stealth assassin

Hi,
I need some ideas on the most drag free shape for a belly pan on a flat surface. This will be a drop down compartment from a flat surface. Some way to create lift from It would be a plus.

Pic of simular but, I want to Improve the design with less drag as possible and I whant the pod/belly pan to be shorter.


Thanks,
D

allanflowers

My guess is that lift = drag. If it produces lift, the drag will increase.
The more air that goes UNDER, rather than around or over, the more drag will be produced. Therefore try to shape the sides so that the air will split in such a way as to direct air to the sides and top.
The outlet at the back should probably be slightly larger than the inlet. Wind tunnels sometimes do this to prevent tunnel "buoyancy".
A movable surface at the back would be a good way of tuning the lift-drag balance on the water.
These are all just hunches - I have no actual experience with these hulls.

BMatthews

The lower side of a central pod of a catamaran planing hull of this sort defies normal aerodynamics. The very close presence of the water and the spray off the outer hulls contaminates any attempt to bring analysis by normal airplane means into play. It comes down to a question of how often will that center pod contact "hard" water vs not having a center pod at all.

Lift from the center joiner will play a part but for model boats I think you'll find that the last thing you want is too much lift. Somersaults are only a slight nose lift away on the best design from the couple of boat races I've seen. Much of the benifit of the "picklefork" design to my way of thinking is that it limits the "wing" area to the front of the hull to help limit lift rather than add to it.

Bottom line is that there's not much help to be had on this topic other than try to use as little a central pod on the exposed lower side as you can manage. If you MUST have a well for the crankcase of the engine or motor to lower the angle of the drive shaft then a sharp leading edge airfoil bubble with a knife like keel behind it would be my choice. Sort of a tadpole shape as seen from below.

stealth assassin

The pod definatly slows the air In the tunnel, no doubt. The tadpole or rain drop shape are my original thoughts but, after doing a airfoil test on this online simulator I was thinking about sort of a mirrored tadpole with the point coming Into the belly and going out.
I want to make this pad/pod as small as possible mostly rounded over. I like the leading sharp edge. Then again, the pod will need to cut through the water when It touches water... Hmmm...

stealth assassin

Here Is a couple other designs,





D

stealth assassin

Really good answers guys. This has really made me re think alot of this Idea.

D

BMatthews

The pod in the first shape in the latest post with pictures looks good. I like the flat wedge shape that will deflect the hull up out of the water and the clean releast at the rear. However the drive shaft shroud is WAY too big. The second hull with the spoon shaped pod has the better idea for the drive shaft I think. But the spoon like shape will generate downward lift much like a spoon's baclside in a tap water flow.

I take back my raindrop shape in light of the suction lift that can occur from water flowing over rounded shapes. A flat or near flat wedge is best but keep it short. And ideally the "step" of the pod should be well back so that it eithef lifts the whole hull evenly when it hits a wave or is behind the center of gravity so lift from the pod does not tend to lift the nose of the hull. Frankly if you can get away witout ANY pod that would be best. And the next best is a pod that runs minimal risk of actually contacting any degree of solid water. Spray and wave crests with no real mass are one thing but if the center pod hits real water that is part of the main body it's going to slow down the hull. Same with any shaft shrouds. Water is hellish at producing drag. The least contact and immersion the better.

stealth assassin

Hey B,
What do you think about a flat bottom tunneel vs. a v shaped bottom? Minus the pod?

Thanks,
D

BMatthews

No pod at all would be by far the best option. The whole point is that if there's nothing to hit solid water then there's nothing that'll slow the boat down. Water is a far harsher mistress than air in this regard as I'm sure you know. Just look at how small and simple it is to produce a brake for a hydroplane.

BUT... there's always a "but" isn't there..... life is all about compromises. Without a center pod to sit the crankcase or magnet bell of the engine or motor into you may end up with a drive shaft angle that is way too steep to work well. In my very limited RC racing boat knowledge I'm seeing that pod as a way to clear the engine crankcase to optimize the prop shaft angle. The other option may be to use a universal joint down near the prop shaft support to slightly alter the angle. Then it becomes a tradeoff between the engergy sucked away by the crankcase pod hitting the odd bit of solid water and the mechanical drag of the universal joint. A typical universal joint such as used on model boats used to even slightly "bend" the power line produces a HUGE amount of drag on the engine. The key is to find the best single or combo of design traits that balances all the needs.

And as you've seen from all the solutions from various boat makers it's not an easy set of factors to balance out.

stealth assassin

The angle Is no problem In this set up without a pod. The main reason I was thinking of a pod was to lower the cog and keep the canopy low. Now, I am trying to decide on a flat tunnel bottom like on a F-1 or put a slight v In the shape. The F-1 guys seem to think that a flat surface creates better lift but, I am also thinking about the tunnel coming In contact with the water and cutting threw thew the cleanest... Make sense?

D