15 or 17 pitch on a 89' 125 HP Force?

[dmas]

Re: 15 or 17 pitch on a 89' 125 HP Force?

Hi Hwsiii


That seemed weird to me also, the one I had before was a Michigan Match aluminum 13.5 x 17 and new one I bought it off ebay, it was a brand new Stainless Steel 13.5 x 15, they are both thru hub exhaust.

I didn't buy that 13 3/4x17 aluminum I ended up buying a 13.5x15 stainless steel.


I don't know the brand on the SS one because it only has a number that says P699, another thing that was happening that day I was having problem with my control cables, the shift cable had rotten and I was having a hard time getting it into gear, but the throttle cable was fine, could this be the cause?

I just replaced both cable and I will be testing it again next week, I'll probably use the 15" and take the 17" with me to do a on-sinte compassion.

 

[dmas]

Re: 15 or 17 pitch on a 89' 125 HP Force?

here's the link to the one I bought on ebay.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=370170744584&viewitem=&sspagename=STRK%3AMEWNX%3AIT

 

[Frank Acampora]

Re: 15 or 17 pitch on a 89' 125 HP Force?

HWSII: Pretend for a moment that the propeller has no pitch. (like a test wheel) That means it will not move the boat forward at all when spun. HOWEVER: It still takes horsepower to spin the prop. Since water has resistance, the faster you try to spin the prop, the more horsepower it takes to spin it. This translates to less horsepower available to drive the hull forward. Sure, you are increasing the RPM of the prop to offset the lower pitch, but you are also increasing resistance. In the case of a prop with pitch, it appears as increased slip.

On a light fully powered hull, this is not so apparent though. For example: On my 15 foot deep vee rated for 90, my 90 (with 2 to1 gearing) will turn 5500 with a 19 pitch stainless and 5000 with a 21 pitch stainless. Top speed is within 1 MPH (42-43 MPH GPS) with both props. But here, this hull only weighs 700 pounds and less than 1000 with the engine--Different animal than a 19 footer at 2300 pounds with a 125. AND HERE, changing from a 1.73 to 2 to 1 ratio and going from 17 pitch to 19 pitch raised the speed from 36 to 43. All props were Chrysler brand 13 3/8 diameter stainless manufactured by Michigan.

Changing the lower unit ratio on my 21 foot cuddy also raised top speed from 34 to 38 MPH.

Believe me: With my Chrysler and Force engines, a speed difference of 5-7 MPH is not unusual.

I don't usually state this because if someone does as I recommend and doesn't get the 5-7 MPH, then they think Frank is full of s**t.

While mathematics is a science and the laws are immutable, there are so many variables on a propeller in the water, that straight math can not predict the absolute outcome unless you have a supercomputer to crunch the numbers. Sometimes an educated guess and some empirical experience is also necessary.

 

[hwsiii]

Re: 15 or 17 pitch on a 89' 125 HP Force?

Frank, I have seen many of your posts, so I know you are very knowledgeable in this field. And I do agree that at some point in propping a boat it becomes an Art and not a Science.
Theoretically, the most efficient prop is 2 blades with a very large diameter turning as slow as you can make it go to meet your speed objectives. With outboard motors the aperture for the prop opening is only so large, that is why as outboards became more powerful so they could be used on larger and heavier boats 3, 4 and even 5 blade propellers became required to handle the loads and decrease the prop slip.

Since water has resistance, the faster you try to spin the prop, the more horsepower it takes to spin it. This translates to less horsepower available to drive the hull forward.

This statement does not take into account that because of reduced pitch there is less horsepower required, and in my opinion an increase of 10% in RPM getting closer to maximum RPM also gives you more horsepower because you are getting closer to the designed operating horsepower when you get closer to the maximum RPM and horsepower. Sure you can overprop a boat and get some extra speed at times, but when I have seen this done in the past the wrong props were being used and what is that doing to the engine and longevity. But normally if you get the right prop running at the top of recommended RPM range it will be faster.

Sure, you are increasing the RPM of the prop to offset the lower pitch, but you are also increasing resistance. In the case of a prop with pitch, it appears as increased slip.

If a boat has a prop with the a geometry and pitch that is close to right you will see a decrease in prop slip all the way to WOT, it does not have a real low prop slip and as it gets faster the prop slip increases as it gets closer to WOT. That matters not whether it is a 17" pitch or a 19" pitch if they are the same make and model on the same boat. If what you are saying was true about increased prop slip then that would bear itself out in the numbers, and it definitely does not do that.
Changing lower unit gear ratios at times does help speed, on that we agree. Just like for high speeds it is usually best to have props that are based on the diameter is half of the pitch.
Propping a boat is an ART in the end, that is why when people get ready to buy an expensive SS prop I defer to Dhadley, as i know he has the knowledge from many years of prop testing.