Limiting speed factor

[Ben_Sorkin]

Hi all,

I'm thinking about building a SAW cat somewhere down the road (maybe summer) and was wondering what the main limiting factor is concerning top speed? My guess would be blow-overs but you all know much more than I do!

Many thanks,

Ben

[Fluid]

Getting a catamaran to very high speeds is a balancing act of power, weight, lift and drag. Hull design has a lot to do with speed potential, both the sponson bottoms and the tunnel shape (often ignored). I have the fastest single-motor cat in NAMBA (89 mph pass) and had to fight excessive drag and high lift - hot electronics or blow-offs. I was eventually speed-limited by motor torque since the little 1515 motor had saturated windings over 250 amps on 6S...bigger props just increased the heat.

Watching the big 10S cats try to exceed 130 mph demonstrated the difficulty in going fast. They had plenty of power but they had a lot of trouble harnessing it. Most struggled with starting and often couldn't get on step without spinning out. Here they stubbornly kept their short rudders to reduce drag - but the rudders were too small to keep the transom planted when applying all that power.

Bottom line is you need a system with all components working together to go really fast.



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[jkr]

Also the pocket is a limiting factor when you want to set record.

[properchopper]

Hi Ben,

It's great to see that you've decided to do some research prior to embarking on the quest for max straightline speed. Much respect to you for such an approach. I hope you get some answers and we're here to help.

I'm by no means an expert but by attending many SAW Competitions and learning all I can from the likes of Jay I'm starting to realize a few things which I'll share here as my opinion only

As Jay stated, it's really a combination of factors to arrive at a harmonious system which allows fast passes minus all the factors that prevent full runs. What we're really dealing with, for the most part, is applying crazy power to hulls that simply weren't made to go that fast and keeping them on the water against most all odds. Most every SAW Competition record was achieved by making two back-to-back passes among numerous unsuccessful passes. A good analogy to describe this comes from Douglas Adam's book "A Hitchikers's Guide to the Galaxy" in which a character describes how one gets to fly without a plane as "the act of throwing oneself at the ground, and missing".

I have a question : When you say you want to build a SAW boat, do you mean a boat to enter SAW time trial competition, or just something to go max speed in a straight line for the fun of it? Just curious.

Tony
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[Ben_Sorkin]

I have the fastest single-motor cat in NAMBA (89 mph pass) and had to fight excessive drag and high lift - hot electronics or blow-offs. I was eventually speed-limited by motor torque since the little 1515 motor had saturated windings over 250 amps on 6S...bigger props just increased the heat.

Thank you for the reply! I was also thinking about doing a single motor setup. If you don't mind me asking, what hull are you using? And have you made any aerodynamic modifications?

-Ben

[Ben_Sorkin]

Hi Ben,

As Jay stated, it's really a combination of factors to arrive at a harmonious system which allows fast passes minus all the factors that prevent full runs. What we're really dealing with, for the most part, is applying crazy power to hulls that simply weren't made to go that fast and keeping them on the water against most all odds.

I have a question : When you say you want to build a SAW boat, do you mean a boat to enter SAW time trial competition, or just something to go max speed in a straight line for the fun of it? Just curious.

Tony

Thanks Tony!

I've learned in many ways that prior research and preparation usually has good results! I like your description of SAW, really hits the mark in my opinion. For now, I would like something that just goes very fast in a straight line (maybe 70-80mph) but still has untapped power. Then, next fall, I am hoping to model whatever I make on 3D CAD programs and run aerodynamic simulations at higher speeds to see what might happen. Then make modifications to the hull/setup and simulate again. I'm also going to try to get access to a windtunnel at my school to get some more testing info so I can be as prepared as possible when it's time to hit the water. After all this maybe I'll have to enter a SAW time trial competition!

-Ben

[Fluid]

....If you don't mind me asking, what hull are you using? And have you made any aerodynamic modifications?

I used a low drag version of the RiverCat/Agitator (limited number of scoops). The aerodynamic modifications I made were limited to an air dam at the front of the tunnel.

DSC00017.jpg
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[properchopper]

Ben,

You got my attention when you posted the magic words "wind tunnel". Given that the power needed for upper-limit speed is mostly available, the big factor in reaching these upper limit speeds is staying on the water. This video that Jay compiled should be required viewing to illustrate just what happens in this realm :

https://www.facebook.com/photo.php?v=697450810273839&oid=448732121813569&saved

If I'm not mistaken, Joerg M., who still holds the fastest electric NAMBA SAW record (set in 2006) had used wind tunnel testing.

As of now, most aerodynamic mods are done by educated guess plus trial and error. Do jump on that wind tunnel if you can- that's a Master's Thesis in the making

[Fluid]

Note that the usual 'airplane' wind tunnel will not work for a boat or car. The latter two operate in ground effect and need to take the solid surface into account. I had a short conversaton with Arne (designer of Joerg's boat) about this some years ago - he wouldn't say much but I well knew what he was talking about. Not to throw water on the wind tunnel idea (no pun intended) but it should be the correct type tunnel or the data may be suspect.



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[Ben_Sorkin]

Well that's somewhat of a demoralizing video! -but also motivating :D

I have a lot of reading to do regarding how to incorporate the ground effect into my tests, but I'm confident that I will figure it all out. We have several wind-tunnels on campus as well as a water channel that I can use for testing. Plus I'm taking a fluid mechanics course so I'll be learning about the nitty gritty details of airflow.

Just some forward thinking speculation- maybe adjustable downforce tabs on the top and front of the hull could work out? food for thought....anyway, this should be a fun project!!