Prop advice for 1990 Force 90 on 17' Bayliner Capri

[jerryjerry05]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

1 inch either way can be as much as 100 rpm' up to 500 rpm's.
Depending on the boat,motor,load.Trim/tilt???
Still wondering about the compression #s???
Now I'm gonna pout till I get ANSWERS!!!!

 

[snowbrd84]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

1 inch either way can be as much as 100 rpm' up to 500 rpm's.
Depending on the boat,motor,load.Trim/tilt???
Still wondering about the compression #s???
Now I'm gonna pout till I get ANSWERS!!!!

Ordered a compression test kit yesterday from Amazon, will be here tomorrow. I needed it anyways as I want to check the compression on my Jeep engine after changing the head gasket. Should be able to check it tomorrow. And I am pretty sure that the +/- RPM due to change in diameter and pitch is a very ballpark figure and varies by motor and situation. I am a civil engineer, and thinking about the way an outboard operates, its basically a variable speed pump. Instead of an impeller spinning in a fixed location and creating a pressure differential that moves a column of water from the high side to the low side, the prop spins and creates a pressure differential that moves the boat through the water. If you had an equivalent of pump curves for each prop size for the outboard, you would select the prop that outputs your desired top end speed at 90hp and 5500 RPM WOT and falls within the highest efficiency band. Now you could vary the pitch and diameter to come up with different combinations of props that would output that same speed at 90hp and 5500 RPM WOT, but they would be operating at lower efficiencies, so burning more gas and more stress on the engine. The maximum HP output is 90hp at 5500 RPM right? As you move away from that point, you lose efficiency and begin to damage the motor. So, your options will be limited. The force necessary to move the boat through the water is required to calculate the speed at which the boat will move at 90hp output. So I have a hard time believing that there is a reliable increment for +/- RPM when you change pitch/diameter. I can see there would be a consistent relationship within a narrow band surrounding the optimal size, but once you move farther away from that, the loss or gain of RPM would become less consistent. Someone who has no clue, like the guy i bought the boat from, probably just thought you could pick as long of a pitch as you want as long as you just decrease the diameter. Just doesnt work..

So all in all, there are a lot of combinations of pitch/diameter that would give you the right max speed at 5500 RPM WOT, but there is one exact combination that will do it the most efficiently.

 

[dazk14]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Snobrd84 - could you clarify your two previous statements...

" Now at the age of 18, finally bought my first boat" and " I am a civil engineer".

Thanks

 

[snowbrd84]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

haha didnt notice that, typo, I'm 28.

 

[dazk14]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

haha didnt notice that, typo, I'm 28.

Sounds good. I was beyond impressed when an 18 yr old wrote about the pitch/rpm change relationship when it falls out of the rather narrow powerband. I'm still impressed.
There are lots of internet experts that haven't figured that out and that diameter can significantly influence the equation.


You'll be glad you have a compression tester. Ground the spark plugs when cranking for the health of your ignition system. Next time around, just buy a metal socket from Napa, etc.. The plastic wrenches are junk (presuming that's what you ordered). We no longer stock them.

 

[snowbrd84]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Yeah I remember sitting there in my Hydraulics class thinking to myself how boring it was, but now its actually kind of interesting. I did get the prop wrench, and agreed, its junk. But it will just go in the boat in case I ever need it... I bought a socket. I would rather torque the prop nut correctly anyways...

 

[Tail_Gunner]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

I love diameter talk...The only known outcome of diameter is revolutions will go up with a small diamter and down with a large. So i will ask a question that will lead to a pretty easy answer. Question what prop.. large diameter... high rake..... heavy cupping can anyone point to...

 

[Frank Acampora]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri



Well, You can argue prop size ad nauseum, but the fact is that the engines are old school and were designed for a rather narrow size of boat---except in the case of a pontoon--- they were not designed to move the Queen Mary. In fact, the maximum diameter that can fit on the prop shaft is 14 inch because there is just over 7 inches from centerline to cavitation plate. However, lower units designed to use a 14 inch diameter prop have a little over 8 inches from prop centerline to cav plate. Stainless, aluminum, diameter, pitch, rake, etc. ---doesn't matter how you argue it! The fact still remains that the design engineers want the engine to use a 13 inch (nominal) diameter prop. Same thinking applies to rake. There is simply not enough room between thrust washer and exhaust snout to use a high rake prop (like the ballistic). To use these props you must do modifications. Look at the clearance after my mods.

Thus, for this 17 foot pleasure boat the only correct diameter is 13 to 13 1/2 inch and the pitch will be somewhere near 17 to 19 inch, most usually with a normal rake, cupped or uncupped prop.

My numbers may be slightly off, but I believe the 90 was still in 1990 rated 90 hp at 5,000 RPM with the power band or operating range at wide open throttle between 4500 and 5500. Thus, the OP would look to prop the engine to acheive maximum boat speed at some point between 4500 and 5500 (if he wanted to maximize top speed.)

 

[jerryjerry05]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Add this to the mix.
A 17p prop is supposed to move the boat ahead 17inches in 1 revolution
A 19p is supposed to move it 19 inches.
But a motor will not work as efficent if the load is too small or big.

 

[jestor68]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

I love diameter talk...The only known outcome of diameter is revolutions will go up with a small diamter and down with a large. So i will ask a question that will lead to a pretty easy answer. Question what prop.. large diameter... high rake..... heavy cupping can anyone point to...

These are 15.25 inch diameter.



These (15 and 18" dia) fit Bravo 1 XR Sport Master drives.

 

[Tail_Gunner]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Is the bravo high rake??? That cleaver is but its a small diameter.

