AOA question for the rigger experts

[Xzessperated]

My question is will changing the sponson angle of attack from 2.5 to 7 degrees make much difference to the speed of a boat? If so will it change it by a large amount or just a few mph?

[Simon.O.]

If it is any help, i found that a change in AoA is more about the handling as opposed to outright speed. In saying that the two go hand in hand to some extent in that if it handles poorly then you will not realise the full speed of the hull. Coversely if at speed the handling begins to suffer then will a change in AoA make a difference.
I found that whn i went from brushed to BL in my balsa rigger the AoA was too aggressive, in fact it still is a little. the problem i now have is to much width on the sponson bottom so the sponsons are due for a major rebuild.In my rigger design reasearch thread it was noted that most hulls work with a 3 to 5 deg AoA.
If your hull has the ability to alter the AoA easily the you may be able to answer the question yourself and indeed for us all.

Always trying to help.
Have a good Christmas there Paddy.

[Xzessperated]

If it is any help, i found that a change in AoA is more about the handling as opposed to outright speed. In saying that the two go hand in hand to some extent in that if it handles poorly then you will not realise the full speed of the hull. Coversely if at speed the handling begins to suffer then will a change in AoA make a difference.
I found that whn i went from brushed to BL in my balsa rigger the AoA was too aggressive, in fact it still is a little. the problem i now have is to much width on the sponson bottom so the sponsons are due for a major rebuild.In my rigger design reasearch thread it was noted that most hulls work with a 3 to 5 deg AoA.
If your hull has the ability to alter the AoA easily the you may be able to answer the question yourself and indeed for us all.

Always trying to help.
Have a good Christmas there Paddy.

Thanks Simon and a Merry Christmas to you also. I love your own design boats and they really are what our hobby is about.

I realise it is a bit of a suck it and see question and I will find out at the lake when I next test but I just thought someone might have some first hand exerience with it. Mr. Krabs is handlng great but I just can not get him past 108 kph. I have been told it is a 160 kph setup. There has to be a major problem there somewhere. I realize I am no expert but I would think that I should get to 130 kph if everything is close to ball park settings and setup.
To tell you the truth I have an Xs2 with a 6XL motor that is as fast and another Xs2 with a Neu 1515 1.5y that is faster.
Me thinks I have done something wrong somewhere.

[raptor347]

It can make a huge difference if you're dragging the sponsons. The flip side is it's easy to make a boat very flighty if you go too far. 7 degrees is extreme.

I almost need to see it run to get an idea of how it's running.

To give you an idea, here's my N2 boat setup:
3.5 pounds, ride pads are 5/8 wide and running at 4 degrees. The tub is dead neutral and creates no lift. All lift is created by compression under the ride pads. At speed there is no spray from anything but the prop.

The boat made 90+mph passes with both 1509/0.5Y and 1512/0.5Y neus on 2S. Modded 2140 octuraon the small motor and a modded 2050 on the big one.

There's always the possibility the Mr. Krabs aero design is fighting itself and robbing you of some speed.

[Xzessperated]

It can make a huge difference if you're dragging the sponsons. The flip side is it's easy to make a boat very flighty if you go too far. 7 degrees is extreme.

I almost need to see it run to get an idea of how it's running.

To give you an idea, here's my N2 boat setup:
3.5 pounds, ride pads are 5/8 wide and running at 4 degrees. The tub is dead neutral and creates no lift. All lift is created by compression under the ride pads. At speed there is no spray from anything but the prop.

The boat made 90+mph passes with both 1509/0.5Y and 1512/0.5Y neus on 2S. Modded 2140 octuraon the small motor and a modded 2050 on the big one.

There's always the possibility the Mr. Krabs aero design is fighting itself and robbing you of some speed.

Thanks very much Brian. I was about to slit my throat... well not quite.
There is a guy on RRR that gets 95+ mph with the same setup 4s, 1521 1D, V947/3. I would like to see more of the sponsons out of the water. To me a fast rigger should run as you said, with no spray from the sponsons.

[raptor347]

Paddy,
Yup, there is a difference between light and flighty. No throat slitting allowed.

I really think the boat may be fighting itself up at the speeds you're trying for. My understanding is the Mr.Krabs was designed as a heat racing boat. If it behaves anything like my heat race riggers, it probably hits a speed where it starts driving itself back down on the surface.

You could always build up a new set of sponsons and see what happens.

Another possibility is the prop stages as it comes up to speed. What I mean is at some point during acceleration the prop starts to climb out of the water. When it does that the sponson AOA it decreased, cutting out some of the compression lift. At the same time it increases the downforce produced by the tub (or neutralizes some of the lift). An extreme indication that this is happening is the boat will accelerate and all of a sudden the back end starts to hop (prop blows out).

