Help needed to identify an old Mercruiser prop

I want to replace the prop on my Mercruiser which I believe is a 230hp small block V8 around 1984. The prop has the characters "48 78118 A40L 17P" (With a small vertical mark after the 48). It is a black aluminum 3 blade prop, probably a black max.

Does anybody know where I can confirm the make/model/specs of this prop so that I can look for a replacement? I am looking for something that will give me a little more power out of the hole. The boat is quite heavy for this engine - a 23 foot deep V cruiser (Bertram 23 like) and weighs around 5000-6000lbs laden. Does around 38mph. I am thinking a 4 blade Stainless Steel replacement.



Also, here are some photos of the leg.



Is that an Alpha leg or pre-Alpha?

Re: Help needed to identify an old Mercruiser prop

Your 230 hp shows as a 1985 with Alpha 1 drive.

The old Mercury prop is a 14.5 X 17. The current equivalent is a 15 X 17 Black Max/Black Diamond and is a better (design) prop than the old one.

We'll need your WOT rpm with the 17 pitch to determine if that is the appropriate pitch to use on your boat.

A SS prop will only perform marginally better than a modern aluminum prop on your style boat.

Re: Help needed to identify an old Mercruiser prop

Thanks jestor68. That was very helpful.

I am having some difficulty getting the WOT rpm spec (And others) on the 230. Is it 4400-4800rpm, 4800-5200rpm or something else? You are right in that I need to take the boat out and run it WOT to see how the current prop is performing as a baseline.

The freshwater cooled engine has been very gently used and well maintained since we bought it new in 1984 (Funny you would say it is a 1985 as I could have sworn we have paperwork somewhere showing a delivery date of around Christmas in 1984 - but I will have to look that up. It was delivered in Australia as well so even more surprising). We cruised on the plane with the tacho reading around 2500rpm. Sometimes up to 2800rpm. The engine seemed very smooth at that range. But I never did pay much attention to what the WOT rpm was as we never ran it over 3000rpm - I think maybe once or twice while trying to resolve some problems. But the tach has since become very inaccurate so I think that's the first thing I need to to - replace it or whatever powers it or both. No good trying to change props when we are basing the selection on inaccurate information.

Interesting that you think we won't get much more performance onto the plane with an SS prop. It is a big premium and frankly the soft aluminum prop has saved our hide many times over the years with submerged objects. A lot cheaper to fix/replace than gears. Here is a photo of the boat so you can see the size.



So being 23' cruiser with the 230hp small block V8 Mercruiser, SS is not worth it? Should we just stick to the new Black Max eh?

Re: Help needed to identify an old Mercruiser prop

The reason I said 1985 was because my data base does not show a 230 hp in 1984; instead that hp starts in 1985.

The recommended full throttle rpm range shows as 4000-4600 rpm. For best all around performance, it is recommended to prop it to run about 4300-4400 rpm at full throttle. The motor must be propped to operate within it's recommended WOT rpm range to prevent over stressing the engine's internal components.

If you have not purchased a new propeller in a few years, you will be pleasantly surprised at the improvements made.

I believe that for general applications, expensive SS props are literally a waste of money.

If you believe that your current prop size is performing well, you might simply replace with another of the same size; or as close as possible. A small variation in diameter won't make any perceivable difference in performance.

If Solas props are available there, they perform as well and usually cost less.

Re: Help needed to identify an old Mercruiser prop

Leaving the SS option aside, there is of course, the perennial question of whether a 4 blade (even just in aluminum) will improve performance out of the hole and with more people on board. Power is fine right now with 4 people on board, but on the occasion where I have 6 adults, the boat struggles a bit to get up onto the plane. There is a small cost difference between 4 and 3 blade aluminum props (Unlike to SS), but is there any point? The marketing blurb stresses that this is where the performance advantage lies, but again, on my type of boat, do you think it would be a marginal difference or a quite noticeable difference?