build #352

[masc]

hi, building community!

my build 352 has already finished some time ago, during the pandemic lockdown right now i'm gonna find some time to document the constuction. first splash was on lake traunsee in 2017, with very helpfully people of AGS. we sailed there some races and modified some things from time to time (until now). and finally i got the sailnumber AUT1.

the build started in spring 2011, so it lasted almost six years. i underestimated the small time window of possible glueing-temperature as i haven't a heatable site. in combination with dense work in my job in june and october and lots of family holidays in summer it has been a bit longer than calculated. luckily i was able to do some preparations and works (panels cutting, rudder, keel, etc.) in a warm workshop during cold months.

there had been some decisions as we all had to take: i went for double rudders, long and wide and flat cockpit with straps, closed bottom inside, sprit on one side. a big cockpit was essential for me: at the time of purchasing the plans there was only the option of a long or short cockpit, so a made a custom shape - it is very close to the A4 version.

i choosed the twin rudder setup because of the wide transdom, but it added some complexity with the connections to the tiller. one advantage is that the gudgeons are now close to the chine at a very stiff point of the hull (remembering the problems/cracks of some fellow builders), and the blades itself are much smaller. by the way all the metall works made a friend, which is a really good mechanic and is very helpful solving different problems.

that's for the moment, don't hesitate to ask or add whatever you think.
so long...martin.

[admin]

Outstanding! Your boat looks terrific.

Your comments about the cool weather hindering build progress really struck a chord. Let's face it, you beat me, mine took 7.5 years!

Have a blast and thanks for giving us a glimpse of AUT1. Cheers,
--TF

[Warren Nethercote]

Clean looking build Martin. There is logic in twin rudders for wide boats like this, but some people have struggled with higher drag in some points of sail. Have you had to adjust the 'toe-in' on the rudders to get them aligned to the flow?

[masc]

i tried my best to get them right i got the positions of the joints between the blades and the tiller from drawings when turning around (fulcrum at the keel), and parallel in the middle. do you have some tips or sources for getting it better? until now i haven't seen a considerable drag.

[masc]

the plywood i used are sheets with 310x153cm (13 pieces at all), so i only had to do one scarf joint for the longest (hull-)panels. the joints were smooth, the bevelled connection was about 50mm. with the maximal length of 620 i was able to change the position of the joint of the bottom and side hulls, so they don't meet at one position (at the chine). at the sheerline i used strips of hardwood (beech) glued together.

sticking it all together wasn't very difficult, with zip ties and distance rings it worked well. one thing i noticed it was hard to get side panels to the straight edges of the frames - so i made a light curve (6-12 mm max height) on the sides and it went really smooth. i used plenty of cords to adust the form of the hull before glueing the frames and chines.

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[masc]

the sprit is running in a tube made of plywood. i learned from two fantastc colleagues making pipes with small strips, cutted at the right angle, and glueing it together on a paper tube. i cutted two rings at each end in the paper tube, so i got 'noses' at that position in epoxy. these are for two teflon rings as sliding bearings for the aluminium sprit, sticked into the wooden tube. of course there is some glass inside and outside of the tube. for the hole in the hull i used a paper pipe of the same diameter and cutted it at the desired angle, the hole was nearly perfect, hadn't to fill much.

motivated of the nice outcome i decied to make the mast compression post in the same way. with enough glass it should be as stiff as the suggested aluminium post.

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[masc]

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[Warren Nethercote]

Martin, most cases I have heard of aligning twin rudders have been powerboats where they could set a constant power level and then monitor RPM as they adjusted toe in. It sounds like you have used a geometric approach like Ackermann on a car and the rudders are 'quiet' as you sail along. You may have got it right, or close to it, on your first attempt!

I don't know how it is normally done for sailboats, but for something like an i550 a misalignment of the rudders would likely generate sideforce that would be seen as a bigger wake from the rudder.

