rake angle-pitch-blade count-how to pick for best all around performance

[muskyfins]

Details:
1993 Stingray 606 (20' open bow)
188/888 mercruiser (not original to boat)
current prop 13-3/4? x 19P 3 blade ss (model 48-88440-19)
runs 42 mph (GPS) at 4200 rpm trimmed up with about 600 pounds of people and 15 gals of gas

Problem:
Cannot take off from idle. Even if I slowly advance the throttle it would suddenly rev fast and no forward motion as if the hub was spun. If I VERY SLOWLY (couple hundred RPM at a time) advance the throttle, eventually she'll get on plane and all problems are gone.

Prop shop says hub not spun but old prop has too low of rake angle (8 degrees) causing prop to slip. Reccomends 4 blade 21P alum "for more stern lift." I didn't think I had a stern lift issue?

I am looking for a compromise of both top end and hole shot. We're on the Fox Chain in Illinois and if you're familiar with it you know you need to be able to get on plane in a tough chop caused by the sheer number of boats out there. I also like to run fast but would settle for 42 mph if that was the compromise. (Many say the stingray hull with a V-8 is capable of more like 55 mph-Singray even advertised it as such with a 23P prop)

So I sort of have a benchmark with my old prop but have no idea how my actual hole shot would be. Is there a semi-scientific formula to get closer to a starting point? 3 Blade/4 Blade? Aluminum/Stainless? How pitch factors into either of the first two equations? It's starting to make my head hurt....

 

[Bondo]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Details:
1993 Stingray 606 (20' open bow)
188/888 mercruiser (not original to boat)
current prop 13-3/4? x 19P 3 blade ss (model 48-88440-19)
runs 42 mph (GPS) at 4200 rpm trimmed up with about 600 pounds of people and 15 gals of gas

Problem:
Cannot take off from idle. Even if I slowly advance the throttle it would suddenly rev fast and no forward motion as if the hub was spun. If I VERY SLOWLY (couple hundred RPM at a time) advance the throttle, eventually she'll get on plane and all problems are gone.

Prop shop says hub not spun but old prop has too low of rake angle (8 degrees) causing prop to slip. Reccomends 4 blade 21P alum "for more stern lift." I didn't think I had a stern lift issue?

I am looking for a compromise of both top end and hole shot. We're on the Fox Chain in Illinois and if you're familiar with it you know you need to be able to get on plane in a tough chop caused by the sheer number of boats out there. I also like to run fast but would settle for 42 mph if that was the compromise. (Many say the stingray hull with a V-8 is capable of more like 55 mph-Singray even advertised it as such with a 23P prop)

So I sort of have a benchmark with my old prop but have no idea how my actual hole shot would be. Is there a semi-scientific formula to get closer to a starting point? 3 Blade/4 Blade? Aluminum/Stainless? How pitch factors into either of the first two equations? It's starting to make my head hurt....

Ayuh,.... My guess is,.... That prop is for a Mercury Outboard, mid-sized gear case...

If so,... The barrel of the prop is too small, 'n it's ventilatin' itself outa water...

Put a 19" pitch prop for an Outdrive on it, 'n Try it....

 

[QC]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Is there a semi-scientific formula to get closer to a starting point?

Sort of, but not really . . . props are 50% physics, 50% luck and the rest is Voodoo . . .

Let's try and work on it anyway Do you know your gear ratio? I am checking some stuff for that, but that is the one piece of missing data to try and see this from the physics angle. One thing I can tell you, if you are maxing at 4200 RPM with a 19 three blade, and your complaint is hole shot, then a suggestion of a 4 blade 21 is downright stupid. Backwards. But let's give them the benefit of my massive doubt and try and work this out. Back in a few mins.

Edit: Bondo's quicker, good catch.

 

[Bondo]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Edit: Bondo's quicker, good catch.

Ayuh,.... Whooped by a single digit typist,..??....


Now, I'm laughin'...

