1.84 ratio on Alpha 1 Gen 2 5.0 L/X

[z00m]

Ok, so you might have seen my thread about leaking transoms and worn out props. The boat has had all of the engines, transoms and drives removed and everything that lived in the water looks in terrible shape. The only used parts I can find and look good enough to use are with the yard doing the work. Transoms and drives are from a trailer boat that had 170HP V6's in it.

I took the liberty of noting the serial number on one of the drives and it is a 1995 1.84 RH rotation drive. The ones on my boat were 1991 1.50 RH rotation. My question now is what to do about the drive ratio difference. One of the objectives was to fit 4 blade props. My 3 blades were 14.5" x 17P. If I fit 4 blade 14" x 20P Alpha 4's, will I be in a sensible rev range?

Not really sure what old WOT was as I never used it (new to me old boat) but when the bottom was clean I saw about 23 knots at 3100 RPM if that helps. Since there is a 19% difference in the drive ratio, does that equate to 19% difference in RPM for the same speed? (3100 + 19% = 3689?)

Using mercury's prop selector, for my type of application on a 5T boat they say a 14" x 20P Alpha 4 would be appropriate for a 1.81 drive (doesn't allow me to select 1.84) to achieve 43odd MPH at 4500RPM. This would be based on a 2000 model 5.0 carb engine at probably 260HP? My LX was 230HP when new so probably some ponies have escaped since then.

So the question to the experts - am I jiggered and should go to the expense of changing the gears or would it be ok to use the 1.84s with the 4 blades? Replacing everything has eaten a big hole in the budget (doing manifolds and risers while its apart) and this is an additional cost I can't really bare right now.

 

[DaveG55]

I just had to replace a 1.45 drive with a 1.84. At this time I have went from a 19p to a 21p (both 3 blade stainless) and all seems well. Since you don't know what your WOT was before you won't really be able to extrapolate what you might need for a new prop. I can tell you, from my short experience with the 1.84, that the drive itself will be fine, you'll just have to reprop to get the WOT within spec. Whether or not you ever go WOT during normal running you should definatly do it when testing for a new prop. I believe that WOT for the 5.0 is 4400-4800 (at least that's what mine is). My guess is that the prop selector you used has gotten you pretty close and that's where I'd start.

 

[z00m]

Thanks Dave, hopefully it works out ok.

 

[achris]

Completely concur with Dave. I like using higher ratio and larger (pitch) props as it spins the props slower and they become more efficient. Just be a bit careful, the old drives and transom assemblies were Alpha One, the new (1995) are Alpha One Gen II. You need to swap EVERYTHING. The engine should just settle back down on the new inner plate and the alignment shouldn't be any problem. And yes, 4400-4800 is your recommended WOT. I also like to go right to the top end (and over a little doesn't worry me) when propping. I have a 17" on my boat and she pulls to the rev limiter, and with a 19" on I get 4600 (right in the middle of my range), but the boat is so much more 'driveable' with the small prop. And the fuel consumption is EXACTLY the same, boat speed for boat speed.... I say go with the 1.84s and prop to the top...

BTW, the Merc 4 blade props are very nice. For my old engine (4.3LX) I had 1.84:1 and a 20" 'Offshore' series prop... I'm still looking for a good secondhand 17" or 18" Vensura for the current engine (4.3MPI) with 1.62:1 (wish I'd specced a 1.84 with that too)...

Chris......

 

[z00m]

Thanks Chris, we boat in the same waters so your experience is very well received. The originals were alpha 1 gen 2's (0D492823) but very early series without the oil bottle. At least now with everything rearwards of the engine being replaced I can be confident of what I have.

 

[z00m]

Thanks for the replies Dave and Chris. With the new transoms and drives on (along with full services and new manifolds and risers, antifoul, polishing, etc etc) the boat went back in the water yesterday. Ran 10 miles up the coast to my marina in 2 to 3 metre seas at 17 knots so won't know about WOT performance until the weather settles down a bit. Seemed ok manoeuvring around the docks. So far so good!

 

[achris]

I expect to be home around the 16th. Send me a PM around then and I'll see what I can do about setting up a meet... Would love to see inside that ship.

That photo looks like somewhere around Aquarama, yes? And '10 miles up the coast' suggests Hillarys?

Chris.........

 

[z00m]

I'll send a pm around then. Although the inside needs as much work as the mechanicals so it will be a process!

Lift was at Hillarys and the run home was up to Mindarie.

 

[achris]

Cool, and I've been 'invited' to stay another week.. So make that PM around the 23rd... Cheers.