1988 force 125 prop question

[bamaboatn]

Guys Ive got a 1988 18' bayliner trophy bass with an 88 force 125. prop 13.5"x21p 3blade aluminum. I had the boat out for the first time this weekend, boat is slugish to get on plane. tried pulling a tuber and it struggles even more. I had two adults and three kids in the boat roughly 650lbs of people, and maybe 175lbs of gear. AT WOT using gps top speed was 30mph, rpm's kinda hard to tell factory gauge jumps around alot. But I thing about 4000 rpm's was the average. The RPM range of this engine is 4500-550 rpm's. I was thinking of getting a 13.5"x17p 3 blade aluminum prop. This is a fish and ski boat but in the summer months it will primarily be used for ski\tube. I want to get the most performance I can get for skiing. I weight 240 and I would like to ba able to get up on a slalom ski with this boat\engine. I dont think that will be a problem I just need the right prop.

I will be getting a "tiny tach" before the next outing so I can get an acurate rpm reading.
So what do you think, should I go 17p or 15p?

Next question is what prop is compatable with this engine. I would like to be able to get a used prop to try, maybe get a 17p and a 15p and do some testing. I dont know much about boats, but I was thinking as long as I got the correct spline, rotaion, and size any prop would fit, but somehow I know that is going to be wrong.

I would appreciate your help

 

[bamaboatn]

Re: 1988 force 125 prop question

bump

 

[steelespike]

Re: 1988 force 125 prop question

A 21 isn't even listed here at boats.19 being the biggest. Being a bass boast it might normally run a prop with somewhat higher than normal pitch.I would try to confirm the rpm before a change. You need an accurate tach to confirm both your present rpm and the new rpm. A 1" pitch change will nety about 100 to 200 rpm change.While o do think you need less pitch I think you may have motor or setup issues as well. Is the wot 30- mph with a light load?

 

[bamaboatn]

Re: 1988 force 125 prop question

A 21 isn't even listed here at boats.19 being the biggest. Being a bass boast it might normally run a prop with somewhat higher than normal pitch.I would try to confirm the rpm before a change. You need an accurate tach to confirm both your present rpm and the new rpm. A 1" pitch change will nety about 100 to 200 rpm change.While o do think you need less pitch I think you may have motor or setup issues as well. Is the wot 30- mph with a light load?

what do you mean a 21p isnt listed? And why would you think i would have engine issues? 30mph WOT is with heavy load like I said up top. maybe close to 900lbs of load. engine seemed to run very good, no hesitation or anything like that.

 

[steelespike]

Re: 1988 force 125 prop question

My bad, I skipped over that expecting a more normal load.
Its easier to tell about your setup with a lighter load.Need to know if the boat can reach its wot rpm with its present prop.
Your load is beyond what a 21" pitch would be expected to handle comfortably.
With the distorted numbers its difficult to estimate the results with a pitch change.If your 4000 is accurate; to get to 5500 you would have to drop to about a 14" prop. Not likely to work with a lighter load. Of course you could change props for those occasions and some do.
With numbers from a lighter run the results become more predictable.
I did a brief check here for 88 125 hp Force props.the largest prop listed was a 19 meaning your 21 isn't a typical selection.
It is intended for speed and won't do well with a load.Obviously 21 is available or you wouldn't have one.

 

[bamaboatn]

Re: 1988 force 125 prop question

My bad, I skipped over that expecting a more normal load.
Its easier to tell about your setup with a lighter load.Need to know if the boat can reach its wot rpm with its present prop.
Your load is beyond what a 21" pitch would be expected to handle comfortably.
With the distorted numbers its difficult to estimate the results with a pitch change.If your 4000 is accurate; to get to 5500 you would have to drop to about a 14" prop. Not likely to work with a lighter load. Of course you could change props for those occasions and some do.
With numbers from a lighter run the results become more predictable.
I did a brief check here for 88 125 hp Force props.the largest prop listed was a 19 meaning your 21 isn't a typical selection.
It is intended for speed and won't do well with a load.Obviously 21 is available or you wouldn't have one.

I have no problem changing props out for the occasion. Do they make a 14? I thought changes were 2" increments. I will take the boat out tomorrow with no load and see what kind of numbers I can get. Can you help me with my other question about prop compatability? Read OP

 

[steelespike]

Re: 1988 force 125 prop question

Based on your 4000 rpm 14 was a guess using the need for about 5500 rp a guess 200 rpm per inch.
I'm pretty sure when we get the lightly loaded numbers the prop required will change.
They do make even numbered props but selection is slim and can be more expensive.
I'm not expert on Force prop compatibility. I can tell you if you have a non through hub exhaust you can use some through hub exhaust Force props.As far as I know only Force props fit an 88 Force motor.Some much newer Forces use a Merc gear case so Merc props fit.
Frank may be along with more Force prop details.

 

[bamaboatn]

Re: 1988 force 125 prop question

Ok so I tokk the boat out today with no load, just myself. I didnt get the numbers I thought I would. I got 37mph playing with the trim. I also found out that my prop is not a 13 1/2". I think its a 12". I measured from center of hub to tip of blade and it looks like 6". I still wasnt able to get a good rpm reading. I bought a teleflex tach and when I went to install it I found out that the force engine uses a 10 pulse rectifier, and of course no one makes a 10 pulse tach anymore. I'm going to order a "tiny tach". Are they anygood? Could the new found info about a 12" prop be the problem?

 

[steelespike]

Re: 1988 force 125 prop question

Using a lot of guess work ,not a good way to measure rpm,it appears your rpm is around 3700.
Lots of people on here have used the Tinytach successfully. You need to get all the information on their site before ordering.
It will work on any motor As a hand held or installed.

 

[bamaboatn]

Re: 1988 force 125 prop question

Using a lot of guess work ,not a good way to measure rpm,it appears your rpm is around 3700.
Lots of people on here have used the Tinytach successfully. You need to get all the information on their site before ordering.
It will work on any motor As a hand held or installed.

I ordered a trail tech tach. Its just like the tiny tach but alittle cheaper. I will wait until I get it in before I proceed.

 

[SkiDad]

Re: 1988 force 125 prop question

i know this is an old thread but it looks like it never completed - anyhow I carry a similar load and I went from a 17p to a 15p last year and it made a huge difference - my rpm went from 4500 to 5100 with a full load. I can really feel the difference when skiing too - boat sounds more relaxed too.