Props and fuel

[gcapehart]

Gas is $3.20/gal. and "rising" as Johny Cash would sing. I have seen this expressed before, "I don't care about top-speed, nor holeshot". But I do care about cruising efficiency! I have a 1982 25 XF w/225 VRO. I have two props, one SS 15x16 and one  aluminum  15x15, both three blades. Both are OMC. I have read that four blades give better mileage at cruise. At this site it has been said that AS hulls are most efficient at 25mph. Recently, on the Penobscot with spring flood running out I hit 35.8 knots at 5200rpm. Returning with the cans still bent to seaward 45 degrees I just made 30.1 at again 5200rpm. These runs were on the SS prop.  Would four blades work say at 3600-4000 rpm and does anyone have experience with such? Do I reduce diameter with four blades and pitch? I can go up to 15 1/2 diameter. Do I then increase or reduce pitch? Do cupped blades factor in here? My OMC manual recommends a 15x17 ss double cupped(?). We'll run out to 24-30 miles in good weather fishing for cod, halibut, haddock, and may try tuna. We also do stripers and blues and then others when we head south to the Cape. Thanks for any help. Gary and Suzy

[John Jones]

I wish I knew.  Props are a science.  Complicating the issue is the fact that it's hard to find someone that will take back or swap a $350+ prop if it's not the right one.  We are luck to have a company called Prop Gods down here.  They will work with you until it's right.  I hear that Overtons will swap props but I would check with them before buying.

The few things I can tell you:
1" pitch change will vary RPM about 200.
Cup gives more bite.
4-blade give better hole shot and more stern lift.
The starting point is having a prop that at wide open throttle with your usual load will allow your motor to max out right at redline on the tach.  After that you play with diameter, pitch, cup, rake, etc. to find the best holeshot, efficiency, etc.

There is a marine surveyor on the FL Sportsman forum that is great with props.  I have emailed him a couple of times and he was very helpful.
Give him a try.  He goes by "Dunk"  dunk79@comcast.net  Give him all the details that you can.

[GoneFission]

Props are a science and an art.  In general, you will find a 4 blade prop will be a little less efficient at cruising, but will get the boat on a plane quicker due to more blade area.  Blade area helps with torque, but also causes drag.  This is why really "go fast" props - think racing boats here - usually are only 2 blades.  Race boats usually don't care about hole shot...

3 blade props are a nice blend of efficiency and power.  You want to typically prop the boat to hit top speed at close to the upper operating limit of the motor.  This will also usually give a good cruise economy at a lower throttle setting.  To optimize, you can run speed vs. fuel flow if you have the equipment, but it's also a general summary that AS boats seem to like about 25 MPH.  This should be well below top speed for your boat and top RPM for your motor if it's propped right.  

If you want to tweak a bit more ecomony (at the expense of hole shot and time-to-plane) you can "over-prop" the boat a bit.  Let's say you get 40 MPH at 5500 RPM with a 17 pitch prop.  That would be pretty well propped, assuming your engine has about a 5500 RPM upper recommended operating RPM.  Going to a 19 pitch prop would probably give more speed, but you may not get over 5000-5200 RPM, and your hole shot and time-to-plane would be longer.  But - you would probably find a 10% improvement in fuel economy at 25 MPH due to the engine running a lower RPM in a low-load condition.    

Another consideration is cost/benefit.  We all HATE $3.00+ per gallon fuel, but let's say you get the mileage my CCP gets at cruise - 3 mi/gal at optimim cruise, or 8.3 gallons per hour at 25 MPH.  A 10% improvement is .3 miles per gallon - or 7.6 gallons per hour at 25 MPH cruise.  That's .7 gallons per hour, at $3.00 per gallon, only $2.10 per hour.  It would take you 150 hours to make up the cost of a new $300 propeller.  Most folks don't run much more than that in a season.  

Here's where the REAL EASY gas saving is - trim!  Trimming the motor out to just before caviatation can get you an easy 10-20% increase in fuel economy - and it's free (except for some ride suffering in choppy water).  So try to remember to trim that baby out when you want to travel a distance and save some $$$ !!!    

Hope this helps!  See ya on the water...