Logo
FULL SITE MENU

Small boat prop for 1975 Merc 50hp

S

SaltyScot

Regular Contributor
So I have this motor on my 11' 250lb hydroplane and It pushes 35mph, but at 7000 rpm with a 11x15 prop. I want to get it down to 5500 rpm and I'm pretty sure an 11x17 isn't going to get me there. Are there any other props available for this motor? Or do I need to have one custom made/modified?
.
Stephen
 
I actually just realized that I have a Flo-Torq hub kit that fits my hub. So there should be many more options for me. What sizes should I consider?

I know you drop rpm 500 by going up 2" of pitch. But what about increasing diameter? I have 2" of extra clearance(radius) before I hit anything. :D
 
Try a 19" to start and if you need to increase performance or reduce rpm further add a cup to the blades, a 2 blade prop is better for your application.
 
Quicksilver said:
Try a 19" to start and if you need to increase performance or reduce rpm further add a cup to the blades, a 2 blade prop is better for your application.
Click to expand...

Thanks. Do you mean an 11x19 with the matching 3" hub? Because I don't see anything in 19".

Or a larger size hub?
 
No the prop has to fit inside the gearcase housing. A 13x17 could work, since this is a "custom" installation after you buy a prop that allows you to turn near the 5400 rpm range a prop shop could help fine tune it with cupping or diameter changes.
 
Quicksilver said:
I found this site accidentally http://dansdiscountprops.com/Discou...-history-of-outboard-boat-motors-part-eleven/
Click to expand...
Thanks. I called them and they don't know of anything bigger than an 11x17 for my motor. I think i'm going to try the 11x17 and see what i get. I'm still not totally convinced I'm running 7000 rpm. Actually it's pegged at 7000, so it could be more. When i had the tach set on '6' , it read 2900. It doesn't really make sense that moving it to '3' wouldn't just give me double the 2900. Or does it? Maybe a bad tach like you said.

Besides, how long could you run a 1975 Merc 50hp at 7000(+) rpms without cooking it?
 
I took it out again today briefly. I'm still topping out at 7000 rpms, verified by both tachs. At least it's not pegged. But now at only 30mph. I'm wondering if my prop is slipping. I'm going to put the motor on my 15' Glasspar I just bought and see how it does there. I just can't believe that 50hp on a such a small is not going over low 30s. Maybe it's the keel screwing things up? I'll probably cut it off and try again at a later date. Too many variables at once. Giving me a headache. I'm going to play with my new toy for awhile. Might get an 85 on there in a couple weeks. Thanks for all the help Quicksilver!:)
 
Thanks for the good info. I should probably mention that my transom is at 15 degrees. The motor has always been set on the third hole. I have a large whale tale(SE Sport 300). And the cav plate is even with the bottom of the hull. I also have a center keel (3/4" W x 1 1/2" H) that terminates 30" before the transom. I'm going to remove that keel and go with two anti-skid rails probably before I even take it out again. Maybe I should trim it all the way in when I start over?
 
So I tried out this same 50hp motor on my "new" 69 Glasspar Avalon and I got basically the same result. It has a tall top on it which probably cost me 1 or 2 mph. I ran 26mph at 7000 rpm. So I thought this can't be right. I switched the pole setting on the tach to 4 and then I was idling at 800rpm and topping out at 5600rpm. So somehow I have to confirm that 4 is right setting(if it indeed is). Then I can put my 17" pitch prop on there and be a happy camper. I did run it for about an hour with max rpm bursts of 5 or 10 minutes spaced out and then I would stop and feel the motor and it was barely warm on the water side and not too hot to hold my hand on on the exhaust side. So maybe that is the right setting. I'll see if I can contact Faria. I'm also going to buy another tach for my Glasspar so I can cross check it. The little digital one I have isn't worth the $11 I paid for it.

Stephen
 
I contacted Faria and they got back to me just a couple hours later telling me that my motor has 12 poles and it should be set to 5. I put it on there and it cut my indicated rpm by about 20%. So now I would be maxing around 4550 instead of 5600 on the 4 setting. Boy, I really didn't think I was running below 5000. I have a hard time believing that. The instructions say that 4 setting is for 10 pole. It seems to be the rarest of the motors on the list. So like I said, I'll get another tachometer for my Glasspar and cross check it by that one.
 
I put the new tach on there and it's saying the same as the Faria. Then i realized that I wasn't actually opening the throttle all the way. So this might just have a happy ending.
 
Back
Top