using larger props to lower slip #'s
 

Boat is a 98 42 fountain, #4 drives 1.5" above the bottom. Boat planes great, never blows out the props. Slip is alittle high with 17" props. Have another set to try that are 17" but ran out of weather. I know i can space the drive down but would rather try to get it to hook up better with props. These are all 5 blades so far. What negative effect would 17.5" props have?

Are you on the limiters with that much slip? From what I've experienced a higher pitch will drop slip a little, maybe a couple percent, maybe. If you don't want to space down try bigger diameter or more blades. Both will take more power to spin, lowering rpm, but with that much slip I would think you'd have some rpm to lose.

Yea that's what I was getting at larger diameter. 17" vs 17.5" diameter. I was curious if it will cause any ill effects on the handling of the boat, too much stern lift Etc. In theory in my mind larger props could possibly equal lower speeds due to surface area and drag. Same reason that all things equal a 4 blade is faster than a 5 blade because of drag.

One of the prop shops around here will be able to give you a solid answer but being 1.5" above and that much slip I wouldn't think .5" bigger would hurt.

It becomes a trade off and search for the right balance of slip and speed. I think if you give up a little pitch, increase diameter to lower slip you will be ahead a few mph. Not giant gains like we'd like to see but a bit. You won't be able to spin as big of pitch, but slip will be down (good for the drives) and more efficient. A while back I tried 22-28 pitch 4 blades on my hawk, bravo and hydromotive, and 1" spacers. The best combo was no spacer and 24 bravos, the 26's pulled the same speed but took forever to get there. As did the 1" spacer. Slip only went down a little bit (1-3% I forget ) and took a long ways of hanging it out to get the last few mph. The race boat, 23 warlock has 3% slip at wot, but it is significantly slower than other warlocks with similar power supposedly are. Could a little more slip allow a bigger pitch and some more speed???

Thanks for the input Jd. I'm bummed out I didn't get to finish prop testing before it got cold. Spring can't get here soon enough!!

Indysupra,

I don't mean to hyjack your thread, I also have a question on diameter, which applies to your post, I'm just looking at going down in diameter. Ive tried a bravo1 26p and 28p on my setup. The 28 runs the best as far as cruise speed and WOT gps numbers and the slip is very good. My problem is that I'm running 200-300rpm low at WOT. Could I have the prop blades trimmed slightly to try and pick up the extra rpm to try and pick up a touch more speed?

Lab Finishing with the correct recipe can increase rpm by 100-400 rpm and increase speed depending on the recipe used. The recipe determination depends on current performance numbers.
If you would like we can discuss the options in detail on the phone. We do not charge for prop and set up discussions. We can more than likely find you a alternative you desire.

Brett

The easy answer to this question is you will get more stern lift. The "X" dimension on your set up is high. There are options for you to debate. Your boat was more designed for 4 blade props. We were just producing 5 blades at Merc Racing when your boat was built.
Knowing more performance information on your set up will help to determine the best route.
Call me anytime and we can discuss the options.

Brett

On my boat (even with bottom) 17.75x 27 4 blades = 60mph at 4000rpm.

Tried a 17x30 4 blade . took 5 minutes to get on plane and to my surprise no difference still 60mph@4000rpm but slip way up.(That prop had way less cup also ) My boat likes a lot of diameter and cup.

If you called Herring (about a straight bottom cig) they`ll tell you need at least 18" diameter... more blade in the water