rookie prop question....

[country6220]

I have a 1988 17' bass tracker, it has a mercury classic 50 on it. The weight is around 1300lbs with me and one other in it. running 5000rpms speed is 25mph, dont know gear ratio or prop slip or how to calculate it. I would like to increase speed a bit if I can. It will plane out at about 3/4 throttle. Please help if you can. It has a 13 degree prop on it now.

 

[RicMic]

Re: rookie prop question....

Its a 13 INCH pitch, not degree, its the theoretical distance the prop would travel in one rotation. Is 5000 rpm, the wide open throttle max rpms?

 

[country6220]

Re: rookie prop question....

yes 5000 is wot.

 

[steelespike]

Re: rookie prop question....

Actually max rpm rating is 5,500. Dropping 2" in pitch ,same brand prop, should result in about 400 more rpm.Better holeshot better load carrying .

 

[country6220]

Re: rookie prop question....

If I drop 2" will that effect speed much? My top speed is 25 and Iam happy with that but I dont want to slow it down too much. But I would be happy with holeshot being better. I guess its give and take but I dont want to slow top speed too much. Isnt about 30mph for this setup about normal?

 

[junior1113]

Re: rookie prop question....

12" pitch would be ideal. 12 " of the same type prop would put you real close to 5500. speed will be close to same as for smaller pitch but more revolutions.
rule of thumb
old pitch x old rpm divided by new pitch =new rpm
13x5000 divided by 12=5416(new rpms)
13x5000 divided by 11=5905

 

[1kruzer1]

Re: rookie prop question....

Actually max rpm rating is 5,500. Dropping 2" in pitch ,same brand prop, should result in about 400 more rpm.Better holeshot better load carrying .

I agree with Steelespike, dropping 2" in pitch with the same type of prop will get you closer to 5500 RPM and you'll be happier with the overall performance at any speed. I don't think you'll see much affect on top speed, as 5000 to 5500 RPM is all well within the powerband of that outboard. 25MPH sounds about right for your setup on that 17' boat. You only have 45 horsepower at the propshaft. I owned the same 4 cylinder 50 merc for a while on a similar weight boat, and 23MPH is about all I ever got. That motor is a gem and is one of my all time favorites. Very smooth and idles like a watch.

 

[country6220]

Re: rookie prop question....

thanks for the feed back everyone.

 

[junior1113]

Re: rookie prop question....

before buying props check to make sure setup is maxed out (is motor as high as possible on transom. rpm change is more dramatic when were talking 13 to 11 as too 21 to 19 which would be about 400