# Thread: 100MPH+ RXP How do we do it????

1. ## 100MPH+ RXP How do we do it????

Well we got a couple of engineering types here and others that have had ALOT of MOD experience so the question is:

1. What would it take power wise to make an RXP go 100+ MPH and from a design standpoint can the hull support that speed WITHOUT getting airborne????

My totally unsupported theory is that we need 450-500HP and that the RXP HULL Would NOT go that speed without becoming unstable . Feedback please with specific mod suggestions if possible.

2. My question is if this question is worth the bandwith that is being wasted?

3. If you were going to build a 100 mph ski do you really want an RXP hull to start with?

4. The RXP hull won't do it. I hear that the pump over stuffs at 92 mph.

5. That is an interesting question.

We know that aerodynamic and hydrodynamic drag are both proportional to the square of the velocity. So let's assume that it takes 200 hp to go 67 mph on the RXP. Going up to 100 mph would take 446 hp then.

The only other problem is this pump "over stuff" issue. I've never heard of it and I deal with pumps every day at work. In general when working with a pump, increases in suction pressure will only yeild an increase in flow. I'm trying to get my head around why this may not be the case with an 8000+ rpm axial flow pump.

Mike

6. Well NCHEEL, not sure about the question but your response is certainly not worth the waste in bandwidth.

7. how do we know the pump overstuffs at 92? has somebody broke 90 yet or was it firgured with math

8. The quest to go faster is always a great topic regardless of what you may think as being a waste of time or "bandwidth." Is this not what we are all about here? Sorry NCHEEL but I'll have to ignore that response and place it in the "I don't know what I was thinking section." God knows I've said some stupid things before.

9. I can remember just a few years back 60 was fast and then we hit 70 and now several are over 80 and getting close to 90 with a few who have already hit that so I don't see how anyone could think that 100 is out of the question,I predict that that will be accomplished in the next five years.I was jacking with a jet boat awhile back and I outrun him for about 1/10 of a mile and he went by me like I was sitting still when talking to him he said he could GPS 130 MPH.

10. The only other problem is this pump "over stuff" issue. I've never heard of it and I deal with pumps every day at work.
Quite common on a watercraft. Overstuffing the pump, on most any craft will almost always happen on ultrasmooth water, while you are at the top of your power band, cooking along at top speed. Somewhere in that equation, an imbalance happens (for lack of a better term) and the water coming INTO the pump overcomes the impellor, and pump. The water flow burps the water back out the intake, lifting the ski. When it becomes very bad, the ski will lose hookup, and come WAY out of the water, turn left or right, and give you a personal lesson on the joys of self-flight.

Grates with scoops on them, are generally mis-concieved as shoving more water in. Nothing could be farther from the truth. In reality (or theory in this case) they divide the pump tunnel into smaller chanels, as well as take up valuable space within the cavity. This allows actually less water. Plus they tend to keep the ENTIRE impellor fed, and not just the top or bottom.

**Remember, the differences can be measured in mm or less**

As the scoop grate maker cuts the wing deeper into the water's forward motion, the hull will slow down, as it drags the hull deeper... This will actually speed up the burping of the pump, if you have the power to overcome the wings momentum. However in rough water, it serves to keep the pump hooked. It's a trade off.

The following is something I've been very curious about and a LOT of conjecture on my part. Forgive me if my ideas don't match up to yours...

Pump tunnel length, very well may be another way to increase pump efficiency. At some point at the rear of the vertical grate, and in front of the prop is a very strange little area. This little area can be effected by things such as pump shoe design, nose cone design (or lack thereof) length of the grate and depth of the cavity itself. Two very different grates act VERY differently on two different pump tunnel lengths. One may overstuff, the other may spin out of the hole. In general the longer the area, the better the hookup. However, as you move the pump backwards, your pivot point for the nozzle goes backwards too, changing the angle of HOW MUCH of the hull can you get out of the water. Again, a trade off.