Prop Pitch, High Altitude...help?

thefu

Cadet
Joined
Sep 15, 2005
Messages
10
All, I just bought a 1998 Glastron 205SF with a Volvo Penta 5.7GS SX. The boat is from California and now it is back here with me in Utah. I operated the boat in Lake Mead with the prop on the boat and I think it was a 19 pitch. I have been told that I need a new prop for the lake altitudes here of >5000ft (most of them in northern UT). So, I thought it prudent to go with one of the ProPulse adjustable props since not only is it better for when I travel to lower altitudes with the boat such as Mead, but it is also sacrificial. <br /><br />So, I set the Propulse at 19 and went out yesterday and the whereas the boat easily did 57MPH on Mead, it only made 42MPH at WOT here at Jordanell Reservoir in the SLC area. <br /><br />Can someone tell me what the best setting would be so that I don't have to play around with it too much. I would like the best balance between get up and go and top speed. So long as I am doing 50+ MPH I would be fine. <br /><br />Finally, do you think I will need to have any other adjustments made to the engine due to the altitude change? <br /><br />Thanks
 

NateCCIE

Seaman Apprentice
Joined
Apr 3, 2005
Messages
48
Re: Prop Pitch, High Altitude...help?

Need to know what RPMS your turning at what pitch and at what altitude to adjust for optimum performance.
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Prop Pitch, High Altitude...help?

This belongs in the Prop forum.<br /><br />Moving it there.
 

thefu

Cadet
Joined
Sep 15, 2005
Messages
10
Re: Prop Pitch, High Altitude...help?

I did not log it that last time. Is there someone out there with a similar setup that can give me a good starting point. The thing is, I don't know if the reason for the speed drop is the engine in the high altitude or the wrong prop. If someone else with a similar setup told me the pitch they run at this altitude, that could help
 

trebor0301

Petty Officer 2nd Class
Joined
Jul 1, 2004
Messages
166
Re: Prop Pitch, High Altitude...help?

Hey Fu I dont have your setup but I can tell you this.If you dont know what the manufactors suggested(W.O.T) wide-open throttle is for your boats motor then you had better find that out before you go messing with your pitch.The speed difference is much too great for me to believe that your RPM's are not getting too high.If your not careful the prop will be the least of your problems!If you have trouble finding out what your W.O.T RPM's should be maybe asking on iboats will help.There are some Volvo experts always looking in to help guys like us.
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Prop Pitch, High Altitude...help?

Every prop acts different on 2 Identical boats anyway...that's the Bass ackwards approach.<br />We NEED RPM figures with a given pitch.<br />50MPH+ may not be "your" choice if the RPM isn't in the range it should be..And, when trying to prop, forget about the speedo, or atleast look at it second, after you look and record what the tach says.
 

thefu

Cadet
Joined
Sep 15, 2005
Messages
10
Re: Prop Pitch, High Altitude...help?

OK, well I took it out with the pitch set to 20. The recommended RPM for my engine is 4800 at WOT. With the pitch at 21 I was only turning 3900 @ WOT and so the rule of thumb I read was 200 RPM for each 1" of pitch. So, I adjusted it to 17" expecting WOT to now be at 4700 given that Rule. Well, it now went to about 4850 and still had a little more to go. So, I guess the magic setting is 18"...next time that is where I will set it. Still at 17" and turning 4800 I could only get it going 42mph according to the GPS and there was no wind and the water was smooth.<br /><br />What would be an expected degradation in performance between seal level and 5500 ft.? If it does 60mph at sea level, should I expect more than 45mph at that altitude? Or, is it because the composite prop is not as efficient as the aluminum?
 

walleyehed

Admiral
Joined
Jun 29, 2003
Messages
6,767
Re: Prop Pitch, High Altitude...help?

About 16-17% less than sea-level (power loss).<br />I lose 7MPH from 600ft to 3500ft with my 200.<br />It also takes 4" less pitch to get the same RPM at 3500ft.<br />The composite prop is very flexible when under a load equating to lower efficiency.<br />At 3900RPM you are nowhere near your max Torque, so what happens is the engine gains more RPM with a prop change when you come in under the power curve and then drop pitch getting the engine to a point it will develope more power and in these situations, the 200 per inch doesn't apply.<br />With it set at 17, you need to see how high it will go...you are not going to hurt it if it goes to 5000RPM, but that is the info we need.<br />I would also recommend you do this test with a normal load, whether it be 2 people and gear or 6...test with the normal load..we want 4800 at that point, and yes, it may go higher when you have a light load, but pull back to 4800-4900 and go.<br />Post back what you get for numbers... :)
 

thefu

Cadet
Joined
Sep 15, 2005
Messages
10
Re: Prop Pitch, High Altitude...help?

OK, thats great info. I was moderate-light loaded when I tested it at 17". It would get to 4800 fairly quickly and then it would creep up to 4900 when I decided not to take it much farther. At the rate it was creeping I doubt it would have exceeded 4900 by much.<br /><br />Next trip out I will set it at 18" and see what I get and then report back for feedback.<br /><br />So, 16% less power huh, well that would mean about a max of 50MPH or so given the near-60MPH speed at about 1500 feet in elevation. Thats good info to keep in mind.<br /><br />I really like the idea of the 4-bladed composite in terms of the replaceability of the blades and the sacrificial nature of the prop. I just hope I don't have to give up too much in performance. I am mostly a fishing person so the launch is not my #1 importance.
 
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