53mph?

[Gaviao]

I just bought a 2002 X7 with the Predator 310hp engine. At full throttle with two people in the boat I show 53mph on the speedo and on the PerfectPass. My rpm is 4700 and my prop is a 4 blade 13x14. Now, I'm new to boating, but based on what I have heard, the speedos must be off. I don't have a slalom course or a GPS (I plan on getting a GPS unit soon), so I can't verify the speedos. What do you guys think? Is it really off or is it possible?
Thanks.

[east tx skier]

Welcome. That sounds off. The 02 isn't hooked, so you might get a little more top end than an 02 PT/03 and newer 197/X7, but 53 sounds too high with the 310 hp engine and a 4 blade.

[Leroy]

I want to know what you are waxing the hull with! ;)

[Gaviao]

I kind of suspect that the speedos are off. If the weather cooperates I'll take it to the lake next week with a GPS unit to verfy it.
Hey Leroy, I put some KY jelly on the hull! :D

[PendO]

I kind of suspect that the speedos are off. If the weather cooperates I'll take it to the lake next week with a GPS unit to verfy it.
Hey Leroy, I put some KY jelly on the hull! :D

what pitch prop is that?

If you look at rpms and calculate it with no loss due to cavitation or friction that would help come up with the theoretical velocity ... the pitch of the prop is the theoretical distance traveled in one revolutions (in inches) ... when we get done watching Anastasia for the 1000th time I'll do the math

[Leroy]

:D What a waste! :eek:

Hey Leroy, I put some KY jelly on the hull! :D

[PendO]

now, this would assume no loss due to friction with water or cavitation

((pitch/12)*RPM) = ft/min ... X 60min/hr .... X 1mile/5280ft

is this right in theory

[Jesus_Freak]

Yes, correct in theory for prop speed in column B. This sets the upper limit for speed. Cavitation is minimized with correct design and condition, but who operates a boat without frictional energy losses? 8p That only comes from a wake-less boat and prop. I want me one of those.

[Leroy]

Very good Pendo!

I consider the loss due to slip of the prop. If you spin the prop with no loss your calc is good regardless of friction, in water the prop only goes 80-90% based on slip.

[Gaviao]

Based on the spreadsheet above, 53mph represents about a 15% loss due to friction/cavitation. This seems consistent with what I have heard about people with a 13x13 prop. (also based on the spreadsheet above) So I guess that 53 is not that far off after all. I'm still going to check with the GPS....

[edwinfuqua]

I just bought a 2002 X7 with the Predator 310hp engine. At full throttle with two people in the boat I show 53mph on the speedo and on the PerfectPass. My rpm is 4700 and my prop is a 4 blade 13x14. Now, I'm new to boating, but based on what I have heard, the speedos must be off. I don't have a slalom course or a GPS (I plan on getting a GPS unit soon), so I can't verify the speedos. What do you guys think? Is it really off or is it possible?
Thanks.

I just fixed my second spedometer on Friday. I took the boat out and had some fun. I have a radar gun that works ok as long as you are heading toward something within 1500 ft. My spedometers were on the money or not more than 1 MPH off at any speed.

I only saw 49 MPH top but I wasen't shooting the gun, heading toward a bridge and holding the boat wide open either. When I hit a wake at that speed it was time to slow down guickly. I'm running a PS190 '85 w/ 454HO @425HP. I'm getting a max of 5500 RPMs now with a 4 blade 13.5x19.5 CNC OJ prop.
Edwin

[Jesus_Freak]

Very good Pendo!

I consider the loss due to slip of the prop. If you spin the prop with no loss your calc is good regardless of friction....

I am only proposing that friction causes slip. The resistance (friction, viscosity, whatever you want to call it) of the water to sudden acceleration, keeps the prop from moving the theoretical volume of water. Also, some of the water surrounding the prop gets entrained by the prop and simply swirls around instead of being used as thrust. Are we saying the same thing?

[PendO]

Yes, correct in theory for prop speed in column B. This sets the upper limit for speed. Cavitation is minimized with correct design and condition, but who operates a boat without frictional energy losses? 8p That only comes from a wake-less boat and prop. I want me one of those.

