Best store for boat prop advice?

[Bduck]

I used Willard as my lowest elevation plus it helps being a couple of miles from house to see where rpm's were with the 20 pitch. All is based on sea level performance utilizing a standard day condition which is barometric at 29.92 @ 59°. Yamaha top end rpm should be 6000-6200. I was getting 3800R's and 33mph. New prop put me at 6000R's and 40mph depending once again standard day condition. I hit a good day that particular day. Bear L I drop back maybe from 5700-5800 speed varies a little, Strawberry I hit approx 5500 with speed varying again. 
First you need to know where the manufacturer specification top end rpm should be. Then you can figure or get help to figure the prop size to help achieve this with your current info at elevation.

[N.E.T.O.]

Looking for a new or rebuilt prop for my 90 HP evinrude. The one I have does alright at Willard but not so good at Bear lake, the RMP's are not going up as high as they should, especially at BL, so I was thinking that a smaller prop would do the trick. My prop is a 15 & 1/2 x 13, I was thinking a 15 x 13 might do the trick.

Props...been there done that way too many times! 

Although my current boat is a simple 16' Aluminum boat, I've owned 7 or 8 larger/faster/bigger/better family and fishing boats as that was our favorite family activity before my kids all grew up and moved away.  We literally went boating/waterskiing every weekend in the summertime (or spring and fall on warm days) for about 15 or 20 years in a row.  It always amazed me when buying a new (to me) boat how the prior owner NEVER had the right prop on it!

Here is what I found out about re-propping a boat after having to do it to every single boat I ever owned:   It's not a science.  There is no chart anwhere that will tell you what pitch or diameter you need to get to gain speed or gain power.  There are general guidelines  that might help.  Like, if you want to go faster, get a higher pitch. If you want more power, get a lower pitch.  You can also thow some diameters and rake designs in there.  Don't forget prop material!  Stainless and Aluminum act differently.   The only way to find out if a prop is going to do what you want it to do is to try it out on your boat, on the body of water that you are trying to optimize it for.  Period.

There used to be a great shop called Jacks Prop Shop here in Salt Lake.  For a small deposit, he would loan you a few props all at once to try out.  Most of the time I ended up buying the perfect prop from him (used or new) but in those instances that I couldn't find one that didn't work, there was no pressure to buy.   Of course it helped that he knew me by name and had a revolving door just for me installed in his shop!    Seems I visited him at least half a dozen times per summer.

My boating friends and I always wanted more power to pull up a water skiier (back when people used a slalom ski, not a surfboard) or more speed if we were going to Powell or Mead, etc.  I usually ended up owning at least 2 and sometimes 3 props for each boat I owned...not counting the repairs I had done on numerous occasions.

Your best bet is to go to one of the shops mentioned above (Jack is no longer in business), and ask if they will work with you to find a prop and let them know you might need to test a few, but will likely only buy one.  Hopefully you can find someone that will work with you otherwise it could get expensive and you'll end up with 3 or 4 new props that are worthless to you and one good one.

However, based on your post and your current prop size of 15.5" diameter and 13 pitch, and wanting more power/RPM's (not speed), sounds like you might want to try a 11 pitch prop (usually props are odd numbers...13, 15, 17, 19, 21, etc.) , same diameter to start with.  Although I have to admit I've never seen an prop pitched that low.    That SHOULD give you more RPM's, and sometimes at no loss of speed, depending on the power curve of the motor and how it's loaded and set up.   Sometimes a smaller diameter works too for that, but I've seen more cavitation happen when doing that. Sometimes the lower pitch gives you too many RPM's, so they can then CUP it for you to make a 1/2 size (ie: 12 pitch) which sometimes is just right. Sometimes shifting the weight around (heavier stuff up front...ie: batteries) is just as good as a re-prop, and sometimes people just have to take out a bunch of weight that is usually not used to get some power back.  And sometimes, it's not the prop at all...it's the engine...or the gears in the lower unit!  (High altitude gears/low altitude gears, etc.).  I've seen a lot of V8's that are really only V7's, etc.  Outboards can have a dead cylinder and most people don't even know it.

Hope this helps.  It can get frustrating, and time consuming, and sometimes you end up just using the same prop you started with...sometimes that ends up really being the best prop.  Watch out and don't get one that gives you TOO MANY RPM's, that is just as bad as too few. Sometimes a prop that you didn't think would work will be the magic prop even though it defies the rules!

One other last thing I found out when trying props...try it the way you are going to be using it.  If you usually have 8 people on board trying to pull up a slalom skier...test it out that way.  Fishing at Bear Lake with 2 people and gear aboard?...try it out that way.   Good Luck.   Props and Wheel bearings were the two main reasons I was happy to sell my last big boat!

Randy

[Anglinarcher]

Looking for a new or rebuilt prop for my 90 HP evinrude. The one I have does alright at Willard but not so good at Bear lake, the RMP's are not going up as high as they should, especially at BL, so I was thinking that a smaller prop would do the trick. My prop is a 15 & 1/2 x 13, I was thinking a 15 x 13 might do the trick.

You will find that most boat dealers have about as many opinions as there are colors in the rainbow.  I often use to hear, give this one a try.

I now suggest the following web site.  If you fill in all of the information, it will give you a correct diameter and pitch, then you find something locally that fits inside that direction.  It takes a lot of information, but it is the best way to save money and time by not just testing what a shop has in stock.

https://www.mercurymarine.com/en/us/prop...!/step-one