save money and labour and buy "Gecko" for 60K $ from Kurt Hughes, who lives in Seattle too.

http://www.multihulldesigns.com/designs_stock/f40shtri.html

By the way, Kurt Hughes is well known for his really fast cruiser racers. I think, becide of the pure raceboat designers like Irens and VPLP, Kurt Hughes is one of the best multihull designers for fast boats around. For sure he is able to give you a design that makes the melges look like a sumpter mule on the race course. In light winds as in a breeze.

If a new boat is to much effort for you, some wheight reduction for races will do a good job. You can increase your light air performance dramatically if you make your f31 as light as possible before race day. This includes a lighter outboard motor (4hp will do), all cushions and berths boards, a lighter batterie, anchor gear and so on. Everything what is not is nessesary for racing shall be at the habour, not on the boat. How many crew do you have? Two or three will be enough for light winds.

Is your mainsail size with 254 sqf correct? It seems very small to me. it should have 420.

Grreeetings from the North Sea Coast, Michel

 

Yes SA/wsa, so up your main sail, lighten the boat, will help a bit. Bet the C31 sucks in heavy air upwind in big seas too? Had a 20+yr-old-everything Dragonfly25 sail circles around a F28 new-everything upwind in big stuff.


Slim the vaka down and everything will be fine, 14:1 minimum for a racing multi.

 

If we are talking light winds only, then sliming the mainhul will not help, maybe making it even slower due to higher wetted surface, right? Wetted surface is the largest speed reducing factor up to 60-70% of hullspeed (not totaly sure about that). That would be up to about 5 knots?

 

Yes, slimmer for the same disp, ie longer or deeper, but slimmer for less disp (same len) has less everything including wsa.

We have a blank piece of paper, go lighter.


Take 3 boats, one a barge (square 7X7X1), a melges32 other a canting keel mono of 14:1 semi-circular wetted area, all of equall disp. Give all same sail area and correct righting moment. The barge will have the highest SA/wsa ratio (higher is better) the melges next and canting mono the least, IMHO she still will be the fastest in all conditions (5+K wind), she might be 50ft too.

Now if we were comparing a 9:1 canoe with a 14:1 cat of similar disp (probably similar len), then 9:1 will have the upper hand up to around your 60-70+% hull speed.

But a true mono, melges of 4:1 (planing bottom) has a fair amount more drag than an appox 9:1 (semi circular) mono of similar disp and length.

 

It's an interesting conundrum the fastest racing boats address the issue by piling on a huge rig I think the original wingmast orma60's were looking at reefing in anything over 10 knots so having the ability to power up the boat in light winds is one approach the negative is you would have to look at beefing up the whole structure and you start to run into the issues the guys who have built Cheekee Monkee have run into of having to address every area of the boats construction, it's almost a new boat.

 

Thanks for all the thoughtful replies.

Corley, I know that you are not suggesting this, but piling on sail area is exactly what I am not interested in doing. Muli lovers tend to have a bad case of greed for speed, as do I. But that greed is often directed at the headline top speed number, not at the overall performance-- and there are so many times (at least where I sail) when light air performance is hugely important.

Sure you can get there with a huge rig, but that is so inelegant. Much more beautiful to get there with an efficient rig, light weight, and easy lines.

Btw-- I just saw something on the web which listed Adagio, Meade Gougeon's 35 foot wood epoxy, tri at 650 square feet of sail area, and 2200 lbs. The boat is 40 years old. Inspiring, if that weight is accurate.

 

Hi Vincent,

Good Point! I think you are on a good way of thinking with easy lines, efficient rig and light weight to optimise overall performance. Do you really plan to build a new boat or are you just thinking around?

I think you should talk to Kurt Hughes and have a look at "Gecko" for inspiration. As you are in the same (Seattle) area .... have you ever seen "Gecko" sailing? It must be lying there somewhere.

Greeetings from the North Sea Coast, Michel

 

I think the guys in New Zealand really have some smarts when it comes to multihull performance on a budget a lot of their 8.5 class boats are using tortured ply construction the net result is a very light boat that is quite strong, a good place to look for inspiration I think.