Hey, your the supersmart guy :) :) I'm just the guy who stayed at the holiday inn ...

I'll say this at 4700 rpms in an X-star w/ a 22 pitch you get 43mph (44mph at 4850 but unable to get more rpms)
and at 5400 rpms w/ a 17 pitch you get 42 mph


the friction increases exponentially above the speed where the shaft angle starts to turn the nose of the boat into a plow

[Jesus_Freak]

Hey, your the supersmart guy :) :) I think you might have me confused with someone else.

[PendO]

I think you might have me confused with someone else.

I am still reading the exhaust thread ... my budde is a Chem E. and he makes me feel the same way

[edwinfuqua]

That's a cool chart but I don't seem right even though it has been carefully projected. I had a 3 blade 14x20 ss prop and it would run 49MPH at 6000 RPM. I changed it to a 14.5x22 4 blade and it ran 49 MPH at 4200+RPM and would not develope more RPMs. I changed it again to a 13.5x19.5 4 blade and it runs 49 MPH at about 5000 RPM and will go to about 5500. That 4blade 14.5x22 should have sent me 90 MPH by your chart. I don't get it. I like the prop I have now. It will get up to aboutr 4000 on a deep start but if the driver don't back off as soon as the skier gets up it will turn 'em end over end every time.
Edwin

[Leroy]

We are saying the same, I just consider the loss at the energy source, if the prop spins 4700 rpm and at specific pitch should do 50MPH then friction, skier, wind or anything else is loss. There are many losses possible. I'm a EE, maybe I'm talking from my backside :)

I am only proposing that friction causes slip. The resistance (friction, viscosity, whatever you want to call it) of the water to sudden acceleration, keeps the prop from moving the theoretical volume of water. Also, some of the water surrounding the prop gets entrained by the prop and simply swirls around instead of being used as thrust. Are we saying the same thing?

[Jesus_Freak]

Thanks for the compliment. Chem E was my undergraduate work. I moved on to focused turbulence research in grad school. Now, if I could just ride my ski 1/100th as good as I can find flow solutions....I would be a happy guy.

[PendO]

That's a cool chart but I don't seem right even though it has been carefully projected. I had a 3 blade 14x20 ss prop and it would run 49MPH at 6000 RPM. I changed it to a 14.5x22 4 blade and it ran 49 MPH at 4200+RPM and would not develope more RPMs. I changed it again to a 13.5x19.5 4 blade and it runs 49 MPH at about 5000 RPM and will go to about 5500. That 4blade 14.5x22 should have sent me 90 MPH by your chart. I don't get it. I like the prop I have now. It will get up to aboutr 4000 on a deep start but if the driver don't back off as soon as the skier gets up it will turn 'em end over end every time.
Edwin

the shaft angle and the hull design apparently require more horsepower to hit 50mph ... your experiences would actually be interesting to plot

[Jesus_Freak]

We are saying the same, I just consider the loss at the energy source....

Thanks. Dude, I gotta get me some sleep, but I will end by humbly adding that part of the resistence of the entire system in responding to thrust involves losses around the hull. These increase the entropy of the water (lowering the total pressure, or the ability to perform work) and do not allow the prop to get theoretical bite.

Good night gentlemen.

[Leroy]

I'm all ears and also ready for bed. I'm in Boulder this week.

[shepherd]

now, this would assume no loss due to friction with water or cavitation

((pitch/12)*RPM) = ft/min ... X 60min/hr .... X 1mile/5280ft

is this right in theory

As a former naval architect, I can say without doubt that is a very very very big assumption...

[6ballsisall]

I just bought a 2002 X7 with the Predator 310hp engine. At full throttle with two people in the boat I show 53mph on the speedo and on the PerfectPass. My rpm is 4700 and my prop is a 4 blade 13x14. Now, I'm new to boating, but based on what I have heard, the speedos must be off. I don't have a slalom course or a GPS (I plan on getting a GPS unit soon), so I can't verify the speedos. What do you guys think? Is it really off or is it possible?
Thanks.


I doubt it.......My 03 w/ MCX and CNC prop wont go that fast w/ just a driver. I bet your speedos are off.

