What trim do you have your motor when trolling. Where do you position your weight in boat. How many people do you troll with. And where do you normally troll.(do you cruise out to trolling spot for an hour each way ??) Do you have hydrofoils on your outboard leg??

Cheers mate. (PS Cant beat the 237 for comfort in the rough)

I had a Honda 225 on my old boat (2.5t) and got 7lph trolling, 20 lph at 22 knots. Pretty sure that was propped the same as RB.

Have a Honda 250 on my new boat (3t) and get 8 lph trolling 22lph at 22 knots.

Prop diameter and pitch: 17p 14 1/4"

Are they the same props.. brand?   3 or 4 blade? same cupping and/ or rake?
ie If RB has a prop with a little more cupping that will give him a little more 'grip' at lower speeds... and therefore a few rpms less maintaining a prop HP requirement between that and the hp curve of the motor at those rpms.
Sort of like this....take a car with a auto and has a 3000 torque converter  and another with a 1200 torque converter
At say 40 mph both cars require say 20hp to move up a slight slope
The 1200 torque converter car will so say 2000 rpms and no slip
The 3000 torque converter car will do say  2500 rpms and be slipping...so also requires more hp to over come that slip, and more throttle.
500 rpms doesnt sound much.... but at 2000 rpms that 25%

Also..both engine require a given amount of fuel mixture at a given rpms and load... ie the mixture amount is the volume of the engine over a given distance...therefore more rpms means more volume which means more fuel to maintain that fuel/ air ratio.

Either one of these, if start crunching actual numbers becomes quite significant in economy... put them both together and will be quite dramatically different