prop question

[smokeonthewater]

I disagree..... a 4 blade 17 would replace a 3 blade 18 or 19 (rule of thumb) but stainless to alum there is no automatic pitch change.... I had a 19 stainless and a 19 alum for the same boat... I got about the same holeshot with each but I got about 100 more rpm and 2-3 more mph with the stainless

[smokeonthewater]

Read this

http://www.formulapropeller.com/propellers.html

[Destroyer]

Originally Posted by smokeonthewater

I disagree..... a 4 blade 17 would replace a 3 blade 18 or 19 (rule of thumb) but stainless to alum there is no automatic pitch change.... I had a 19 stainless and a 19 alum for the same boat... I got about the same holeshot with each but I got about 100 more rpm and 2-3 more mph with the stainless

Perhaps, but a 4 blade prop is not the same as a 3 blade prop. I said twice in my post all other things being equal. 4 blade props will give you a much better hole shot since there is 1/4 more blade area in the water...but 4 blade props will also result in more drag and a slower top end. They're great for tubing and water sports, but generally not recommended for racing or going fast..

As for an automatic pitch change, that's true, there is no such thing, because there are so many diferent variables, even between two props from the same manufacture. BUT, as a general rule of thumb, I've always found that when you switch from an Alum to a SS prop you wind up dropping 2 inches of pitch and usually get better preformance. Not all the time, not always, but generally speaking.

The reason you were getting more RPM and 2-3 more mph with your stainless is probably because at top end your aluminum prop was actually flexing slightly and you wound up running a prop with about an 18P at WOT. Stainless props don't flex like that so it was performing better.

[smokeonthewater]

well that's possible but I think it's more likely that it was due to a better designed prop... I don't believe that the alum flexes that much (some, yes) but rather the manufacturers don't generally put near the engineering into a $100 alum prop that they put into a $400 SS one.... As I understand it 90% of the performance difference is generally in the engineering

I understood what you were saying but the point is that if I had dropped 2" of pitch I would have been over rev'ing....

The reason I mentioned the 4 blade is that I think you mixed up the rule of thumb that I've always heard for 3/4 instead of al/ss

DEF not trying to point fingers... I guess you and I have just had different experiences w props....

as it is, he's not buying a prop right yet anyhow so it doesn't much matter

[Destroyer]

Originally Posted by smokeonthewater

well that's possible but I think it's more likely that it was due to a better designed prop... I don't believe that the alum flexes that much (some, yes) but rather the manufacturers don't generally put near the engineering into a $100 alum prop that they put into a $400 SS one.... As I understand it 90% of the performance difference is generally in the engineering

I understood what you were saying but the point is that if I had dropped 2" of pitch I would have been over rev'ing....

The reason I mentioned the 4 blade is that I think you mixed up the rule of thumb that I've always heard for 3/4 instead of al/ss

DEF not trying to point fingers... I guess you and I have just had different experiences w props....

as it is, he's not buying a prop right yet anyhow so it doesn't much matter

Yep... totally agree. The fact is that props are so infinately variable that it's almost useless to say "this is what you have to do" vs "That is what you have to do". The ONLY way to know what prop is right for your boat is to actually know what you want your boat to do, and then try several different props until you get the one that's dialed in for your needs.

[aussie]

you guys still haven't answered my question anyone else on this forum can help lol AUSSIE

[smokeonthewater]

sure we did but I'll translate to kiwispeak.....
No it would not be correct to say ANY of what you said

read the link I posted in my first comment....

[spareparts]

I've been over this time and time again, pitch and diameter are a starting point, blade composition is not that important till you get above 60 mph. Design and profile are the most important thing about props. I can take 10 different 19 pitch props and all ten will perform differently. Blade surface, rake, number of blades, blade design, ventilation holes, over hub or thru hub, cup, tip roll, etc.......They all make a difference. In lower speed props, generally the aluminum wheels are more of a generic design and perform good all around, lower end stainless props duplicate the performance of their aluminum counterparts. Every put a stainless prop on a Volvo single prop out-drive? It will run slower than the aluminum one. When you get into higher end props, they generally have more specific design goals(higher prices too).Don't always go just on the pitch either, there are several prop manufactures that rate their pitch differently. Mach was real bad about this(they are out of business now), their 20 pitch four blade checked out at 24 pitch on the blocks. I've had some props that none of the blades were even close to the same pitch(Eagle and Turbo). I've sent props to have them lab finished(by Merc racing) and the boat slowed down afterwards. Point being, they are very loose guidelines, until you bolt it on the shaft, you will not know for sure. Aussie, from my experience with the 3.7, you'll be hard pressed to find a better wheel for what you are doing than a Mercury Mirage plus. you need to base line the engine with a known unaltered prop to get a feel for where you need to be, but from there own, its trial and error. BTW, its hard to beat mercury props.