FlatBottom Tubs on Outriggers are they now Needed

[NZMPBA_22]

I am wondering if it is necessary to to have the centre tub of a outrigger hydro, to have a flat bottom ? or can we move to a shape like the front of a jet aircraft for example .

flat bottoms may have been needed a few years ago when engines weren't making the power they are today , to help them get on plane, by acting like a water ski in conjunction with the sponsons, and due to the fact most hydros were built of wood .

as nowdays there seems to be a move to narrower tubs e.g SG Eagles , Hummingbirds etc. also newer construction materials are now more readily available and more powerful motors are arround too ...any thoughts on this idea?

 

[Slideblues]

Man I am glad somebody asked that question, I have one on the buildup board right now, Broke the side out on the first tub gettin the motor mount in so decieded to try something different this time, still has a flat bottom thou :huh:

Gene

 

[AndyBrown]

Yes you can make the front 1/3 of the tub rounded. My SAW boat is done that way. You will want to be careful about rounding anything on the rear half of the bottom or you will create something that will suck to the water. Even if the boat planes off it will do weird things in the turns and in rough water conditions.

P.S. Don't believe half of what I say. I'm just a Newbie!! B)

 

[Slideblues]

What about this, a totally flat bottom, reversed airfoil shape, with sides angled in 2 degrees and two canards half elipse shaped at the cg angled down bout 5 degrees, also serve to strengthen the engine bay,

Gene

He,He, Newbie...... Yeah right Andy :lol:

 

[AndyBrown]

I once made some inverted Clark Y wings that we mounted to the sides of the tub at the CG. They were set at the top so water would'nt interfere. We angled them down about 5 degrees. They were about 2.5" wide each and about 6" long on a 67 rigger. Had some interesting results that could work, but I didn't like it enough to coutinue working with.

You certainly will find good results with a little refining.

 

[Slideblues]

Yeah mine are at the top too, out of the way, interesting info, got some sanding to do and I'll snap a pic,

Thanks Andy,

Gene

 

[NZMPBA_22]

andy's thought is along the lines of what i was thinking ... make the front slippery and flat rear to aid keeping the rear end up in the corners an try to take some of the boxy look out of the boat

 

[NZMPBA_22]

what started me thinking on this was a friend of mine a couple years back built an outrigger with a rounded a curved top enclosing the pipe, motor and all but it had a flat bottom . i was wondering if it could be taken a stage further

 

[Will Gallagher]

The Octura Wingding had a very interesting shape to the bottom. Flat in the rear, then it had a flat center strip that tapered to a point 4 or 5 inches from the nose with sides that angled up starting just in front of the rear sponsons. It’s worth having a look if you get a chance to see one. I still have my 21 size I built in the late 70’s. It was a very labor intensive boat to build, but the results were worth it……

 

[Glenn Quarles]

Very good question...yes there are reasons for making something other than a flat bottom on an outrigger. Several years ago (1988) when I really began playing with ways to get rid of weight on my time trial boats I found that I also needed to get rid of lift. The idea I came up with was to build about the front 1/3 or so of the bottom as a "V" (kind of like a mono). Those of you who have ever seen one of the few caron fiber "Texans" I had molded or any of the fiberglass boats Maus sold after he got the molds will know how it looked. What I found was that it worked really good on my designs at the time when I had a strong motor, but actually hurt performance when I was down on power.

What we have to remember with these boats is that we actually want them to have lift and to "fly" (that's why they are called hydroplanes), and if we take too much lift away by getting the air out from under the boat or by not having enough area to provide lift (skinny tubs, etc) then we have lost some of the advantage of running hydros. Our goal is to get our boat to "weigh" less while running on the water than they do on the post office scale. In other words if your new 21 hydro is 4 pounds on the scale we would like for it to weigh something less than that on the water (some of the weight will be supported by the air...lift). If you can support 2 pounds of the weight with lift then you only have the water drag of a 2 pound boat...if you get rid of too much lift and can only fly 1 pound, then you have the water drag of a 3 pound boat. Of course we always have those happy times when our 4 pound boat makes 4 1/4 pounds of lift and we have an airplane, but that is what makes this whole design your own boat thing cool!

