Proposed Prop Work

[msflynn03]

Would like to hear opinions on the following.

Have a 1997 Sea Ray 370 w/ twin 7.4s (new to me this year). Have spent some time getting it in shape and have been making some progress. Have had one continuing issue though which relates to the boat's speed at WOT and the RPMs at WOT.

At survey, the boat was moving (via GPS) at 22-23 knots at WOT and the RPMs were about 3400-3500. Both engines have new plugs, wires, distributor caps and rotors. Compression in both engines was OK (155-165) . Ignition timing was on spec and the distributor advance is functioning correctly. The specs on the boat call for RPMs more in the range of 4000-4400 and speed of about 30 knots. Bottom was relatively clean at survey. Since that time, speed and RPMs have continued to decrease. I believed that this recent reduction in speed was due to growth on the bottom, props so I had the boat pulled for inspection. I also expected to find that the boat was overpropped. (The original props were 3 bladed 18x22 props and I expected to find something other than this on the boat.)

On inspection, the bottom was very dirty with a lot of growth as were the props. However, the props were not oversized as I had expected but were 4 bladed 18x22 props. The hull has now been cleaned and the props sent out for cleaning/overhaul. The marina recommends that the prop angles be modified to 18x18 from the current 18x22 as this (with the cleaning of the hull) will allow the engines to rev within the desired range.

Would like to know what the members think. Would like to know what changes occur when going from 3- to 4- bladed props of the same size. Are the dirty props and hull really the issue or is there something else I should be looking at. Hate to make the changes, put it back in the water and find that my speed is still down and the only real change is that the boat's engines are reving higher.

Thanks in advance for any advice. The group has gotten me this far with their great advice.

 

[Warwgn]

I have seen it said that you should drop your pitch by 2 when going from 3 to 4 blade, so that may be about right. I am trying out my new prop this weekend went from 3 blade 14.5x19 to a 4 blade 14.5x17, will let you know the difference.

 

[bahamabreisus]

With this warm water in the Carolinas, we have a diver every month clean the bottom. It does not take much to slow you down. Hope the props help.

 

[fwebster]

I wouldn't repitch the props until you run the boat with a clean bottom and running gear. You may have a do-over on your hands if you do.

Somebody has already done a prop/rpm analysis on your boat. It came from the factory with 3 bladed props so I suspect somebody had already gone thru the rpm analysis. Typically, a 4 bladed prop will have a little less pitch than a 3 bladed one since you are theoretically moving 25% more water with each revolution. It doesn't quite work that way due to slippage . The 4 bladed prop with a little less pitch may have a little more cup which nets you much better handling around the dock, but not a lot more performance at cruise and higher.

So, I think you are on the right track, but I would eliminate one problem at a time before changing the props. A dirty bottom covered with slime and hard growth could easily cost you 750-1000 rpm.

 

[dpmulvey]

Take Frank's recommendation. If all else fails, Call Wildcat Propeller in Chesapeake, VA. They are the go to guys for the USN and the USCG for prop analysis and rebuild/tuning. They mat already have the specs on "your" boat.

 

[Warwgn]

Ok so as i said i went 19 pitch 3 blade to 17 pitch 4 blade, gained 100 rpm and lost 3 mph. In my case not happy with that at all. Think I should have stayed 19 pitch or went up to 21 pitch, i have a smaller boat and different gears in my drive not to mention a 3.0 so take that for what its worth. But i did gain rpm at the sacrifice of speed though.