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Prop size and operating RPM for 318 Chrysler engines

Posted: Sun Oct 07, 2018 4:47 pm
by HighlandGent
Any input on what the operating RPM should be on my 318’s? It was suggested to me to go from a 16in prop with a 15 pitch to a 12 inch prop/15 pitch. Now I lost a ton of top speed and my RPM’s are topping out over 5,000. This does not feel right to me. What is everyone else running?

1988 32 FB Sedan

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Sun Oct 07, 2018 7:12 pm
by liquidplummer
What was your speed and rpm with the old props

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Sun Oct 07, 2018 9:31 pm
by jralbert
I recall that the book said 4,000-4,400 rpms with 16x15 cupped props which is what my 32 FBS had. I topped at around 4K. I am not going to pretend to be a prop/torque gearhead guy but I think you are carrying the wrong props now. 5K rpms (again based on my recollection) is way too high and could cause engine damage. I hate even thinking or writing this because it's probably not my place to say it but I'll will step in it anyway and suggest, with apologies, you drop that source of advice.
Wishing you well as you get this ironed out.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 08, 2018 3:44 am
by HighlandGent
liquidplummer wrote:What was your speed and rpm with the old props



I was getting 22-25 knots at about 3,200rpm before without a load of passengers. Yesterday I was pushing almost 5,000rpm with just one other person and we were barely touching 20knots. Maybe there is something I was missing. I was told that running such large props has been found to cause long term engine damage and that reducing the size and raising the rpm at top speed was a good idea but I feel like we missed the mark somewhere.

I’m trying to use one of those prop size calculators but I don’t have some of the specs I need to get an accurate result.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 08, 2018 7:30 am
by Fastjeff
Ouch! You are WAY under propped. As I recall, 16 x 14 were about normal for your set up.

Jeff

PS: I'll bet she leaps out of the water!

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 08, 2018 12:10 pm
by jralbert
You said you were hitting almost 5k rpms. Was that wide open throttle? If not, imagine how much faster the props could be spinning. A sure fire way to wreck those rebuilt engines (or any engine). The correct term Jeff used was "underpropped". Sorry, but it's very much underpropped.

I'm curious (and no reply is required, of course) and wondering what's the source of info that 16x15's cause long term engine damage.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 08, 2018 12:16 pm
by EWRice
I think somebody made a suggestion based on 1:1 gears and I am gessing you have reduction gears. Way too small of a prop. If you go down in diameter then you must increase pitch.

16x15 is the factory size for a 32 with twin 318s and 1.52:1 ratio.

14x10 for 1:1

3 blade for both.

14" is the smallest prop even listed for any BigM . 12" is what my 18ft Thompson uses.

You want to prop the boat so that you hit as close to 4400rpm at wide open throttle as you can with an average load (fuel, gear, crew, etc).

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 08, 2018 3:35 pm
by Joe
16" 15 pitch cupped.Is the right prop. 1988 32 sdfb 318s

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Tue Oct 09, 2018 3:51 am
by HighlandGent
Thank you all for the input. I think the change was in the right direction but way overshot the goal. I’m thinking a 15 or 14 inch prop would have been better. I’ll take all this back to the Marina and see how they can straighten this out.

Thanks again!!

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Tue Oct 09, 2018 7:41 am
by Fastjeff
Good. DON'T run it long with that set up.

Jeff