Prop advice for 1990 Force 90 on 17' Bayliner Capri

[snowbrd84]

I've been boating my whole life (dad's boat). Now at the age of 28, finally bought my first boat. It is a 1990 Bayliner Capri 17' with a 1990 Force 90 Outboard. The guy I bought it from was, to put it lightly, full of you know what. I get it out on the water (after fixing a bunch of stuff he "forgot" to mention) and open it up and I am WOT at 3500 RPM pushing 26knots on the gps. This is obviously way wrong. Should be WOT at about 5000-5500 RPM and pushing (im guessing here) 40 knots ish? Just had me and my gf (300lbs total) and about 15 gals gas... The guy i bought it from said the prop is a 10x23 stainless steel prop and the boat goes 50mph. Well I have already came to the realization that everything he said is complete bs. I am guessing he put on the 23 pitch prop having no clue how boat motors actually work. There are no markings on the prop other than what looks to be a "GM" or "CM" stamp, I cant take off the prop until my prop wrench gets here in the mail on thursday, I have every socket size known to mankind except for a 1 1/16th".

So yeah, long intro, but my question is, after reading for hours and hours, it seems I should be using a 13x19 prop for this engine??? And my second question is, I found a guy on CL that has a 13x19 aluminum and a 13x19 stainless that he says fits a Mercury Inline 6, he thinks its the same fit as the Force. He is a 45 minute drive from me, so I would rather not go out there if it isnt going to match up. Anyone know if these are the same? He only wants $40 for the aluminum and $125 for the SS (both are Quicksilver), but would the stainless be better? Dont mind paying the extra money, but want to make sure I get the right one, my understanding is you should get the next size down if going SS? Any help is much appreciated...

 

[emoney]

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

I don't think the 6cyl Merc prop will fit. They're a different sized hub. Mercury props from 4 cylinders work, but not the 6's, I'm pretty sure.

Definitely pull the prop before you buy anything to confirm what's on there, but it does sound overpitched. It also seems like it's rather narrow if it's a 10x23. I'm running a 19 prop on my 120 and it hits around 4800ish RPM, pushing a pretty heavy boat in the mid 30's to as much as 37 when it's calm. My Trophy 1802 is a lot heavier than your Capri, so you should be getting better top-end speed, even with your 90 I'd think just due to the weight. If nothing else, you should be more than where you are.

I've always heard you should go down in pitch with a Stainless prop, so if the 19 is the best in Aluminum, a 17 in SS should work. Definitely want to get the RPMs up, but make sure your tachometer is working correctly too. A mini-tach isn't very expensive and is handy to have.

 

[SkiDad]

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

it should fit - i would get the Aluminum one - that way if you don't like it it can be your spare. I think 19 is good if you don't plan on a lot of people. if you do want to take 5 people or ski alot you might need a 17 pitch. I think you might need a thrust washer or spacer - but I'm not 100% sure on that year - my force is a different style.

 

[snowbrd84]

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

Thanks guys, I do plan on have 4 or 5 people on the boat from time to time to go skiing, but that wont be till next year since its a tad bit cold out here in maryland now. So I will get a 13x19 aluminum I think and try that out this weekend. I will definitely pull the prop that is on there to make sure the one I buy will fit.

 

[SkiDad]

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

maybe you should have him measure the hub before you go out there. I'm pretty sure the force's take a 4 1/4 " hub - he might have a 4 and 3/4.

 

[jestor68]

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

It should fit if it's from the older in line 6.

 

[jestor68]

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

maybe you should have him measure the hub before you go out there. I'm pretty sure the force's take a 4 1/4 " hub - he might have a 4 and 3/4.

The 1995-1996 Force motors of that horse power( 90, 120) are listed as having the intermediate 4.25 inch gear case.

 

[jestor68]

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

For perspective; I had a '94 18 ft Capri with the 120 Force.

It came with a 21", but wouldn't pull skiers very well. I switched to a 19" and it worked well, turning 5500 rpm.

My feeling is that your 90 MAY do OK with a 19", but would be better all around with a 17", especially if you intend any water sports.

Your WOT rpm range is 5000-5500.

