1985 Force 125 prop choices

[Troncd]

I’m thinking 17 will be best for me as we will mostly cruise and tube with maybe occasional skiing. Thoughts?

 

[SkiDad]

17 sounds good as long as you mean just skiing on 2 skis - if you mean slalom skiing I would go with the 15

 

[heypawpaw]

No expert here, but getting a tach to determine current wot rpms before getting another prop will help you choose correct the prop. Just going to a 15" may get your skis up quicker, but may also may spin up too many rpms wot.

 

[Troncd]

17 sounds good as long as you mean just skiing on 2 skis - if you mean slalom skiing I would go with the 15

Ok. I’m gonna call and see if there is any chance of swapping to a 17. I think that will be the most well rounded prop for me. Thanks for your help!

 

[WesNewell]

A 125hp engine should push a 1250# boat at least 50mph or close to it. My 60hp pushes my 1000# 17.4' boat up to 40 mph after remounting the engine to the proper height and proping it properly. Now if you are going to pull skiers, you will definitely want to go down in pitch. A 19p prop at 5000 rpm calculates to 52mph with 0 slip. And properly mounted and trimmed, one can get very close to 0% slip.

 

[jimmbo]

Get a Tach and see what the numbers are. I suspect you are going to need something in between 15 and 16 inches to get the the rpms into the upper end of the the range. 40 to 43mph would be a respectable speed for an 18 ft with a 125hp. My 15ft with a prop rated 115, with SS prop and engine lifted so AV plate was an inch above the hull bottom did 48 mph. it gained 1 mph from where the AV plate was flush with the hull bottom.

 

[Troncd]

I’ll likely be order the 17 pitch tomorrow. Any suggestions on a cheap tach?

 

[SkiDad]

unfortunately force engines don't use a typical tach - you need a 20 pole tach - so probably something used on ebay. If you are just trying to prop the engine you can get a cheap digital tach on ebay or amazon that you wrap around the 1st spark plug wire.

that being said a 17p will get you around 5000 rpm and that is going to be close enough for what you are trying to do.

 

[wahlejim]

Also keep in mind that Tri-Hulls like yours were built for stability and maximizing useable square footage for the length of boat. They were not built for speed. I think that the 40-45 mph range is the peak we got out of our old tri-hull with a 115 Johnson hanging on it, and that was in smooth as silk water with the onboard speedo as gps units weren't available like we have now.

Trihulls displace more water than v-hulls and are greatly affected by any chop you encounter. I wouldn't be chasing after that 50mph mark and would side closer with the 15 pitch than the 17.

 

[QBhoy]

Think Ski dad is spot on with his experience of what speed to expect. No way it will do 50. Not gps at least. Loads of guys tell me their boat does 50 and 60. Truth is, in real gps terms. They are a long way from it. Not many boats will do 50 with modest power in real life.

 

[Troncd]

Think Ski dad is spot on with his experience of what speed to expect. No way it will do 50. Not gps at least. Loads of guys tell me their boat does 50 and 60. Truth is, in real gps terms. They are a long way from it. Not many boats will do 50 with modest power in real life.

I don’t expect to or care to hit 50. Just looking to reach the best overall performance for family cruising and efficiency.

If it ever very stops raining here In north Alabama, I’ll get it out on the river and play with trim as well as check where motor rides in the water. From there I’ll look at getting a prop. All the while I’ll be searching for used props...

 

[Troncd]

I found a 15 pitch prob for a decent price used. I’ll install once it arrives and see what that does for me.

Untill then, any advice on the tilt and trim system? Works fine at idle or on trailer but doesn’t seem to want to work while underway. Checked fluid level, and no leaks. Hoping to resolve that as I think a large part of my problem w speed is not being properly trimmed.

 

[SkiDad]

I found a 15 pitch prob for a decent price used. I’ll install once it arrives and see what that does for me.

Untill then, any advice on the tilt and trim system? Works fine at idle or on trailer but doesn’t seem to want to work while underway. Checked fluid level, and no leaks. Hoping to resolve that as I think a large part of my problem w speed is not being properly trimmed.

i've had to rebuild the valve body on both of our engines in the past. It's fairly tricky to do but doable.

Does it hold position all night on the trailer or does it leak down ? My dad for many years just put some large bolts and washers in the 1st or 2nd hole (can't remember which) so they it would never fully trim down (it would rest on the bolts) - that held it at a good position. I know every boat is different but I usually just trimmed my 1985 bayliner so the engine was just about strait up in down sitting at rest and that seemed to be fine overall.

 

[Troncd]

i've had to rebuild the valve body on both of our engines in the past. It's fairly tricky to do but doable.

Does it hold position all night on the trailer or does it leak down ? My dad for many years just put some large bolts and washers in the 1st or 2nd hole (can't remember which) so they it would never fully trim down (it would rest on the bolts) - that held it at a good position. I know every boat is different but I usually just trimmed my 1985 bayliner so the engine was just about strait up in down sitting at rest and that seemed to be fine overall.

I’ll set it to a position and leave it overnight and see if it stays. I suspect it will because it’s all in great shape and works fine just not under load. I think I remember from my first short trip out that the trim motor wasn’t even running when I’d try to use it under load.

 

[heypawpaw]

If after leaving engine trimmed up overnight and it doesn't leak down, next time out trim engine up before putting under load. Trim engine up a little and run at 3/4 throttle to prevent over revving. If all ok, stop and trim up a little more and try again. If porpoising or cavitation occurs trim down a little at a time to correct condition.. Once you determine optimum trim position then increase speed to WOT unless rpms seem too go high. If power trim leaks down either while sitting or while under load, seals or o rings inside the unit are leaking and need replacing. (oil leaks around piston inside trim cylinder) oil leak doesn't have to be visible for it to leak down. If trim doesn't hold engine in place, use a tilt pin through holes in transom bracket to hold engine in desired position until trim unit is repaired.

 

[jimmbo]

Try opening the manual valve a bit and operating the pump, then close it, there might be bit of contamination allowing the fluid to bleed.

 

[Troncd]

Try opening the manual valve a bit and operating the pump, then close it, there might be bit of contamination allowing the fluid to bleed.

Manual valve?

 

[jimmbo]

Manual valve?

There has to be a valve that can be opened to allow the engine to be tilted up or down if there is a failure in the Hydraulics, or Electrical components