solar electric boat needs more boyancy

Hi

I have built a small solar electric boat that is fun to use and seems to put a smile on everyone's face. It's a small remote controlled boat that is steered by differential thrust. (see pictures)



The problem is that it has barely enough buoyancy for one person. I would like to add a third pontoon and swap the captains chair for a couch so that I could take soul mate 1 along.

I have a few questions

1. should I install the third pontoon a little higher so that the boat is not high centered and rock side to side? if so, how much higher?

2. if the center pontoon extends out the back a foot or so will it disrupt my steering via differential thrust? note that the motors are slightly cantored to provide more steering thrust.



thanks so much for your help.

scott harris

 

Welcome to the forums,
Depends on how much extra buoyancy you need. It looks like the hulls are significantly down by the stern, meaning a lot of buoyancy forward is wasted.
1) Adding a third hull will greatly affect steering, especially at slow speeds.
2) Extending the existing hulls aft 1 or 2 feet will not affect the steering preceptably due to the location of the props. It will effect the steering due to the lengthening of the hull though.
Think of it this way, you haven't changed the moment between the the props, but whatever you do will have increased the force needed to rotate the hull(s) in the water. At speed it should make little difference, rotating at a stop may change significantly due to decreased lever arm (i.e. the props are closer to the center of mass).

 

hi jerhardiman

thanks for your quick reply.

the boat is built from a hobie 16 and the box is centered on the frame. the box would be hard to move forward since the frame bounds it on the front end.

you are right that it sits low in the stern. but even if I easily could move the box forward (which i can't) I'm afraid that I would still not have enough bouncy to carry another adult.

my thought was to add a third pontoon mostly to the back. the original pontoons are 16.5 feet long. my idea is to add a third pontoon that is about 10 feet long so that is even with the front of the box and would extent about 2 feet behind the box.

would that add buoyancy to the stern without messing something else up?

 

I'd run the centerline of the center hull deeper than the amas. Look for an old Hobie 18, Prindle 18-2 , or Nacra 5.8 or 6.0. I think they are all symmetric hulls. What is the net weight of your mods? I think the H16 had a crew weight of about 300 pounds. A H-18 hull should gain you 250 pounds or so and get the amas up a bit.

 

It might be worthwhile considering adding a layer of xps foam and glass over the existing Hobie hulls.
The forward half just bonded to existing hulls and faired down at the stems. The back half extended a couple of feet aft and with less rocker.
Very rough calculations puts added buoyancy at about 100# per hull with 1" foam.

 

Are you using the standard Hobie 16 tubular aluminium structure for connecting the two hulls together, and supporting the bridgedeck, or did you literally start with just two Hobie hulls?
I am wondering if you might be able to take some weight out of the bridgedeck structure? How was it built?
How heavy is each yellow box of batteries?
Apart from crew weight, the Hobie 16 hulls also had to support the weight of the mast, boom, sails and rigging, which might be another 50 lbs (?)

 

After a gutcheck review with zero data, I’d recommend you get a little serious about weight calculations and establish the hydrostatics.

Or, you will be always guessing it all.

For the guessing game, move everything forward starting with the solar.

Rather than adding a heavier third pontoon, I’d be more inclined to extend the hulls. Very little added weight and almost one for one added drag and added length aft which would help efficiency. The boat would perform better and you could probably leave the motors about where they are. Of course, you add to the center or widest part of the hulls to simplify construction. The displacement calcs are pretty simple, just take the volume added that will end up below the waterline and add for say 250-300 pounds or maybe 3’ each side?

 

if I added a centerline hull that is deeper than the amas, then would the boat tend to rock side to side?

 

that is an interesting idea. will look into that

 

yes, the original tube structure holds the pontoons. the mast is still with us. I cut it in 3 pieces and used it as support struts under the deck. I cut holes in the mast sections and threaded them on the square supports that hold the pontoons.

I build the batteries from 280ah LiFePo4 cells I bought from China. they are lighter than lead acid but at 7.5 kwh each they still weigh 120 lbs apiece

 

what is the easiest way to add to the hulls?