Simon's home-brew hydro

[JimClark]

where is the CG on that boat?

[Simon.O.]

About 1"aft of the sponson line.
This equates to approx 8% of the afterplane length if measured to the prop.
With the excessive lift I currently have any move of the CoB to the rear loosens up the front too much regardless of strut adjustments.
That lift has got to go !!

I have fitted a temporary and adjustable air dam near the front of the tunnel and will trial that tomorrow morning.
If that is sucessfull then I will remove it and try venting or reducing the airtraps.
I want to know more about these two methods before I commit myself to adding another skin to the bottom to reduce the tunnel depth and therefore the lift that it produces.

This is a whole new ball game for me and I love it !!

[JimClark]

Sponson line? It should be probably .75 aft of the back of the sponson

Jim

[Simon.O.]

Ooops I did not make this one clear!
It is approx 1"aft of the trailing edge (wet) of the sponson.

[JimClark]

try moving it forward to .5 or .75 of and inch behind the sponson

[Simon.O.]

As some of you are aware I am running this same thread on rrr so I get as many opinions and ideas as possible. I like to use a big net

The air dam trial was a sucess, it showed me what reducing the airflow into the tunnel does. Guess what guys ? It reduces the lift, wow !.....o.k I'm being smart now, of course we all knew that it would affect the lift. Good !!
It does not take mush of a restriction to dump a lot of lift.
I was able to make some CoB changes to tame the rear down too.

I trialed another prop too. It began life as a 534 and after a spin in the dremmel and an emery board it is now detounged and 30mm dia.
THAT make a difference. This dt "530" appears quicker than a stock 531.
I am waiting a small package of these plastic octuras so I can try some new cuts.

I have decided to refit the superstock motor 23t (27krpm) and send the 30krpm to the cat for trials.

After reading all of the replies regarding this hull I am happy for it to have a little skipping, dancing, loosness, well it is a a hydro after all so i should expect it to be a little flighty.

The rigger will be here soon.

[phase]

well weree is the video..lol.. its a good lookin build.. have fun and please post a video or some action pics.. thanks

[Simon.O.]

Today I ran it with the slightly slower can ( 27krpm ) with an untouched 531 prop and all was clear to me.
I ran my 6 cell pack that has slightly less punch than the IB's and all was good.

There is still a little skip and I will tame this out with small reductions in the air-trap skirts. I believe this is the place to make alterations.
My reason for this is that it is better to dump the lift than to restrict the air intake which in my mind equals more drag.
Given that aerodynamic lift produces drag in any case it may 6 in one half a dozen in the other.

I'll go for a full pack run with the existing setup to see how it runs now.
During the first 2 mins or so if there is undue skip I will have to reduce the airtrap skirt.

Given that I can sand out a little of the a-t skirt and run again in less than 24hrs I see this as a viable tuning method.

It'll be a few days before I can run any boats but as alway I am here reading.
Catch ya all soon !!

[Simon.O.]

Here are a couple more pics, including the new strut arrangement and carbon rudder, and a bit of a crap video of it running. I just set the camera on the tailgate of my truck and let it go, this was with the test cells that I use. I left my good ones at home on the bench. #$%&#@

http://s15.photobucket.com/albums/a392/simono/?action=view&current=P6260716.flv

I am now satisfied that I can design, run and evaluate a hull enough to implement design keys into other hulls.

I will never be one to go to the LA saws and take records………..but I will continue to design and build hulls, even against common conventions.

Given the design methods I use I believe that I am now ready to do a Canard type of hull.

There are other threads on this and I will post there soon.

[Jeepers]

If you can get a close up video of it running it would help you tune the hull better.

[Simon.O.]

It has been a while since I have written an update on this hull so here it is.
Since I have installed the BL motor the performance has been wild.
Given that this and the other balsa hulls were originally designed around a brushed 23t car motor it is no surprise that the handling has changed due to a massive power increase.

I am currently running 540 8S with a 531 plastic Octura prop I am waiting for a few more 431’s

I am now in the position where I think I have too much lift from the sponsons.
I have dammed the tunnel and removed the air-traps and still it dances from side to side in an effort to dump air. I can either dam the tunnel more or rebuild the sponson bottoms.

I have adjusted the strut up and down a lot and gone from blow over to stuffing the nose into the water and at the same time all it wants to do is dance from sponson pad to sponson pad so some of the frontal lift has to go.

I am leaning towards adding a ride surface to the bottom skin which will be approx 1”wide and ¼”deep 25 mm x 6 mm. This should now reduce the sponson wet surface and thus help to reduce the hydrodynamic lift that is there.

Spectacular to watch as it may be, I am not getting full speed out of these hulls with the power that I have available.

The OB tunnel is another story. Blow overs are a blast as 9 out of 10 will land on its feet and drive away.

[Simon.O.]

It has occured to me since writting the above suggestion of adding a ride surface that I am heading in completely the wrong direction.
This will only provide more tunnel volume which I believe is where my problem lies.

Since I put a small dam near the front of the tunnel inlet and removed the airtraps have got more speed before it does the sponson tap dance, keep in mind that my speedometer is each side of my nose.

It is now my intention to seriously drydock this hull for some sponson surgery.
I will remove approx 12mm or 1/2" from the sponson depth to reduce the tunnel air volume, at the same time I will add in 1 deg of deadrise.
This should tame the little bugger and give me a smoother ride at wot.

If this is sucessful the the rigger will get some similar surgery.

[Dr. Jet]

I had a discussion with Brian Buaas some time ago regarding reducing the amount of trapped air in a shovelnose hydro. As you may know, the early hydros had “wet” sponsons, meaning they were open in the back and were allowed to flood with water when the boat was at rest. At speed, the sponsons emptied and ran “dry”.

Brian suggested adding a “NACA-Styled” air inlet on the inside (towards the tunnel) surface of the sponson and ducting it to a “scale” opening at the back of the sponson.

Maybe you could try something along those lines as sort of a “Guinea Pig” for the rest of us.

[Meniscus]

That's an intriguing concept Jet. I think you guys watch a little too much LaMans series racing. However, I'm all for it! LOL

[Fluid]

From reading the thread it appears to me that the issue is not the sponson shape or tunnel depth. It is partly the lack of mass to offset the lift. It is FAR easier to add weight to the hull to balance the lift. I know the OP did this, but how much did he add? Most likely not nearly enough. I'd add a minimum of 8-10 ounces lead (not wrenches) to the inside of the hull at the CG.

Next keeping air out of the tunnel. The full scale Slo-Mo-Shun did this with the addition of a solid airdam placed at the nose of the boat. It served two purposes, blocking air and assisting getting on plane. You can make it out in the photo below, the dark shadow in the tunnel is the back of the airdam.


Finally, when you set the strut I assume you are doing it on a flat table and not "by eye". If the latter STOP. Make sure that the strut is flat on the table when the sponsons are resting on the table. You can try slight down angle it help plant the sponsons. None of this is very invasive, and if it doesn't work then on to plan B.


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