John Shuttleworh Yacht Design.


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Questions and Answers

Technical questions about John Shuttleworth's designs


What building options are there for your designs?


The options in order from the lightest to the heaviest are....

  • Nomex sandwich Prepreg Carbon or Kevlar/carbon. Build male mold. Vacuum inside skins onto mold. Heat whole part to set resin. Vacuum Nomex and outside skins onto mold. Heat whole part to set resin. Fill and sand the surface, paint, lift hull off mold, fit bulkheads. Join deck to hulls. (deck is made on the same mold as the hulls.) Resin - epoxy prepreg.

  • Nomex sandwich Carbon or Kevlar/carbon. Build male mold. Vacuum inside skins onto mold. Vacuum Nomex onto mold, vacuum outside skins onto mold. Fill and sand the surface, paint, lift hull off mold, fit bulkheads. Join deck to hulls. (deck is made on the same mold as the hulls.) Resin - epoxy.

  • Foam sandwich Prepreg Carbon or Kevlar/carbon. Build male mold. Vacuum inside skins onto mold. Heat whole part to set resin. Vacuum Foam and vacuum outside skins onto mold. Heat whole part to set resin. Fill and sand the surface, paint, lift hull off mold, fit bulkheads. Join deck to hulls. (deck is made on the same mold as the hulls.) Resin - epoxy prepreg.

  • Foam Sandwich GRP or Carbon or Kevlar/carbon. Build male mold. Vacuum inside skins onto mold. Vacuum Foam and outside skins onto mold. Fill and sand the surface, paint, lift hull off mold, fit bulkheads. Join deck to hulls. (deck is made on the same mold as the hulls.) Resin - epoxy.

  • Foam sandwich GRP. Build on a stringer male mold. Screw or stitch foam to mold. Glass the outside, fill and sand the surface, paint, turn hull over into frame. Glass inside. Fit bulkheads. Build deck on temporary frames. Resin - isophthalic polyester,vinylester, or epoxy.

  • Strip foam sandwich.(onto male mold frames) Build mold frames. Strip plank in foam onto frames. Glass outside, fill and sand the surface, paint, turn hull over into frame. Glass inside. Fit bulkheads. Build deck on temporary frames. Resin - isophthalic polyester,vinylester, or epoxy.

  • Strip foam sandwich. (into female mold frames) Build mold frames. Split hull on vertical centerline. Strip plank in foam into frames. Glass inside, Fit half bulkheads. Turn hull halves onto support. Glass outside. Fill and sand the surface, join hull halves. Paint. Resin - isophthalic polyester,vinylester, or epoxy.

  • Duracore wood/balsa sandwich. Build mold frames. Strip plank in Duracore onto frames. Glass outside, fill and sand the surface, paint, turn hull over into frame. Glass inside. Fit bulkheads. Build deck on temporary frames. Resin - epoxy.

  • Cold molded Wood epoxy. Build mold with stringers. Cold mold veneers onto mold. Turn hull over into temporary frame. Build in stringers bulkheads and furniture. Build deck directly on to hull. Resin - epoxy.

  • Strip plank Wood epoxy. Build mold frames. Strip plank in western red cedar onto frames. Glass outside, fill and sand the surface, paint, turn hull over into frame. Glass inside. Fit bulkheads. Build deck on temporary frames. Resin - epoxy.

  • Note that foam is Divinycel, Airex or Core-cel PVC closed cell foam. In all cases the bulkheads can be built in the same method as the hulls, or in plywood, with wood being the heaviest option.


    I note that in your formula for the stability of a multihull you use the figure of 9.48 as a constant, but the static stability is higher than this.


    That is correct - the constant in the basic formula for static stability is higher. The actual figure is 15.8 and not 9.48.

    However since I am only giving one number for all boats and all stabilities, it is appropriate to take into account gust loading and wave action, and hence the reduction.

    In actual fact when I specify reefing for my designs I use a different figure according to the wind strength that the boat has to reef in, and according to the sea state that might be encountered , and I explain that to the owner. The heavier cruising designs might have a static stability of 40 knots and so reefing for them will take place at about 28 to 30 knots. Another design where the static stability is say 20 knots, then reefing will occur at around 18 or even 20 knots if the boat is racing. Because the skill of the crew can be taken into account and the sea state will be less than in the stronger wind. Cruiser racer designs fall between this.

    When to put the second and third reefs in will vary as well according to the wind speed and the sea state. But as a general rule as the wind speed increases I decrease the factor from 15.8 towards 9.48.


    Do you calculate the diagonal stability for your designs, and what do you give to boat owners to inform them about the stability of their boat and when to reduce sail?