 

[jestor68]

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

Thanks guys, I do plan on have 4 or 5 people on the boat from time to time to go skiing, but that wont be till next year since its a tad bit cold out here in maryland now. So I will get a 13x19 aluminum I think and try that out this weekend. I will definitely pull the prop that is on there to make sure the one I buy will fit.

With that load, pretty sure you will need a 17".

 

[Frank Acampora]

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



This is a 1990 three cylinder 90 horsepower Force on a 14 foot flat bottom hull. It does 55 MPH with a 21 pitch prop. (it was later swapped out with a 125 that reached 60) SO: Anything over a 19 pitch on your hull will be too much AND 50 MPH is out of the question--40? Maybe.

Your engine has the small gearcase (variously described as 4 inch or 4 1/4 inch) and you must be careful about which Mercury props you buy. Your engine SHOULD have the dual exhaust lower unit. If it does, then you must also buy thru-hub exhaust props as shown in the photos below..



If you want to drive up here ( Near Philly. More than 45 minutes.) I have a bunch of the correct props in stainless for your engine. But, they are not cheap. Send a private message if interested. I do go to the Chesapeake a couple of times a summer so if still interested next season perhaps we could meet, say at the Inner Harbor

 

[jerryjerry05]

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

The Mercury props probably won't work.
You should have a 17/19p prop.
Get the 17 and then go for a ride???
Michigan wheel sells a good replacement.
Your local Mercury Dealer can get them too.
This site sells props too.
Stainless props are nice but not for the boater that's gonna find sandbars or shallow water.
New boaters usually find these things with no problem and end up chewing the up prop.

But first do a compression spark test.
You should be doing way more rpms than 3300 with a 23p.
Look between the blades for #s or letters to help identify your prop,under the nut too.

 

[snowbrd84]

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

So my understanding is that for every 1" pitch increase, you lose about 200 RPM. Additionally, a SS prop (which is what was on there) equates to an aluminum prop 2" larger in pitch to achieve the same RPM. So I was essentially running a 25" pitch aluminum prop. So that would mean a loss in RPM of about 1600 from 5000, so about 3400 RPM. Which is what it was running at WOT. However, decreasing the diameter to 12" which is what that prop was, should gain RPM at about 200 RPM/.25" decrease. So "theoretically", the increase in pitch would be offset by the decrease in diameter.

My guess is, that is what the guy thought when he put it on there. In reality, Im thinking that the relationship is exponential, not linear, and once you exceed the limit of the pitch the motor can handle, you lose more RPM per unit increase in pitch.

 

[dazk14]

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

So my understanding is that for every 1" pitch increase, you lose about 200 RPM. Additionally, a SS prop (which is what was on there) equates to an aluminum prop 2" larger in pitch to achieve the same RPM. So I was essentially running a 25" pitch aluminum prop. So that would mean a loss in RPM of about 1600 from 5000, so about 3400 RPM. Which is what it was running at WOT. However, decreasing the diameter to 12" which is what that prop was, should gain RPM at about 200 RPM/.25" decrease. So "theoretically", the increase in pitch would be offset by the decrease in diameter.

My guess is, that is what the guy thought when he put it on there. In reality, Im thinking that the relationship is exponential, not linear, and once you exceed the limit of the pitch the motor can handle, you lose more RPM per unit increase in pitch.

Yes, you're starting to get it. When a motor falls out of it's peak HP powerband, the 150/200 rpm's per inch most often will not apply. The motor is making less power(and other factors), so the effect tends to be greater, although I wouldn't call it exponential.

The diameter equation is typically 1" change in diameter changes RPM's by approx. 400 with all other dimensions remaining essentially equal, which is not common.

As an example, switching from a Laser II (normally ~14"diameter) to a Mirage (normally ~15" diameter), the diameter "equation" will very often come into play for props of equal pitches.

If you're looking at props in a similar diameter family (3/8" diff), pitch is where you need to focus your attention.

 

[jestor68]

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

The diameter equation is typically 1" change in diameter changes RPM's by approx. 400 with all other dimensions remaining essentially equal, which is not common.

Make that 150-200 rpm; same as an inch of pitch.

A 1/4 inch of diameter change has very little effect on rpm; certainly no where near 200 rpm.

Aluminum to SS is only about one inch difference; not 2 inches.

