Prop size and operating RPM for 318 Chrysler engines

HighlandGent
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Prop size and operating RPM for 318 Chrysler engines

Postby HighlandGent » Sun Oct 07, 2018 4:47 pm

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
Highland Lady
1988 32’ Sedan FB
Essex, MD on the Chesapeake Bay
Twin Chrysler 318’s (rebuilt this winter)
Westerbeke 6.5kw Genset
Used as a houseboat for 16 years.
Resurrected in 2016!

liquidplummer
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby liquidplummer » Sun Oct 07, 2018 7:12 pm

What was your speed and rpm with the old props
Silver Queen St Louis Mo
1982 37 ft flybridge sedan twin 360 chryslers 1.91 velvet drives

jralbert
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby jralbert » Sun Oct 07, 2018 9:31 pm

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.
-joel-
former owner 1988 '32 FB Sedan
Chesapeake Bay
twin 318 / 240 hp
Potomac MD

HighlandGent
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Location: Brunswick, MD

Re: Prop size and operating RPM for 318 Chrysler engines

Postby HighlandGent » Mon Oct 08, 2018 3:44 am

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.
Highland Lady
1988 32’ Sedan FB
Essex, MD on the Chesapeake Bay
Twin Chrysler 318’s (rebuilt this winter)
Westerbeke 6.5kw Genset
Used as a houseboat for 16 years.
Resurrected in 2016!

Fastjeff
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby Fastjeff » Mon Oct 08, 2018 7:30 am

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!
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington

jralbert
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby jralbert » Mon Oct 08, 2018 12:10 pm

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.
-joel-
former owner 1988 '32 FB Sedan
Chesapeake Bay
twin 318 / 240 hp
Potomac MD

EWRice
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby EWRice » Mon Oct 08, 2018 12:16 pm

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).
Muskegon Lake
1972 32' Express flybridge
"AL13"
Twin 318s
On board air & prime 920
1963 Thompson Super Sea Lancer
Graymarine 327
1961 Alumacraft 12'
'55 10hp Johnson

Joe
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Re: Prop size and operating RPM for 318 Chrysler engines

Postby Joe » Mon Oct 08, 2018 3:35 pm

16" 15 pitch cupped.Is the right prop. 1988 32 sdfb 318s
1988 sedan flybridge. Black / White. 318 240 hp Aluminum aft deck hard top. Louisville Ky . Tartans landing. " KNOCKEFELLERS"

HighlandGent
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Location: Brunswick, MD

Re: Prop size and operating RPM for 318 Chrysler engines

Postby HighlandGent » Tue Oct 09, 2018 3:51 am

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!!
Highland Lady
1988 32’ Sedan FB
Essex, MD on the Chesapeake Bay
Twin Chrysler 318’s (rebuilt this winter)
Westerbeke 6.5kw Genset
Used as a houseboat for 16 years.
Resurrected in 2016!

Fastjeff
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Posts: 1027
Joined: Sat Jul 12, 2014 5:06 am
Location: Rock Halll, MD
Contact:

Re: Prop size and operating RPM for 318 Chrysler engines

Postby Fastjeff » Tue Oct 09, 2018 7:41 am

Good. DON'T run it long with that set up.

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington


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