prop help ...please

Can anyone who knows about props and pitch help me? I currently have a three blade prop with 19 pitch. I am concidering a similar prop with 15 pitch as i think this would make the boat accelerate faster and maintain cruising speed somewhat cheaper/easier.. Am i wrong??

Going from 19 down to 15 is a radical change.

In general, going to a lower pitch prop is like down shifting a car. First gear will get you going but it isn't what you want for the freeway.

If you assume no change in prop efficiency (which is not a good bet) then for a given speed your RPM will change inversly with prop pitch. So, let's say that you currently get an engine RPM of 4000 RPM at your desired cruise speed. If you change to a 15 inch pitch prop your RPM will change by 19/15, which will be 5066 RPM.

I don't know about your area (are you really in Norway?) but around my home town there are boat shops that have "loaner" props for exactly this purpose. Put a deposit down and you can take a few props with you and try them out. It only takes a few minutes to change a prop on an I/O so you can try a few out and decide which works best for you.

Need to know boat, motor, usage, WOT rpm, cruising rpm (or towing), and what you really want to achieve.

- yeah Rod, im really in Norway.. thanx
The boat is a 21 ft Regal cuddy with a 4.3 merc 175 hp. It does about 35 knots max however i usually run at about 20-25 knots. For wakeboarding we ride at appx 18 knots. At towing speed my engine does just around 3000 rpm.
The issue however is whether or not a prop change will make pulling wakeboarders easier on the motor and concequently lower the fuel consumption. Also i'd like to achieve towing speed faster when i pull from zero. We only ride on "open sea" over here.

Bob, In my old boat I had the 4.3 merc/alpha combo and I originally had a 19 pitch prop and i went to a four blade 18 pitch which made all the difference for me. I actually used a prop called piranha props from the local boat dealer and it was a composite plastic that allowed you to change blades in a few minutes by taking the hub apart. If I hit something I could swap it out in minutes in shallow water and be back boarding or I could change to a different pitch in minutes as well. You would have to carry the extra blades which were not to expensive if I recall. Do an internet search and check them out.

thx for the tip wakeboardin. Apart from the blade swapping thing- what differences did it make?

It held speed way better (wife was my driver) helped plan faster with weight. I did loose top end speed but really didn't care as we primarily boarded. As for fuel consumption I really never noticed a change that boat always ran forever on a tank of gas. Let me know if you have any other questions.

thats kinda what i was expecting. although i am concerned going from a 19p all the way to a 15p might be too much.. Don't think there's anyone on this side of the atlantic that'll let me test drive new props...

Bob, go with this prop. It's made close to you, so it should be available, and you can change to different pitches with it. As said earlier, it is made out of a composite plastic, but with this prop there is no need to buy extra blades for you to change your pitch. It is adjustable from a 15 to 21 pitch.

http://www.propulse.se

thanx mike. it seems these props are actually available at my local parts and accessories dealer.
Anyone know the effect of going from a 15 1/4" to a 14 1/4" by the way? Seems most replacement props have a smaller diameter than the one i use now.

Nice to see fellow snowboarders in here too

I've been riding since about 1986-87 but have had to cut back from competitive riding as i blew both knees on a 40ft cliff back in 97... Somehow wakeboarding helps the cravings..

This is actually pretty easy to figure out.

The props pitch is the number of inches the prop will move forward in one revolution. Your 15.25" pitch prop will push the boat forward almost 15.25" each revolution.

Boat speed = prop pitch X revs per minute / gear ratio X slip / 12 (inches per foot) X 60 (minutes per hour) / 5280 (feet per mile)

My boat:
35mph = 16" X 4000RPM / 1.46 X 0.85 / 12 X 60 / 5280

Your boat:
Assumption 1, gear ratio = 1.5
Assumption 2, slip = 20% (0.80 multiplier)

running the numbers with these assumptions:
19" prop -> 23mph @ 2400RPM
15" prop -> 23mph @ 3000RPM

Boats reach maximum speed roughly when horsepower equals drag. This means that your motor will spin much higher RPMs when you get close to maximum speed potentially overrevving your motor risking throwing a piston through the head.

Assuming that your boat's previous top speed was 50mph at 5500RPM your new max rpm could be 6500RPM in theory. You should check with your engine manufacturer to see what the maximum RPM is and check if it has a rev-limiter.

(Message edited by mikeski on May 13, 2006)