This is my scratch built rigger built out of 2mm ply and .5mm carbon fibre sheet. I think i'll call it the scorpion because it kinda looks like one. I don't know if it works yet, and whether my calculations for the angles and distances were ok....but i did check to see that it floats and doesn't leak. Hardware is home-made (rudder needs to be changed because i tried to clean it with acetone, and this melted the plastic.

I might need to take the strut up a little - we'll see about that after the first run.

It's powered by a 3000kv outrunner that i took out of my iceberg (she deserves better i think) and a 2s lipo pack. All up weight is 410 grams.

Here are the pics:

What is your front sponson AOA? On my newest rigger - 34" long, I designed it to have an AOA of around 4°

The smaller the rigger the greater this angle may be.

Ryan

more like 8-9 degrees on this one. I had to increase the angle somewhat because of the low profile of the sponsons (25mm), otherwise the only way i could get it on a plane would be to hand launch....which is possible, but i would hate getting stuck in the middle of the lake with this tiny thing. Still haven't found time to try it out. It's a trial and error thing that's true - but next time round I'm gonna try an adjustable sponson rig. Basically the carbon rods would be fixed to two aluminum triangle, and the top corner of each triangle is bolted on to the main tub with fly-nuts. That way you could adjust angles and distances when weight distribution possibilities are limited....

I couldn't do this with this rigger because of the curve in the side-walls of the tub....

8 degrees sound just on the high side but should work. How much of a flat spot do you have on the front sponson until the 8 degrees increases or changes? My rule of thumb is approximately 1/2 the size of the sponson.

Also what is the overal length of the hull? 410 grams is extremely light! The motor I plan to use weighs in at 475 grams. [X(]

Get her wet and tell us how she does. It looks awesome!

Ryan

Thanks a bunch Ryan.

It's 23' long (27' if you count the strudder) and 17' wide. In the meantime i went down to the river to see how it goes. First couple of runs were hopeless - till i realised there were tons of alga on the strut. Otherwise it's quite lively. BUT nowhere near the performance i want. As regards attitude in water, the sponsons seem to be doing their job and come right out of the water but the rear end i'm not too happy with. Basicaly all the prop is still submerged, very very small rooster tail. My options at the moment are 3:

1) take up the strut
2) take the motor forwards - (at the mo it's about 1/3 of the way from the back - yes i know that's wrong - but that was as much as my only flex-shaft would let me go - didn't feel like going to the model shop)
3) use a higher kv motor - i think 3000kv is way too small to begin with....but that's what i had lying around


and 4 - all of the above - i think should be the best option

As regards the flat bit on the sponsons - about 1/3 of the way, but this is countered by the bigger angle of attack - so yes more drag on the water, but I had other constraints....a little bit the wiser for it though...

Wow, I didn't realize is was 23" long,(584mm) I expected it to be around 17" (430mm) or smaller.

Where is your center of gravity at in relation to the back of the front sponsons or in relation to your turn fin?

Your turn fin does not appear to be a hydro fin. Is it hooked at all or straight?

Usually for the placement of the motor I take about 33% of the length. So for a hull around 23" it would be 7" (175mm) absolute minimum from the rear. 1/2 would be much more ideal

All my rigger layouts front the front of the hull are in this order.
1) LiPo
2) ESC
3) Motor
4) Radio/Servo

3000kv on 7.4v LiPo is quite weak for a rigger or even a mono. unloaded - 22000RPM
What size 7.4v pack is this? C rating?
Usually I aim for around 35 000-40 000 Unloaded RPM for a well built rigger hull. Others choose to go with less unloaded RPM which works as well.

What kind of estimate speeds did you achieve?

And lastly what prop was used? Diameter/Pitch

A lot of question, I know, but we'll get this thing moving. If it is in fact 23" long there is no reason it should be running less than 40 mph or 64km/h on 2 cell 7.4v LiPo with the right setup.

