Chapparal SS Prop uprgrade question-350 Mag

I have a 1988 Chapparal 2350SX cuddy cabin with a 350 Magnum 270hp and an alpha 1 lower/ new Sterndrive Eng upper drive 1.47 ratio. The boat came with a Quicksilver Silverado 3 blade SS prop 14x19p (cupped). When trimmed out at WOT it is running 5000-5100 rpm @ 47mph. Last year I did see 48.5mph one time but have not been able to repeat this season. On the engine the label says WOT should be between 4200-4600 rpm. I would like more speed to at least see 50+mph. Don't care too much about hole shot- no skiing etc.
Should I change to a 21P-22P prop and what would this do to my RPM and top speed? Would this bring it down to acceptable range and most importantly would speed increase? I have tried all the online calculators and they say my boat should run much faster (54.8 with 15% slip on the 19P) It is a heavy boat at around 4000lbs but it seems like it should do better. Prop slip seems to be an issue for me or at least it seems like it according to the calculators...Any SS prop recommendations or any other easy performance upgrade ideas would be much appreciated...

PS. boat was tuned up new plugs etc. in spring

Thanks in advance,
Bob
Kingston, RI

Re: Chapparal SS Prop uprgrade question-350 Mag

I disagree that you should get much more out of that boat/engine set up - you're doing pretty good right now! Are you sure your tach just isn't a bit off, screwing up your calulations? Because your rpm is over recommended, a drop to a 21" SS prop may get you tp the magic 50 mph, I guess. It's certainly worth a try. Find a prop designed for cruising and high end speed for sterndrive boats. Maybe someone will come along with more specific info.

Let us know what you find out.

Re: Chapparal SS Prop uprgrade question-350 Mag

How do I check the tach and how do I hook it up? I think this may possibly be the issue...the merc prop calculator states that 47mph with this setup should be turning 4700rpm. (this is still over the motor plate recommended rpm though)

Thanks,
Bob

Re: Chapparal SS Prop uprgrade question-350 Mag

Is that GPS speed, or Dreamometer speed? It seems a bit more then I would expect, but whatever? If your tach is right, I'd say try a 21' pitch SS prop, but here's the dilemma, I know the Merc props are better then most but, there are probably even better designs out there, that I'm not familiar with? Maybe one of our resident prop guru's like Walleyed, or Dhadley, will chime in.

On another issue, if you do change props, and you're not using Mobil one synthetic, Do it, I've seen the Best Gas Mileage, & Power increase in all my vehicles ever since I began using it, over any other oils, or synthetic oils. Though I haven't tried Amsoil yet? You'll probaly see an RPM increase, let me know.

Next, when hot/engine idling, adjust your idle mixture screws out to reach the highest idle speed you can, this will be where your engine runs best! The EPA may not like me though

Re: Chapparal SS Prop uprgrade question-350 Mag

Is that GPS speed, or Dreamometer speed? It seems a bit more then I would expect, but whatever? If your tach is right, I'd say try a 21' pitch SS prop, but here's the dilemma, I know the Merc props are better then most but, there are probably even better designs out there, that I'm not familiar with? Maybe one of our resident prop guru's like Walleyed, or Dhadley, will chime in.

Click to expand...

These are all GPS speeds. You think that I am getting what I should out of this rig? When I look at some similar (but probably lighter boats) rigs on this site they seem to be running low to mid 50's....
As far as props, do you think a four blade Stilletto 20P or 22P might bring up the speed by giving better bite (lower prop slip)?

On another issue, if you do change props, and you're not using Mobil one synthetic, Do it, I've seen the Best Gas Mileage, & Power increase in all my vehicles ever since I began using it, over any other oils, or synthetic oils. Though I haven't tried Amsoil yet? You'll probaly see an RPM increase, let me know.

Click to expand...

This is interesting, the boat came with Redline Racing Synthetic oil from the previous owner when it ran the fastest. The boat yard just used god knows what in there. What weight Oil would you suggest? BTW, in my experience (I have raced autocross/track days in various cars for 9 years) the car gurus don't like Mobil One anymore...I guess there was a formulation change to a cheaper base stock about a year or so ago. And the price went up too! Amsoil and Redline and Royal Purple are the new hot oils to use at least on the car front...But if Mobil One has worked so well for you I just might give it a shot!

Next, when hot/engine idling, adjust your idle mixture screws out to reach the highest idle speed you can, this will be where your engine runs best! The EPA may not like me though

Click to expand...

Ahh, not going to worry about the EPA-they will have to catch me at 50+ hahh

Thanks for the Help!!!

I need a Prop Guru!!! Anyone???

Re: Chapparal SS Prop uprgrade question-350 Mag

I suspect a bad tach reading. Those numbers would indicate 29% slip which is bad high. I would connect a remote tach just to confirm RPMs and go from there.
I just tried the Solas HR Titan 4 on my 4.3 and I'm pretty impressed. Was running a 3 blade aluminum turning 4600 @ 50 MPH (GPS). With the 4 blade 14 X 21" hole shot was better and turned 4400 @ 52 MPH. This was with only me in the boat , fully trimmed ( I weigh 155). Reordered the same prop in a 14.125 X 19 because I normally have 4-5 people in the boat and pull skiers. This should get me in RPM range with boat loaded and still should top out around 50mph. Hole shot should be awesome. Got the 19" today but wont get a chance to try it for a few days. I do recommend this prop. I think you would like it but you need to nail down acurate readings so you'll know what size to get. Cheaper than the stilleto too.

Re: Chapparal SS Prop uprgrade question-350 Mag

Bob, as said, confirm the tach. If it's even close to correct, yes a change to a 21 pitch prop will lower your RPMs and increase your speed.
However, my sister has a 90 2350SX cuddy with a 383MAG MPI Bravo 1 and it isn't any faster than what you have. Plain and simple, those are big heavy 23' boats.

Re: Chapparal SS Prop uprgrade question-350 Mag

TilliamWe said:

Bob, as said, confirm the tach. If it's even close to correct, yes a change to a 21 pitch prop will lower your RPMs and increase your speed.
However, my sister has a 90 2350SX cuddy with a 383MAG MPI Bravo 1 and it isn't any faster than what you have. Plain and simple, those are big heavy 23' boats.

Click to expand...

I will pick up a tach and run in on Thursday to confirm...If correct or even close I think I will try a prop with more pitch. The question then becomes 3 or 4 blade and what brand/pitch????

My boat is an '88 2350SX cuddy! What is funny is none of the prop calculators take weight into consideration in their calculations?? Do you know what prop she is running on her boat? I am surprised th 383 MPI is not faster than my setup but I know there are alot of factors at work here...

Thanks,
Bob

Re: Chapparal SS Prop uprgrade question-350 Mag

I believe that her prop is a 19pitch. I know it's a three blade stainless. The 383MAG MPI was a replacement for a broken 454 MAG.

If top speed is all you care about, you'll want to stay with a three blade for now.