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 ?