Ryan

EDITED motor placement

ok many questions - but if that's what it takes....;-)

HEre are the answers:

COG - 240mm from the back, so it is a wee bit too far back i think no? i think it should be 40:60 rather than 60:40 no?
I'm using 1300mAh long max lipos 20C rating
29mm Graupner k series prop.
Turn fin is completely flat (super sharp but not curved at all - will take your advice there)

An alternative to change the whole thing could be to add another 2s....that would x2 the rpm and change the COG to something nearer what i want...

thing is i don't think the motor could handle the load.

The distance you mention for the motor is from the front right? not from the back as mine is.....

As regards speeds i think it was around 30km/h...yes i know that's slow - but as i said - prop was way too low in the water and was nowhere near reaching a respectable rpm....

Cheers,
Andrew

Your Center of Gravity should be located 0-50mm from the rear of the front sponsons. I like to place my CofG right on the turn fin if possible. The further away your CofG is from the rear of the front sponsons, the more weight is going to be on the prop. Without rear ride pads you will want to reduce this weight on the prop.

The turn fin being straight will work to some degree. Once this gets really dialed in you will notice that the right sponson may be bouncing or lifting in corners. A curved fin will hold the rigger on the water during the turns.

Actually the motor mount distance I forgot to mention is from the rear. Basicly 1/3 from the rear is absolute minimum. If you could get it to work some where around 1/2, this would be optimum.

The largest problem I see is lack of power in the motor and packs. If you really want to watch this thing fly and be on edge while doing so, you will have to reconsider motor, esc, and cell options. Power is needed to get these hull to run dry and out of the water, especially in the rear as you have noticed.

A cheap route would be to go for a Feigao 540 7s motor, an 80A ESC, and 5000mAh 25C 2 cell LiPo. A prop that would work well would be an X430.

Ryan

hmmm - i will sleep over what you suggest. I was thinking more on the lines of a KB20 (180) XL 5000kV, to keep things light. my concern is that since this thing offers so little flotation with so much weight it would really sit way too low in the water, and possibly run too wet. I dunno if you can see it in the pics, but the rear end is actually 15mm wide. With a 540s motor i would have to keep the motor in its actual position to fit in the cells somehow.

In post 5 you replied with a length of 23" (584mm) I believe you are more familiar with mm and this may be the confusion??

I have a 685mm rigger that runs a 540 size motor on 2s. I have tons of room in it. For a hull over 550mm a 540 s motor is needed. A 380 XL motor performs similar to a 540 s motor and may be used. I place my cells over top of the second sponson boom in my 685mm rigger.

The front sponson on my 685mm rigger is about 30mm deep from what I can remember. When the hull is at rest only about 5 mm is above the water. The booms are submerged completely.
How deep the hull sits at rest has no relation to how wet/dry the hull runs when on plane. Riggers will sit very low in the water but they were not designed to look pretty just sitting there.

I don't understand what you said is 15mm wide. My hull has an inner wall to wall dimension of 75mm.

Photo is of my 685mm rigger before anything had been placed in it.

Ryan

ok i understand the misunderstanding. I am using this old measuring tape with funny units - dunno what they are - and these aren't inches - DAMN!!!! - sorry dude - my bad. - god i'm embarrassed! -

THESE ARE THE CORRECT DIMENSIONS!!!!! 370mm main tub length, 70mm width at its widest point and tapers to a point 15mm without the 2mm walls on each side of the transom so really 19mm - my bad again - at the rear. This is kinda compensated for by the relative height at the rear (30mm).

470mm front tip of sponsons to rear end of tub. 540mm tip of sponsons to rear tip of rudder - overall length.

you can't really compare my rigger to yours as the booms are right up at the nose of the tub, and not further back as they are in yours. i do have a 380m 3800kV which i'm tempted to use.
Sorry for the mix up - i'm a right goof sometimes.

Andrew

Ok, for this size a more appropriate goal would be over 45km/h.

A 380 s motor should be all that is needed to move that hull. A kv around 4500 should be plenty. You should be able to turn a 27+mm prop.

Try and get your CofG located in the proper position. Close to that turn fin.

Ryan