 

[QC]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Put a 19" pitch prop for an Outdrive on it, 'n Try it....

I agree to try this as a baseline. Get a 3 blade Aluminum, and worst case you now have a spare for emergencies which you need anyway. Test, and repost data as you have. Good job BTW. I am getting 17% slip with a 1.5 ratio. Seems believable. Fact is that the WOT RPM range for the 888 is 3800 - 4200, so a decent 19 may make her absolutely perfect if the data is right and the ratio is actually a 1.5. Good luck!! Now comes the Luck and Voodoo parts . . . .

Edit: Ayuh

Edit 2: Twice :facepalm:

 

[muskyfins]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Not necessarily maxing out. There was more throttle left, but didin't want to push my luck any further at that point. I trimmed up a little too far at one point (still at 42 mph and 4200 rpm) and it suddenly revved way high. Brougt it back down and didn't approach any other limits.

 

[QC]

Re: rake angle-pitch-blade count-how to pick for best all around performance

BTW, a 4 blade 21 may be right considering the very low WOT range. I assumed (always bad) 4200 - 4600, but waaaaayyy off. Still think the 19 baseline is a good idea. Especially if you want hole shot. Will still make a great spare and give us something to work from.

Was your load typical?

Edit: Need the WOT trimmed out numbers, the reason it revved when you trimmed too far is the same as why it wouldn't go in the first place: Ventilation (prop grabbed a gulp of air).

 

[muskyfins]

Re: rake angle-pitch-blade count-how to pick for best all around performance

As always-many thanks to you guys. I'll get my hands on the correct 19P and run her this weekend and repost the data.

Load was probably on the low side of average. Four people comfortably.

 

[QC]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Load was probably on the low side of average. Four people comfortably.

Then the 19 is the proper starting point based on what we have. I think 42 MPH is very respectable for a 188 hp (crankshaft, maybe 165 at prop) package. If the prop hooks up better then I could see both hole shot and top speed improving. Add in that WOT RPM may actually go down and this is very rare, but possible, result.

 

[muskyfins]

Re: rake angle-pitch-blade count-how to pick for best all around performance

Ok few updates. Highly regarded prop shop told/showed me that my old prop was for a much larger boat from the 70's. It still actually measures 19P, but the blades have zero cup to them. One actually kind of cups backwards/bent. Said it would work better as an anchor.

Some investigating reveals a buddy of mine had a Vengance SS 19P in his garage and said i could borrow it for testing and my brother had a 21P in his garage-same thing. So far I've only tested the 19, but it was like a completely different boat. Better control at no wake speeds and planed pretty fast. As a matter of fact, I need a second outing with this prop just to take some actual times and gather a little more data. Upper end remained the same-42 mph on GPS and 4200 rpm WOT trimmed all the way up load was about 80 pounds less than previous.

Maybe (maybe) I'll get a chance this weekend to try it again and swap out for the 21P. I am a speed nut and was hoping for a bit more on the top end (50 MPH) but I also wouldn't want to have 6 adults out and not plane well or at all. I guess I'll have to see how slowly the 21 gets me out of the water. I'll probably add some tabs over the winter as well which would help. Right?

Overall I'm not UNhappy with this set up and my buddy says I can have the Vengance prop for a case of beer, $50, and a favor to be named later...

 

[QC]

Re: rake angle-pitch-blade count-how to pick for best all around performance

The 21 may be good, but just be aware that 8 MPH in our world is a LOT. Let's hope for 45, and if you do any better consider it a major bonus

 

[jimmbo]

Re: rake angle-pitch-blade count-how to pick for best all around performance

I'd be leary of any 'favor to be named later' stuff

 

[muskyfins]

Re: rake angle-pitch-blade count-how to pick for best all around performance

I'd be leary of any 'favor to be named later' stuff

I'm pretty sure the favor to be named has to do with me helping him move a couch that I swore would have to be sold with his house.