 

[jestor68]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Is the bravo high rake??? That cleaver is but its a small diameter.

The clever available for stern drives comes in 15 and 18 dia. As far as what constitutes "high rake"; that's for you to decide. I would assume that the smaller outboard props need more rake to compensate for their small size; where the big boy stern drive props don't require as much due to their larger diameters and 5/6 blades.

Suggest a visit to the Mercury Racing site. Interesting reading about drives and props designed "for boats capable of 90+ mph.

 

[Tail_Gunner]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Subjective opinion's as to rake...ok But lets try to keep on the effects of diameter..Just basic's we can see... so lets see if there is a design that incorprates large dia....high rake and strong cupping. Left click on the prop jpeg you will see that's from the merc site and they comment on the bravo's rake...Flat rake 15 degree's.

 

[jestor68]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Subjective opinion's as to rake...ok But lets try to keep on the effects of diameter..Just basic's we can see... so lets see if there is a design that incorprates large dia....high rake and strong cupping. Left click on the prop jpeg you will see that's from the merc site and they comment on the bravo's rake...Flat rake 15 degree's.

They are available in 15,18, and 21 degrees of rake as well as varying pitches and diameters from 15-18". Which would you consider "high rake"?

Since you seem to be siting recreational props, and I'm looking at "racing" props, we're in a apples and oranges discussion.

Your example is from the Mercury Marine (recreational)site; not the Mercury Racing site.

You asked if larger diameter, high rake, progressive pitch props exist. Without any ground rules; like what constitutes low, medium, and high rake; this discussion goes nowhere.

How bout this: 15 degrees=low. 18 degrees=medium. 21 degrees=high. Over 21 degrees=higher.

I win!

 

[Tail_Gunner]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

They are available in 15,18, and 21 degrees of rake as well as varying pitches and diameters from 15-18". Which would you consider "high rake"?

Since you seem to be siting recreational props, and I'm looking at "racing" props, we're in a apples and oranges discussion.

Your example is from the Mercury Marine (recreational)site; not the Mercury Racing site.

You asked if larger diameter, high rake, progressive pitch props exist. Without any ground rules; like what constitutes low, medium, and high rake; this discussion goes nowhere.

How bout this: 15 degrees=low. 18 degrees=medium. 21 degrees=high. Over 21 degrees=higher.

I win!

http://www.mercuryracing.com/blog/wp-content/uploads/2011/07/Rake_Images_2.jpg

Surely your jesting jestor high perfromance only take a basic principle farther.

You asked if larger diameter, high rake, progressive pitch props exist. Without any ground rules; like what constitutes low, medium, and high rake; this discussion goes nowhere.

While i do not know for a fact there is no such prop...Think about it High Rake...large diameter heavy cupping....Lots of blade area there... Here's a test site that illuminates large diameters and what they do well and ties into your thoughts on HP racing..High HP and heavy pusher's

http://forums.iboats.com/showthread.php?t=218877

HEAVY LOAD PROPS
MANUFACTURE RPM MPH NOTES
MWC 013052 15X15 6550 42.1 PUSHES 800LBS OF SAND BAGS LIKE THEY ARE FILLED WITH AIR........................
LEXOR 15.75X15 6500 41.9 “............................................
MIRAGE PLUS 15.75X15 6500 41.7 “..........................................
MANUFACTURE RPM MPH NOTES
OMC 431930 15X17 6300 44.6 SLOWER HOLE-SHOT....
LEXOR 15.5X17 6250 45.2..........................................
VENGEANCE 14.5X17 6300 44.8 LITTLE SLOW OUT OF HOLE
BALLISTIC 14.75X17 6100 44.1 NOT A GOOD LOAD PROP
MANUFACTURE RPM MPH NOTES
MWC 14.5X19 5850 47.6 SLOW HOLE-SHOT-NO VENTS
VENGEANCE 14X19 5850 46.9
MANUFACTURE RPM MPH NOTES
MIRAGE+ 14.75X21 5500 48.6 HOLE-SHOT BETTER THAN EXPECTED
LEXOR 15X21 5500 49.1 HOLE-SHOT AS ABOVE WITH GOOD MID-RANGE ..........................

TURBO 1 14.25X21 5550 48.0 VENTED BUT PRETTY SLOW HOLE-SHOT WITH MAX LOAD
VENGEANCE 13.75X21 5600 48.2 NO VENTS AND SLOW HOLE-SHOT, BUT GOOD MID-RANGE


ALUMINUM DEEP-V, 200 EVINRUDE, PERFORMANCE PROPS
MANUFACTURE RPM MPH NOTES
OMC 15X17 6350 48.1 GOOD HOLE-SHOT.....
TURBO1 14.25X17 6300 50.9 “
STILETTO 14.25X17 6300 50.2 “
VIPER 14.875X17 6200 48.8 “
BALLISTIC 14.75X17 6150 46.6 VENTS EASILY WITH TRIM
RAPTURE 14.25X17 6250 47.4 VENTS ON HOLE-SHOT
MANUFACTURE RPM MPH NOTES
RAPTURE 14.25X19
6100
49.9
MWC 14.5X19 6100 52.0 GOOD HOLE-SHOT
BALLISTIC 14.5X19 6000 50.6 HOLE-SHOT SLOWER
TURBO1 14.25X19 6050 53.1
OMC SST 14.5X19 6100 52.1

Do you see any trend's there????

 

[jerryjerry05]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

You get the compression/spark results?

 

[jestor68]

Re: Prop advice for 1990 Force 90 on 17' Bayliner Capri

Do you see any trend's there????

Uh, I'm sorry; wasn't paying attention.

What was the question?