Gear ratio change/unknown oem prop

[pakkolee]

Hey.
So boat is a 79 Wellcraft Sunhatch 189. Has a Mericruiser 898 (5.0l v8). having a hard time finidng info and parts for this boat. Sterndrive was seized and i managed to find another older drive replacement from a much newer 90s Mercury 190 (3.7l inline 4) with an alpha drive. From what ive been able to research is that the original drive was a 1.5:1 ratio and the newer sterndrive is a 1.84. the old sterdrive had a 14x19 prop, apparently used to be the fastest boat on the lake or something so not the original, also all the calculators recommend a 14x15 prop as general replacement prop for this boat.

what prop would i need to buy for the newer taller ratio drive. since calculators recommend a 14x15 prop for good economy and cruising would the 14x19 that came with the boat be good for this new ratio? or would i need to get a diffrent prop?

 

[Chris1956]

So you went from 1.5::1 ratio and 19 P prop to a 1.84::1 drive? You will want a prop with more pitch. Simple math suggests a 23 pitch prop, however, that is just a multiplication of the ratio difference, not considering the power curve of the motor.

If you could try a 21P prop, and see what max RPM are, that would cinch it.

 

[pakkolee]

So you went from 1.5::1 ratio and 19 P prop to a 1.84::1 drive? You will want a prop with more pitch. Simple math suggests a 23 pitch prop, however, that is just a multiplication of the ratio difference, not considering the power curve of the motor.

If you could try a 21P prop, and see what max RPM are, that would cinch it.

thanks for the reply. the boat came with a 19p prop. i dont think its the factory prop since most prop finder tools say to use a 15p prop for this boat type,weight,power, ratio combo

i honestly only want a prop thatll let me cruise around and get good fuel economy. boat will be used for fishing mostly. no need for a super agressive power prop i think

 

[Scott Danforth]

The 19p is most probable for a 19 foot boat and a 5.0. it's also the prop you had on the boat. So starting with the 19p and simple math like stated by Chris gets you to 23p

The drive you have will require a 23p +/- a bit during testing

So where did you pull the 15p that you keep holding onto?

 

[jimmbo]

i honestly only want a prop thatll let me cruise around and get good fuel economy. boat will be used for fishing mostly. no need for a super agressive power prop i think

You choose a Prop that allows the engine to run in the Upper Half of the WOT rpm range, which for your engine is likely 4200 - 4600 rpm. With the Ratio change, a 23 is probably a reasonable place to start. Get on good terms with a Dealer, and maybe he will let you try out a few, with your promise to buy one, once you figure out what you need.(Of course any damage you do to a prop means you buy it as well)

 

[QBhoy]

Do you know it was definitely a 1.5 ratio ? Most 5.0 weren’t, I’d think. Although some definitely are. I have a 1.47 ratio on my 5.0. But it has much more power than that 5.0.

 

[Scott06]

Do you know it was definitely a 1.5 ratio ? Most 5.0 weren’t, I’d think. Although some definitely are. I have a 1.47 ratio on my 5.0. But it has much more power than that 5.0.

Correct most of the 5.0's I've seen are 1.62 in which case the guy would only need to go to a 21" pitch form 19".

 

[QBhoy]

Correct most of the 5.0's I've seen are 1.62 in which case the guy would only need to go to a 21" pitch form 19".

I’d think you’d be right on that. To be honest, mine is the only one I’ve known with the highest gear ratio available. But I’m aware it was certainly an option over the years. It’s a 1.47 I have. Dare say the older option would be the 1.5…if it was back then.

 

[Scott Danforth]

guys, per post #1, it was an 898 drive. they were listed as 1.5:1 for the V8's or 1.65:1 with the inline 6

this is the MC1 / MR / A1 ratios

not until the A1 Gen 2 in 1991 did the drive change to 1.47:1 or 1.62:1