Looking for advice on prop for 21' Chris Craft Bowrider

[dlk4602]

I am looking to see if I can get a little better fuel economy without sacrificing too much performance. From what I have read, switching from a 3 bladed prop to 4 bladed prop may do this.

The boat is a 1998 21 foot Chris Craft bowrider approximately 3500 lbs. with Volvo 5.7 GSI with an SX single prop 1.43 ratio drive. Currently I have 14.5X19 aluminum prop. Using a GPS and an external tach, WOT is ~47 MPH at 4700 RPM (the dash tach shows 3800 RPM???)

I am OK with losing a few MPH on the top end if I can get better economy. From what I understand, a 4 bladed prop will keep the boat on plan at lower RPM giving better economy.

Would a 4-bladed prop get me what I am looking for? What size/brand should I looking at?

Other suggestions?

Thanks, Dave.

 

[steelespike]

If your serious about fuel economy you should get a flow meter. There are meters that can do real time mpg,gph calculations.
It is said that a 4 blade prop can help a boat stay on plane "easier" at reduced throttle.
" Easier" may just mean that the boat is less fussy trying to stay on plane saving a tiny bit of fuel.
Of course there will probably be an ideal speed/rpm that will show a savings.
Keep in mind the your ideal cruising speed may be at just on plane or up to as much as 800 or 1000 rpm higher.
A 260 hp motor will use at least 30 gph at wot.
I think you should get a flow meter and establish your present numbers.
Keep in mind that if you select a prop that loses some top end it may lose some speed at other rpm.
If it takes too long to get from here to there it may also takes more fuel.
A 4 blade will very likely lose some top end but there will likely be other rpm that is more productive.
For instance the 4 blade may get on plane quicker saving a tiny bit of fuel.

 

[Tail_Gunner]

SS or alum
Do you want to prop for the weight or load you took those readings's from

 

[dlk4602]

I plan on staying with aluminum. The readings were taken with only me in the boat. Typically, we would have 2-4 people in the boat.

Right now I only have the one prop and I would like to have a spare. So I am trying to decide if I should get the same size and pitch I currently have or if I can get something that might be a better fit. Maybe the prop I currently have is the best fit for the boat.

Dave.

 

[Tail_Gunner]

Go with a 17 pitch solas 4 blade of course. You will more than likely gain speed and everything in between. Your currently over propped that engine peak is 5000 and that's not pushing the internals at all. But the cam and valve train run out steam at 5000. Good luck and report back if you will, the actual tach numbers make it a easy pick.

 

[Tail_Gunner]

Lost the edit function... again everything spike says is true and in a boat there is little one can do other than a good tune on the engine and clean hull bottom. Four blades are smoother and do have lower planing speed's you have more metal in the water and a more effective push. I believe your engine will be most effective between 3500/3800 rpm. A quick and cheap way to determine that is watch your rpm and speed aka 3000 rpm at 35 mph etc or it might be 3500 rpm at 40 mph.SImply said best econ occurs at the lowest rpm and highest speed.

http://boatpropellers.iboats.com/Vol...t_id=624796648 I believe the hub is included in that price you might want to call them if you go in that direction.

So 1.47@4700@19p gets 47mph that's 20% slip and bad
1.47@ 5000@17p gets 50 mph 10% slip where it should be give or take a few points.