    I do these calculations for my designs. If you look at Fig 1 in my article on Multihull Design Considerations for Seaworthiness, the diagram shows how the diagonal stability is calculated from the hull form. I ensure that the diagonal stability is higher than the sideways stability in this way. I can do this by adjusting bow volume at the design stage.

    I define diagonal stability as the point when the whole displacement is taken by one hull with the deck at the point of burying. The downwind stability is the point when both hulls are taking the whole displacement of the boat, with the bows about to bury.

    In a heavier cruising design like the Aerorig 52 the diagonal stability will be in the order of 15 % higher than the sideways stability, and the directly downwind stability will be 23 % higher than the sideways stability.

    In a cruiser racer like the Shuttle 40, where the displacement is lower, and the boat is likely to be sailed closer to the stability limit, the diagonal stability is 38% higher than the sideways stability and the downwind stability is 28% higher than the sideways stability.

    These calculations are done for when the bow is about to go under. If the bow does go under, then the hull drag goes up, and there is a component of downforce on the deck right at the bow. This means that the stability will decrease much more sharply when the bow buries on the diagonal and downwind, than for the sideways stability where bow burying is not a problem. So the 15% and 23% extra stability represent a reasonable reserve to account for the bow burying effect.

    Therefore I give the sideways stability figures for various sail combinations to my clients, and I know that that includes a safety factor for diagonal and downwind sailing, to account for bow burying. I also give a guide to how much that stability should be reduced as the wind strength increases and the effect of waves is taken into account.


    From the mutlihull discussion list at

    Richard Shipes of Eustis Florida, asks:

    Can you describe what it is like to sail a boat with a hull flair like the Tek 35?

    In the article "The Elusive Cruising Catamaran Performance" of the July/August Multihulls, Mr. Kanter alludes that this type of hull might be slow. The article really bugged me, it seemed incomplete and unorganized.


    Ted Grossbart , who owned the Tektron 35 "Rosebud" for several years, replies

    Yes Rosebud was the prototype of the Tek 35. She is a fantastic boat, one of the rare true performance cruisers. I'd be happy to talk with anyone who has a serious interest.

    Someone asked if the hull flare on the Tektron 35 "Rosebud" was a performance killer like some of the bumps and bulges critiqued in the last Multihulls Mag. Short answer: No. The boat is very fast for a super roomy cruiser (5 doubles, 2 heads, huge galley, etc.) doing about 2/3 wind speed in most conditions. We saw 17-18 kts routinely with a top of 19-20 and I believe later boats have exceeded this. This shape maintains a sleek footprint with the room and safety of flares that you typically do not have to push through the water.

    These are fantastic boats, John is a real genius.

    Gary Pearce answers questions about "Zazen", after his voyage from England to Australia

    Zazen thumbnail click on thumbnail for larger picture.

    For more photos see "Biscay trip photos.


    I'd also be curious to hear how you would rate Zazen's performance under sail to similar-sized cruisers you met on passage.

    Gary Pearce answers....

    Well that's a _really_ interesting question and hinges on how you define performance.

    If I learned one thing on our little trip its that performance is a lot of different things apart from outright speed.

    In terms of how fast Zazen went then we did not encounter anything I would consider faster, but we actually sailed far more within our limits than a lot of people. No spinnakers at night (except for a few amazing evenings), no spinnakers above 15 knots true (didn't need it anyway) and always stayed at least a reef ahead of the weather. And the boat still kept going quickly. Even under this regime we were doing 160-200 mile days.

    Where we did really well was in the light stuff. That boat sails nicely down to 2-3 knots true (sluggish dead downwind in this) and in fact we probably used less than 200 litres of diesel in 16 months and 15000 NM. I still have a jug from the UK. That's important given that a lot of ocean crossing takes place in light conditions and one is limited by weight. Being able to sail in the very light stuff means less diesel to carry means less weight - its a virtuous circle.

    We didn't go faster in the heavy stuff because mostly it was just Jane and I and 2 12 yr olds and we needed our sleep which was hard to get due to noise and motion and for an 18 day passage you need to _really_ look after the crew. Above 12 knots was too difficult for the crew.

    In fact we hardly broke anything. Mashed a couple of Rutgerson blocks when the lines didn't pull fair, an undersized outhaul block fell to bits, the sheave box for the jib halyard fell to bits (must speak to Marstroms about that) , we stripped the case off the code 0 halyard (I'll never buy Liros again), and the leech of the code 0 suffered sun damage from being left up for 15 months.Outboard needed a new starter in Coffs Harbour at the end of the voyage due to salt ingress.

    That's it for breakages. Just as well really as there were no spares available for anything anywhere.

