3 props tested

[Just for fun]

I have a '99 Chapparal 180 LE 4.3 Alpha one Gen two. The prop was damaged when I bought it so I tried 3 different props to find the right one. I used one of the prop calculation tables on one of the websites and it recommended a 17", 19", or 23" depending on use and number of people in the boat. I bought a 19" and 21" thinking the 23" would only be good for a light load.
The 19" aluminum Hustler maxed out the rpm at 4800. The 21" aluminum Hustler also maxed out the rpm at 4800. According to Mercury, the max RPM should be 4400-4800. I was looking for about 4700 to be sure i wasn't on the rev limiter. I wanted to try a 23" so I made a deal on a 23" Mercury Laser II stainess prop. After driving 70 miles to get it, it turned out to be a 25" prop. The price was good ($80), it looked like new and it came with the hub kit so I bought it anyway. I tried it today, here are the results compared to the 21" aluminum.

The 21" ran 30 mph at 3000 rpm. Max was 53 mph speedo, 51-52 GPS at 4800 rpm. Fuel tank was almost empty (3 gallons left at fillup)
The 25" ran 35 mph at 3000 rpm, max was 58 mph speedo, didn't have the GPS today, probably 56 mph at 4600 rpm. The 21 gallon gas tank was full.

I did notice it was not as quick to get on the step, but not too bad, and was somewhat surprised at the speed I was getting. I didn't know it would go that fast.... not that I would stay that fast for very long. I was running it to see the rpm to evaluate the prop, not to see how fast I could get....that's my story and I'm sticking to it.... it was fun though...
My questions for someone much more experienced or wiser than myself:
Would this be a good prop for a cruise up the intercoastal where I would probably be running at about 3000 rpm? It does push the boat 5 mph faster at the same rpm setting. Is is straining the engine with the higher pitch? Would it be more economical on fuel or would the engine load cancel it out?

Would I be better with a 23"? I'm surprised the 25" did so well, that's a steep pitch for a 4.3, although I do have a 1.81 ratio outdrive. Is the 1.81 instead if 1.6 why it seems to work out?

Should I sell it and get a 23"? I don't want to stress anything with the steeper pitch.

I don't intend to use the 25" for watersports, I have the 19" and 21" for that.

 

[steelespike]

Re: 3 props tested

Im a little outside the Prop expert circle but it sure seems good to me your in the middle of your rpm band.It appears your cruising speed will be much more effective.By the way best cruising speed could be anywhere between just planing and about 2,000 rpm higher.When going from aluminum to stainless
the changes can be surprising.And pitch to rpm change can be outside the norm 1"= 200 rpm.

 

[Just for fun]

Re: 3 props tested

I'm going to run it for a few trips to see how it goes. I was very surprised that I only dropped 200 rpm with a 4" prop change, but I was going from aluminum to stainless so other factors may be involved. I may have been bumping the rev limiter with the 21", that's why I wanted to try a 23". Most of the time there is only one or two in the boat. I'll try it with a load, but I'm afraid my rpm may drop below the norman range. When I load it up, I'll probably have to swap back to the 21". With the 21" I would turn 4800 rpm with 5 people, my dog, a big cooler onboard, and a full gas tank. I'll probably score a 23" in the future.

 

[Randybeall]

Re: 3 props tested

Would the 25 prop allow you to reach 4800 rpm? What is your normal load, you mentioned low fuel on one test full tank on the other. A little hard to get good scientific testing when the benchmark is moving. You are looking for a prop, stainless is better, which will allow you to reach 4800 rpm at wot with nomal load. When running light you can throttle back, when running heavy it will come back to you as fuel burns off. Cruise is of little importance in the process, though when you have the right prop it will allow the engine to cruise at less effort, saving fuel. If the prop is too much pitch surprisingly you will use more fuel, and load the engine more than need be.

 

[Just for fun]

Re: 3 props tested

Randy,
The 25" prop only reaches 4600 rpm, right in the middle of the max rpm range. It may fall below that if the boat is loaded, I haven't tried it and don't intend to use it with a loaded boat. Prop changes are a 5 minute job, so I don't worry about that.
The only reason I had a low fuel tank on the first example was I was diagnosing a fuel shutoff problem and wanted to see if it was fuel level specific. I wanted to pull the fuel tank so I was running it low for that. It ended up being unrelated to the tank, it was the oil pressure switch.
My reason for mentioning the fuel differences was to point out point out that the speed increase was several mph even with a heavier load (only about 100 lbs). The 21" prop rpm was at 4800 when the tank was full or empty. Before I tried the 25", it would reach 4800 rpm no matter which prop I had on it. That's why I decided it was probably hitting the rev limiter. The only way to know for sure was to try a steeper prop. As I said in my original posting, I went to pick up the 23" and it was actually a 25". Since it was a copped stainless and my previous trials were with aluminum, I didn't know what to expect in rpm and speed. The seller agreed to refund my $$ since the pitch was not what the ad stated. I have that option.
The 25" prop rpm's may increase a bit when the boat is empty, but it would have to be empty with only one person in it, a pretty narrow time frame.
I know prop choices for a specific situation would be easier, but they usually have to serve several different scenarios, ie: different loads, cruising at 30 mph vs top speed, getting on the step with and without watersports involved.
My concern was if running the 25" for cruising would be a good idea. I have a 19" and 21" for watersports. I think the 25" will be one of those narrow range of opportunity props. I just didn't want to keep it if I had no use for it. The good news is that I only have $80 into it including the prop hub adapter (worth about $35-$40 of that $80..) That said, I would trade it for a 23", which is what I was looking for.
Thanks for your response, I am getting an education on props thanks to guys like you.
Fred