[east tx skier]

the shaft angle and the hull design apparently require more horsepower to hit 50mph ... your experiences would actually be interesting to plot

That's my question. As top end goes, so much depends on wetted surface. Maybe I was glancing to quickly, but does the chart have a set allowance for drag? When you pull the throttle back on the boat, it slows down in a hurry as opposed to a car. The newer boats have a lot of wetted surface (althouth the 02 without the hook should get a littel more on the top end than the later hooked hulls. Plus, traditionally, you subtract 1 or 2 mph off the top end for a four blade prop.

I'd guess that that boat (with 310 hp and a four blade) will grab 46.6 mph on the gps).

[3event]

I have a 2002 PS197 with the 310hp Predator, and the most I got was 48.9 on Perfect Pass, analog speedo agrees. It's a couple of ticks less now I believe, I will check this weekend. Stock 4-blade prop, when I wandered under the lift to try and read the pitch on it, I could not.

In any case, I would like to know what you're feeding yours to top out above 50 !

[PendO]

As a former naval architect, I can say without doubt that is a very very very big assumption...


Hey, I just need one of you to figure in the real life friction, although it is not constant and changes with the attitude of the boat ... I've done the easy part, someone else do the hard part.

So, someone figure in friction with water :)

Someone figure in wind resistance :)

[east tx skier]

Don't forget about water viscosity. Still, the way it's always been explained to me is that wetted surface is the biggest variable/obstacle to increased top speed. This makes sense since I can get 45.5 mph with under 300 hp.

[phecksel]

Hey, I just need one of you to figure in the real life friction, although it is not constant and changes with the attitude of the boat ... I've done the easy part, someone else do the hard part.

So, someone figure in friction with water :)

Someone figure in wind resistance :)

At what water temperature, LOL?

[Gaviao]

Well, a lot of good input and in the end it seems to match my initial suspicion that the speedos must be off (coincidently, by the same amount). With so many variables, such as water resistance, which in turn varies with speed, temperature and water properties. Then we can throw in elevation, which affects engine performance. Furthermore there is the prop itself, which are not all created equal, and boat weight (I have a headache just thinking about solving this problem on paper:) ). So, I'll be testing with a GPS unit next week and will report back.
BTW, what is the "hook" that you guys are talking about?

[east tx skier]

BTW, what is the "hook" that you guys are talking about?

It's a small downward lip on the trailing edge of the hull that causes the boat to pitch forward a bit when underway (gets a bit of the rear end higher in the water). Supposedly reduces porpoising, softens up the wake, and improves tracking if I remember correctly. It does, however, increase drag a bit.

Check this thread (http://www.tmcowners.com/teamtalk/showthread.php?t=5213&highlight=hooked+hull) out.

[Gaviao]

Thanks for the info "east tx skier".

[shepherd]

Hey, I just need one of you to figure in the real life friction, although it is not constant and changes with the attitude of the boat ... I've done the easy part, someone else do the hard part.

So, someone figure in friction with water :)

Someone figure in wind resistance :)


Let me stress the "former" in "former naval architect." ;)

I once tried to write a spreadsheet at work to estimate the speed/power requirements of planing boats but gave up because it is an iterative process and there are so many variables -- wetted surface (big one like Eastie says), propeller design/efficiency, weight, wind resistance. The easiest thing to do would just take the boat out with a gps and measure the speed. But even that only gives you one side of the table because you'll never really know how much power you're putting out at a given speed.

[rcnjson]

But even that only gives you one side of the table because you'll never really know how much power you're putting out at a given speed.

Great point (I've been reading this thread and it is very interesting) I think it is important to recognize that an IC engine makes its maximum power at a specific RPM level. Meaning that a 400 HP motor only makes 400 HP at say 5000 RPM's and less than that at any RPM above or below 5000 RPM's. So when you try different props with different pitches and get the same max speed with different RPM's it could be three things. One is the maximum output (HP) of the motor happened before the lowest RPM listed and from there it is just falling off. Or the horsepower required to accelerate to a higher speed is not available in the RPM range that you are operating in with the different props. Or you have hit maximum RPM's (rev limit) and can't go any faster as a result.
k