PDA

View Full Version : RPM vs MPH


gts-20
10-11-2010, 03:17 PM
ok, I'm sure this has been covered before, but i can't seem to find it.

My shoulder/pec muscle is stll recovering from recent surgery, so I'm the designated boat driver for the rest of this season or longer. That means I'm spending a lot of time tinkering and playing with settings and watching the instruments. We enjoyed a great day on the water yesterday, 85 degrees air temp, 78 h20 temp.......while driving I was playing with the PP and just watching RPM vs MPH.

On my old boat, '97 Gekko gts-20 direct drive, black scorpion 5.7 315hp, 1:1 velvet drive, the RPM matched almost exactly to the MPH. At 3000 rpm it was running 30 mph, 4k 40mph, and 5200 running 51 mph, all GPS verified speeds...

The new boat, 2008 prostar 214 direct drive with the MCX engine, as far as I know a 1:1 trans., The speeds are totally different. At 30mph it is turning 3600 rpm, at 34 mph its turning 4000 rpm, and at wot, 4998 rpm @ 45.7 with 3 people and a full fuel load.

basicly my question is, was the old boat's speed vs rpm matching up just a fluke? Or could it be possible that my 214 has a power slot?

thanks in advance for your help.

flipper
10-11-2010, 03:29 PM
The prop is going to play a part too as well as weight, wet surface area....

Jim@BAWS
10-11-2010, 03:34 PM
Tough question to answer without you getting us an answer first!

Jim@BAWS


o

The new boat, 2008 prostar 214 direct drive with the MCX engine, as far as I know a 1:1 trans., The speeds are totally different. At 30mph it is turning 3600 rpm, at 34 mph its turning 4000 rpm, and at wot, 4998 rpm @ 45.7 with 3 people and a full fuel load.

basicly my question is, was the old boat's speed vs rpm matching up just a fluke? Or could it be possible that my 214 has a power slot?

thanks in advance for your help.

Miss Rita
10-11-2010, 03:36 PM
Comparing the Gekko and ProStar is apples to oranges. The boats are very different; the Gekko is smaller and lighter. Since the boats have similar power I'd expect the Gekko to be noticeably faster. If the MC has a powerslot it would have lower top end.

Forty six mph with three people and a full tank of gas is pretty darn good.

My '89 TriStar runs 30 mph at 3100 rpm. It's just coincidence, but my 1991 SkiPro I/O (with nearly identical power) runs 30 mph at 3000 rpm. The SkiPro is 10 mph faster though.

Thrall
10-11-2010, 04:10 PM
My 190 1:1 was alot like your old boat rpms matched mph to about 36-38 mph then 5000 rpms = 44-45mph.
My New boat, like your X14 is ALOT heavier with about the same hp on tap, so even if you have a 1:1 trans, I'd expect somewhat higher rpms than speed.
And yes your prop can have a big effect on that too.

gts-20
10-11-2010, 04:43 PM
Ok, I went to my boat storage at lunch to check the trans, but i can get the motor cover to lift high enough to see the trans lable. If I remove the lift supports from the cover, will it open more without hurting anything? If so, Ill check that and check to prop size.

The boat performs flawlessly, so this post is by no means a complaint. The hole shot is amazing, it holds speed and tracks perfectly, and is plenty fast enough. I'm just frustrated/bored since my injury, not being able to ski, so I've been spending a lot of time looking at the little things i don't usually pay too much attention to.

on a side note, if anyone in the Houston/surrounding area wants a pull behind a PS214, say when and where and I'll be there. Any excuse to get on the water works for me since I can't ski.

east tx skier
10-11-2010, 06:50 PM
Comparing the Gekko and ProStar is apples to oranges. The boats are very different; the Gekko is smaller and lighter. Since the boats have similar power I'd expect the Gekko to be noticeably faster. If the MC has a powerslot it would have lower top end.

Forty six mph with three people and a full tank of gas is pretty darn good.

My '89 TriStar runs 30 mph at 3100 rpm. It's just coincidence, but my 1991 SkiPro I/O (with nearly identical power) runs 30 mph at 3000 rpm. The SkiPro is 10 mph faster though.

