Re-prop question...

[soggy_feet]

As far as I know, when propping a boat, you pair it to the max working rpm of the engine.

Lets say you never run up to max working RPM?

I have a 40 ft houseboat, if I run it wide open (a little under 4000 RPM), I can hit the neck breaking speed of 8.5 knots, and empty the gas tank in no time.

More often, Im running it at about 2000 RPM. Any faster and I start building up a bow wave that I can never break thru. Im interested in holding that speed, but with just a little less revs out of the engine to see if I can extend my fuel range.

Shouldn't be a problem, right?

 

[dazk14]

Re: Re-prop question...

What engine/drive are you running. What's your recommended WOT range.

 

[180shabah]

Re: Re-prop question...

You prop to the top end of the WOT range so your engine will still like you in the morning.

I don't know what engine you have in that thing, but if WOT is under 4k, you are already overpropped and lugging the engine at ALL rpms.

Lowering cruise RPMs does not give a corresponding drop in fuel consumption. This is not a car with a very light cruise load.

Overpropping also tends to make the boat less responsive at docking speeds, inconvenient on an 18' boat - dangerous on a 40'er

 

[soggy_feet]

Re: Re-prop question...

Its a 165hp mercruiser straight 6, early 70's, couldn't tell you exactly what the outdrive is, there are no stickers, stampings or what have you left on it.

Think the engine was supposed to originally run up to 4200 RPM. I'm sort of short on documentation with this thing.

 

[soggy_feet]

Re: Re-prop question...

Overpropping also tends to make the boat less responsive at docking speeds, inconvenient on an 18' boat - dangerous on a 40'er

Good to know, thanks.

 

[180shabah]

Re: Re-prop question...

A little research and I find that your I6 WOT range is 3900-4300RPM - lower than I thought it would be. A little clarificaion on "I run it wide open (a little under 4000 RPM)" and we'll know where you stand. You might be at the bottom of your WOT, you might be over propped. Either way, any changes in pitch should be lower NOT higher.

If oyu are serious about squeezing every drop of fue you can, a flowscan is the only way to know what you are actually using. I don't have any experience with boats that large, but I will tell you most guys in smaller faster craft are usually surprised about their most economical cruise. It is frequently several hundred RPM higher than expected.

 

[steelespike]

Re: Re-prop question...

House boats produce a lot of prop slip.A 4 blade should help reduce slip and should improve handling at the dock.Would be nice to be able reach closer to that 4,300 figure if you can be more specific about your present max rpm we may be able to suggest a prop size.As suggested your best cruising rpm might surprise you.I would think a gps would help. A 165 will use about 16 gph(gallons per hour)@ wot(wide open throttle) Whether pushing a house boat or a runabout.You need to find the least gph at the most speed.You could top up run say 15 or 30 minutes out and 15 or 30 minutes back and top up. or if you have a favorite destination do out and back
at a logical rpm.Otherwise a flow meter and gps is the best solution to watch gph.