Ultra Light Displacement Powerboats

[tom28571]

Will, I'm not great at moving images around and even less so at B&W drawings but let's see if this works.

[ Admin Edit: image on remote server seems to have gone offline unfortunately ]

I see that it is a bit weak, but maybe you can make it out.

Tom

[Willallison]

Tom,
I have to say that you did a much better job of posting your drawing than I did!!

[Scott]

If you attach a .gif or .jpg format image it will post inline like the above.

If you attach a .bmp, .zip or other file format, it will post as a link since not all browsers can display these.

It is usually pretty easy to save as either a .gif or .jpg from an image editing program when you go to Save As - to save as a .gif you will have to reduce the number of colors to 256 or less (a paletted image) and 16 or 32 colors is usually sufficient.

P.S. I also really like your boat Tom, and I think your project is very interesting too Will. And definitely Stepen's as well. I'm glad you got this discussion started... 3 great projects - what a great thread!

[Stephen Ditmore]

The answer to the inquiry about the jet drive is yes, I'm taking an efficiancy hit. I think some jets would be better than others, but I've yet to find one that's really right for my purpose, and the Legend Jet is so much cheaper than the larger alternatives that I'm thinking it's cheaper to just throw on some extra horsepower. My reason for a jet is that launches that operate around moorings need to be maneuverable and not get lines tangled in their props. I could do a prop version, but I like the idea of buying the drive as a single unit. Am I completely happy with the solution? No.... but I have a freind who got a slow turning propeller in him when he was young, and had to talk his companion through shutting the motor down without killing him as the blades ripped into his flesh. So I'm inclined to favor jet drives.

[Willallison]

A little explanation goes a long way.....
Actually, looking at your design a little further, the bottom shape is reminiscent of those found on some of the newer displacement catamarans - it may be worthwhile taking a closer look at the shape of their underwater sections.
Do you think that the abrupt shape change will create turbulence in the area of your jets intake?

[Stephen Ditmore]

Optimally the slope of the intake would be the same as the slope of the hull surface just forward of it, so the water would not have to turn up at all, but would simply continue straight into the intake.

The idea here is to create a "pressure bubble" under the stern in the vacinity of the intake. See
http://www.sciam.com/1097issue/1097giles.html
http://www.dt.navy.mil:8765/query.ht...&submit=Search

[Willallison]

I'll take a look at those links, but in the meantime, I've managed to track down the photo's of the dive boat that I was talking about. Unfortunately, now that I see them again, they dont't show the aft sections of the hull all that well (I took them about 8yrs ago - on my honeymoon!). But the deep sections of the bottom abruptly stopped and the two shafts ran horizontally out from this. The aft sections of the boat were completely flat. Note also the way that the forefoot was warped downwards giving the hull a distinctly hooked bottom shape. I had my reservations then about problems with bow steering - unfortunately, I never got to see the boat in the water, so I don't know how it performed.