    There was zero damage to crew (although they were a bit stressed after the longer passages - 3hrs on 3 hrs off is tough). Access to the mast from the cockpit had a lot to do with that IMO.

    Oh and in port performance was great. Our awning would catch 60 gallons in a couple of minutes - we were always the "party pontoon" which was nice, especially for the kids.

    Oh top speed ? we often exceeded 20knots on a surf but you cant be pushing that hard in the middle of an ocean with your wife and kids unless you are a psychopath.

    I hope that goes some way towards answering a very complex question.

    Gary Pearce


    Next question..

    Thank you it does most excellently, and sounds like you made a happy choice of vessel and gear. How would you rate Zazen's "performance" in the ARC, amongst an admittedly wildly varying fleet and crew abilities?

    Gary Pearce replies


    You are indeed a cruel man, the ARC was one of the lowpoints of my life but a (longish) tale worth telling. You can make your own assessment of performance at the end :-)

    We started from Gran Canaria and I was told "don't go near all those boats" - but thats the start line "I don't care". OK so we started near the back, put up the code zero and were near the front by the South end of the Island when the few boats ahead were laid flat. Ahh that will be the front that they said would come nowhere near us in order that everyone would start and provide a nice spectacle. Sadly didnšt get the code zero away in time and shredded it.

    That didn't matter much as it blew like stink all night but in the morning we looked around and could see the Kevlar sails of the big racing monošs on various horizons. Then it went light - no light sail no longer.

    I had been advised to go South. I took the rhumb line as I fancied something more on the beam than from dead behind. Wrong move big time. 2 days later and suddenly the seas were huge and during the afternoon radio schedule this guy about 45 miles North of us calls in to say his brother has gone off the back, is being dragged along and and its his brother who is the sailor. Zazen is surfing nicely down these huge waves hitting 20 knots at time but the autopilot it too hot to touch (no really, too hot to touch), the crew are all pretty shaken by news of the guys death and fear worse weather might be heading our way and suggest we might slow down and let it pass. So out with the drogue and the world is a different (though still eerie) place. We did a day and a half averaging about 2 knots with drogue and wingmast alone. Not good for the average but safe and comfortable. (Oh yes 2 drogues actually, we lost the first one when the rode parted and I had to make a second)

    Then we have a few more light days with no light sails except this huge spinnaker which is a bit big to carry at night.

    Then we get in the trades and discover that our 70 something crewman's cataracts are a bit worse than we thought and he cannot see the squalls coming, Good as we are getting 3 or 4 big ones a day. Decide it might be best to be a tad more conservative and be alive at the end.

    We arrived around a day behind the TRT1200 Sister Skrit (there was _nothing_ inside that boat just a few nets on the bulkheads) crewed by 4 young Scandanavians so in the end I wasnšt unduly upset but was still pretty cross with myself. On the other hand we were fairly well rested for a crew of 3 (not counting the kids) after 18 days (I think it was). Oh and we did not use a drop of diesel. Oh and the only breakage was the code 0 from day 1.

    Of course the nice ARC people unpolitely snipped off the overall placings of the multihulls with scissors so we never knew where we ended up overall and if that was their attitude could not be bothered anyway. I would probably not bother with the ARC a second time.

    The correct thing to have done was forget the main, use wing and wing jibs of varying sizes according to conditions. We tried this later on and it was easy and fast. Not maximally fast but overall you do more miles as you can keep up a more optimal sailplan without fear. Brigand did this and went South and caught some _huge_ fish and still came in days ahead of us despite being on paper a "slower" boat.

    It was a different story across the Pacific. It was lighter, we knew the boat better and had a plan, but there were few directly comparable "times"

    Hope this shed a little light.

    Gary Pearce


    Some more notes from Gary on sail combinations for long downwind legs across the ocean.

    Just to be clear we can of course achieve a wing and wing setup with our rig .... provided you don't shred one of the jibs first day out.

    This is also only useful for fairly dead downwind in conditions where the boat is not pulling the wind forward too much (e.g. 9-10 knots boat speed in 25 knots from dead behind) This just happened to be the conditions we experienced across the Atlantic. The Pacific was _very_ different as was Biscay.

    I would not change a thing about the rig, the rotating wing mast, the self tacking jib, the big roach main, the code zero rolling on a spectra luff and hanked on jibs. Nope the combination was spot on.



    Another Question..

    I don't have a problem with the self-tacking jib, the code zero on spectra, hanked on jibs. And I can REALLY respect a rotating mast. I love what they do for a mainsail. BUT have you had the rotating mast in a REAL BLOW, and particularly downwind. And have you had a fat-headed, roachy main in a real blow, even reefed, and particularly downwind.

