Subject: Re: Flat bottom hulls?
From: Mike Crawford
Date: 11/3/2018, 9:51 AM
To: harryproa@yahoogroups.com.au

<< I guess this makes the trailer set up and take down time be offset by hull scrub time. Did you hear that Mike?>>

  I that Mike me?

  If so:  right on!

  I love my Stiletto 27 catamaran, and still enjoy daysailing after 14 years, but it has five major flaws:

    - Half-day setup and takedown trailering time requiring the strength of three grown men while struggling with a custom expanding trailer.

    - No real interior, requiring camp-cruising with a boom tent.

    - Aft-mounded outboard ventilates in seas over 2.5 feet.

    - Patently unsafe in high winds.

    - Everything that's wrong with a unidirectional boat using a standard rotating bermuda rig.


  My goal is the largest transportable boat at 10' to 12' wide on the trailer (no lead or tail "wide load" vehicles required), that can be folded safely while on the water, and trailered by two adults.

  A Dragonfly 32 or longer meets most of the criteria, but it is way heavier than an ex40, much more expensive, and has that dreaded rotating bermuda rig.  Not only are there 70+ points of failure, but you simply can't depower on some points of sail, reverse course, or shunt instead of tack in either big winds or very slight winds.

  The proa would be much faster, safer, more seaworthy, and also less stressful to trailer with mast-raising assist poles that hoist the masts up at their center of gravity and allow you to drop them into their casings.  I like the mast-raising setup on the trimarans, but it only works on the trailer, and it involves huge horizontal stresses on the mast bearing in the early phase of raising it up. 

  And, of course, being able to store the boat anywhere in the winter instead of paying $4,000 is a huge plus.

  Alas, I keep spending the proa money on the ongoing house construction project.  For a while I couldn't make the leap because the designs weren't there, but the latest VR renderings of the ex40 are so close to my dream boat that I no longer have that excuse.  Any remaining details can be resolved during the build.  Now it's just down to funding.

  I had no idea how many details would call for funding, which is odd, because I've worked in construction in the past.  Oh well, it's easy to slide down that slippery slope when it's my own house, and things are so close to optimal that I can't resist the call to go the extra mile.

        - Mike




'.' eruttan@yahoo.com [harryproa] wrote on 11/2/2018 9:36 AM:
 

| 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.

Chlorine generator sounds interesting. Any bag with dosed chlorine will burn through said chlorine relatively quick, unless somewhat form fitting and opaque, limiting sun exposure and water volume. But I guess a few pucks might last between sails. I know pools here burn through a gallon or more of chlorine concentrate a day.

| 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.

Great point. Sail, trailer, or scrub. I guess this makes the trailer set up and take down time be offset by hull scrub time. Did you hear that Mike?

| 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:
| https://1drv.ms/v/s!Aq1iAj8Yo7jNgw1Xx8jXch9HROgO

|
| 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.

Help me to understand the performance cost of a larger corner radius.

Like if that 'chine' radius is 2, 10, 20, or 30mm, what are the effects?

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Posted by: "." <eruttan@yahoo.com>
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