Serge,
Your english is better than a lot of people who actually speak english!.
Your explination and the example of the tightrope walker was a perfect way to express the concept I was thinking of with the mast helping to keep the boat from rolling.
-Red
--- In electricboats@
>
> Hello,
>
> You are right about the inertia of masts. Even if they get de CG a little higher, they are so far away from the roll center that inertia does in fact stabilise the boat. A dismasted sailboat is much less stable, having only its ballast, which is only efficient when you have more than 10 degrees of heel. Going for a hard chine boat can do the trick, with form stability.
> So, for small angles, the mast help, then it does help a little less when you reach an angle at which the CG of the mast start to pull it down. At 15 degrees, you get:
> RollMoment = force * lever =
> RollMoment = MastWeight*sin(
>
> For a 25 kg mast with a CG at 6 m, that is 9.3 kg.m at 15 degrees. In Imperial, that is like having a 20 pounds weight placed on your deck at 3 feet port or starboard of the mast. Not much heel generated by that!
>
> But the inertia helps you much more because it is related to the SQUARE of the distance between the weight and the axis of rotation.
> This is why the guys who walk on a wire use a long piece of wood to stabilise, even though it gets there CG higher. The tradeoff is vastely in there favor.
>
> Sorry for the bad English, I speak French... but used to teach Physics ;o)
>
> Hope this helps,
>
> Serge
>
> --- In electricboats@
> >
> > Denny,
> >
> > I get so many different opinions when it comes to the physics of how a boat works. The understanding I had was that the vertical line from the mast to the keel helps initially keep a boat from rolling due to the inertia of having that much weight up high and the resistance of the keel going broadsides through the water. That is why I am a little cautious of pulling the mast off and trimming the keel. It would not surprise me in the least if I'm wrong in my assumtions and understanding. That's why I'm asking so many questions in this group. From what I've read, this bunch seems to have the broadest range of the knowledge I need to decide if this project is worth investing my limited resources.
> >
> > -Red
> >
> >
> > --- In electricboats@
> > >
> > > Red,
> > >
> > > If you get the weight and center of gravity for your conversion at the same place as the boat as-designed it will have the same stability.
> > >
> > > Removing the mast will greatly increase roll stability, even though it may only weigh 50# or so it's cg is so high that it has a significant effect.
> > >
> > > I would bet that if battery weight = removed ballast weight and they are mounted as low as possible and roughly in the same fore-aft location as the ballast and you remove the rig you will have a more stable boat.
> > >
> > > It's your boat, though.
> > >
> > > Denny
> > >
> > > ----- Original Message -----
> > > From: peoria_diver
> > > To: electricboats@
> > > Sent: Thursday, April 08, 2010 12:44 AM
> > > Subject: [Electric Boats] Re: Pure or mostly solar powered boat?
> > >
> > >
> > >
> > >
> > > Denny,
> > >
> > > Following sugestions from this group, I did a little online shopping today. Powerboat hulls just don't seem like good candidates to me. Even small trawlers have engines in the 100HP range. I switched back to sailboats and found a couple of centerboard boats at 28'. I am still a little concerned about the stability. I'll have to give it some thought.
> > >
> > > -Red
> > >
> > > --- In electricboats@
> > > >
> > > >
> > > > On the notion of revamping an old fg sailboat hull for electric power: I think is is a fine idea. Is it the most efficient shape for the new low-power boat mission? No. Is is cost effective? Absolutely.
> > > >
> > > > A purpose designed, custom built 30' power boat would be at least $150,000 - probably more. An old 28 - 30' sailboat with clapped out sails and a non-working engine could be found for a few thousand dollars. I have a friend who just bought a nice sailable 32'er at a bank reposession auction for $1000.
> > > >
> > > > A boat having a centerboard with internal ballast would be the best as its is designed to have the ballast inside the hull, not hanging down 3' below the boat. It will have much less draft than the keel boat, too.
> > > >
> > > > Load it up with 1000 lbs of batteries, fix a Torqeedo 4.0 in a well and steer with the rudder. You would have a very nice E boat with a new high-tech propulsion system for less than $15k.
> > > >
> > > > Just my $0.02.
> > > >
> > > > Denny
> > > >
> > >
> >
>
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