The amount of lift any particular hull needs is determined by it's weight (and intended use) and the amount of lift a hull makes is determined by it's lifting area and speed. By putting the "V" on the bottom of the tub I effectively changed the amount of lifting area by letting most of the air "slip" out from under the tub.

I have found by much failure and a little success that it is possible to get rid of too much lift and have a 2lb 4oz 21 SAW boat run much "heavier" (and slower!) on the water than a 2lb 11oz boat just because I was not keeping enough air under the tub. I have found with my latest 21 and 12 boat design (they have a fairly wide tub...about 2 1/4" for the 12 and 2 1/2" for the 21 in about the center of the boat getting narrower in the front and rear) I have been able to build as light as I can and still not have much of a blowing off problem while still using a flat bottom on the tub. I say this only to show that that shape of the bottom is not the only way to control the amount a lift a hull makes...there are a million possible ways to come up with a great solution! It is good we can have discussions like this to help spread a bunch of different ideas around and keep the sport engerized.

It is late at night and I hope I didn't ramble on too much to make sense :huh:

Glenn

 

[PHIL THOMAS]

Glenn, Very interesting. I wonder if this Vee bottom hydro idea is good to dump lift on a threepoint hydro design? Do you think the vee will direct air to the sides and out from under the boat. What about some strakes to direct the air.

The vee would end just behind the motor and the rear would be flat. I may work this idea into a gas sport boat.

 

[Ian Inverarity]

Phil,

For a sport hydro or scale type design I doubt the V shape will reduce lift. On a rigger tub the V shape encourages air to spill out the sides rather than staying under the tub, so with less air under the tub the lift is reduced.

With a sport hydro type design the air woul dstill have nowhere to go because of the sponson sides etc, so I dont think you would notice an improvement.

Ian.

 

[Ian Inverarity]

Good info Glenn.

From a heat racing point of view, how do you feel about the balance of aero lift and hydrodynamic lift? I would have thought that relying less on aero-lift would be better to make the boats less sensitive to wind etc.

Ian.

 

[jaso]

Ian, I think that you always should have a little extra lift rather than too little for those days of no wind. I guess this is a lot more important for saw events as you dont want to lose any speed that is directly related to drag caused by too little lift. For example, if you have a skinny tub you have to run a very high angle of attack on the front sponson and a high lift prop just to get the boat up out of the water to the correct attitude. Now you have just used how much of your power to do this when you could have been using some of this power for more push rather than lift. I think that you need to run this fine line to have the best of both worlds and adjust for the wind and the water conditions . My 02. cents. :huh:

 

[Mark]

Jaso,

If you've seen Andy Brown's SAW boats you would soon realise that getting to much lift with a flat bottom is a problem, I think his hull on that rigger was 2" wide and still had a tapered nose to reduce lift.

 

[jaso]

Mark, Yeah I believe that but, how much Hp was that Mac putting out to get that sort of speed . More than enough to over come the drag and also I believe that boat was very small all round ( power to weight and all) ?

Jaso.

 

[jaso]

Just another theory also, I'm thinking as the speeds of these boats get up around 100mph, they must be getting a lot more effected by aerodynamics simply due to the pressure of the pushing through the air. What I mean is I think there is probably a point where this un wanted lift becomes much more of an issue and flight is much more likley. Anyone out there got a wind tunnel ?? He he he :blink:

 

[Glenn Quarles]

Phil, sorry it has taken so long to get back to you. Ian hit it right on the head. Yes the "V" bottom would move the air around, but without knowing about any airtraps, sponson sides, etc. I can't say if it would actually get any air out from under the boat and reduce lift. Even if it doesn't reduce the lift, it may help with "roll" stability by moving the higher pressure to the outside edges of the tunnel. This is just a guess on my part and someone that runs more 3 point boats or even cat boats may have more experence than I do. I do know that my Sprint Cat had a "V" shape to the bottom of the tunnel...could have been done for many reasons, but it sure worked good.

Glenn