Simon's home-brew hydro

[Simon.O.]

As a follow on to my hybrid hydro I built this one.
The tub is a little wider, and the boat is a little longer to allow me to get 8 cells + in there.
So far it has run very well.
Given that it has a straight shaft I expected a little tail-hop at max speed and thatis fine for me. This thing flies !!

Basic setup is
Graupner 75A esc
Jamara 600 brushed can
8 x Camelion NiMh
All handmade hardware.
All balsa construction, varnished.
Nice!!

Estimated speed would be 40 kmh.

Now that I have figured out how to make wire drives I will develop the smaller hydro and design a small rigger to go in the fleet.

[Simon.O.]

some more

[Simon.O.]

last ones

[Diegoboy]

Very nice Simon. I'm just a bit concerned about the bend in the water pick up, where it enters the hull. It seems a bit too sharp of a bend there in the pic.

[Simon.O.]

That was a concern during design.
I use the smaller hydro as a test rig. It was sent out with a variety of pick-ups and this one passed the test.
There is 1 each side. Right side feeds the esc and can coil. Left side is brush tabs only.
Once running there is a nice steady flow out the two exhaust tubes.

[10gauge]

Bravo Simon! This is how our hobby started out by scratch building boats on the bench. Either you have the patience or the time to build the hull yourself, not to mention the great talent. Hope to see more creations in the future...

[Simon.O.]

Skippy the hydro

I have had a lot of fun with this hull over the last few weeks.
The current setup is.
Tamiya 23t can that gives me 27.5krpm (free)
6 x IB4200 Nimh
Proboat 2114 Esc
Octura 531 prop.

It now has a stand-off on the transom to allow a fully adjustable strut for me to run a wiredrive.
This was fitted as it had some serious tail-hop issues.
Now I have full hull-hop issues !
I have had the strut is almost all positions imaginable. Level with and above the sponson line ( I haven’t been below yet ) Flat through to Positive Thrust angles.
I have had the CoG from the rear of the sponson to 1.5” aft with no discernable difference.

It is a very fast chatter and from the rear and side looks like tail-hop but from front on it is clear that the whole boat is skipping parallel to the water, sponson and tail-hop combined.
My feeling is that there is too much lift under the tunnel or the sponsons or both.
I don’t regard it as a major issue as it looks quite cool, but it would be nice to see if there is a way to settle the little bugger down, and no, a smaller motor is not one of the plans.

[Simon.O.]

Last night I fitted a 2mm strip that is about 20mm / 3/4" wide to the lower surface of the sponson.
This has seriously calmed down the hop that was previously there at the front.
There is still a bit of tail hop that I believe can be ironed out with CoB / CoG adjustments.

Today I lost my first tail shaft due to wire drive failiure.
It was not unexpected so not a catastrophe.
The pond monster now has about $8 of goodies and it took me less that half an hour back home in the workshop to replace it.

It is now quiet time at home and I am on duty for the next few days so I'll be back then with more progess reports on this and the Cat developments.

There rigger is nearly ready.

[Simon.O.]

Right !
I did a few more tests today with some very interesting results.
I have currently got a Reedy 17t ( 30krpm) can in there spinning the 531 prop. 6 IB4200’s push the electrons round.
The strut is set so the shaft is level with the sponson bottom line and at 0 degrees.

Even with the new ride surfaces that calmed down the skipping with the 23t can there is still a bit of skip going on up front.

If I pull the CoB back to tame down the rear end the front skips even more.
I taped a couple of heavy screwdrivers to the lid to give it some more weight and this calmed the whole boat down, and slowed it too.

I have come to the conclusion that I have too much lift up front, both aerodynamic and possibly hydrodynamic too.

I can see a couple of ways of reducing this lift.
1. Air dam in the tunnel
2. Venting the air traps to let the high pressure air out
3. Fitting a roof rack to carry spare tools.

Anyone got some other cool ideas ??

One other thing. When I pull it into a nice wide turn I get a nice tail off the tf and the whole boats sticks to the water. The revs appear to rise and it looks quicker through a turn than on the straight.
Does this sound right, or help with any diagnosis of the above problems?

[JimClark]

rework the sponsons? They look very deep which could mean the strut is too deep also.
Jim

[Simon.O.]

Given that they have 2mm skin on the bottom surface and the sponsons are a part of the main hull as such, I am reluctant to reduce thier depth as this would require more structural work than I am keen to do.

I do understand where you are coming from and I appreciate your knowledge and input as always.

I am looking for a slightly easier solution, otherwise I will live with the skip and hope that the new rigger does not exhibit these tendancies too, i am confident that it will not as there is no tunnel and therefore much less aerodynamic lift overall.

Simon.

[Simon.O.]

Jim.
I have just had a brain flash.
We agree that the tunnel may be a little to high / deep…. yes?
You suggest that I raise the sponson bottoms…..yes?
This is to reduce the height / depth of the tunnel.

What if I were to lower the tub bottom.?
This could be done easily by adding a second skin about ¼” lower
Options include running it full length or even partial length, such as to the rear of the air traps. This could be left open or closed off and would produce a step.

This would be about half and hours work plus varnish drying time.

Am I barking up the wrong tree here??

A quick rough sketch to illustrate my idea.

[JimClark]

Depends if you think it can handle the extra weight

[Dr. Jet]

I’m not entirely certain that the sponsons skipping are a problem. My Campbell shovel almost seems to “hover” above the surface of the water and skips from one wave crest (ripple crest?) to another. It does have considerable weight (12 4200s) and that dampens the quickness if any motion; but the hull is doing what it is supposed to do: Float on a cushion of air, and only touch the water when unavoidable.

A laminar sheet of water, flying off the turn fin, is desirable in the turns. Speeding up in the turns seems counter-intuitive.

Skipping, as you have described it, should only be a problem when it leads to a blow-over, unless I am missing something here.

[Simon.O.]

Depends if you think it can handle the extra weight

The weight will not be an issue.
I have had this hull scream round with 8 cells on a 600 that gave similar handling problems.
I'll give the air dam a crack first and if that proves my theory of reducing the lift in the tunnel then I'll go for the new bottom.

On the turning thing, I spent an hour in bed last night going over and over the article on turn fins etc from rrr.
I realise now that my inwards canted tf pulls the right sponson and indeed the whole front down in the turn and smoothes it out. Further evidence that I must lose some of this frontal lift. I can then rest the tf for a less agressive pull down.

Today is housework day so no boating until tomorrow.