Denny,
Combining your response and Serge's makes me feel more comfortable with my ideas of going to a pure electric powered boat. I took into consideration the advice from a lot of the other members of the group and started looking at trailer sailor type boats. I'm thinking a water balasted boat like the Macgregor 26 might make a good test bed. It's a little smaller that I really want but there seem to be a lot of them around. The ability to haul it in and out of the water easily while I tinker with it is a big plus. I'm not sure I'd want to cruise it to the Bahamas, but I understand some people have.
If I can find one cheap enough, I might have my summer project to play around with without dipping too far into my savings.
-Red
--- In electricboats@
>
> Red,
>
> You are mixing up roll stability (technically righting moment) and roll inertia (technically polar moment of inertia). Having the weight distributed away from the center of gravity slows down the RATE of response to a disturbance, say a big wave hitting the side of the boat. The boat will tip just as far but will move to the tipped position a bit more slowly. I believe this really only has sigficance to a blue water boat that might spend days on end with a beam sea.
>
> Roll stability is more relevant to us. To a sailboater it means how far the rail will go down under a steady press of sail. To a power boater it means how far will the boat tip when a heavy crew steps on the gunwale or waves hit the boat from the side. It is determined by the position of the center of gravity (regardless of how far the weights are spread out) as compared to the hull's center of buoyancy which is dependent on the shape of the hull, it's displacement (weight of everything) and angle of heel.
>
> The take-away is that the boat will tip the same amount whether its weight is distributed as with a deep keel and tall mast or concentrated as with no mast and higher ballast. The conventional sailboat will rock back and forth more slowly due to its greater roll inertia and hydrodynamic damping of its keel or deployed centerboard forcing aside the water.
>
> Converting a centerboard boat with internal ballast is a safer bet because the designer already figured its stability with the ballast in the bilge, not 4' under water in a fixed bulb keel. You can put batteries in almost the same position as the ballast so the stability will not be reduced much. Removing the mast will increase the righting moment disproportional to its weight as its center of gravity is so high. Remember the original boat was designed to withstand a pretty big tipping force with the wind on the sails close hauled. Most sailboats can be driven to where their rail goes under water and its often exciting but not dangerous. I can't imagine a power boat putting its gunwale under water in anything less than a huge breaking sea.
>
> It's still your boat, though.
>
> Denny
> www.wolfEboats.
>
> ----- Original Message -----
> From: peoria_diver
> To: electricboats@
> Sent: Friday, April 09, 2010 9:53 PM
> Subject: [Electric Boats] Re: Pure or mostly solar powered boat?
>
>
>
> 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 -----
> >
>
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