I THINK, I get it? Prop/RPM question

[Mikehdn]

Hi all,

Took the "New-to-me" 80 Gulfstream Montego out for it's first test-run this weekend. Boat came with 2 Props.
** 21P/14" Diam. Aluminum 3-blade
** 18P/15"Diam. "" "" ""
With the 21 Pitch/14" Dia.... I noticed that at WOT, the RPM's were excessively high (4500-4700) Top speed approx 38MPH
Do I understand correctly that If I change over to the 18P/15" I will achieve a more acceptable, lower RPM (4200Max) at WOT?? Albeit at a lower Top Speed? Is that NOT how it works?

Boat info:

898 Mercruiser 198hp sterndrive 5.0L V8
19ft length Approx 2700 weight
Typically 3 Adults on board
Primary use for run-about/ Tubing

Thanks in advance!!
Mike

 

[steelespike]

Re: I THINK, I get it? Prop/RPM question

First to make things a little easier and avoid confusion. Prop size is all ways expressed Diameter X pitch.
Pitch being the primary concern.You do have it right usually a lower pitch will be slower but rpm higher of course depending on where the rpm was in the first place.
You have it backwards as you increase pitch you lower rpm.Increasing pitch can improve speed if the motor can twist it up to the rated rpm range. 4400-4600 is the rated rpm.
Your numbers don't add up I believe a 1.65 gear ratio was standard with a 1.50 as an option.
Speed should be about 50. at 4700.
Is the speed by gps?

 

[Mikehdn]

Re: I THINK, I get it? Prop/RPM question

Steelespike,

Specs in the 898 Operators manual shows Max of 3800-4200 WOT. (Was I wrong in thinking 4600 was too high regardless)?
Speed was NOT GPS but Speedo gauged... and now that I recall, speed at CLOSE to WOT was more like 43mph but I didn't want to twist it up further because of what I considered excessive RPM for the 305ci.
I think that is the flaw in my thinking. My thought was the RPMs could be LIMITED with a different prop but It's more about finding the relative sweet-spot of speed/RPM?
Therefore...according to what you said, lowering to a 18p would just exasperate my problem!

 

[mpsyamaha]

Re: I THINK, I get it? Prop/RPM question

yes, dont go to the 18p prop. Ideally, you would want to hook up some sort of shop tach to confirm that your boat tach is reading correctly (as they often dont), and verify speed with GPS. Then you can be sure before you go spending money on a fancy new prop.

 

[Sea Rider]

Re: I THINK, I get it? Prop/RPM question

Ideal rpm would be to have it dialed at 4 K at wot with minimum load with boat/engine tested on flat calm waters. As stated by Mpsyamaha, test with other tach, can get a cheap induction Tiny Tach, Hardline Tach to know for shure actual rpm achieved at wot with both props. Then decide any other prop option.

Happy Boating

 

[steelespike]

Re: I THINK, I get it? Prop/RPM question

Stick with the manual numbers 3800-4200. As suggested you do need to confirm the tach.

 

[BonairII]

Re: I THINK, I get it? Prop/RPM question

Confirm speed with gps....speedos are not reliable. Def get a tach(portable or permanent).

Sounds like you're going to be going UP a bit in pitch to RPM down to where you need them.