Prop sizes ?

[GbayGiant]

Any prop experts on here ? I need to get a new prop for a 1991 70hp Merc. The old one was 13.25" and 17pitch.

 

Does it just have to be the same 17 pitch or both. Could I get a 17pitch 14" for instance ? I'm just trying to figure out my options.

[aplumma]

Give this site a check it should get you in the ballpark as far as reproping the boat. If you are not having any problems with the old prop as far as performance then stay with the same style and size. If you change the diameter or the pitch you will effect the speed or acceleration/load carrying ability of the boat.

 

 

mercury prop selector

 

 

Art

[GbayGiant]

Thanks Art. Wow, I checked that site but it's crazy the results came out as 13.5" , 23 pitch but the latest models it does is 2000. That's a big pitch difference, I guess I'll stick with the old size.

[linweir]

Yes, high pitch # may reduce RPM. The top speed will be lower.

Also, hub size is important. For example, if your old prop hub is 4", you must use same size hub prop.

[Woodsman]

GbayGiant:As you already have a prop that works here's how you can test if it is the proper size.

Load your boat as to your most typical situation. On a day that is not to windy or rough drive the boat at WOT ( Wide Open Throttle) and check your RPM. Compare to the manufacturer's WOT specs for the RPM. If it is not reaching within about 200 Rpm of specs. you have too much prop. Either reduce diameter or pitch. If you are over the WOT Rpm you need more prop. Increase diameter or pitch. Also 4 blade props compared to 3 blade props of the same diameter & pitch will give you more prop & less Rpm. Changing prop material (ie: steel, stainless steel or plastic) will effect the power of the prop.

Lots of variables. Once you know how your current prop is preforming a good dealer should be able to point you in the correct direction.

 

Good Luck: Rick

[aplumma]

due to the age of your motor I would disregard their recommendations. You can also call their tech support with the info that woodsman was nice enough to post. I also have heard that some places that sell the props have a free testing props program I am not sure if it is in Canada or not though.

 

 

Art

[GbayGiant]

Thanks guys, good info. I just found the old motor manual it says 13.25"-17p as well, so I'll just stick with that and have no worries .

Edited January 17, 2008 by GbayGiant

[Guest skeeter99]

are you going to stainless? or aluminum?

 

if the one you have is a 17 pitch aluminum under load it will bend quite a bit and be running (estimate) 15.75 to 16 pitch **hard to believe**

 

you could go with a 14 by 17 stainless or 13.75 by 17 stainless and the performance would be better on turns and take off

 

but for better top speed if that is what you are after you could put a 13.75 by 19 pitch I dont see a problem

 

as a rule of thumb for every 1 pitch increase your rpm will drop roughly 200

 

 

unless your rpms right now are lower than what the engine should have at wide open throttle, does it have power trim cause it will make a huge difference running wide open and trimmming to get rpm up just before to much cavitation?

 

 

i had a 60 hp merc on a 17 foot bass boat running a 13.75 by 19 pitch prop(stainless) no problems and gained 1.7 miles per hour, hole shot was not as good though top speed of 39.7 mph

 

and if you change your prop get a flo-torq 3 hub assembly from merc(makes hub interchangle with other makers if they dont have old style rubber bushing in them)

Edited January 18, 2008 by skeeter99