OMC-Alpha one prop help!!

[matt407]

Hey Guys,

I know theres a set of questions you need answering but this is where I am stuck...

I purchased a four winns freedom 18" with an OMC 4.3 2 barrel carb 1990's ISH engine in it (175hp???) . It had a knackered leg on it so I set about a conversion to a merc alpha one leg with the SEI conversion. This has worked VERY well however. I didnt have enough to buy the kit and a new merc leg so a friend has given me a merc leg which has very little use.

According to SEI I required the 1.81 gear ratio to be rougly where the old omc leg was geared too however the leg I now have on the boat is a 2.00 gear ratio. The old OMC prop doesnt fit the merc leg so I havent been able to run the boat to get any kind of bench mark. I am looking for good hole shot as the boat will be mainly used to wakeboarding.

Any form of help would be great. i am sorry I cant provide a boat weight or anything like that.

Cheers matt

 

[QC]

Re: OMC-Alpha one prop help!!

Welcome aboard matt. Tell us more about the previous prop (OMC). Was there one? I am guessing one or two inches more pitch with the lower gear ratio. Without a benchmark I think I'd start at 21". I had a 140 with a 1.81 and used 17" in a light 17 footer.

 

[matt407]

Re: OMC-Alpha one prop help!!

I dont know what the old prop was but searching through the forums it may have been a 21" before. I have had a trawl through ebay and found this :

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=320551857453&ssPageName=STRK:MEWAX:IT

Its a prop which i can change the pich on from 21" to 14.5" is that the way I want to go?? Because once I find what size prop I need I can buy a metal one so it doesnt flex!

Thanks for the quick reply

 

[180shabah]

Re: OMC-Alpha one prop help!!

I would normally say pass on the plastic toy prop, but at that price - your plane just may have merit - and you'll end up with a spare too.


4.3 with 2.0:1 in a small boat will probably put you in the 21-23" range FWIW - I have a 2500-2600lb 18 footer with the 4.3 (4bbl) and 1.84:1 drive spinning a 21" prop all the way to 4800 - have seen a tic over 5k running solo and light on fuel.

 

[matt407]

Re: OMC-Alpha one prop help!!

Hi thanks for the reply.

So its worth a shot with this plastic prop to get some form of base line then! As its a 4 blade prop at 21" is that the same as a 22" 3blade prop??

 

[180shabah]

Re: OMC-Alpha one prop help!!

I am no expert on pro-pulse props, but they have relatively small blades. I THINK this allowed them to match pitch with a normal aluminum 3 blade.

 

[matt407]

Re: OMC-Alpha one prop help!!

Hi,

Thanks for the continued help. I have been offered a high five prop with a 21" pitch. I am mainly doing wakeboarding and have seen that the high five keep you on the plane at slower speeds. Will this be suitable? do I take the risk as its quite alot of money to waste if it doesnt work! How many inches do you normally drop when going to a 5 blade prop?

cheers matt

 

[matt407]

Re: OMC-Alpha one prop help!!

Hi I have managed to find that the original prop was a 14 ,1/4 X 21"

This high five I have been offered is a 13,1/4 X21" will it work?

 

[180shabah]

Re: OMC-Alpha one prop help!!

High fives run a smaller diameter so they typically swap for the same pitch as a 3 blade design.