1985 Force 125 prop choices

[Troncd]

Hello all!

im new to the boating world. While growing up my dad has a Bayliner runnabout that we used all the time but I’ve never owned a boat of my own until recently.

I jist bought an old 1973 winner Trihull 18’. It’s a pretty awesome boat in my opinion. I’ve been digging old crap as of late. I’ve taken it out twice so far. It floats and moves along under the power of the 125 force motor at about 30 mph. My whole family together probably weighs in about 650 pounds or so. From what I’m reading 30 mph is probably on the low end or normal speed for a bout of this general style, weight etc. sound about right?

the prop is a 13x19. I know cause on my first ride out I destroyed it! Found a replacement of the same size and pitch before I began reading about fine tuning a boat with prop selections. Should the 13x19 service me well for general boat/tube/family fun? I need to get a tach, I don’t believe I’m running at 5500 rpm at wot.

Looking forward to learning from you guys!

 

[jimmbo]

Do you know what the WOT range for that engine is? It might be 5000 - 5500, or it might be 4250 - 5250
Gear Ratio is 15:26. Based on the gearing I would say a 19 is too much prop for an 18' boat. With that gearing, 5500 rpm with a 19 inch would put you around 50mph.

Regarding the boat, is the hull clean, straight with no Hook or Rocker? You never mentioned the weight of the boat and motor. What about the floatation foam being saturated with water? Water weighs about 8 lbs/US gallon.
When you calculated load, did you account for the weight of the battery, fuel and tank.

 

[Troncd]

Not sure if the weight of the boat. And the weight I noted above was just my family. Does not include the 10 or so gallons of gas, battery, etc. the hull is in good shape. Everything seems solid and dry below the floor but access to view is minimal.

According the the motor plate, rpm range is 4800-5500. I know high end was 5500. Low end may have been 4500... Either way, without a tach I’m out of luck as far as diagnostics goes am I right?

From what I’ve read, the prop was original to the motor and a common prop used with some of the bay liners that had this motor?

thanks for your help.

 

[WesNewell]

30 mph is a 40% slip from a 50 mph theoretical, which is terrible. I wouldn't worry about the prop until you check your engine out. Have a mech check it out and/or get a tune up first. And you'll need a tach and gps speedo to really dial in the prop.

 

[Troncd]

So, 50 mph is a reasonable expectation? Everything I was reading over the past couple of weeks had me thinking 35-40 as a top mph would be about right and that my 30 may not be all that far off...

I will check over everything mechanically and try to get my hands on a tach. A few notes on how it runs

1. Starts pretty easily from cold start.
2. Idles fine/smooth
3. Accelerates well
4. Planes easily
5. Re starts once warm well
6. Responds to throttle adjustments


I’ll check plugs, and compression. I know it has good gas, and all other characteristics lead me to believe the carbs are clean.

 

[Troncd]

I found some specs on this website for the Winner Minikai 1973 model which says the overall length is 17.58 feet and 1250 pounds dry.

 

[jimmbo]

I said "With that gearing, 5500 rpm with a 19 inch would put you around 50mph".
The engine has to rev that high to get that speed.
An 18 footer is probably too big of a boat for a 125hp with a 19" prop to rev that fast. I suspect you will need a a prop with about 15" to 16" of pitch to get the engine up to 5500 with your boat. Which will put you in the low 40s mph wise

One other thing to check, make certain the throttle cable is properly adjusted so the engine is actually getting full throttle

 

[QBhoy]

I think you’d be very lucky to see 50mph with your boat and engine, no matter what prop is on it.

Lots of sound advice here. What I might ask is if you are running with lots on board and perhaps the motor trimmed down ? This would make all the difference and dramatically slow the boat.

With my own boat, even with only me on board. The difference in speed between trim full down and up making the hull air out...is an easy 10mph difference l.

 

[Troncd]

Aaahhh I did have it trimmed all the way down...

 

[QBhoy]

Aaahhh I did have it trimmed all the way down...

Ah. Makes a huge difference if you have the bow low and ploughing through the water. So much more hull in the water. I’ll bet my life you’ll pick up so much more revs and speed trimmed well. I’d try that before anything else. Best of luck.

