Where would you start?

[Rapid Racer]

Our racing we use 9.9 hp outboards, we have two people, flat bottom, hydraulic jacking transom. We have alot of stopping & starting with some long straights.
We have to use the manufacturers original props.
So they come in 9.25" x 6" up to 10" pitch. (there seems to be alot of 9" used)
Once you have your prop any modifications are allowed to it.

I thought I would throw it out there to you all, where would you start with modifying a new stock prop and what pitch to start with?

Then we test test test.

[Tomtall]

Our racing we use 9.9 hp outboards

What manufacturer?

So they come in 9.25" x 6" up to 10" pitch.

Three blade? Two Blade? Alum.? Stainless?

[JohnsonM50]

Our racing we use 9.9 hp outboards, we have two people, flat bottom, hydraulic jacking transom. We have alot of stopping & starting with some long straights.
We have to use the manufacturers original props.
So they come in 9.25" x 6" up to 10" pitch. (there seems to be alot of 9" used)
Once you have your prop any modifications are allowed to it.

I thought I would throw it out there to you all, where would you start with modifying a new stock prop and what pitch to start with?

Then we test test test.

The best answer here is see what the front runners are doin. Seems like traction & acceleration are important even more than top end. Id recommend stainless steel once you know what prop is best. As far as mods, its risky to mess with a good prop, find out whos doin the props for the competition.

[Rapid Racer]

Ours is a Suzuki, three blade aluminum, thru hub exhaust. All the manufactures here only supply three blabe, aluminium prop for this size outboard. The sports class can use any prop they can find and all go with a stainless racing style chopper 3 blade. We don't qualify for this class yet. We have one good prop and it seems to have a fair bit of grinding and shaping from a standard. What we intend to do is play with the shape on a couple and do speed/acceleration tests. Hole shot is very important, practically with 2 people onboard. We fight our way thru trees and over rapids, so we are slowing and going lots. That said there is a still long straight where top end comes into play. One race has 30km of flat water at the end of 100km of broken river. Everyone stops and changes their props for the run home. Some races are purely flat water with 8km straights.

So we need to develop top end and acceleration props for different races. I haven't looked at getting a prop god to hit one for us, where would I find a good one in this corner of the world. I'm still looking for suggestions as to what you would do if you had to.

When you say 'traction' what is that?

[JohnsonM50]

[QUOTE=Rapid Racer;74310]Ours is a Suzuki, three blade aluminum, thru hub exhaust. All the manufactures here only supply three blabe, aluminium prop for this size outboard. The sports class can use any prop they can find and all go with a stainless racing style chopper 3 blade. We don't qualify for this class yet. We have one good prop and it seems to have a fair bit of grinding and shaping from a standard. What we intend to do is play with the shape on a couple and do speed/acceleration tests. Hole shot is very important, practically with 2 people onboard. We fight our way thru trees and over rapids, so we are slowing and going lots. That said there is a still long straight where top end comes into play. One race has 30km of flat water at the end of 100km of broken river. Everyone stops and changes their props for the run home. Some races are purely flat water with 8km straights.

So we need to develop top end and acceleration props for different races. I haven't looked at getting a prop god to hit one for us, where would I find a good one in this corner of the world. I'm still looking for suggestions as to what you would do if you had to.

When you say 'traction' what is that?
[/QUOTE ]By traction I mean a prop that will take off w/o much slippage or cavitation & will hold in turns w/o blowing out. When you choose a prop to work with it should start as a good 1 but Id still keep your best as is. The 2nd best can then become the best. If you go into this a little at a time it might go well.
Take the prop & 1st thin it, leaving the rear [drive surface] alone & cut material from the front. Use a caliper to measure from the hub outward about every centimeter from about a cm in from the leading edge & so on across each blade. Mark with dots as ya go then connect the dots with lines per row. A pattern will develop [like a topographic map] to show thicknesses. The prop should taper evenly out from the hub. You may be surprised how precise a prop is not by doing this accurately. Then put a piece of paper on a soft surface, press the prop into it & trace each blade.. # them. Now your ready for the 1st cut. Start with the smallest blade, clean it up so the measure lines show an even progression along the blade. Carefully cut the other 2 to be an exact match. Balance is important & can be done by putting a well fitting shaft thru the prop & make up a parallel bar for it to roll on, the heavy blade[s] will go down. [when you can set it in any position w/o rolling its good] Test time. You should of course know 1st what the prop does & test on the same setup. Next step would be more thinning by that proceedure. For the most part a stock prop can be improved on its mass production condition but beyond that cupping, changing pitch or dia can be done but then becomes experimental based on what 'should' work. There are threads here on BRF that you can look into more.

[Rapid Racer]

Thanks Mike, that’s exactly what I was after. I'll try and take some photos as I go.

When we get to the stage of looking at cup, do we ‘gently persuade’ (read: hit with hammer) the blades over something round or do you have a better method?

Thanks

Daniel

[JohnsonM50]

Thanks Mike, that’s exactly what I was after. I'll try and take some photos as I go.

When we get to the stage of looking at cup, do we ‘gently persuade’ (read: hit with hammer) the blades over something round or do you have a better method?

Thanks

Daniel

When you cup Id say be careful about hammering, the aluminum tends to weaken & crack Its probably the 2nd most economical prop material [comp-props are cheeper] but not the best. A couple lines down from your post is the Thrust Factors thread, reading that over & a look at the links would help, waaaay more knowledge than I bring as Im only talking from seat of pants experience that includes making some ok props into duds. [why to keep the best as is ]

[Rapid Racer]

Well, time is a tickin and we are running out of time before our first major race. So needing 5 or 6 new props just to get us to the finish line (ie 20-30km per prop!), we are getting them cut by someone else. $60 (AUD) each for cutting and $170 (AUD) for the base prop, is not to bad but it's also something else adding to the rising cost........ now where did I put those sponsors I had laying around????

[JohnsonM50]

Well, time is a tickin and we are running out of time before our first major race. So needing 5 or 6 new props just to get us to the finish line (ie 20-30km per prop!), we are getting them cut by someone else. $60 (AUD) each for cutting and $170 (AUD) for the base prop, is not to bad but it's also something else adding to the rising cost........ now where did I put those sponsors I had laying around????

Havn't seen em