Why does my boat surf so fast?

My 30' Sagitta catamaran surfs too fast - 18 knots in a force 7 with no sail up. I have had even greater speeds I think, crossing the Atlantic this winter, though at those times I was too busy to be looking at the speedo. In the end, I solved the problem with a drogue in the form of a bight of rope with a chain slid down, the rope easily adjusted for length. I was then able to adjust the drogue to suit the waves and to limit surfing speeds and to adjust the sails to suit the wind speeds. With this arrangement, I was able to make good speed without risking excessive surfing speeds.

When I crossed the Atlantic, I set off alongside a Shuttleworth 31 open bridgedeck boat, and then I sailed through the Windward islands, frequently in company with the same boat. The Shuttleworth was clearly faster, being at a guess something like half the weight - though we did the crossing in very similar times.

Despite being faster overall, the Shuttleworth never reached such high speeds going down waves. At times when they were comfortable with just a storm jib up, I would be going too slow between the waves and sometimes far too fast going down the waves. So I'd have a bigger jib up AND the drogue out. Comparing our tracks, I was able to catch up with the Shuttleworth in the rough weather using these tactics. I think the Shuttleworth hasn't sailed at more than 14 knots (which is enough for me too, actually!)

I can't understand why the Shuttleworth didn't find it necessary to tow stuff behind them when I did. Their boat was clearly faster, so why didn't they surf faster or more frequently than me? The main differences between the boats seems to be that my boat is 1.5 - 2 times as heavy and mine has a bridgedeck. Underwater hull profiles seem similar. Could it be the windage of the bridgedeck? I'd have thought this wouldn't count for so much, as the apparent wind is so much diminished when the boat is surfing, and there isn't much wind anyway close to the water under the lee of a big waves.

Both boats use daggerboards.

I'd be interested in any theories about this.

 

You have the designer of your boat here in the BD.net forum, he might know the answer better than anyone else.
Check here: http://www.boatdesign.net/forums/profile/richard-woods.html
Cheers

 

I'm aware that Richard contributes to this forum, and of course a reply from him would be welcome, but I don't think he has any obligation to answer such things. That's why I'm asking the question generally to the forum members, rather than to him specifically.

 

Without knowing all the data relative to both boats (shape and layout of hulls, mass of the boat, CoG position, windage etc.), I am affraid that it will be nearly impossible to answer your question... That's why the opinion of the designer of the boat would be the most indicated one.

 

This is just a random guess, but it may be the case that your boat has more buoyancy aft and/or more weight forward. As a general rule for surfing, moving your center of mass forward on the board will cause you to accelerate down the wave front, while moving back will result in slowing and moving back up the wave front. Two identical boats, one with the weight forward will likely surf faster than one with weight aft.

The same result could possibly be created by having the hulls shaped with greater buoyancy forward or aft.

 

Jetboy,

With that in mind, I moved my anchors and chain from the forward lockers in front of the bridgedeck into one of the hulls, about 2/3 of the way aft. It certainly helped a bit, but not as much as I would have liked.

I guess it could be, given similar hulls, more a matter of weight distribution than windage. I'd have difficulty moving much else further aft.

 

Perhaps boats need surf brakes. Something (flaps?) that would create lots of aft drag but only get activated when speeds get too high (ie, when surfing down the front of a wave).

Supposedly a "Burke Sea Brake" is similar, but there are times when you don't want anything dragging behind the boat.

 

If it wants to go fast did you try keeping it sailing fast? The Piver big wave method was to keep weight aft and enough sail to keep the boat moving quickly without the big surf/sail speed difference. This worked well for the slender hulled CSK cats.

A slippery shape and enough weight could be the reason, like adding weights to the nose of a soap box derby car to get more speed and a longer roll. What you are already doing sounds like the best way to keep a lower constant speed as the drogue adds directional stability. A faster overall speed may work well too.

 

20 knots or so single-handing in mid-Atlantic, steered by an Autohelm - that was a bit hairy! And once, the front of the bridgedeck hit the green stuff at such a speed, sending a foot or so of green water over the top. None landed in the cockpit - it went straight overhead and landed in the sea behind the boat.

Slowing down was essential! With some drogue out, and sail up, varying amounts of each according to the wind and waves, I was able to maintain 6 knots or so in the troughs and surf comfortably at 10 or occasionally 12 knots or so. Unfortunately, the wind speeds were quite variable (for the trade winds!) and I was frequently making adjustments to both sails and drogue.

I found having no sail up at all to be not such a good idea. Sometimes on the back of a wave, the boat could slow down so much that it would lose steerage a bit, and if it was then hit on the quarter by a wave, it was sometimes knocked side on to the waves - even with the drogue out. It would then of course have no steerage at all, and side on wasn't much fun. I settled on this tactic - I left the storm jib up on the inner forestay at all times. When the wind was OK, I'd have a jib or two on the forestay. When a squall came, I could lift the jib halyard clutch and drop the foresails, and leave the storm jib up to provide good speed and steerage in a squall.

