1971 Trojan F31 Hardtop - Twin 1971 Chrysler 318's

[Seafarer]

Hello to all,

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

[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.

[Seafarer]

So they published the wrong data.
https://www.mopar1.us/engines.html
Ok. I'll recalculate and see how it plays out.
Thanks

[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.

[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.