High Altitude Gearing

[Grodyman]

I am new to this forum, but really enjoy the prop information. I recently bought a 20'6" Campion bowrider with 5.7EFI/Alpha. The boat was orig. purchased in Salt Lake City area. I have seen a few posts from people saying many boats from that area are geared differently for high altitude (abt 4000-5000ft). My alpha says 1.47 on outside, but what is high altitude gearing and how would I know if the drive has a different gear?

I plan on getting a new propeller, just purchased an alpha4 14X20, to use until Merc. expands X7 to other lines (Vensura/Rev4). Thank you for any help.

Grodyman

 

[jimmbo]

Re: High Altitude Gearing

Merc and Volvo use higher numerical gear ratios in boats used at higher altitudes. The reason for this is simple. At higher altitudes the air is thinner and engines make less horsepower. Using a lower pitch prop would allow the engine to reach the proper rpm range. But using different gearing enables these engines to use the same range of prop pitches as engines at lower altitudes. Doing so reduces the range of prop pitches needed to cover most boats/loads

 

[Crownie2]

Re: High Altitude Gearing

Grodyman - the 1.47 ratio is not the high altitude ratio. High altitude is 1.62 or 1.81:1.

If the drive says the ratio is 1.47 than that should be it unless someone went in and changed it afterwards.

You want to be propped to be near the top of the WOT recommended RPM range when going flat out.
I believe this should be around 4,800 RPM's. That info should appear on the flame arrestor.

Bob

 

[Grodyman]

Re: High Altitude Gearing

Thanks, didn't thinks so. Boat came stock w/Black Max aluminum 3 blade 21", I just didn't know if all boats sold at altitude had different gearing, or if it was special request only.

RT

 

[Crownie2]

Re: High Altitude Gearing

Normally the boat Mfgr. would order the drive (w/high alt. gearing) based on the dealers request, assuming he knows where the boat is going to be used or at the customers request...

If the 21" Black Max stocker was right for your boat, then the 20" Alpha 4 should be a good choice.

Bob