As usual I am really impressed by the depth of knowledge and quality of workmanship posted in various threads in this website, thankyou to all those who who contribute their expertise.
For me, its been a while since i updated this build log so heres a short report.
I have completed the shaping of deck plugs for both hulls and am now about to fair/paint fair/paint.
Built cavity into aft of main hull for steering servo and rudder post. Cut recess into centre of main hull for hatch and finally cut rebates to accept crossmembers…these need little more work yet.
One suggestion, and only a personal opinion - consider a more highly peaked foredeck from bow back to first beam to help shed water if you stuff it into the back of a wave or an “almost pitchpole”. Can be sharply peaked - or a mirror of lower hull shape. Once you get back to front beam with deck, not much further help since the front cross beam will act as a brake to stop much forward progress. If you get this far, you are probably heading down the “mine shaft” ! Again, only an opinion.
My plan was to try to get both crossmembers flush with the main hull deck…until i start buildling the beams I am not sure how close to flush i can get, likely will have the rounded top section curving above deck…we will see!.
At this point I would like to call in the big guns because its time for some big decisions, so any one out there who has ideas/experience about this- please step forward now.
I have set up the hulls on the jig/cradle.
The pictures show the hulls positioned according to plans
In this set up, the bow of ama hull is 3cm above main hull bow waterline and 24 cm back from bow on Ama hull is the lowest point of Ama rocker, at this point the ama hull is 2 cm above waterline.
The Ama hull hull has 5 degrees of toe out, so when an ama hull is resting in water with virtually no pressure in sail then the ama hull would be at 90 degrees to waterline.
Question one; is 2 cm above waterline to high for Ama…kinda feels right but is taking some getting used to visually?.
Question two; should I increase ama toe out slightly or plenty?.
Hey Gary,
Yes, you are definitely on a point of “big” decisions!
Why I designed the amas to be 20mm above CWL at rest:
Especially due to the experience of Karl Schmidt (Skaut AUT 07) -
a) If the amas are placed at this level, the heel of the boat at light wind helps the sails to fall in their right position and shape easier after tacking.
b) At stronger winds the mainhull is lifting out of the water easier, cause centerboard and rudder are not at 90 degrees in relation to the waterline anymore, but something in between 100 and 110 degrees (not a “sharp edge” so to say).
The mainhull comes “softer” out of the water, not so “suddenly”.
You also have to consider, that this ship would be 40 feet long and 40 feet wide at scale 1:1 and between 2.5 and 3.0 tons! There the mass resistance of course would be quite different and you could place the amas right AT the waterline, not 20 cm above it - but a model at 1:10 simply reacts different!
Toe out:
Please make a drawing of the complete boat - view from front (suggested scale 1:2 for the
drawing/model = 61 cm wide):
Main cross section mainhull + main cross section amas at the given position 20 mm above
CWL and draw the CWL. Then an angled waterline from center mainhull radius (60mm)
to center ama radius (48 mm) - heel at sailing condition in light winds.
Should be around 7 degrees in relation to CWL now.
Now tilt the ama out/ toe out 7 degrees MORE. - 14 degrees in relation to CWL!
Cause now the ama would be 90 degrees to CWL, as soon as the mainhull is clearly out of the water - heel at sailing condition in stronger winds.
And at this heel level the straight main part of the crossmembers should be parallel to CWL!
Socalled “Ackermann-Effect”:
Something you may have not considered at all yet.
Due to the different keel lines of mainhull and amas at heel the keel line of the amas are pointing outward now in relation to the mainhull!
You need to “correct” the centerlines of the amas therefore -
they should point inward about 20-30 mm!
Stern moves outward a bit and the Bow inward - by keeping the overall width not wider than the given 1220 mm for the Mini40 class.
Think about that a bit.
Of course you could/can do that as you please - I´m only suggesting this to you.
But with your new - well done - cradle you could do that with ease.
You could also ask SIRI or Umut Korkmaz for help - they know their CAD stuff well.
I can´t do that - still working with millimeterpaper and a triangle.
If I get a chance to pick up the digital camera from my girlfriend, I could place a photo of my own “old fashioned” drawing(s) here today.
I’m a bit confused on toe-in vs. toe-out. I can understand that the center hull, being wider at the stern and pointy at the bow, would see the center of it’s waterline flow/displacement shift toward the direction of heal at the stern ever-so-slightly. To compensate so that in light wind conditions the centerline of the submerged sections of hull are parallel, wouldn’t the outer amas need to toe-in?
Must have missed the earlier post from August. Sorry if too late, but let’s look at the issue and forget about hull shape. If the keel lines are all parallel, there should be no toe in/toe out. On mine, I simply work only via center lines or keellines and don’t worry about the hull shape, since each hull and/or ama could wind up being completely different from one design to the next. If you plan on only one ama in the water for most of your sialing, everything will change when the main hull drops back in the water. In similar mode, if you planned for mainhull and leeward ama in the water, if the main hull comes out, it will hange your perspective. Give a 48 inc (1.2 meter length) I would suggest (opinion) any small amounts of toe in or out would probably have a negligbile effect on the performance. On a full size cat (F-18/18 Square/etc.) the rudder toe in/out has more effect on performance than the hulls. In light air - go forward on bow to raise sterns and you’ve effectively changed the waterline form of the hull. Get the windward hull out of the water, and you have a basic “symetrical” shaped foil of the leeward hull in the water, so planning for one scenario completly negates plans for a different scenario. On the beach cats (above) one could argue the points - but unless you have some really sophisticated analysis software, I’m not sure a + or - distance between bows of the hulls and amas really makes much of a distance. I’m suggesting fractions of an inch - obviously not much more.
For cats with identical hulls, agreed it is not really an issue, but tri’s are a very different story(unless you have three identical hulls).
Typically ama’s have a very different hull form to main hull, introduce some tilt out on ama’s and then factor in the centre board and rudder on main hull , once sailing in optimum position with main hull just out of water then your ama is no longer running parallel to foils. They will be fighting with each other. Even with main hull and one ama in water the effect is apparant
I have spent many hours playing with adjustments on my three hull positions and unless the bow/ stern of ama is adjusted bow in then it is obvious optimum hull efficiency cannot be reached.
The other factor I discovered was the centreline of ama and at what height on centreline (planview)to take your position (measurement to main hull).
As you tilt the ama out then the bow and stern centrelines at deck height do not move corresponding amounts…in my case because my decks are not flat planes but feature a curved shearline at gunnels…
make sense??..hard to explain and took a lot of thunking for me to resolve . but the effect does exist…in my case I brought my ama’s bow in about 1.5 cm to compensate for ama tilt out.
Gary, I’m pretty sure that makes sense but it depends upon which axis you rotate the hulls relative to the cross beams. If you rotate them about where the beams attach I can see the effect. However, this angular affect would be reduced (or eliminated) if the foils were in the amas (rudders and boards) and the under-water shapes of the amas were designed with a specific canted nature in mind - no? naturally, in two-hull flying state, those foils and that hull are all that are giving the directional control.