 

Sounds sensible. Anything over 15 knots should be hand steering anyway. They didn't have auto pilots in the Piver/ Choy days.....

 

I thought you'd be pleased to have an overweight (I know the boat) 25year old design, bridgedeck cabinned 30ft boat that was faster under bare poles than a lighter, longer, open deck Shuttleworth with it's sails up!!

Clearly the Sagitta has much lower hull drag at higher speeds than the Shuttleworth, otherwise it couldn't go faster. It obviously generates less power from its barepoles than a boat does with sails hoisted.

As you know (but others may not) I have built/owned a Sagitta (which was much lighter than your boat) and also ocean cruised it's larger sister, the 32ft Eclipse. I never had problems like you reported, but the most I did on my Eclipse was 21 knots, and 16 while crossing the Atlantic, that was just under reefed jib. But I never got the bows under when sailing. Maybe I don't drive the boat as hard as you.

I can understand the extra weight will increase the chances of the bows going under, but I'd have expected the boat to be slower, which reduces the chances of bow burying.

I agree, I wouldn't want to sail under bare poles, you do go too slow in the troughs or climbing up the back of a wave. But I think you don't have roller furling headsail - just hanked on foresails?

How big is your storm jib? Usually a multihull storm jib is smaller than on a monohull. But I have never used one in anger, although always had one on board. Instead I have deeply reefed the genoa. Sometimes very deep, as in this photo (not of Eclipse but a 33ft Crowther) when we were sailing along the coast of Colombia which as you know, is one of the windiest parts of the Caribbean.

You crossed the Atlantic singlehanded in a home built 30ft catamaran and are now sailing in the Caribbean, I wish I was with you! Not sure what I can suggest from an office desk. Enjoy your cruising.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Hi Richard,

Overweight? Really? I thought the waterline looked like it was about right, comparing it with various photos I've seen of the Sagitta. I'm very aware of the weight I'm carrying with the ground tackle, 2 engines, 2 batteries, 2 dinghies, the never-used series drogue - and I try to minimise the weight of everything else I'm carrying. The waterline at the stern is about level with the end of the hull.

Don't get me wrong! I'm pleased with the boat - I was just a bit surprised at the behaviour I've described. I've found a workaround to deal with it, and can now confidently get good but not excessive speeds in strong downwind conditions.

I do have hanked on sails, and I like that. I've previously had boats with roller furling, and didn't like trying to get upwind with a half-rolled jib. With my method of having two sets of sails up - a jib or two on the forestay and the storm jib on the inner forestay, I could drop the big sails in a couple of seconds when a squall came. And there were sometimes a dozen a day of them. I marked all the hanks of the genoa and jibs with coloured numbers, so that I could attach any two sails to the forestay with the hanks interwoven without snags. I attached a block to the jib halyard, and put a short strop through the block so that I could attach the heads of two sails to the halyard and tighten them both up evenly. That worked nicely. As in the photo (no storm jib raised there though).



I don't think I drive the boat hard. Those high speeds were with the storm jib up or no sail at all.

Storm jib: 10.4 square meters
Jib 1: 33.5 square meters
Jib 2: 20.4 square meters
Genoa: 57.9 square meters

Hey, where else to be in a squall other than in the saloon (or on land)? The Shuttleworth no doubt has a higher monetary value, but if both boats were to return across the Atlantic, I know which I'd prefer to do it on!

I've just flown back to the UK to do a bit of work, leaving the boat in Antigua, stripped of gel coat and drying out (osmosis isn't a design problem!). Though I'll have a bit of fibreglassing to do when I get back, I'm still looking forward to my return - autumn sometime, as soon as possible. Plenty of easy cruising options from Antigua.

 

It is a good reminder that increasingly changeable weather is becoming the new normal and good seamanship requires constant vigilance. Being able to cope with the variables isn't a work around but a necessity for safe boating.....I'm going to try auditioning ballerinas on water skis for speed regulation. I'll call the performance the Wave Cracker!

 

Here is a quick idea. How about a rotating drogue? There would have to be a mechanism for deploying rotation brakes as velocity increases the spin rate. I can think of a number of methods of doing this that could be automatic but the thing could use and generate electricity with digital control too with the proper cable set up.

As the boat slows the drag on the spin is reduced, as speed increases the spin is braked for more drag keeping boat speed more constant. Interesting to ponder especially with the generating possibilities as that stormy weather will make the solar panels less efficient.

 

Or the "Drag Leg", an outdrive style leg mounted on the sterns or aft beam with a large variable pitched prop hooked up to the appropriate things for braking and electricity generation. It wouldn't give as much directional stability as the tether but would certainly be easy to deploy and could function as an auxiliary drive.