 

[dazk14]

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

The general rule of ~400 rpm per inch of Diameter has been around since time immemorial.

It is commonly misquoted since off the shelf propellers are ordered by pitch.

Recreational propeller geometry has become very sophisticated, it's very difficult to know the other variables that are changed when moving up or done a pitch size for any given propeller model - never mind switching brands.

While certain prop "shapes" tend to yield certain characteristics (bow lift with rake), trial and error eventually becomes the order of the day for maximum results.

Taking a ride to see Frank A. above would certainly be a good avenue. I'd also suggest jumping over the the Bayliner owners forum and you have received some good information above.

I would think a 17" pitch would be in the neighborhood.

 

[jestor68]

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

The general rule of ~400 rpm per inch of Diameter has been around since time immemorial.

I would think a 17" pitch would be in the neighborhood.

I believe that I recommended a 17" earlier in this thread. You are wrong concerning the 400 rpm/inch of diameter. But I won't attempt to change the belief system that you have held to since time immemorial.

The fact is; an inch of diameter is equal to slightly LESS than an inch of pitch. If an inch of pitch is generally accepted to change rpm by 150-200 rpm, how in the world can an inch of diameter adjustment result in a 400 rpm change?

Answer: It can't.

 

[dazk14]

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

I believe that I recommended a 17" earlier in this thread. You are wrong concerning the 400 rpm/inch of diameter. But I won't attempt to change the belief system that you have held to since time immemorial.


The fact is; an inch of diameter is equal to slightly LESS than an inch of pitch. If an inch of pitch is generally accepted to change rpm by 150-200 rpm, how in the world can an inch of diameter adjustment result in a 400 rpm change?

Answer: It can't.

Let's not get worked up here. You're certainly welcome to take full credit for the 17" pitch. My suggestion was just another data point for the OP.

I won't say your wrong about your feelings on diameter, simply misinformed. We've had a dealership since the 60's and the numbers I quoted you have been published since then... For the term time immemorial...

I think these strapping gents from Rundquist propellers would fit the bill...

http://www.rundquist.com/history.htm ... their equation is as follows
1" Pitch = 200 RPM
Lower pitch to gain RPM
Increase pitch to lose RPM

Single cup = 200 RPM
Double cup = 300 RPM

1/4" Diameter = 200 RPM
Decrease diameter - gain RPM
Increase diameter - lose RPM


If you don't consider Rundquist competent, here's the next listing from google...(I looked at the 1st 2)

http://youboat.net/boatPower4.aspx and their quote below...and what looks like a nice article and chart.

"The same propeller can't deliver both high speed and maximum power. A propeller sized for high speed has a small diameter and maximum pitch. In contrast, propeller that has high thrust has a large diameter and lower pitch. Both diameter and pitch absorb the torque generated by the engine. As a rough guide, 1 inch increase in diameter requires decrease of 2 - 3 inches in the pitch for the same load."

 

[jestor68]

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

You can quote the internet (we know that's always right), or you can can provide examples of real world tests.

Here's a couple(of several) conducted by myself. A 14.5 X 17 Solas runs 4900 rpm. A 15.5 X 17 Solas runs 4800 rpm. These numbers are on my boat. A 14.5 X 19 Solas runs 4650 rpm, while a 15.5 X 18 Merc prop 4600 rpm. It seems that inch of diameter is being canceled by the inch of pitch. All props were aluminum of identical or similar design.

There are other examples, but I don't want to fill the page with this silly debate. Real world tests simply do not show this 400 rpm change, or anything even close. In fact, all my tests have resulted in only 50-100 rpm difference with an inch of diameter change. Sorry, but my tests do not bear out what you heard or read on the world wide web.

You can get yourself a few props (please do)and conduct your own tests, or depend upon what others say.

Not that this anything at all to do the OP's question.

 

[jestor68]

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

1/4" Diameter = 200 RPM[/U]
Decrease diameter - gain RPM
Increase diameter - lose RPM

That equates to 1 inch =800 rpm. You said 400 rpm.

Seems you can't get your story straight. Better go read some more internet . :facepalm:

 

[jestor68]

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

Let's not get worked up here

I'm not the one using bold type. And even your bold type is questionable.