 

[QBhoy]

To put an example to it. My boat might be on the extreme end of it, but if my trim is fully down the boat will only see about 50 or just over with the engine lugging. Trim her up and some air under the hull and she will be mid 60’s or better and be right at the top of her rpms range. An extremel example but I’d bet you will gain at least a few hundred rpm and perhaps nearly 10 mph. It’s mich easier to push your hull through air than water. Good luck.

 

[QBhoy]

Have a look at the video I just posted in the general section for the extreme end of trimming up, haha.

 

[Troncd]

Thanks guys. This brings me to another thought, and what is actully the only known problem that I forgot to mention in original post.

The tilt and trim system work just fine in the trailer, or at idle but does not seem to wanna work while under load. I think it will trim down while under load, just not up. I did top off the pump but it wasn’t really low and seemingly made no difference.

 

[WesNewell]

With the engine mounted at the proper height and in good condition, you should see very close to 50 mph if all the info provided is correct. If the engine is mounted too low, you can expect to reduce that by as much as 20%. Properly mounted you should be able to your slip down to 5% or less.
http://www.go-fast.com/Prop_Slip_Calculator.htm

 

[SkiDad]

I'm late to this thread but I can tell you that 19p is not what you want - having had 2 of these boat and engine combos they do really well with a 15p prop if you want to pull skiers, etc. If you are just into cruising and tubing a 17p is OK. The reason for the lower pitch props is the force 125 has a 1.73:1 gear ratio. The WOT range is 4500-5500 but you really want to prop it for at least 5000 rpm but my boats really came alive if I propped them for 5500. I've taken my boat all the way to 6000 rpm so I wouldn't worry if you are a tad over 5500.

my last boat would do 38 with a 15p prop. Would do 40 with a 17p. Around 40 is all you can expect from this engine and that size of boat.


https://www.iboats.com/shop/michiga...rotation-3-blade-aluminum-boat-propeller.html

https://www.iboats.com/shop/michiga...rotation-3-blade-aluminum-boat-propeller.html

https://www.youtube.com/watch?v=o91jaTO6Pq4

https://www.youtube.com/watch?v=3ufBl6IcZPI

 

[Troncd]

Skidad,

what type of boat were your force 125s on?

i wish I knew all this before I ordered the new 19 pitch prop...oh well!

 

[Troncd]

Skidad, another question.

According to the maual that came with the motor, the 19 pitch prob was originally what came on it. Was there a certain boat/motor combo that did well with that set up?

 

[jimmbo]

A 19 would work well with a 14ft, maybe even a 15ft boat. The motor was shipped with a 19, the Dealer, when installing the motor was expected to chose a more appropriate prop. Mercury, many years earlier, stopped shipping larger engine with props.

 

[SkiDad]

Skidad,

what type of boat were your force 125s on?

i wish I knew all this before I ordered the new 19 pitch prop...oh well!

maybe you can reject the shipment and ask them to send a lower pitch ? Does your engine still have the original white lower gear case ? they put 19p in the engine manual but most of the time the dealers would put a 17p on 18-19' boats b/c 19 was just too high.

my dad's boat was a 1988 Bayliner cobra - 18' and we had a 15p on it. Originally had a 17p. Was about 1200 lb boat.
my boat was a 1985 Bayliner Capri - 19' with a 15p on it - originaly had a 17p on it. Was about 1250 lb boat. Add 300 to each for the force motor weight.

 

[Troncd]

maybe you can reject the shipment and ask them to send a lower pitch ? Does your engine still have the original white lower gear case ? they put 19p in the engine manual but most of the time the dealers would put a 17p on 18-19' boats b/c 19 was just too high.

my dad's boat was a 1988 Bayliner cobra - 18' and we had a 15p on it. Originally had a 17p. Was about 1200 lb boat.
my boat was a 1985 Bayliner Capri - 19' with a 15p on it - originaly had a 17p on it. Was about 1250 lb boat. Add 300 to each for the force motor weight.

It’s already been installed and used once although it’s in perfect shape. I’ll call the people I ordered it from.

Everything on the motor, lower unit, prop etc was oem untill the prop met the log...