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Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Tue Oct 09, 2018 3:01 pm
by Joe
Hello ,Why did you take 16" 15 pitch Off ? Was their a problem? 16" 15 pitch cupped is the correct size.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Sun Oct 21, 2018 9:31 pm
by HighlandGent
UPDATE!! - I wasn’t crazy!

Here is an email from the service advisor at the Marina in response to my concerns about the engine speeds with the new props.

Hi David: As per your conversation with Slim: he said that the new props are fine for now. The RPMs are at 4,900: but DON'T run the boat that high, keep the rpms lower for now. With the higher rpms, it is going to loose speed, that is expected. Use it this way for now--until you haul out for the winter--and then the props need to be sent out to be 'cupped'. That will put the rpms within specs. This will be exactly as it should be...……………..Sandy

So, fortunately I only have one more weekend available this year to take her out. I’ll go easy and when we get in the water next year it’ll be perfect. Thanks you all again for your input.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 22, 2018 7:34 am
by carl
Propeller sizing is a skill, somewhere between a mathematician and an art form. Cupping your tiny 12" props are not going to get you perfect. You need to stick with the original equipment. While you have your original 16 x 15 props off consider sending them out to be serviced, typically a good prop shop will repair any damage such as nicks and bends, they will true up the blades, balance and polish them. This way they will be ready to install for next season.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 22, 2018 11:08 am
by EWRice
I have to agree with carl. Cupping the props will only reduce slippage, not drastically change rpm. I also think that you should be running faster than 4900rpm with the props you have on there. I wonder if your valves are starting to float (chryslers have very weak valve springs in their industrial engines, which the marine engines are considered) and limiting your rpm before the props can. Be careful running with those props as engine damage could be quick.

If you want a smaller prop, I re-propped a 32 sedan with 4 blade Acme CNC props and had great results. I think we dropped down to a 14".

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Mon Oct 22, 2018 11:57 am
by jralbert
I will hold back using the same adjective as above, but will enthusiastically agree the advice you're receiving isn't sound. I am not an expert by any means in boat propellers but have examined the question for years as an owner of an M dating back to the 1980's and now a casual observer as a former owner. Nowhere in the literature is there the slightest mention of 12" props, cupped or otherwise, as a unit for your boat. Sixteen, fifteen and (rarely) 14 - ok. But never 12.
Obviously, it's your call and your boat so do what makes good sense for you. My two cents is probably worth just that.
An idea is to call an established prop specialist company and ask about their recommendation. Some used to provide an online form to submit for a prop plan.

Re: Prop size and operating RPM for 318 Chrysler engines

Posted: Tue Oct 23, 2018 7:40 am
by Fastjeff
Must disagree a bit here.

On my old boat, at one time. I had a 16 x 16 on one side and a 16 x 15 on the other. I had taken the 16 x 15 to a prop shop to have it repitched to a 16, but the guy said he couldn't do. He added a lot of cup (using a hammer and a wooden post), and it worked great! RPM and synch were right on.

The bottom line: A 15 heavy cup equals a 16 normal cup.

Jeff