Mirage plus 18 or workshop my 17 pich

[erick11s]

Hi, I have a cigarrette 29 feet year 1979 wiht two optimax 225 hp propeller mirage plus 17 pich... The proplem is a full trotle i take 5750 rpm and only 48 mph... The engines recomendation is to max 5600 rpm..... The Mercury charter said my propeller will be 18 pich mirage plus.... My question: What is better take the mirage 18" or with my 17 proppeller send to some good work shop propeller and make the modification to 19 pich and take more performance, my Goal is better top end.

 

[Sea Rider]

If you go for a 18 pitch, will lessen max achieved 5750 wot revs, should gain slight more top end speed. The issue is that one more pitch, 17 to 18 not always lessens - 200 rpm to be near 5600 wot revs, could be way less, this is not exact rocket science. On the other hand (2) 225 OB's will double the HP, but not the speed. Was 5750 wot revs achieved with you alone and lightly loaded boat ?

Happy Boating

 

[Texasmark]

My experience with where you are is that the pitch change will improve your speed while lowering the rpms. Really it all depends on your boat/engine dynamics, but if you are at 48 now, your dynamics ought to be in your favor. Had you been back around 30 then it's a crap shoot. 2" at a prop shop is more than is recommended per my experience with tweaking props at shops. They feel (the shop I used.....big one) comfortable with one inch but asking can't hurt. Then you don't have to deal with other issues:

On changing prop types/shapes/brands/material/blades....etc., you are throwing an unknown into the equation. Take rake for example. If your replacement has more rake, expect more bow lift which translates to less drag and more speed AND rpms simultaneously. If going in the opposite direction expect less. Cup is also a consideration; cupping or not, if so, how much cup did the prop designer find was the right amount for his prop.....you can have too much (reference my readings on the subject from prop mfgrs. postings) as well as not enough.