need prop help for 78 merc 500

[s.hadley81]

hi i am fairly new to the forum i need help finding what prop i should run i took old one to find out pitch because there were no stamped numbers on it i believe it was 10 3/8 but no idea on pitch thats what they were supposed to tell me they sent it out and lost it so while i wait on them i would like to buy brand new one and keep old one as spare if they find it or give me another one my motor is a 1978 merc 500 (50hp) on a 1982 glasstream probass 1550 (fiberglass 15 1/2ft) dry weight is 500lbs motor 254lbs persons and gear probably 600 with old prop boat had no problem on hole shot even with 3 people and ran about 30mph 5500rpm and 36 at 6500 rpm at wot i would like to get at 5500rpm wot (where motor should be) with at little more top end i would appreciate any help

 

[86 century]

Re: need prop help for 78 merc 500

Sorry for your miss fortune

I would go to the place you took you prop to make them give you a prop like your old one.

Any prop man worth his salt could tell you what you have.
Dont know where you are from but deleware propeller in muncie IN has a few guys that can get real close on pitch just looking at them.

The one I took in was faily common the guy never picked it just walked over to the shelf picked up its twin.

Good luck

 

[steelespike]

Re: need prop help for 78 merc 500

For future reference the numbers usually appear under the propnut.
A couple of things,My info indicates the boat is 700lbs.
I'm suspicious of your tach;6500 is very high for a motor that is
rated at 5500. It is possible that your very good loaded hole shot could be the result of being underpropped
which would account for the high rpm but not the speed
All that said I think an 11" prop is a good place to start.I think they should let you try props till you find the right one.
If you get us the rpm and speed with the new prop we canget an idea if the setup is right and the tach makes sense

 

[s.hadley81]

Re: need prop help for 78 merc 500

i know my boat was under propped this is why i took prop off in the first place and there are no numbers on prop at all not even under nut or washer i was told prop was redone and they probably changed pitch this is why there are no numbers they guy at the shop sent prop out to have pitch checked for me they are a reputible shop so im sure they will resolve problem but in the mean time i was wondering if anyone would have idea what pitch i should be running so i could look into buying a brand new one and keep other for spare and my boat does weigh 500lbs not 700lbs

 

[steelespike]

Re: need prop help for 78 merc 500

I have the cousin to your motor a 1965 Merc 500. Esentially the same motor.Its on a 57 Crestliner 18ft utility dry weight 330lbs.
All up weight with a 220 lb driver about 850 lbs. It is running an 11" prop.
By the way my info indicates 78 Merc 500 is about 180 lbs.

 

[s.hadley81]

Re: need prop help for 78 merc 500

from what the manual tells me motor is 254lbs im not concerned with size as much as pitch

 

[corm]

Re: need prop help for 78 merc 500

Hi,

The 10 3/8 is the diameter of the prop. The pitch is somewhere, usually in a -13 form on the outside of the prop or on the thrust hub end of the prop if it is a Mercury Marine prop. Other brands use different numbers or letters and numbers that relate to the pitch in their literature.
I looked up your boat on the net. I would think you will probably need a 13 in pitch prop. The motor is !80 lbs dry, no oil, prop, or controls. Your boat appears to be 700 lbs, not much different from one of my boats with a 83 50 hp merc/PT on it and I run a 13 on it. With every in in pitch you change you will gain or loose 150-200 RPM depending on which way you go, up or down. Some shops will rent props to try and see how your boat and motor work. Without knowing what pitch you are running it is hard to tell what you need to buy to make it run at 5500 RPM.
If you have a Merc Quicksilver Tach have the shop test it to make sure it is reading correctly too or borrow a friends tach.
Good Luck.

 

[corm]

Re: need prop help for 78 merc 500

Sorry for the double post

 

[s.hadley81]

Re: need prop help for 78 merc 500

there are definitely no stamped numbers on the old prop i know the diameter of prop i 10 3/8 just by measuring and there is no way my motor weighs 80lbs i have had it off and i know its a 2 man job just figured maybe someone would have a good knowledge of props and would be able to give good insight of what pitch on the specs i provided i should run and i have ran 2 tachometers first thing i tried and i can tell the motor is screaming wot thats why i dont run wot

 

[corm]

Re: need prop help for 78 merc 500

Sorry to make you irate over my typo !1!1. I'm to retarded to help ya out. Good luck with your project

 

[steelespike]

Re: need prop help for 78 merc 500

I still feel the numbers don't add up prop calculator says your info indicates a 10 1/2" pitch prop produces 36mph at 6500.
By the rule of thumb 200 rpm per inch of change we would have to go to a 15" prop to get 5500 rpm that indicates a speed of 42 mph not possible for that setup. A 14" would be 41 mph. Your 36 mph is normal for the setup.
Your 1.70 gear ratio would use props in the 11-13" range for your setup. While the iboats chart only goes to 77 the gear ratio is the same The largest prop is 17" and would be on a very light boat.
You might go to Johns Old Mercury Web Site and explain your problem and known specs.
I'm pretty sure they can give you an idea of your prop size range.It will be difficult to be exact.

 

[s.hadley81]

Re: need prop help for 78 merc 500

Well they finally found my prop the prop shop wound up tagging it wrong and rebuilt it instead of just telling me pitch long story short got a prop rebuilt that didn't need it but it wound up being a 14" pitch so I just bought a 15" pitch for 30$ on ebay I know it won't get me down to 5500 rpm but I didn't want to go to drastic plus tech at shop said my old prop is not cupped so a the new one being cupped should knock Rpms down a little more I just wanted say thanks for all the input can't wait for weather to break so I can give it a whirl