3 blade to 4 blade conversion

I have a 2008 Maxum 1800MX with the 3.0L and a 3 blade aluminum 14x21" prop. I found a great deal on a 13.25x21 stainless 4 blade prop. What should i expect this change to do to my WOT RPM? I know if it was 3 blade to 3 blade I would expect slightly lower RPMs due to stainless props not flexing like aluminum but what's the difference going from 3 blade to 4?

Edit: I'm not trying to fix any particular performance issue but the aluminum prop does have some dings and is in need of repair and the price is right on the 4 blade stainless.

Re: 3 blade to 4 blade conversion

the 4th blade will hurt you even more. 150-200 rpm loss from the 4th blade, and another 200rpm loss from the stainless. The smaller diameter will help you gain back some rpm, but not much.

Re: 3 blade to 4 blade conversion

The deal was better than I thought because I ended up with two right hand 13.75x21 stainless 4 blade Mercury Offshore Series props with hubs for a little more than I would have paid for one hub kit. I thought it was going to be one RH and one LH so it was a pleasant surprise.

I think that the change will be good when I'm in salt water as I typically run at the very top end of the rpm range in salt water. Fresh water, it may not work out so I'll likely be looking to trade one for a 19 or 20" pitch or a 3 blade 21" pitch depending on how the RPMs are. I've got two weeks vacation in the salt to test it out so we'll see what happens.

Re: 3 blade to 4 blade conversion

rule of thumb from alluminum to ss 4blade is to drop 2" pitch. what is wot rpm right now?

Re: 3 blade to 4 blade conversion

In freshwater about 4700 and in saltwater almost 5000. The range is 45-4800 so I'm in the ballpark for saltwater but will likely have to change for freshwater.

With any luck, I will be able to do a test run this evening on the river and next week in the brine.

Re: 3 blade to 4 blade conversion

Boy were my numbers off! I did some testing in freshwater yesterday and had some very interesting results. It seems that the difference between the 3 blade aluminum and 4 blade stainless of the same pitch is almost non-existant on my boat.

I made 4 runs with the aluminum prop followed by 4 runs with the stainless prop. All were done between the same two bouys and the current was running about 3mph. Two runs upstream and two downstream for each prop.

In every case, I was able to exceed 4900rpm with top speeds within 2mph. In all but one run, the aluminum prop was faster and the stainless prop had lower slip. The stainless prop was consistently faster to plane and had a lower planing speed although I must admit that the GPS jumped from 6.xMPH to 13.xMPH on every run so the actual planing speed is inacurate.

I attached a text file that should be the output from the Excel spreadsheet that did the calculations. It contains the raw data.

Testing was done with about 3/4 tank of fuel (about 15 gallons) and 170lbs of person which is light for how the boat is used.

Re: 3 blade to 4 blade conversion

4 blade is better on gas. your doing good if this was a light load. get a coulpe people and a big cooler and youll be right on the money. if this is your normal load maybee a bit high.

Re: 3 blade to 4 blade conversion

You've got a good point about the weight. When I stated 4700 for my WOT rpm in fresh water, that's based on my normal load. I rarely go out alone and almost never run at WOT so it will probably be fine.

Re: 3 blade to 4 blade conversion

yes 4700 is great. i'd run the ss 4blade for sure for optimum performance. they are better taxiing around docks, esspecialy in reverse. better hole shot, slower plane speeds, better fuel eff. gas milage way better in mid range. the 4 will go faster than 3 at any given rpm except wot. wot they will both suck about the same fuel @ wot. pull a tube, ski or another boat better with 4 blade also. happy boating.

Re: 3 blade to 4 blade conversion

I'm going to repeat the same tests in saltwater next week and see but I'm fairly certain I will be sticking with the ss 4 blade. The only thing I might do, since I have two of the same prop and only one drive, is see if I can trade one for a 22 or 23 pitch to try it out. I'm keeping an aluminum prop as the spare because the SS is at least 4x the weight.

I can't wait to tow the kids on the tube with it. I'm sure they'll love it even more!