Alpha 1 Gen 2 Outdrive ratio choice, Please Help!!

[opentraffic]

Hi there Im new to this forum but have found your other posts very informative and helpful. So my dilemma.... I have a 1994 Wellcraft eclipse 196 sports cuddy boat that has recently required a new sterndrive. The boat was originally a 305 but at some point has been repowered with a 350 using the stock 1:1.65 ratio sterndrive. My question is.. Do I replace the sterndrive with the similar 1:1.62 or go for the correct match to the 350 and use the 1:1.47 ratio?? On paper the boat weights 2750 lbs Reached 42 knots on a 19 pitch stainless @ appox 4800rpm, maybe a little higher. Thank you in advance for any help

 

[flipbro]

If the boat was stock with a 305 the gear ratio should already be 1:47. I would say some one changed the drive as well Or the boat originally had a 4.3 V6. Yes you should have the rite gears( 1:47) But running 4800 with a 19 pitch with 1:62 just don't seem rite are you sure the gearing is 1:62? You cant always go off the casing numbers as sometimes drives get rebuilt with different gears! I would dig a little deeper

 

[achris]

Sorry flipbro, the 2 bbl 305s used a 1.65:1 drive.... Opentraffic. If you're using a 19" prop and getting to 4800 (+ change ) with the 1.65, I'd definitely stay with that drive ratio.... The higher the prop pitch the more efficient the prop... (mainly because it's spinning slower)....

 

[flipbro]

My bad! Guess i learned Something

 

[Maclin]

4800 rpm at 48mph (42kts) with 10% slip shows ratio of 1.62 in prop calculator. Might be best to remove drive and count revolutions to be sure.

If the 350 is in a good state of tune I think I would go with the 1.47 in that size boat.

 

[achris]

4800 rpm at 48mph (42kts) with 10% slip shows ratio of 1.62 in prop calculator. Might be best to remove drive and count revolutions to be sure.

If the 350 is in a good state of tune I think I would go with the 1.47 in that size boat.

My calculator gives an angle of attack of 3.5 degrees, and a slip of 13.8% Which is a perfect scenario... I can see how you arrived at 10%, you didn't add 1" of pitch for a cupped blade (as all propellers are now)... If you used the on-line calculator on BAM, put in a pitch of 20" (which is the effective pitch of a 19" cupped propeller at speed)....

Based on those calculations, I would definitely stay with the 1.62. It has the propeller spinning at just under 3000rpm. A move to a 1.47:1 would need the propeller to be changed, most likely to a 17" and that would mean a propeller speed of 3300rpm... A slower propeller is less likely to ventilate/cavitate and is more efficient...

 

[Maclin]

achris, thanks for the extra info. I just wanted to do a quick calc to see if the ratio came up close to what the OP thinks is in there now. My ratio recommendation to go to the 1.47 was more to what Merc would have installed with a 350 in that size boat, since it sounds like the OP is definitely going to replace the drive.

 

[opentraffic]

Hi guys Thanks for the detailed info, I had to make a choice at the beginning of the week and I've gone with the 1.47 as I was advised it will provide the 350 with the extra cooling needed. I really hope I haven't messed up by making this choice . I'll try it with the 19 pitch lazer 2 but I'm guessing that the wot is now going to be too low. What prop recommendations would you guys give? I like the idea of 4 bladers, getting on the plane at a lower speed!

 

[achris]

If the engine isn't currently getting hot, why change? And going from 1.65 to 1.5 is less than 10% change. If that makes a difference, there is something else wrong with the engine. I've seen a350 running with a 1.84 drive (in a VERY heavy boat, guy thought he'd save fuel by powering a 28 footer with a single small blockV8 :facepalm: )... Just to get into a usable prop range we couldn't put anything less than 1.84... Engine (cooling) was fine with it.

 

[opentraffic]

The the engine was not overheating, I just presumed that because mercruiser spec that drive for the 350 it was a safe bet. Hopefully I can find a prop to make it awesome again

 

[achris]

The ratio is 'specced' based on the most likely usage. It takes no account for fitting the engine into a boat of a different range than the 'norm'... If you have a look at the 'spec' it also calls for a lower ratio to be used at higher altitude... When ordering a new Mercruiser, one of the items on the list is 'Drive ratio'... The factory is quite happy to send any ratio with any engine... It's up to the dealer and the customer to decide what they want for their application. Just because 1.5 is the most common used with the 350 doesn't make it the right ratio in every application. And even if the boat IS in the 'normal' range, nothing stopping anybody using a lower ratio. I prefer the lower ratio anyway, it spins the prop slower and thus the prop will be more efficient, and be less likely to cavitate/ventilate... And if it pushes you up into the 19" to 23" range, there are SO MANY amazing props, that just don't exist in the lower pitches...

Anyway, that's my opinion based on 30 years experience...

Good luck with whatever you decide.

Chris......

 

[DaveG55]

Gonna resurect/hijack this a bit but it's related to the original question.

My boat; '94 21' BR, 5.0l (200HP), A1/G2, 19p SS prop @ about 4600RPM with a top speed of 48-52MPH depending on load and water condition.
I got water in the drive and ruined the gears. Having to replace it. Don't have the cash for a new SEI so the mechanic went shopping for a used drive. What they have found is a late model 1.81 drive from a similalarly sized boat. I assume it's a 4.3l V6 which was an option for my hull. Mine is a 1.62 and the way I'm reading this thread is I should be ok with the 1.81 drive as long as I reprop to keep the RPM between 4400-4800. This is the information my mechanic is giving me and claims that a tech at SEI confirmed that to him as well.
To me it seems the same as the OP with his original outdrive but a step up on the engine. Also, I have a good friend that just repowered to a 5.7 from the 5.o w/o changing his outdrive. He is quite happy with his results.

I guess my question is, will the 1.81 be ok?

 

[flipbro]

Why not start your own thread? Probably get better results!!

 

[DaveG55]

Yeah, you're probably right. Just thought that since the subject matter was so similar it would tie in.