Old prop can not plane, new prop high rpm's

[Dae06]

I bought my 16' Lowe F/S 165 with a 2005 Mercury, 90 horse 4-stroke carburated motor. It came with a 13.25" diameter 3-blade 17° pitch prop. With 4 adults (~750 lbs.) sitting behind the windshield, I could not get the boat to plane without having my wife (~135 lbs) moving up to the bow of the boat. I decided to try a new prop. I went with a 13" diameter, 4-blade 15° pitch. It planes much better, but at WOT I'm at 6200 rpm on a 90° day. To me this is a little too high and on cooler days it will get worse.

Maybe my original prop was stretched a bit and not performing well. I'm thinking of a 12.75" diameter 4-blade, 17° prop. What's your opinons on this?

Thanks

[REW]

Dae,
I would suggest that you go with a 15 pitch 3 blade prop with the largest diameter that will fit under your cavitation plate - maybe 14 inches?

REW

[REW]

http://www.amazon.com/dp/B000XBFOHU/...SIN=B000XBFOHU

http://boatpropellers.iboats.com/Mer...t_id=890035370

[Dae06]

Originally Posted by REW

Dae,
I would suggest that you go with a 15 pitch 3 blade prop with the largest diameter that will fit under your cavitation plate - maybe 14 inches?

REW

I'm new to boating, can you explain why you suggest this? I realize the 15° pitch will help me plane better, but what will the 14" Diameter do (if it will fit; lower rpm's?)? Also with the 4 blade, it seemed to me, that I had better low speed control.

[REW]

Dae,
You are having an issue with a prop that does not give your motor enough load. i.e. with the 15 pitch prop, you are overrevving your engine.

In propeller design, even a small increase in blade diameter can change the load on a motor a great deal.

Since you went to a 15 pitch 4 blade prop and had overrevving issues, I am recommending a 15 or 16 pitch that has a lower pitch than your 17 to let the engine rev up, but also to go to a larger diameter to lot let the engine rev up too much.

You will find that with the lower pitch prop, you will have the better control and hole shot, whether it is a 3 blade or 4 blade.

I think that if you take a piece of cardboard and trace the outlines of each of the blades of your current 3 blade 17 pitch and do the same thing with each of the blades on your 4 blade 15 pitch prop, you will find that each of the blades on the 4 blade prop are somewhat smaller than the comparable 3 blade prop.

Again, prop designers do this to keep the motor load about the same - when going from a 3 to a 4 blade prop.

Good luck
REW

[Dae06]

After doing some reading I have found this out:

These are a "GENERAL" rule of thumb and I do realize each prop design is different.

But, it makes sense to me that each degree of pitch change will change ~200rpm's. (17° to 16°= 200 rpm increase)

Also each 1/4" of diameter will change rpm's 100 rpm. (13" to 13.25"= 100 rpm decrease)

Reffering in your suggestion, I believe I can go up to a 13.75" for my motor from a 13" diameter, keep the same pitch, and in theory drop 300 rpm's which should get me to 5900 rpm's WOT. I like that better.

[staylor]

...you are changing several variables, anyone of which can come back and haunt you- especially if you go to a different make or model of prop. But consider that with your present 4 blade you seem happy with everything but the top end rpm. So rather than buy a new prop, just take your 4 blade to a prop shop and have the pitch increased to 16 inches. Run this and if your need to reduce rpm further, have the prop shop add a little bit of additional blade cupping. I currently have 3 props I run during the year on my 150 Opti- all in the 21-23 pitch range when I bought them. Each one has had the pitch increased to get me to 5600 rpm, which is the max rpm my Opti likes for best top end speed.

The one flaw in the theory you list is that each prop design you look at has certain design aspects that are interrelated. Thus, if you could just scale your current 4 blade up to 13.75 diameter you may find that you now have too much blade area for best efficiency of the blade- and this will kill your hole shot and probably cost you a mph or two at top end. This is why many of the 4 blades for a given engine have less diameter than the 3 blades. Note that prop efficiency is not the same as slip- efficiency being the power being put into the water to push the boat forward vs the power available at the propshaft. The other design aspects to consider are blade rake angle and the amount of progressive pitch in the blade- since most props today are not fixed pitch. Instead, current props have the pitch actually change from the leading edge of the blade to the trailing edge. This is often why 2 props from different makers of the same pitch and blade type will end up with different top rpms. For example, my 23 pitch Trophy Plus revs to within 50 rpm of my 21 Trophy Plus- but the 23 actually has more like 24 inch pitch, since I reduced blade area with a leading edge cut- taking off the lower pitched section of the blade, but reducing blade area at the same time. My goal was to hold slip, but change the blade's angle of attack to the water surface- thus improving the blade efficiency. On this prop it worked....and it worked on a 22 Raker I had modifed back in the 1990s, but don't ask me about the rare and expensive 23 pitch OMC SST-RX V-4 Chopper that I screwed up totally with the same theory!
Doug

[Dae06]

Thanks, Staylor

[REW]

Dae,
Careful in modifying a brand new prop as you currently have in your possession.

Before I would take a multi hundred prop and modify it; it would be if you were positive that you knew exactly what and why you wanted to do something with that particular prop.

Rather, I would just buy a new aluminum $100 prop of what you think is the right pitch and diameter and see how it goes from there. It is really surprising the excellent performance that one can get from a simple OEM aluminum prop that is the exact right prop for the motor and rig that you are trying to push. Sure you might get 1 mph out of a stainless prop - but most of the time that is about all that you gain, unless you have a very high powered motor that can flex an aluminum prop too much.


If it turns out that your current 4 - blade prop is really not the right prop for you, you can likely resell it for essentially what you paid for it. That may not be the case if you go and modify it from its factory new condition.

REW

[Dae06]

My local Mercury dealer suggested that I use the 4-blade, 15° prop when ever pulling tubes, skiers or when the boat is loaded. Whenever I fish with just one other person with me, use the 3 blade, 17° prop.

This could work, but also could get to be a pain because I may fish in the morning and pull skiers in the afternoon and then go back to fishing that evening. I don't really see me changing props each time, but I appreciated his suggestions and the fact thar he didn't try to sell me a prop, just to make a little money.

The most important thing to me is to be able to plane out without moving people around the boat, so I will just leave the 4-blade prop on and limit my rpm's to under 6000. Losing a couple of mph doesn't bother me at all.