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Greyhound37 03-25-2012 03:37 PM

Fixed vs folding prop
 
Any guess as to the speed advantage of a 37' boat with a saildrive fixed vs folding prop. The fixed blades have a surface area of 35-38".(each blade is 7" x 3" +/-) What do you think, reaching 12 knots of wind... Fixed is 6.7knots on the polars.

msmith10 03-25-2012 04:12 PM

Re: Fixed vs folding prop
 
Not a simple question and as far as I know there's no simple answer or formula. Folding prop will be faster, that's about all you can say. 2-blade, 3-blade? Wind speed?
At hull speed it won't make any difference. In light air it will. If you spend most of your time racing, you'll want the folder, but then again if you race a lot you probably have one already. If you race occasionally but have to motor 3 miles up a river against 4 knot current to get back home, you may just decide to eat the speed loss. If you'd prefer to motor than sail when the weather gets rough, go with the fixed blade.
If I recall, PHRF gives you 8 seconds per mile for a 2 blade folder over fixed, 12 seconds for a 3 blade. It will cost you more than that, otherwise every racer would have a fixed prop.
If I had the money for a saildrive, though, I'd definitely spring for a feathering prop (slightly more drag than folding but better performance).

sailingfool 03-25-2012 04:13 PM

Re: Fixed vs folding prop
 
Half a knot.

msmith10 03-25-2012 04:17 PM

Re: Fixed vs folding prop
 
I was wrong. There is a simple answer, as good as any.

Greyhound37 03-25-2012 04:44 PM

Re: Fixed vs folding prop
 
8 seconds per mile or .5 knot, either way that is plenty. I was trying to decide if i want to replace the fixed prop now and guess i better.

msmith10 03-25-2012 04:58 PM

Re: Fixed vs folding prop
 
After replacing my 2 blade fixed with a folder several years ago, I went from 2nd-3rd place finishes to consistent 1st-2nd place in our club races. The folder doesn't back well, and it doesn't drive well in a chop, but I'd rather sail with rough seas anyway. It was worth the sacrificed performance under motor.

Greyhound37 03-25-2012 05:06 PM

Re: Fixed vs folding prop
 
Wow that is impressive. The Flexofold has cruising folding blades or racing using the same hub. I noticed that is the prop on several boats hauled near my boat Mumm 36, J-111
After spray applying VC offshore, would not be practical to be dragging a bucket under the boat. I hope to do 4 races this year but mostly cruise (as fast as i can go)
Thanks for the input!

Faster 03-25-2012 08:58 PM

Re: Fixed vs folding prop
 
We recently switched from a 2 blade fixed to a 3 blade Max prop.. the difference is remarkable and percentage wise much higher in the light air 3-4 knot boatspeed range than the powered up 5-6+ knot range. We probably reach our optimum upwind speed now in 5 knots less wind than before, and can ghost along at 3-4 knots where 2-3 would have been it before.

The feathering props do away with the folders' reverse issues, and 3 blades is much less vibration under power too... but at a cost, of course ($$$)

HUGOSALT 03-25-2012 10:16 PM

Re: Fixed vs folding prop
 
Simple answer of 1/2 knot seems about my experience with my
Flex o Fold, but as stated above by Faster equal advantage is
sailing in light conditions.
Have cruising blades attached to saildrive very good forward speed
and no apologies for reverse, can and have stopped on a dime.
If you go flex-o-fold make sure you or your installer, properly
installs "tab washer" (simple but important).
Also I used red loctite on my install, I will have to heat things up to remove, but hopefully will not be telling any stories about the
time I lost my $1000+ prop in the middle of nowhere.

blt2ski 03-27-2012 01:00 AM

Re: Fixed vs folding prop
 
If racing, the credit of 9 or 15 secs locally for a 2 vs a 3 blade prop is NOT any advantage, you lose more than that, and gain way more speed with a folding/fixed prop than the credit is worth. .5-.75 additional speed seems to be the norm in speed gain. As faster said, you hit X knots in less wind etc etc.

Marty


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