fuel used for same load, bigger engine

[simonl]

If I am proposing a 150hp vs a 200hp for the same boat, what difference would you expect in economy at the same speeds?
I'm thinking that the 200 is likely to use a bit more at low speeds, but then start to become the winner as the load approaches 150Hp.
Clearly if the 200 is heavier then it'll have an impact.
My thoughts aren't backed up by any data though. Any thoughts/experience?

[Last Tango]

Your 5mtr boat will be doing well over 50mph by the time the engine's producing 150hp.
If you ski (I see it's a ski boat you have) at around 30mph then it seems kind of pointless putting up with the fuel, maintenance, weight and buying cost of an extra 50hp you'll probably never use.

[simonl]

Originally Posted by Last Tango

Your 5mtr boat will be doing well over 50mph by the time the engine's producing 150hp.
If you ski (I see it's a ski boat you have) at around 30mph then it seems kind of pointless putting up with the fuel, maintenance, weight and buying cost of an extra 50hp you'll probably never use.

All very valid points. I should have added that this is for a different boat, sorry.

[Last Tango]

Originally Posted by simonl

All very valid points. I should have added that this is for a different boat, sorry.

Ah.... In that case it kind of depends on the boat and what you're going to use it for.

No doubt there'll be responses advocating "the biggest the transom can take" and this definitely use to be the case, however, that formula can now produce some totally impractical, dangerous and ludicrously expensive solutions.

If the 150hp is going to be "working hard" most of the time then regardless of fuel consumption I'd go for the bigger engine. It's likely if a 150hp is having to run 4500 - 5000 rpm to "earn it's keep" the 200hp won't be any more expensive on fuel but you'll probably have to go into this in quite a lot of detail to get actual figures. Info on what you're thinking of and it's application might help if there's folk here with something similar.

[HUMBER P4VWL]

Also if a 150 and 200 is the same block, same make, whilst it may peak at 200, it’s probably just a different cam or ecu or injectors etc, therefore it’ll be down on something (torque) at different rev ranges. So what will you use it for?.

If it’s a different block, bigger cc, it will “probably” be heavier. And then it’s v complicated. We assume they’re both a 2S or 4S.

[jonp]

Having nmea connected to my Lowrance hds I'm able to constantly keep tabs on my fuel burn and how simply having two extra people onboard makes quite a difference. Also I need to keep my engine at around 4700-5000 rpm to sit at the hulls most efficient cruise. My engine is just a Yam f70, compared with the Suzuki f90 leanburn I sold before this outfit the bigger engine was actually more economical because I could sit on 3800-4200 rpm to maintain the same speed.

A friend recently repowered his boat weighing double my outfit, he also more than doubled the power to mercurys latest 4 stroke. With that weight and power difference his fuel burn at my cruise speed of 23knots is much the same as my f70. He sits on 3500 rpm but also has heaps of power in reserve. Unfortunately my boat came fitted with the 70 and isn't worth upgrading for the small benefit.

Things to cost in are the bigger engines cost far more to buy and often more to service to, this takes thousands of hours to recover in fuel savings.

[Pikey Dave]

If going down the 4s Lean Burn route, the Lean Burn is most economical at around 4500rpm, so this needs to be your target RPM at cruising speed. An underpowered boat maybe over revving at this speed, conversely an over powered boat may not reach the ideal rev range at cruise. Just thought I’d chuck that into the mix to complicate things[emoji6]