    Gary Pearce replies..

    Depends on your definition of BIG. In the Atlantic we were running off in 30 + knots and big seas doing about 2-3 knots with the drogue. We had the mast locked central there for a while. The autopilot kept us sailing dead downwind (and downsea).

    Off Portugal we hit some sort of white squall thing but had no functioning wind guage at the time. We dropped sail and were making 7-8 knots just slightly cracked off on just the mast. The sea conditions were relatively calm however.

    Off Coffs we had just under 50 knots and ran off with just the mast and the heavy duty number 3 jib against the drogue. During the worst we reduced to just the mast and were happy with the result. Here we had large confused seas breaking over the boat.

    I was always far more concerned by the sea state than the wind strength.

    We never felt for a moment that we were or were likely to be overpowered by the wind on the mast. 28 foot beam on a 40 foot boat obviously helped. I guess we did not experience BIG enough winds.

    Another Question about the wing mast.

    I'm sure with a little research back thru various articles one could find a number of such real life stories. I seem to recall one involving a Tektron 50 racing cat a number of years ago.

    Gary Pearce replies..

    I know that story ! The boat was sailing downwind with full sail (full main and genoa) in 40 knots of wind .... averaged 27 knots over 2 hours. Was doing fine until he reached Shipwreck point and the notorious standing waves. After 3 standing waves the boat found itself stationary with full sail and a full gale from dead aft and - not surprisingly pitchpoled. Nothing to do with the wingmast.

    A further question on the wing mast

    On a relatively heavy cruising cat that is much more likely NOT to scamper forward with the sudden gust, presenting a rotating mast of any sizable dimension sideways to a storm should be a real concern.

    Gary Pearce replies..

    a) Even at around 6 tonnes fully loaded I would rate us as light so perhaps your comments do not apply.
    b) I cannot think of any reason to present the beam of a boat of any kind to a _real_ storm. If you are even remotely likely to experience those sort of conditions you should have both a good drogue and a good parachute IMO.

    Just my opinions of course.



    Were you pleased you went on an open bridgedeck boat with the performance potential of "Zazen"

    Gary replies

    Way back at the beginning of our adventure I was reminded that its easy to slow a fast boat down but difficult to speed a slow boat up.....

    Having said that then in our experience it was sea conditions that dictated speed. In most conditions 12-14 knots was acceptable in terms of noise and motion. Zazen can do that sort of speed effortlessly in most conditions. That gives you 200 mile plus days without trying which is a pretty good _cruising_ speed. We were more conservative than this initially.

    When it gets very light we could sail down to 2 or 3 knots true wind provided it wasn't from dead astern. This keeps you ahead of some fairly large lush boats and requires no diesel. Noone would argue that this is Bad Thing. This is more important in the Pacific than the Atlantic.

    Apart from the noise and the motion I was uncomfortable allowing my relatively inexperienced wife or 12 yr old kids from cruising above 14 knots. We always reefed early and were rarely disappointed with our choices in this regard.

    I would not regard the extra windage of a wing mast as unacceptable and if you have spectra sails neither is the weight.

    I do not think we ever turned the autopilot off :-) We have a HUGE motor that links into the bevel drive of the Whitlock Mamba system and yes the harder it worked the more power it consumed. This was from between 20 - approx 60 amps per day. The worst was dead downwind with a big following sea (of course). It was too hot to touch most of the way across the Atlantic. You underspecify this at your peril. A couple we cruised with hand steered for 5 days and the result was not pretty.

    We met a number of people cruising in heavy multihull caravans and they were fairly universally unhappy. We owned a Prout ourselves before Zazen and would not have been happy.

    Bottom line is I doubt we would do anything much differently in this regard.



    Do you consider that you have a forward cockit and how would you compare that with sitting behind a bridgedeck cabin to steer the boat.

    Gary Pearce replies..

    Well I guess we have one. Its good because:

    a) Its huge, we were always the "party pontoon".
    b) You do not have to climb anywhere to get to the base of the mast which makes control lines easier, esp for a rotating mast.
    c) When covered with an awning its a lovely place in the tropics and the awning catches huge amounts of water.
    d) We have plenty of shelter under the spray hoods on either side (although a bit bigger would be better)
    e) Excellent access to sheets and such.

    Its bad because: a) Its a big area to fill with water -> need lots of drains b) Its cold in the UK in winter (but we have large warm areas elsewhere)

    We had a bridgedeck saloon and now are undecided on what we would have next.

    On balance we reckon its about even unless you to to very cold places but we have sworn off them :-)

    Last updated 20th Oct. 2010.