From my experience the only way to beat fouling is to store the boat out of the water. If moored in the water then get used to cleaning the hull. A lot of the area off the hulls on the 18m proa can be cleaned without getting wet because the hulls are so narrow.
I have got to the point of actually making a chlorine generator using a solar panel and electrodes but have not tried that. Not sure how the marina managers or authorities would view that. The proa owner has also thought about bagging the hulls and throwing a chlorine tablet into the bag - requires a big bag though.
If the boat is used often, say weekly, and gets to decent speed then ablating antifouling stays reasonably clean.
If you tolerate fouling on a multihull then forget about performance. You will have a wet rag and finesse in hull shape is meaningless.
Ease of build usually equates to reduced weight and again that will be more important than finesse in the hull shape. A 28 degree flare gives the lowest wetted surface for a 3-panel hull but zero gives the lowest waterline beam and lowest wave drag. A minimum drag hull will have more flare in the ends than the middle. For high speed to length ratio there is near zero flare in the mid-section. So zero flare is consistent with high speed relative to length.
I allow the thickness of the core to dictate the radius. On the pedal boats it is difficult to get 200gsm twill weave cloth to stay down on a 3mm radius chine seam. For that reason I prefer 4mm core for 200gsm cloth. For anyone building a hull using my internal frame technique, I recommend they monitor the chine seam during curing to ensure it does not bubble (It is not bagged). Making the radius an elliptical arc rather than a circular arc lowers risk of bubbling.
My latest hull has 8mm core and I had no problem getting 400gsm biaxial cloth to follow an 8mm radius:
If you are building in a mould you can force a tighter radius but the sharper you make the radius the more prone to tearing along the outside cloth on the corner. Chine seams are vulnerable to damage on a flat bottom boat because they are rigid. In my light hulls I use a longitudinal bulkhead to force a bit of curve into the bottom. You could have a slight curve in the mould to make the contact point along the middle of the hull rather than the chines. The middle will flex so not quite as prone to damage. Also an extra layer of tape along the centreline for scuffing wear can be more easily blended than adding tape along the outside of the chine seam. Adding filler to make a fillet on the inside of the chine seam is also worth the effort as that strengthens the join.
Rick
On 1 Nov 2018, at 8:05 am, '.' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:| The 18m proa has evolved slightly. It started out with two under hull rudders that were 900 span and 450 chord. With those, the successive tracks for best VMG were about 110 degrees apart. If the hulls had any slimy fouling that would get out to 120 degrees.
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| When we were working out how best to sail it I would not bother trying to get any data unless I knew the hulls were clean; usually by spending about 80 minutes in the water with a scrubbing brush.
I searched the list for fouling and found how you have regularly pointed to the gentle fouling of a hull easily doubles the drag of a hull.
It seems defouling tech is, perhaps, more important than hull shape. Although hull shape won't be fixed by rubbing.
I did have another question regarding hull shape. Given the simple mold of the HP has 90° sides, how much does that cost a long slender hull vs 23° sides, in your estimate?
Given your experience actually building boats, would you consider adding a 23° side, or stick with the simple sides? I would guess adding the angle would basically double the complexity of the mold.
Finally, does the radius of the corners matter? I am looking at building molds now, and was curious about your thoughts.
If you have already answered these, feel free to suggest what I might search for.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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