Lower Unit Ratios

[snooks2005]

Given the same horsepower for both engines, identical boats, one engine with a 2.3:1 lower unit and the other with a 2:1 lower unit swing the same props which boat would be faster.

 

[ob]

Re: Lower Unit Ratios

In theory the 2:1 would be faster than the 2.3:1 at the same rpms.Problem is the 2:1 may not be capable of the same rpms as the 2.3:1.If it were capable of the same rpms the 2.3:1 would develop quicker rpms and be faster out of the hole than the 2:1.

 

[snooks2005]

Re: Lower Unit Ratios

so what you are saying is that you would have to reduce the pitch of the prop on the 2:1 lower unit

 

[ob]

Re: Lower Unit Ratios

Not necessarily.It will depend on whether the engine will eventually develop the recommended WOT operating range.Messing with gear ratios is the same as changing prop pitch and cupping.Most outboards are engineered with an optimal ratio that will allow several prop applications to suit the rig it is attached to.

 

[snooks2005]

Re: Lower Unit Ratios

The reason I am asking this is because i want to change from a 2.3:1 to a 2.1 ratio and I don't know what effects it will have.

 

[ob]

Re: Lower Unit Ratios

If you're after more top end speed have you tried changing to a prop of higher pitch?Bare in mind you'll only be able to incerase so much before you start overloading the engine and lower rpms , which inturn will equate to slower speed.Same with gear ratio.If you gear it down your engine will not develop torque as quick and may very well not be capable of the same WOT rpms you expeienced before changing ratio.The cure. Reduce prop pitch .The effect.Slower speed.Lot easier to play with pitch and cupping than gear ratios.<br /> Trust me.If I thought I could gain say 5mph top end speed using the same prop setup I now use by changing ratios in lower end I'd be tearing mine down as we speak.

 

[snooks2005]

Re: Lower Unit Ratios

well I have tried alot of props and still can't get my boat to WOT. I am currently running a 16' flats boat with a mariner 50 4 stroke big foot.

 

[ob]

Re: Lower Unit Ratios

If your rig is not capable of recommended WOT operating range with current prop you need to lower pitch to incerase rpms or you are essentially lugging engine.Lowering pitch in some cases will actually gain top end speed by allowing your engine to develop its full rpm.I'm figuring a prop of 15" pitch is a good starting point for the 16ft boat that your 50 is driving.

 

[snooks2005]

Re: Lower Unit Ratios

5450 with the 15 pitch. I also have a jack plate on the boat but when I get to about 2 1/2" the prop looses bite

 

[ob]

Re: Lower Unit Ratios

Sounds to me like you're set up is just fine the way it is.What does your manual state as the WOT range?

 

[snooks2005]

Re: Lower Unit Ratios

my wot is 6000

 

[ob]

Re: Lower Unit Ratios

If WoT is 6000 and you only get 5450 trimmed out then you can try one of two things.Change to lower pitch and pick up the additional 500 rpms or have some cup removed from existing prop.Is this a three or four blade prop?

 

[snooks2005]

Re: Lower Unit Ratios

this is a three bladed prop

 

[ob]

Re: Lower Unit Ratios

Reason I asked is that you mentioned that it cavitates sooner than desired when lifting on jack plate.Generally a four blade will allow you to run higher without cavitation.However first things first.Need to get your rig propped to turn in the upper end of 6000.If you lower to a 13" and find it overrevs slightly you can always add some additional cupping to fine tune your prop for your application.

 

[snooks2005]

Re: Lower Unit Ratios

So I guess I need another prop . Who's prop would you suggest. The problem lies in the with the lower unit I am running the don't make alot of props for my engine.

 

[ob]

Re: Lower Unit Ratios

Checkout this site<br /> WWW.a1discountprop.com

 

[snooks2005]

Re: Lower Unit Ratios

yes I have seen that site but at this point I don't know what blade style to look for anymore. Right now I am running 13 3/4 x 15 scd Power Tech prop which is the smallest they make in that blade design any lower in pitch you start getting into the pontoon style props.

 

[ob]

Re: Lower Unit Ratios

Snooks here's a couple of props available that fit mercury bigfoot 50 hp<br /><br /> Mercury vengeance s/s 13 3/8 x 14 p/n 17314A46 $350.00<br /> Mercury black max aluminum 14x13 p/n 77340A45 $133.00<br /><br /> WWW.marine-outboards.com