3 Blade vs. 4 Blade test results

I asked on this board and a few others what the performance differences woudld be with the two different props. Others have chimed in and gave some guidance. Without boring you with all the details I will post the basics below. Yes, I am well aware we are supposed to use knots intead of mph on the water (and the air), just trying to follow Yamaha and Grady's performance reports.

2006 Grady Marlin almost every factory option (less generator), bottom paint, curtains, 24' outriggers, 100 gallons of fuel, 15 gallons water, four adults, about 200lbs in misc gear (tools, flares, etc). Twin counter rotating 2006 Yamaha F250 outboards.

With freshly 'tuned' Yamaha 15 1/4 x 19" Saltwater Series II props I got the following:
At 4000 rpm I burned 16.8 gallons per hour at 25.3 mph netting 1.51 mpg (this is the best I could get at any rpm above 2000rpm/8.9mph)
At 6000 rpm (wot) I burned 44.5 gph at 45.4 mph netting 1.02 mpg
From 4000 to 4600 I was able to get at least 1.4mpg (speed was 25.3 to 33.7 mph)


With new Powertech 15 x 19" OFS stainless four blade props I go the following (equal load)
At 3500 rpm I burned 15.1 gallons per hour at 25.4 mph netting 1.68 mpg (this is the best I could get at any rpm above 1500/8.3mph)
At 5700 rpm (wot) I burned 44.5 gph at 45.3 mph netting 1.02 mpg
From 3500 to 4600 I was able to get at least 1.4 mpg (speed was 25.4 to 36.3 mph)

On a recent canyon run (with the three blade props) we burned a total of 195 gallons, this included a rough ride out into 4' seas and many hours of trolling. With the four blade props on the same trip I am expecting to burn about 175 gallons. In the ranges we typically cruise at we are doing about ten percent better on fuel burn. If anyone wants my fuel burn charts (MS word) as a reference just shoot me a pm or email.

Holeshot with the three blades was pretty good, now it is better.

I have only been in 0-3' seas with the new four blade props, so far so good. I have been told a lot of people switch to four blade props becuase their three blades blow out easily. I can honestly say mine did not blow out with either prop on this boat. This may be because the outboards are bolted through the second lowest hole (meaning they could be raised three holes or so).

My main objective was to decrease my fuel usage while cruising/running offshore, I am very happy so far. My second goal was to increase the speed I could run and still get good economy in relatively calm 1-3' seas, again so far so good.

Hope this helps, I will report again when I get a few more hours on these props.

Guys, boat is kept in water till the end of November, so raising engines is not feaseable at this time. I am leaning to raising them one hole over the winter while the boat is on land. Maybe I could go two holes, but I don't want to push it.

Yes, I can trim the engines higher without blow-out with these four blade props. But like I mentioned, the Saltwater Series II rarely if ever blew out when they were on the boat. As a matter of fact, I have used Saltwater Series I, Michigan Rapture and the black Yamaha stainless on my old boats (24' Grady Offshore and 24' Grady Explorer) and they blew out often. I suspect between the twin engines grabbing twice as much water, the hull design on the 06 Marlin and good props it makes a big difference with regard to staying hooked up.

Hope this helps.

With regard to the trim tabs, they work a little different with the four blades. Before I purchased the new four blade props, I was informed that these four blades raise the stern of the boat. Most props I have owned and read about are designed to raise the bow. I have not used the boat in weeks (high winds, rough seas, hurricanes, and busy with work), so I cannot comment with more details.