Blade area & diameter vs. height
 

I just raised my Ilmor Indy drive up from 1" below the waterline to even with the bottom of the boat. For various reasons, Hering and I had been doing prop development at the 1" down position. Now, I see prop slip numbers have increased from less than 9% to almost 12%, and the prop seems to be searching for bite in rougher water, although it recovers quickly. It's a 5-blade 15.75" x 29" x 18 degree Hering with lightly rounded ears. It was cut down from it's original full-cleaver 16.5" x 29" configuration, and much of the cup at the hub removed to reduce transom lift.

Since I am down about 500 rpm at top speed (5800 @ 103 mph), I am considering going with a 16.5" x 28" x 20 degree full cleaver prop next. my thinking is that the increased rake will provide enough bow lift that the cupping and round ear modifications will not be needed, and the 1" reduction in pitch will get me the rpm plus a bit more speed (6300 @ 108?). Hopefully having more blade area from the increased diameter will help the prop hold better in the rough.

Does this make sense to you prop gurus?

I dont know much about cleaver style props in surfacing applications, but I dont think that 1" reduction in pitch is going to pick up 500rpm. Especially if you are increasing diameter by 3/4".

Good luck, cant wait to see it on the bay next season.
Phil

The pitch reduction works out to about 3.4 per cent. Even if the boat doesn't pick up any speed, the rpm will increase to 6,000. That alone will give me another 25-30 hp.

If it bites too hard, I can have it cut down. It's hard to go the other way...

The increased diameter will compensate for the decrease in pitch. I believe you will be hard presses to see your rpm gain. In case you were not told, higher rake can rob R's as well if the boat doesn't loosen up more. You may need to go down further in pitch with the added diameter.

Nice job on the round ear. BBLADES was the only shop doing that until recently. Can't keep all our secrets (especially when they work). I wouldn't necessarily give up on that route.

Holy Chit a Ilmor Batboat !!!! What size propshaft is that and how many splines?
That has got to be a neat ride!!

It is the same as a late Speedmaster/#6: 1-7/8" diameter.

Yep - It's a ride and a half: Batboat build thread

I would be careful with the high X and large diameter prop. I changed a setup an ocke going 1/2 inch above the bottom and large diameter prop. When testing flat out I didn't realize that the large prop was walking the narrow beam sideways causing me to 'crab' the drive to maintain a straight line. I was approaching a bridge and chopped the throttle. The drive was turned (crabbed) enough to veer the boat sharply to port , leaning out. The stbd wing caught the water and acted like a turned rudder. I remember flying out of the boat upside down and then later being pulled from the water with many a broken bones. I spoke to my buddies over in Sweden who advised the combo was not a good idea. They told me to add pitch not diameter if going above the bottom. I should mention that during that and all setup test runs I ALWAYS wear my helmet and jacket.

I also notice you too are running a left hand.

Just another note. Winrace props in Norway were doing my prop work. I was running a bravo but the prop started out as a hydromotive with the same rounded tip cleaver style but thru-hub. Winrace swore down that the rounded tips did not work on the Ocke's and cut the diameter to give a true cleaver style blade. The other thing that they did was the bravo gearcase. Their method was to install two hydromotive nose cone, bullets only, one welded in front of the other. This led to a very long gearcase. They also mounted them 1/4 inch above centerline which gave me much more bow lift compared to a sportmaster (boat didn't like it at all) and a single hydro nose cone installed in the standard location.

Trust me; I've been sneaking up on this one all summer. I'm pretty sure that the increased skeg area of the Indy is providing more stability than a Bravo could. The gearcase and lower unit are MUCH larger. Thanks for the input - it's good to know.

I am using similar cleavers on my boat. What is the advantage behind the round ear at the ends of the cleaver ?

Apparently, it makes the prop act as if it has more bow-lifting characteristics/more rake.

For years while I was at Mercury Racing we only produced 15 degree rake propellers. The rake angle allows the propeller to hold water to it at steeper trim settings. Therefore leveraging the stern more and creating better bow lift. By creating the round cupped tips, we in effect, trick the propeller into thinking it is a higher rake angle.
We were doing this before there even was a Hering propeller. Indeed Hering did this via Merc and BBLADES technology. That's just fine, we all share in the technology of the propeller world to make everybodies evperience as best as it can be (there are still radius and cup tricks we keep to ourselves LOL).
We now have higher rake angle casting to work with as well. However, there are still certain situations were not only the higher rake angle is a advantage, but the round ear just makes the boat carry better.

Brett

Yes, but a true cleaver just looks soooo bad-a$$....

He*l yeah :evilb:

LOL!!

Yeah but beauty is in the eye of the boat b'HOLDER"..........

Chrome bumper polished propellers look awsome on the trailer during the 4th of July parade but without the right touches in the right places, or satin polishing.. they just look good.

Each his own.
Merry Christmas.

Brett

contact me
 

Brett, I sent 3 emails through your web site. I need assistance in picking out new props for my boat.

Rick Taylor

I only recieved the 2 emails sent late Fri and Sat night (3rd never came through).
Sorry for the delay. 3 days out traveling on business gets me behind. Expect a response in the next 24 hours.

And, thank you.

Brett

No big deal. Just wanted to make sure you got them. The
3rd one was a response I got back from the original owner. It had some technical info that I thought would be very helpful. I will re send the 3rd. I am in no rush to get this boat out into Lake Tahoe now..... Especially considing that as I write this, it is probably about 19 Degrees F. on the lake....!