Prop Pitch

[gone_wishin]

Is it possible to calculate on average, the rev changes on a motor with each change in prop pitch?

For example, my motor has an 18" prop and max's out at 5,200 rpm

What rev's could I expect with a 17" prop? or 16" prop?

I'm just looking for a rough guess like each change gives you 500 rpm as an example.

If you need to know specifics it's a 2005 115hp 4 st Merc on a Quintrex 530 spirit.

My manual says recommended rev range is 5000 - 6000 rpm.

Hull weight with fuel, gear and 2 people would be around 800 - 850kgs (guesstimate).

[johnno]

not sure on size but

a mate told me with a pitch change the rpm would lower/raise around the 250/300rpm mark

you would probably want to be more around the 5700 rpm mark

my outboard has a 5000-5750 range and i run mine at 5600

75 mercury 2st

5200rpm sounds a little low to me

but then you are still in the recommended range

depends really what you are using it for

[gone_wishin]

Thanks Johnno,

That's the type of answer I was looking for.

The sizes I've mentioned are the pitch sizes.

Yes I agree 5200 is lower than I would like it to be, I'm thinking of trying a Solas prop but didn't want to waste time trying a bunch of different props.

Edited February 4, 2008 by gone_wishin

[johnno]

if you can find out if the 2001 merc 2st props will fit your 4st

i think it might not

i have a 17pitch as a spare i could give you to try out if it fit.

i use it towing

it is the kind that uses the internal sleeves so the spline is on the internal sleeve rather then the prop

if you know what i mean

[a boat]

Hi Guys, all else being equal you should see about 200 RPM increase going down a pitch and Wishin at only 5200 at WOT you are way overpropped, with your engine much happier when propped to reach about 5800. Johnos prop will work becasue you guys have the same gearbox, and it would worth having a run but I am thinking you will really need a 16 inch prop with your combo and hopefully the engine has been rigged at least on the second hole, but better yet the 3rd hole is what we run most 75-115HP Mercs with the Vengance props.

Give it a try and see what happens,

Cheers,

Huey.

[squizzytaylor]

This is cool.

http://www.csgnetwork.com/marinepropcalc.html

Cheers

Geoff

[gone_wishin]

Thanks guys,

Huey, yes I was aiming to get it around 5,700 -5,800 rpm When you say the 3rd hole in reference to what? is that raising the motor higher with each hole?

Squizzy, I used the calculator on the Mercury website and it suggested a 13.125 x 17" prop

[a boat]

Hi Wishin, your Merc will have five engine mounting holes with the top hole on the transom brackets being hole No.1. We have found that the 2nd or 3rd holes work best with most boats running the engine you have and the higher the outboard is out of the water than the less drag you have resulting in better performance.

Going too high can cause cavitiation problems, but with years of experience we have found that the 2nd or 3rd work best. If you are on hole no.1 raise the engine at least 1 hole and try a 16 inch prop and your revs will be where they need to be to give your outboard the best chance of surviving. What we have found with any 4-Stroke is that they like to rev and having them labouring the whole time is not good for longevity. Unfortunately you are one of many boat owners that are running around without the best prop on the boat and really that boat should never of left the dealer with an 18 inch on it. The prop you have is what a 115HP 2-Stroke Merc would run on your boat and they were only rated to 5250RPM at WOT.

Cheers,

Huey.

[gone_wishin]

Thanks Huey,

I have not taken my boat back to the dealer and I would never go back there or recommend anyone else to.

This is just another reason why.

.......and what is suprising to me is they were voted the No. 1 Quintrex dealer for 2007, obviously not an award given by customers.

[a boat]

Hi Wishin, I know who you are talking and yes I do not think they deserved it also. I think they pump the boats out the door and do not really care about the end result.

Get yourself the 16 inch prop, raise it a hole or two and it will be what it should of been from day 1.

Cheers,

Huey.

[bolan]

what dealer are you guys talking about?

Mate, Rule 3 on Fishraider prevents us from slinging off at dealers or businesses etc.... Naming the dealer would be a big no no!

Cheers,

Stumpy

[gone_wishin]

A big thanks to Chris (lightning2K) and Huey.

Chris has loaned me his 16" prop and the results were good, the motor feels happier (if that makes sense) revs easier and I got to 5700 rpm. A lot better than the 5200 I was getting with the 18".

Once again thanks Chris and Huey.

[a boat]

Hi Wishin, good to hear and cheers to Chris for loaning you his 16 inch prop. The revs are now where they want to be for better performance, fuel economy and most importantly the longevity of the outboard. Worse thing you can do to any outboard no matter what brand or technology is over prop it causing it to "lug".

Cheers,

Huey.

[LIQUID NIRVANA]

Hi Wishin, good to hear and cheers to Chris for loaning you his 16 inch prop. The revs are now where they want to be for better performance, fuel economy and most importantly the longevity of the outboard. Worse thing you can do to any outboard no matter what brand or technology is over prop it causing it to "lug".

Cheers,

Huey.

Another VERY BAD thing to do is to run you boat half on plane pushing a great wall of water for long periods. It stresses the motor big time which in turn causes it to use a lot more fuel & because of running at low revs under stress, it prevents the motor from breathing properly. What this means is that the exhaust gasses cannot be scavenged from exhaust efficiently which in turn allows soot & carbon to build up. This applies to ALL outboards. This is a MAJOR source of engine wear because soot is very abrasive. Prop sizing is VERY VERY important. All engines should run at the UPPER end of its rev range, not at the mid or lower end. If a motor is propped correctly according to the boat load it will perform far better from idle through to full throttle. Please make this information known to all your boating buddies. Incorrect propping is VERY common & reduces the life of your motor.

Edited March 3, 2008 by LIQUID NIRVANA