Prop top speed boat calculator - accurate?

[Speedling]

http://www.go-fast.com/boat_speed_predictions.htm
This is their estimated top speed for a boat calculator.
If you put in 4000 lbs for our boat (mine is just over 3000, but with gear and people I assume 4000) and 320 hp, with the least efficient hull design, then we should be doing like 64 mph. I don't know about you, but I'm not!
How accurate is it for prop boats?
I know before I bought my boat, I had looked at the Stingray 225. In ways, I was glad I didn't get that, but it's a good comparison at 21 feet with 320 hp.
http://www.stingrayboats.com/products/models15/models.php?model=225sx
Tested top speed was 62 ish mph on that boat. Can't imagine that i'm hugely overweight compared to that iron V8.

Just food for thought.

 

[jcyamaharider]

Do you think not being able to trim our boats is leaving some on the table??? I think it might be something to look at which actually might benefit all areas of watersports. Being able to trim down for a hole shot or trim up for speed. Sure would be nice.

 

[txav8r]

That stingray is a perfect example. That is a true speed hull and rides line crap except on smooth water. I know, because I had a 21' stingray...FAST! But not quick. When I had mine, it was 10 mph over the nearest like competitor, and 15 mph over most. I a coveted long ago that to get one thing, you give up something else. I don't think a calculator made for prop boats is going to apply to jets, but maybe some dynamics apply

 

[woodard1983]

you shouldn't be getting what those calculations spit out because those calculations are made for prop boats, which we are not....your never going to get as much speed out of our boats as you are a similar weighted/powered prop boat....the dynamics of the boats hull, trim, and the difference between impellors and props are the reasoning behind it....

 

[Speedling]

you shouldn't be getting what those calculations spit out because those calculations are made for prop boats, which we are not....your never going to get as much speed out of our boats as you are a similar weighted/powered prop boat....the dynamics of the boats hull, trim, and the difference between impellors and props are the reasoning behind it....

Would love to hear your explanation of why. We have no outdrive in the water. You could argue that the surface area of the inside of the pump equals the outdrive, but i doubt it exceeds it. A prop also sends water in a cone where a jet sends it directly behind. It looks like comparing the boats that:
They weigh about the same
Same hp
Jet is slower
Jet accelerates faster
Jet gets better fuel economy

Of course it's all where you get your numbers too.
I bet that boat gets 55 in real life.

 

[woodard1983]

1-hull-the Yamaha hulls are not made for speed...look at our boats and a go fast or even a general bowrider when out of the water
2-trim-we don't have it, prop boats do. this allows them to keep more of the hull out of the water, less hull in the water=less drag=faster speed
3-prop-you are correct, that prop hangs down into the water where it spins to pull/push through the water....we force the water into an intake grate and then shove it through our impellor, then out of the nozzle...just like the trim issue, more drag/more water turbulence=slower speed

this isn't to say a jet can't go fast, many custom built have, but that is not the market that Yamaha is in

 

[Speedling]

I agree @woodard1983 , the Yamaha boats are just simply made for an overall nice ride. Next time you ride along side one, watch where the water it hitting the hull at speed. Any speed on plane will work, as it's all the same. Instead of getting it so the boat planes with as little hull in the water as possible, Yamaha (and the other mainstream Jet boats) have put the nose down a bit, and gave us deeper hulls in order to give us a smoother ride that is favorable among families and testers. In fact, the water typically hits about 4/5's of the way up the hull so most of the hull is in the water even on plane.

In the equation I am ATTEMPTING to make work for jet boats, I take into consideration what speed the boat comes on plane. It's just one of those variables that can't be calculated perfectly, but once you have that information, you have a ballpark starting point in the terms of efficiency and speed of the hull. Higher speed boats will typically plane at a higher speed.