One way of reducing that tendancy to climb out of the water is to add some cup to the tips. Look at the trailing edges of the two props in the picture. Notice how the outer 1/3 is bent/curved forward. That allows the blade to take a scooping bite of the water (sorry, best description I could come up with) that counteracts that lifting tendancy. Where you start that bend is how you adjust how high the prop will climb. You can actually over do it to the point it will draw the back end down rather than lifting.

Don't know if that helps, but it's a little more info for you.
BTW, that full bladed V935 is the prop that pushed my O hydro to a one way of 100.8mph on 8 NiMH's at the '06 LA SAWs.

[Xzessperated]

Paddy,
Yup, there is a difference between light and flighty. No throat slitting allowed.

I really think the boat may be fighting itself up at the speeds you're trying for. My understanding is the Mr.Krabs was designed as a heat racing boat. If it behaves anything like my heat race riggers, it probably hits a speed where it starts driving itself back down on the surface.

You could always build up a new set of sponsons and see what happens.

Another possibility is the prop stages as it comes up to speed. What I mean is at some point during acceleration the prop starts to climb out of the water. When it does that the sponson AOA it decreased, cutting out some of the compression lift. At the same time it increases the downforce produced by the tub (or neutralizes some of the lift). An extreme indication that this is happening is the boat will accelerate and all of a sudden the back end starts to hop (prop blows out).

One way of reducing that tendancy to climb out of the water is to add some cup to the tips. Look at the trailing edges of the two props in the picture. Notice how the outer 1/3 is bent/curved forward. That allows the blade to take a scooping bite of the water (sorry, best description I could come up with) that counteracts that lifting tendancy. Where you start that bend is how you adjust how high the prop will climb. You can actually over do it to the point it will draw the back end down rather than lifting.

Don't know if that helps, but it's a little more info for you.
BTW, that full bladed V935 is the prop that pushed my O hydro to a one way of 100.8mph on 8 NiMH's at the '06 LA SAWs.

I think you have helped with not just one problem but two Brian. I have an Xs 2 rigger that hops with a V937/3 and thus never really gets to it's top speed. Only once did that prop bite and it took off like a rocket. Just one time that boat done a little over 75 mph but now it just hops. I will get the prop cupped as you suggested.
After reading your comments about the Mr. Krabs problem I will start at 5 degrees AOA as I do not want a blow over if I can help it. Yeah I know it will happen but....
Thank for your help Brian. The cheque is in the mail.

[raptor347]

Not a problem. More fast riggers is a good thing.

Also, you need to make sure you haven't folded the blades back slightly. Assentialy the reverse of cupping, creates a ton of lift. If the props aren't hardened a bit, the 1521's will bend them pretty easily. When you add cup, you also add to how hard the prop bites and increases the load on the blades.

Do we need a post on prop hardening?

[tylerm]

already know how to do mine, but seeing your way Brian would be interesting.

[raptor347]

I've done it a couple ways.

#1. If you really want them HARD: Anneal them at 1225F for an hour and quench in water. Do all you prop work: sharpen, balance, thin, cup etc. Then 600F for 2 hours and turn off the kiln and just leave them to cool overnight. This yields very hard props, but you can no longer work them at all.

#2. Just do the 600F step after yo work the prop. Still very hard, but you canbend them a little if you really need to.

[RandyatBBY]

Paddy you need to look at the tub bottom and AOA of it. It needs to be at zero for the center section front and rear are different when you raise the strut in the rear to increase the lift on the sponsons the tub will katch air and pack it causing the front to lift and blow over. Increase the lift on the ride pad not the tub center

On the rear of the boat hopping out of the water, this is caused by large amounts of pitch. there is two ways or three to correct this #1 drop pitch, #2 cup outer edge of blade close to the tips.#3 add weight to the back of the tub to hold it down.

[Xzessperated]

Brian,
Ok I will print the prop hardening bit out and send it to my prop guy. If he can not do it I will have to take them to a foundry and see what they can do.

Randy,
Mr. Krabs does not have a flat bottom but Andrewg (I guess you know who I mean) said he sets everything off the rear end when it comes to strut depth and that would keep the nose in the air a little as far as tub AOA. I will stand at about mid run distance so I can see if the nose is lifting. I hate blow overs. I have had many with one of my Xs 2 hulls and last time out I cracked a sponson when the other one went over.

Thanks to both of you for all your help. Now I have something to work on with two of my boats. I know from something that Joerg said that the Mr. Krabs will do 100 mph as many of them do it in Europe and John (m4a1usr) said his does 95+. Mine gets to 65 real quick but then the sponsons are glued to the water and appear to be holding it back. Next time out will let him run free I hope.

A merry Christmas to you both and Randy I hope next year is a better one for you. No need to tell the Snowman that as he will just go set some more records no matter what happens.