2006 Chaparral 246 prop change and max RPM drop (videos attached)

pmtom1

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Jun 21, 2010
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New to us 2006 246 SSI (4900 lbs) is under powered with 5.0 / Alpha. (this will change!) The SS prop it came with was in bad shape, mainly a few too many back yard strip and repaints. No way to tell what it was besides being old and aluminum.

Here is a video with the old prop. 5200 RPMs best case was 38mph. Empty boat, super calm water. https://photos.app.goo.gl/rVt1asyLScszpLU57

Did the whole prop selector thing on a few different sites and went with Solas HR Titan 4-Blade Pressed Hub / SS, 14.3 x 17. Whole shot improved and now I get 40 mph but cant get over 4200-4400 rpm with throttle to the floor. https://photos.app.goo.gl/6rwKEp5uBT9zK2k19

For where we boat and what we do, I don't need much more speed then that, but 45-50 would be nice now and then. Im sure Im saving some gas and the engine life will better at 4400 then 5000 but thinking about going up a size or two.

Thoughts on 18 vs 19 or something different all together? I appreciate the help / advice!
 

tpenfield

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Do you have a pitch & diameter number for the old prop? Not sure you will get much over 40 mph, but maybe get the RPM up a bit.
 

pmtom1

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Jun 21, 2010
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I sanded the hell out of it and got a number. mercury 832828a45p17 which looks to be a mercury black max 15 x 17.

Wonder how much speed would be gained going to a 15 which should get my RPMs up to 4800ish, correct?

Any downside to running at 4200 all day?
 

porscheguy

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Better to run slightly over the limit as opposed to way under it. Only maxing out at 4200rpm is very hard on the engine and will shorten its life.

Is that an MPI or a carb? What’s the gear ratio? It should on the outdrive.

If you search google, they did a test years ago on that model. Their boat had a 5.7 VP DP @ 280hp. Their top end was 47. If you’ve got the mercarb 5.0, then I doubt you’ll see much more than 42-43. If it’s an MPI which should be rated at 260hp, then you can maybe see 45/46.

With that said, you won’t know for certain until you get the right prop that gives you full operating rpm.
 

pmtom1

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porscheguy It's MPI so 260hp. I did search all the tests and reviews and saw the one you are referring too. Never expected 50 but felt that 36 was too low. Don't have a trailer yet and drive & prop are just under the swimdeck so EVERYTHING is harder to do / figure out.

Running that few (4300-4400) RPMs makes that big of a difference, huh? I knew 3500-3800 was bad but thought I was close enough. Looks like I need to try a 15.
 

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jkust

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crazy to think that someone ordered that with 5.0 in it. That's the engine that the 190ssi got which was 18' 3".
 

tpenfield

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You went from a 17" Pitch 3-blade to a 17" pitch 4-blade of slightly smaller diameter (right?). Your best bet would have been to go down 1 step in the pitch, so a 15" pitch 4-blade might do the trick.
 

jimmbo

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What’s your engines WOT range, 4600 - 5000 rpm? If you are running 4200 right now, you are going to need less than a 15 to get to the upper half of that rpm range.
Your old 17 was probably in bad enough shape to allow it to lose its grip on the water, allowing the engine to attain 5200 rpm. A new 3 blade 17 would probably have put you in the middle of the rpm range. Adding that extra blade lowered the rpm and boat speed

In the first paragraph of your initial post, you say it is an SS, then you say it is Aluminum. So which is it?
 

pmtom1

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You went from a 17" Pitch 3-blade to a 17" pitch 4-blade of slightly smaller diameter (right?). .

Yes, exactly. Didn't know what was on it initially. Pulled it sitting on a sand bar dodging wakes and couldn't see the marks from being painted so many times. Ordered what I thought would be best then swapped them at the same sandbar a few days later. Ordering the 15p 4 blade today. Will update results!
 

pmtom1

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Jun 21, 2010
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What’s your engines WOT range, 4600 - 5000 rpm? If you are running 4200 right now, you are going to need less than a 15 to get to the upper half of that rpm range.
Your old 17 was probably in bad enough shape to allow it to lose its grip on the water, allowing the engine to attain 5200 rpm. A new 3 blade 17 would probably have put you in the middle of the rpm range. Adding that extra blade lowered the rpm and boat speed

In the first paragraph of your initial post, you say it is an SS, then you say it is Aluminum. So which is it?

Yes 4600-5000 WOT. I am able to hold steady at 4400 right now but pegged.

It WAS totally slipping. Could feel it in hard turns and when trimmed up a bit to far.

Old one was aluminum, new is SS, sorry for confusion!

So what would be the biggest differences between a 3 blade 17p (new and SS) and a 4 blade 15p? I went 4 blade for a little better hole shot since im massively under-powered. Is this a good idea?

...and thank you everyone! I really appreciate it.
 

tpenfield

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You should be OK with the 15" pitch 4-blade. The 17" 4-blade is probably overloading the engine slightly. SS tends to take a better 'bite' of the water than Aluminum. Adding blades generally means reducing pitch.

Since you have a 5.0 engine on a 5K lb boat, you will have to stay lower in pitch to match what the engine can do.
 

QBhoy

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Hi. You aren’t going to see near 50 I’m afraid.
Check your tachometer firstly I’d say. That engine should limit and beep at just under 5200 at about 5150 roughly.
I’ve found that these engines make their max power at about 4800-5000 rpm. Although rated at 4600-5000 anything below 4700/4800 then you are down on power and lugging.
The 4 blade is never going to see the optimum speed either. Might get you out the hole quicker but will never be as quick as a well suited 3 blade.
If you can get her singing at 4800-5000 top end, you’ll be at your best all round. Likely see 40 maybe ?
 
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