1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

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Seafarer
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1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

Post by Seafarer »

Hello to all, 8)

I'm new to the forum and would like to ask some questions about a boat that I'm looking to purchase, (on subject line), and the performance of the engines based on BSFC and it's relationship to RPM, Speed and GPH consumed.

I have put together a simple spreadsheet that includes RPM, HP, Torque, and calculates GPH, MPG and Range but I don't have the GPS speed for corresponding RPM's in order to solve for MPG and Range. So basically, I would like to find out if anyone has GPS speed information they can share for the following RPM's on a Trojan F31 w/ twin 318 Chryslers. It is strictly BSFC for the engines so not concerned about any other variables unless you read GPS while being towed.

RPM
700/ 1000/ 1500/ 2000/ 2500/ 3000/ 3500/ 4000/ 4020/ 4400.

Why 4020 RPM's? Well, the manufacturer has the specifics on the 1971 318's as follows: 230HP @ 4400 RPM and 320Ft-lb torque @ 2000RPM's.
As I was calculating the relationship for torque & HP published, there was a variance. My calculations produced 230HP at 4020 RPM for 300.5Ft-lb to match the manufacturers published T & HP. At 4400 RPM's the calculation produced 251.8 HP for the same torque. So since it may be that their published numbers may be eschewed, I figured I use my calculations instead. For confirmation, I will recalculate with their results once I have reached received the GPS speeds.

Any help would be greatly appreciated and yes, I am positive that there will be many questions to follow.
Thanks
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prowlersfish
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Re: 1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

Post by prowlersfish »

The 318 is rated at 225hp. Trojan info shows 4000 as top rpm. And it is propped that way. I can try and look up the specs for a new 1971 F31, but it is unlikely an old boat would come close.
Boating is good for the soul
77/78 TROJAN F36 Conv.
6BTA Cummins diesels
Life is to short for a ugly boat :D
Seafarer
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Joined: Fri Mar 29, 2024 4:17 pm

Re: 1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

Post by Seafarer »

So they published the wrong data.
https://www.mopar1.us/engines.html
Ok. I'll recalculate and see how it plays out.
Thanks
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prowlersfish
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Re: 1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

Post by prowlersfish »

You can compare an automotive engine to a marine engine with many differences. Like cams, ignition advance curves to start. starboard engine even turns in the opposite direction than the port.
Last edited by prowlersfish on Sun Mar 31, 2024 11:11 am, edited 1 time in total.
Boating is good for the soul
77/78 TROJAN F36 Conv.
6BTA Cummins diesels
Life is to short for a ugly boat :D
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prowlersfish
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Re: 1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

Post by prowlersfish »

Not 100% on what you are looking for. But you can plan on about 1 MPG on plane and maybe 2 MPG below hull speed ( 7 knots or so) While not exact it should be in the ballpark and will vary some on the exact speed, weather conditions, and of course the boat's condition.
Boating is good for the soul
77/78 TROJAN F36 Conv.
6BTA Cummins diesels
Life is to short for a ugly boat :D
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