Prop for Classic 18

[Dilbert]

Looking for a prop for my '97 classic 18 with 260 hp 5.7 and alpha drive. Tried a Mirage 21 from Throttle Up (a great shop!) and got the following results:
RPM MPH(gps)
3000 39
4000 53
4400(WOT) 58

Using the handy dandy calculator, the prop slip is only 7% when assuming the pitch is 22" (1 inch for cup). Matt at Throttle Up said that the Mirage props are true pitch and that I should use 21 as the pitch makes the slip is less than 4%. These slip figures seem way too low. Verified the tach accuracy before the run. Also, is 4400 too low for WOT on this engine spec'ed for 4400-4800 rpm? Any other prop recommendations?

[Ranman]

Dilbert, I used to have a 96 18 Classic with the 260HP/Alpha combo. I believe that boat ran a 21P Vengeance, which I think is what came from the factory. That boat ran 58-60mph at around 4800rpm. You might look into a Turbo Prop. There are several members here who have had success with these props and the Turbo is going to be my next choice. Good luck. Randy

www.turbo-props.com/ (http://www.turbo-props.com/)

[Dilbert]

Thanks Randy. Is it better to be at the high range of the max RPM range at WOT? I'm noticing that your pitch and RPM also seem to indicate around 7% prop slip given the same alpha drive setup.

[Ranman]

Dilbert, I'm coming up with about 10% slip with my numbers. This is just a ballpark figure though. Here's the formula I used, which looks close to what you're using.

Pitch X RPM / Gear Ratio X 1056 = Theo Speed

Theo Speed - Actual Speed / Actual Speed = Slip

21 x 4800 = 100800
1.5 x 1056 = 1584
100800 / 1584 = 63.63

63.63 - 58 = 5.63
5.63 / 58 = .097 or 9.7%

Thing is, this is a bit of a ballpark since the 4800 could be +/- a couple rpm. Calculate cup or no cup and top speed varies a bit due to water conditions. These factors can vary the slip considerably.

I would prefer to be at the upper end of the spec, but do not exceed it. If your between 4600 and 4800 I'd be happy. The trick is to try different props, record the results and go from there. The Mirage + Props work well on an 18. I run a Mirage + on my 18 now and ride and handling is good. I do plan to try a Turbo prop next season as I'm starting to buy Big Grizzley's argument and several members have had success with the Turbos. Cleavers seem to be less desirable on the 18, but may be favorable on the 16's. It never hurts to try try try!

Lastly, there's a lot of information on props archived on this site. If you haven't done so already, be sure to use the search function at the top of the page and look up "props". There will be plenty of reading. Good luck. Randy

[clayman]

I've been working on dialing in my boat and I am stumped. I ran two props today in my 260 mercruiser V8 ,1.5 ratio with approx. 35 gallons fuel, one person:
Mirage plus 21,, wot 4200rpms, 61 gps
crappy aluminum 21x(14?), 4600, 60.9 gps
both of those numbers don't add up do they? I used a few on-line prop calcs, and it said tha would be negitive slip. How do I test my Tach for accuracy?
James
http://www.donzi.net/photos/jspiroff64.jpg

[BigGrizzly]

With all the information gathered and the GPS numbers I would have to say Both of you have tachs that are off by about 200->300 rpms. Clayman don't be surprised about the perfomance of the aluminum prop. I have seen this several times. I took an old aluminum prop for the Corsican to have it re pitdhed and it fixed to use as a shallow water prop. When I tried it, it was only 1 mph slower and 300 rpms less than my Hill stainless prop. The amaizing thing is the shop is in this guys back yard in Gainsville Ga.and is a part time deal. He charged me $25 this included welding a big chip in the blade. It just makes you wonder doesn't it.
IT sounds like the M plus with its aggressive cup is droping the bow and pulling te RPMS down. the aluminun may have a cup byt being a thicker bladeis actually letting the hull lift itself out of the water. My suggestion is cut the stop on the trim cylinders and try it again.
The stop should be cut so it protrudes 3/16 of an inch below the top mount OD.

Randy owner of Donzis since 1966.

[clayman]

I am going to try and test my tach somehow tomorrow. I called CP performance, they said my MSD might be causing interferance with my tach. I guess they make a diode for it. The Mirage plus is a much better performing prop, but the top end is almost the same. the aluminum blows out alot.
james