C'mon all you math gurus!

[Captain Andy]

Greetings,
I saw an interesting thread re a Li Po conversation to a UV-26. This got me started thinking about my seldom used UV-18. I'm one of the old guys with an old scooter that's kinda heavy to lug around the boat and even worse loading it in the truck! Seems like everyone has newer lighter.scoots so, if I could save weight on the batteries and cut the tube down, I could have a more user friendly scooter.

Here is where the math comes in.

The current tube is 10.6" x 28" that's 2462 cu in or 1.429 cu ft
Fw is 62.3 # per cu ft so I should have 89.02# of positive buoyancy ( not sure of how to word that).

The existing batteries weigh 11.9 # s each and the LiPo weigh 3 #s each so I should shed 17.8#s
89.02-17.8=71.22#s
1.143 cu ft
.286 cu ft
494.2 cu in
The tube is 88.2 cu in per inch of length.
62.3 / 1728=.036 # per cu in
494*.036=5.6


So if I take 5.6" off the tube length I should be neutral in fw.

Ok gloves off. Tell me where I screwed up please.

Thanks in advance,
Andy

 

[johndiver999]

didn't do the math, but you also have to figure the loss of weight of the tube material. was that addressed?

 

[Captain Andy]

didn't do the math, but you also have to figure the loss of weight of the tube material. was that addressed?

No, no it was not.
I was using that weight as a buffer in my calculations... Actually I didn't think of it till you brought it to my attention.

Thanks!

 

[treasurehunter]

A UV-18 total weight is about 75 pounds. For every inch you remove you will reduce the buoyancy of scooter by 3.18 pounds + weight of material which is not enough to concern your self with.

The amount of body you can remove will be limited by how deep you can recess the battery. Make sure you leave enough space in the nose to insert ballast weight. I removed 10'' from my UV 26 body, which is now 24'' total from shroud to top of nose cone and I have to insert about 6 pounds of ballast. And the trim is perfect and weight without ballast weight is 52 pounds and 58 pounds with ballast weight added,

You need to verify if your motor is post 2007. If not you will need some sort of speed controller to protect motor. I can recommend the smart-dpv controller as that is what I installed and like it.
If it is 2007 and up you don't need a speed controller but it will be much more jerky at start up because it will be a more powerful scooter.

 

[tbone1004]

Also of note that for every inch you shorten it, the center of gravity shifts towards the back with the very heavy motor. Those batteries are stood off of the motor as far as they are to get it to trim out flat. If you shorten it at all, it will go nose up. It may not be enough to concern yourself with, but please keep in mind that it isn't just about getting them neutral, it's also about getting them to trim flat.
The motor in the back of that sucker is pretty heavy, I don't think you're going to be able to get it down to 50lbs or less and if you do shorten it enough to do that, it will make it even more awkward to carry than it is now.

Coming from an owner of a UV26/18 who contemplated such matters already...

 

[Gone for diving]

I have nose up problems with my mark 3
Can't get enough weight forward and still be neutral...
Thought about hiding a bit of foam or something on the shroud...

no matter what you do... leave it longer than you do the math for.. you can always cut it shorter... harder to add..

 

[Gone for diving]

My brother switched his gavin scooter
To lithium
left it stock
he's got 80ah batteries in there...
problem is... hardly ever have to charge it...

 

[tbone1004]

My brother switched his gavin scooter
To lithium
left it stock
he's got 80ah batteries in there...
problem is... hardly ever have to charge it...

the batteries won't complain about that.... All he has to do now is get a Smart DPV controller from @Jona Silverstein installed and he'll be able to have gears!

 

[Gone for diving]

Does it have soft start?
Not Totally sure it really needs gears...
It's on full pitch all the time...
Other than going slower... what's the advantage?

 

[tbone1004]

Does it have soft start?
Not Totally sure it really needs gears...
It's on full pitch all the time...
Other than going slower... what's the advantage?

does have soft start,
a couple of gears are nice for efficiency so you don't have to depitch the prop.
it also gives you battery cutoff so you aren't killing the battery if you run it until it stops.