Just wondering what the generally consensus would be on how much of a horsepower increase would be needed to be able to turn an additional 100 RPM? Have some room left before the rev limiter that needs to be "used".

Thanks

sounds like you might need to reprop, by room what do you mean......

you dont want to get so close to the limiter that your hiting it all the time

Props are right pitch however need to have then "labbed". I know what to expect from that. Been there before. Still after that will have a little RPM room to work with. Yes, I agree I don't want to be all over the limiter just closer. Thus my question.

then best and cheapest would be to get the most out of what you haveie reprop


what rpms are you pushing at wot with what mrts

Missing my point. Lets say you want to turn a given engine - prop combo 100 RPM more. You want to achieve this with horsepower alone and nothing else. How much horsepower would be needed?

depends on boat type,prop,drive.... there is no easy answer easyer to ask boat xxx does xmph now how much hp do I need to go z mph

rpm is a none fixed variable because there are too many things to take into concideration if you want to change rpm from what you have now its easy to convert to prop size and pitch though

Depends on drag...i.e. the boat.

100rpm for a 21' Superboat takes much less power than 100rpm for a 29' Scarab.

So, what engine and hull are you thinking about?

thank you,

also drive gears, weight,single or twins ...ect

Without going into too much detail, you have two forces working against each other - thrust and drag.

Thrust is a function of (but not limited to) engine rpm, engine torque, drive efficiency, gearing, prop pitch, prop efficiency, etc.

Drag is a function of (but not limited to) weight, hull shape, water surface (from glass to huge waves), outdrive shape, outdrive depth in water (x-dimension), prop, etc.

Just picture the Thrust is trying to keep the boat moving while the drag is trying to slow it down. If they are balanced you are holding your speed. If there is more thrust, then the boat accelerates (F=ma).

So, wayyyy to many variables to answer your question. Again, if you have a specific boat or engine in mind then you can get a ball park number.

drag per mph is a cube of the speed. ie.... increase in mph/rpm vs drag is directly dependent on speed you are goinging when you NEED the next 100 revs.

as exagerated example for clarity.... from 5 mph the next 100 revs might only need 10 hp to overcome whatever the drag is...

from 150 mph you might need 300 hp to overcome the drag that exists at that speed to get the next 100 revs... point is...not linear.

and from experience will suggest that hitting top speeds w/in 100 revs of the limiter is about as good as you can get... would suggest that tachs not accurate w/in 50 -75 revs either way... and as stated earlier, you don't want to be on the limiter very often..even soft limiters aren't good for the hardware.