Polar Moment of Inertia Question

[mcdenny]

I'm building a light narrow 25' electric launch originally designed for a 1907 era heavy (500 lb) low power gas engine located pretty much in the middle of the boat. Design displacement is 2000#.

http://www.imagestation.com/picture/sraid215/p3f041db0d5dff5677ad091f1dceadae2/ecc13e92.jpg

There was no option for the engine location - it had to be in the middle to get the prop shaft angle and keep the fore-aft CG at the Cb of the shallow canoe form underbody.

With my electric boat I have a lot of flexibility where put the 500# of batteries and keep the CG correct. I can:

1)put half in the lazerette and half under the fore deck (high longitudinal
polar moment; medium roll moment, modest 'ballast' effect)

2)put all in the middle under the sole (low longitudinal polar moment; medium roll moment; maximum 'ballast' effect of lowering CG). Pink batteries in lines plan above.

3)put half under each seat along the sides in the middle (low longitudinal
polar moment; high roll moment, minimal 'ballast' effect.)

These options would cause the boat to respond differently to waves in the roll and pitch axes. Option 2 seems intuitively best but I'd be interested in opinions from people who understand boat design better than I do.

TIA

[Keith Wilson]

I can't help you with the battery placement question; #2 seems better to me, but I don't know enough about the dynamics of a hull in waves to say anything useful. A question about the design, though - is that Coyote, originally designed by Edson Schock and modified by Weston Farmer? Lovely boat, anyway. There was somebody in St. Paul that was building one few years ago with a small diesel. I presume it's finished, but I haven't heard anything lately.

[John E Hardiman]

The polar moment of inertia of the mass will not effect the final stability, only the rate at which the vessel responds to load (i.e. the apparent "tippiness" of the vessel). Too low a polar moment and the boat "rocks" too much, too high a polar moment and she does not rise to the wave and can be "wet". IMHO, I would go for the lowest CG to get maximum stability, or if you want a particular roll/pitch period, adjust the locations to get that. Normal target roll periods are ~9-10 secs., pitch period ~30 sec.

[Tom Lathrop]

In that boat, I'd go for the best ballast location although I think that would be best for pitching and dryness also.

Very interested to see how your boat runs.

[mcdenny]

Keith, Yes it is Coyote II.

Finished painting the boot stripe today - tomorrow's the big rollover!
http://www.imagestation.com/picture/sraid215/pc27c64b3bf2509efae6c77fe9e36072b/ecc07d5e.jpg

[Ron Carter]

One thing to consider on the electric is power distribution. Concentrating the batteries in one place will reduce line losses within the battery bank. As copper prices continue to run up the wiring costs, shorter is better from a $ standpoint as well.

[TR]

Denny,

It's an interesting question, option 2 wins out hands down from a safety point of view. Moving the weight lower will mean a longer righting arm throughout her stability range.

Option 1 makes little sense to me as this hull has a very fine bow and nice flattish bottomed stern and thus will damp out (or not start) pitching really well just by virtue of her form. The extra weight forward will (IMO) increase the tendency to submarine and stick her nose into waves. So I would lobby for keeping the weight closer to amidships.

Option 3 is the puzzle, would reducing high angle stability be worthwhile if it increased comfort in a sea by lengthening the roll period? Judging the magnitude of this comparison would require running a series of calculations, which I'm not going to do. But I can make some guesses. Despite John's mention of an 9-10 second roll period as a target, I don't believe this vessel will come anywhere close to that. I would be surprised if her roll period was more than 2 seconds. As a wild generality a roll period equal to the boat's beam in meters is considered okay. Most light powerboats will be less than this though.

So how much would moving the batteries outboard actually slow the roll period? I would guess a few tenths of a second. This might be noticeable, but it won't be for most. If the boat was flat-bottomed and moving the batteries outboard did not cause a rise in VCG I would say do it. But as doing so in this boat will cause a substantial rise in VCG, I would say this is a bad idea. As this is a day boat (I presume) and won't be at sea for days or weeks on end, I don't think the motion will be a major issue.

Another thought might be to combine option 2 and 3 and run the batteries right across the boat under two thwarts. You could get some weight low and centered and some outboard to slow the rolling. But it will impinge on your arrangement .

All the best, Tad

[mcdenny]

Thanks, Tad. Compromising the VCG to get a trivially better roll motion doesn't make sense. I appreciate your NA opinion.