[Watershed]

A beautiful and smart build. I am impressed with the use of paper tubes as forms for the sprit tube and mast post. You do neat epoxy and glass work, sir.
T

[masc]

one option is to have a solid plug - but to remove it after the epoxy gets hardened is more complex (has to be in more parts like tetris). and the easy way (especially if you don't need many similar parts) is to use something destroyable. just keep it separate from the resin (paint to close the pores and release wax) and it's easy.

[masc]

we spoke of rudders: the twin rudders are at 15 degrees from the centerline directly at the chine. i got that angle from comparision with the open5.70 and a possible heeling angle. for the first version i started with somthing little smaller than the original center rudder and a shape i could imagine. after some trials i cut them clearly smaller and at a more round shape at the bottom. right now i'm having a NACA 63A-010 with 295mm width, 30mm thickness and 710mm below (380 above) the chine. the boat is very responsive with the last version and running with no noticeable turbulences.

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the blades are made of 15mm sapeli strips glued together, shaped roughly in profile with a planing machine and doing the rest with a machine grinder (by eye), and after that covering with some glass, epoxy and primer. the first version hung just after the stern and lead to some turbulences in the crossing hull/rudder and water/air, at the lower gudgeon. so i moved the leading edge slightly below the hull, but i got some turbulences at the edge cutted out - i rounded that part below the gudgeon, leading to the shape it has right now.

for the rudderposts i made a square of 60mm out of 12mm ply and some glass around. at the holes for the screws i glued some solid oak inside, the rest is hollow. the height from chine to top is approximately 355mm (135mm above the cockpit floor). it is supported inside with some (ply)struts. for my feeling it seems solid enough. inside of the gudgeons (on the centerside) i had to glue a rubber for maximal inclination - otherwise the blade could get in a locked position (because of the geometry) for the tiller.

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i made the tiller with 4 bended 6mm ply, glued into shape, sticking with a steel pin in a 16mm hole in the bottom (oak reinforcement of course). all the metal work is custom made and strong. i'm using threats with locking nuts to adjust the distances (as we talked before) to get the blades in the right position.

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[Warren Nethercote]

Martin, I just read about a technique for aligning twin rudders. As long as the rudders are not over-balanced, just disconnect one of them and let it trail naturally while you sail using the other rudder. Record the angle between the two of them over a range of headings and speeds (wind speed related I suppose) and from those records decide what angle to set between the rudders, Or perhaps develop a table of angle settings with varying windspeed just like you might do for rigging settings.

[masc]

thank you, warren, for this great hint!

[masc]

i choosed to build a flat cabin floor (from F54 to F169), thought it could be more practicable and add some solidness to the hull. the chambers are with hatches except the ones between F110 and F124. on the side panels i glued some small stringers over the whole length, on the bottom continuous ones from F54 to the stern.

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as mentioned in post 6035 http://i550class.org/forums/viewtopic.php?f=3&t=444#p6035 i noticed some bending in the deck/hull area of the chainplates, even with beginning cracks. i decided to strengthen the hull panels between F89 and F110 (with another layer of 6mm ply and glass) and some minimal bending remained (visible outside on the mirroring surface), with the extension of the slanting panel for the chainplates it seems ok now at tension in the rig.

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under the cockpit floor i added some stringers too, but i didn't felt comfortable so i added a second layer of ply. now i'm having 12mm ply with glass and the stringers, and it feels solid. unfortunately this added some weight in an unfavorable place.

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the deck is made of 6mm ply/glass with two length stringers between F18 and F54, but some bending is present when walking around. i made the heighter part of the deck on a male mold in the workshop and glued it first on the (on position) deck panels, then took it off again and reinforced the seems with glass, and after painting all together went back on the hull.

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[masc]

the chainplates are custom made of stainless steel. the construction is somewhat different. the intention was to get the loading vector straight from the wire to the steel and on to the ply panels with no side-torque (maybe i'm too worried about the forces). the bow fitting is more or less common.

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