Just to add to the numbers. My old 93 205 would do 36 mph at 3,475 rpm with 2 people in the boat.

flipper
10-11-2010, 06:51 PM
I didn't have a tach, but I had a Johnson 9.9 on a fishing boat that would be screaming at about 15mph :D

TX.X-30 fan
10-11-2010, 08:27 PM
I didn't have a tach, but I had a Johnson 9.9 on a fishing boat that would be screaming at about 15mph :D




Fascinating....................

JohnE
10-11-2010, 08:39 PM
I imagine you have a 1:1. My X14 tops out at 42 mph with 2 different props. ZO gps speeds. I never paid attention to the rpms tho.

TX.X-30 fan
10-11-2010, 08:50 PM
I imagine you have a 1:1. My X14 tops out at 42 mph with 2 different props. ZO gps speeds. I never paid attention to the rpms tho.



Wax you bilge and and I bet you'll see 44/45.............:D

ahhudgins
10-11-2010, 11:14 PM
Wax you bilge and and I bet you'll see 44/45.............:D

Ok now, let's not start up the old "wax, no wax.....and dimples" thing! :D

I'm on my second power slot boat and the RPMs still pretty much match my speed depending on the water conditions and the number of fat chicks in the boat. They change the gear ratio and then put a bigger prop on the boat. I realize you are bored just driving (I would slit my wrists) but I wouldn't lose any sleep over the RPMs. But really, try the wax.

FrankSchwab
10-12-2010, 03:07 AM
Early ski boats were set up with 13x13 props that gave pretty much the results you saw - 100 rpm = 1 mph. I would guess that it was a happy coincidence, as much as it was a "gee, this way you can keep skiing even if the speedo gives out".

I would guess that the 214 is set up with a slightly lower pitch prop; as a result, you see slightly higher engine RPMs. General prop set up is to choose a prop to get the engine near redline at WOT; if it hits redline, put on a higher pitched prop, if it doesn't get close, put on a lower pitched prop. It's not rocket science, but there's definitely an art to optimizing the prop you need.

With the increased weight of a modern boat (and possibly with increased drag trying to get a good wake with the extra weight), the prop tends to get reduced in pitch to compensate. This would explain your higher revs. It's even possible that the boat was intentionally built with a lower-pitch prop to increase it's acceleration out of the hole for wakeboarders carrying a lot of ballast.

I wouldn't worry about the difference in engine RPM between boats; worry about the performance. If you need more pull out of the hole, call Eric at OJ Props and let him guide you to a lower-pitched prop. If you want lower revs at speed, and possibly a higher speed if you currently hit the rev limiter at WOT, call Eric at OJ Props and let him guide you to a higher pitched prop.

/frank

Jesus_Freak
10-12-2010, 07:33 AM
Early ski boats were set up with 13x13 props that gave pretty much the results you saw - 100 rpm = 1 mph. I would guess that it was a happy coincidence, as much as it was a "gee, this way you can keep skiing even if the speedo gives out".

Just to clarify...this only works over a particular range of variables. Anyone know what the range is, just for my curiosity?

MCVOLS
10-12-2010, 09:41 AM
If you have your HIN, it will tell you if you have a slot or not. Easier than removing the engine cover shocks, etc. especially if you are not at the boat.

Jorski
10-12-2010, 10:41 AM
Here is a theoretical speed caculator that is fun to play with. Keep in mind, it is "theoretical"...




http://www.rbbi.com/folders/prop/propcalc.htm

TX.X-30 fan
10-12-2010, 05:10 PM
Here is a theoretical speed caculator that is fun to play with. Keep in mind, it is "theoretical"...




http://www.rbbi.com/folders/prop/propcalc.htm


Kinda like liberalism huh...............:D

gts-20
10-13-2010, 09:55 PM
sorry for the long delay guys....Work is killin' me.

anyway, I got the engine cover off this evening. The trans is marked 1.1, and the prop is a cnc OJ 13 x 11.

Any thoughts?

FrankSchwab
10-14-2010, 02:03 AM
Any thoughts on what?

I thought you'd gotten plenty of thoughts, I thought.

Jesus_Freak
10-16-2010, 08:54 AM
Here is a theoretical speed caculator that is fun to play with. Keep in mind, it is "theoretical"...




http://www.rbbi.com/folders/prop/propcalc.htm

The problem with using this as a "speed calculator" is that prop slip is not known a priori, and it is a function of too many things. An alternative is discussed here (http://www.mastercraft.com/teamtalk/showthread.php?t=35593).