Dear Riva,

All things being equal, will a pwc go faster in fresh or salt water?

I am guessing that the salt water craft will have less surface area in the water because it will be more buoyant. That being said, will the propulsion system having to thrust more dense water (salt water is more dense that fresh) mitigate the less surface area effect?
It’s been a while since my Nav Arch 101 days.

Thanks,
Steve

saltwater I think steve

PWC's run faster in fresh water, some have seen as much as a 2mph difference between the two.

Fresher is faster. We've seen -2~3mph in our testing and it has been documented by the magazines when testing with us in the Florida Keys.

Okay, so by that logic, does warmer fresh water produce higher speeds than cooler fresh water?

I ask because I cannot get my RXP-X with the stage 1 kit to 74 mph with 1/3 fuel and trim in the neutral position.

I live on a lake in CT, elevation ~800 ft above sea level and water temp of approx 70 degrees. The best I have seen it on my GPS (Garmin $99 at **** Sporting Goods) is 72.1 mph. I have seen it as high as 73.7, but only with trim all the way up in the "Superman" Position. I weigh 173 lbs. Also, the RPM Gauge says ~8180 at the speeds above. When I lower it to 8000 rpm, I get a slower speed.

Any thoughts?

Thanks,
Steve

General rule of thumb, the colder the better. Cold weather means denser air which increases the psi at the supercharger.

Cooler water helps with speed too. Remember, the ride plate is the heat exchanger. The cooler the engine coolant the better the engine will run. The harder you run your craft the hotter the engine will get.

We offer a Thermostat Kit. Swapping out the stock one for ours drops engine operating temps by as much as 30°F. This kit is a must for modified craft.

I did some further research on the warmer fresh water versus colder fresh water and I found that colder fresh water versus warmer fresh water will result in slower speeds (all other being equal). Why? The same reason the salt water craft is slower than the fresh water craft. The density of the water…mass density is much greater in salt water than fresh water. The same holds true for colder fresh water, but to a less extent.

Riva Test Results (assumed 85 F degree Fresh Water) = 74 mph
*Approximate Salt Water Results (assumed 85 F degree Salt Water) = 71.4 mph
(*based on Naval Architecture Resistance and Powering formulas)

Riva Test Results (assumed 85 F degree Fresh Water) = 74 mph
*Approximate Fresh Water Results (assumed 60 F degree Fresh Water) = 73.0 mph
(*based on Naval Architecture Resistance and Powering formulas)

Please correct me if I am wrong, but I am sure that if the RIVA tests were performed in 85 F degree or greater temperature in fresh water (very likely if tests were performed in the Florida keys).

Steve

quote:

Originally posted by Steve_CT:
Riva Test Results (assumed 85 F degree Fresh Water) = 74 mph
*Approximate Salt Water Results (assumed 85 F degree Salt Water) = 71.4 mph
(*based on Naval Architecture Resistance and Powering formulas)

Riva Test Results (assumed 85 F degree Fresh Water) = 74 mph
*Approximate Fresh Water Results (assumed 60 F degree Fresh Water) = 73.0 mph
(*based on Naval Architecture Resistance and Powering formulas)

Please correct me if I am wrong, but I am sure that if the RIVA tests were performed in 85 F degree or greater temperature in fresh water (very likely if tests were performed in the Florida keys).

Steve

Not understanding your last sentence. Anway, the colder the temperature, the faster you will go, proven fact. The cold water helps the rideplate to exchange heat out of the ski, reducing heat fade. Cold air also means more boost. You will see your fastest speeds on a cold day.

The engine has a rev limiter on it...correct? So, if the engine tops out at 8000 rpm...cold or warm temp is not going to affect that, so the water density will drive the top speed.

How am I wrong?

My last sentence was assuming (not fact) that RIVA performed their testing in water greater than or equal to 85 F deg to obtain the 74 mph for the RXP-X Stage 1 kit.

The testing in the Keys I was referring to was during a magazine test years ago. All of the testing for our kit listings online is performed at our fresh water lake a couple miles from the main shop in Pompano Beach, FL.

quote:

Originally posted by Steve_CT:
The engine has a rev limiter on it...correct? So, if the engine tops out at 8000 rpm...cold or warm temp is not going to affect that, so the water density will drive the top speed.

How am I wrong?

The engine has a limiter at 8100rpms, the optimal rpms for the most hp is 8000rpms. Temp makes a difference, trust me:

Here's where you are going a little wrong, keep this in mind while you read this, the colder the air, the more dense it is. The RXP-X uses a supercharged 4-stroke engine, here is the reason cold weather means you go faster, cold air = dense air = more psi boost. The more psi boost your supercharger makes, the more power the engine puts out. Rev limiters have nothing to do with this, your s/c will make more psi on a cold day at 8000rpms than on a hot day.