I remember reading an article on Timberwolf I think it was, a thoroughly modern and innovative trimaran design when built and inexpensive construction techniques netting a very fast boat in a range of conditions. The constraints of the 8.5 rule mean that they have to build to efficiency not just brute force.

 

Here is JR Watson's response to my question-- what would you do today if building a new Adagio to try to beat an F31 in all conditions. JR has been with gougeons since the beginning and has sailed on Adagio a lot. JR included some photos, which I do not know how to post, but I thought the text was fascinating.

"There is much that can be learned from the photos and with a comparison of Ollie (Stressform). Ollie is an evolution of Splinter a 25 X 25 trimaran that is very successful (two pictures of her). Ollie was built much later than the original Adagio but we will compare it with the evolved Adagio – i.e. new floats, mast and cross arms. Both accommodate 3 people Adagio with sea bunks in the main cabin on the ‘wing decks’ and one aft (all single bunks). Sleeping on Adagio when underway is like being in the back of a speeding pickup truck (an old one with poor suspension) going down a bumpy what we call in upper Michigan a corduroy road. Ollie has a wider bunk forward and one a wide one aft (no wind decks exist). Ollie is more commodious; it has a much wider main hull. Both main hulls were built utilizing developed plywood construction – I’ll discuss this later.

Both are 35’ less on deck, Ollie is 28’ wide Adagio 24. Ollie is heavier. Adagio wins the beauty contest by all accounts no matter what angle you look at them.

Big differences; Ollie has a much higher prismatic coefficient thus higher speed potential, her sections are flatter midship and on the ends and less rocker (that results in higher p.c.). Note end view of Adagio with pronounced vee amidship and forward, less aft and much rocker. Ollies centerboard is way forward, Adagio’s is way aft (under the cockpit). Ollie carries water ballast in the wave piercing floats (amas) (second generation of an ama). Ollie’s cross arms are a plywood section with an I beam (like the 050 wind mast) that she sails with. The struts that join the beams to the main hull are carbon and aluminum tubes that can be dismantled. Adagio’s cross arms were wood box beams, now symmetrical section carbon tubes. Both boats join the beams over bulkheads and carry sea stays to support the beams that join to that bulkhead.

I’ve raced a lot on Adagio and less on Ollie but have had the opportunity to observe the following: In most conditions, when equally sailed Ollie is faster – but is more difficult to sail fast. In choppy conditions upwind Adagio is faster and more comfortable due to the finer ends. Power reaching Ollie is faster (so is a F31) than Adagio. In light conditions Adagio will win by large margins. Ollie’s centerboard forward loads up the rudder (by design) and steers nicely. Adagio carries 4 – 6’ of back rake to rid lee helm but steers nicely, but in squall/survival conditions will bear off and scar you. Ollie’s bigger board pays off upwind.

Ollie was sold to some local guys that still cannot get her going to her potential and have only beat Adagio in one race over a 5 year period.

Trends; both have newer amas which are longer and narrower than the originals and rotating wing masts which are smaller in area with less weight aloft. Sea stays put the cross beams in compression and save much weight over a cantilevered beam. But the stays do hit the water and are a structural element that cannot fail.

I would not build the main hull using developed plywood – too limited in shape. I’d strip (with foam or cedar) in halves on the centerline using female forms and cover inside and out with reinforcing fiber then join the two halves on the centerline. Focus on light amas using foam and carbon. Keep accommodations on the ends to a minimum. I would not get too greedy with a high p.c. Wide beam imposes structural issues but with sufficient ama clearance pays dividends in righting moment so is worth it. Two tube beams with sea stays over bulkheads is a proven good approach. I like the beam connections of Spliter best. Rotate a small, light rig and stay to wide beam to reduce mast compression. Note that Splinter and Ollie incorporated block and tackle shrouds to adjust when sailing. This approach was abandoned and now all three utilize lashings thru polished thimbles. I would not get too greedy with centerboard forward; I’d locate it similar to the F-31. Hope this is helpful."