effect of airdams on hydros?

[befu]

Edit: AIR TRAPS, I AM TALKING ABOUT AIR TRAPS, not air dams. Sorry
so I am downstairs looking at my 1/16 shovel nose sitting on the shelf, and I relize that I never put the small air dams on it that extend back from the sponsons.

I know they are to help hold the air under the hull and provide lift, but since it is just aft of the sponsons does it help lift the mow, stern or both? I also know some have them extend all the way to the transom getting smaller as it goes, but these were not setup that way.

the reason I ask, is the boat submarined on me once. COG was about 1 1/4" back from the sponsons and she is pretty light. bow was riding low and it stuffed once. Put a smaller prop on it and it worked better along with the COG moved slightly further back.

So, will airdams help or do I just need more weight on the prop? Will the airdams lift the whole boat or help float just the nose/tail?

thank you for the help.

Brian

Oh, link to my build thread, video's are at the end.

http://forums.offshoreelectrics.com/...highlight=data

[andym]

Are we talking air dams ie; go across the hull sponson to sponson or like chines running from the rear of sponson to the stern along the outer edge of the tub ?? They do very different things

[befu]

extend back from the sponsons, so like strakes. Not an airdam across the hull to force air out or limit air stuffing under the hull.

Brian

[MarkF]

Try putting a little possitive in your prop to hold the nose up. If you can't then I would get rid of the strait shaft. You might have negative in your strut which is trying to force the nose down. You can also add ride pads to get more AOA on your sponsons to help hold the front of the boat up and keep it from trying to submarine. Nice build on your boat.

Mark

[befu]

Yeah, keeping the solid shaft on this one. will build a new boat first for a flex drive. Wanted to try a more scale setup on this one first. the prop negative is right at 3 degrees which is what is recommended on the plans also, just not adjustable now!

I figure I need more weight on the tail. Was wondering if the AIRTRAPS extending aft from the sponsons help to lift the wholeboat equally, the tail more or the nose more.

Thank you, Brian

[Simon.O.]

Yeah, keeping the solid shaft on this one. will build a new boat first for a flex drive. Wanted to try a more scale setup on this one first.

I figure I need more weight on the tail. Was wondering if the airdams extending aft from the sponsons help to lift the wholeboat equally, the tail more or the nose more.

Thank you, Brian

Brian.
The bits you are refering to are called air traps and prevent the air packed by the tunnel from escaping so fast. The air is held longer, this increases compression lift and moves the Cg aft. On my hull it did anyway.
I had them on my second shovel hydro that was built for brushed power. When I went BL they were cut out.
My experience showed that I needed to reduce lift and to get the Cg forward a bit, removing the traps did both at once.

An air dam goes across the front of the tunnel to reduce the amount of air entering and thus reduces compression lift, as you already know.

Two quite different things that are both a part of setting up a shovel.
I am no expert but I spent many water hours setting up my two shovels even going from straight shaft to wires, brushed to BL, Ni to lipo.

I no longer have the shovels but remember them fondly.

I'll see if I can find some of the articles I read on the whole subject.

[befu]

Air Traps!

Ah, air traps! Thank you for correcting that!

OK, sounds like it would help me out by lifting the front and effectivly moving the COG back some. When I was running the slightly larger prop, it was running heavy on the nose and had an impressive submarine! I will have to fab some up from some 1/16 ply and add them.

sorry about the confusion by calling them the wrong thing, but thank you for pointing me in the correct direction.

Brian

[Simon.O.]

Just realised that all the good articles I saved are on my old PC that died.

It seems to me you have most of what you need to know.
A smaller prop helped by reducing the lift on the tail and sending the bow in.
Moving the Cg back will naturally lighten the front and eventually you may blow over.
I say may as you also may find that the whole boat begins to skip due to equal lift front and rear. This can be fixed with a small amount of weight added or by reducing lift again.
To reduce frontal lift remove the traps and dam if needed
To reduce the rear lift either delift the prop ( not my game) or move Gc back again to load the rear.

I had a lot of fun doing my two small shovels and now I am begining to want one again.

I hope some of this may help.

[befu]

Just realised that all the good articles I saved are on my old PC that died.

It seems to me you have most of what you need to know.
A smaller prop helped by reducing the lift on the tail and sending the bow in.
Moving the Cg back will naturally lighten the front and eventually you may blow over.
I say may as you also may find that the whole boat begins to skip due to equal lift front and rear. This can be fixed with a small amount of weight added or by reducing lift again.
To reduce frontal lift remove the traps and dam if needed
To reduce the rear lift either delift the prop ( not my game) or move Gc back again to load the rear.

I had a lot of fun doing my two small shovels and now I am begining to want one again.

I hope some of this may help.

It helps alot, thank you sir!

Brian