Prop for 96 Force 120 on 20 Trophy

[Trophyman2]

I have had a 1996 Bayliner Trophy 2002 with a Force 120 since it was new. My top speed is about 35 MPH at 5200 RPM trimmed. The RPM operating range according to Specs is 4750-5250, however I can get up to 5600+ but always throttle back in fear of doing damage by overreving the engine. I did see a post that indicated that overreving is not much of an issue with outboards but I feel tht 5600 is high for a small outboard. The hole shot is slow when loaded but good with just one or two passengers. I'm curious if I can get a higher top end without overreving or killing the hole shot.

1996 Bayliner Trophy 2002 (20') with a 96 Force 120
Approx 3500lbs w/ fuel
4750-5250 Top RPM range by Spec
35MPH
Aluminum Prop - Hub stamp 48-77344-c1-17, side stamp on prop 48-77344 A45 17p


Any information would be appreciated.

 

[steelespike]

Re: Prop for 96 Force 120 on 20 Trophy

Your running a 17" prop
Your speed needs to be by gps.35 @ 5200 indicates about 13% slip, high normal.
What is the speed at 5600? I do suspect your tach may be a little high.
Classify in seconds good hole shot with one or two passengers?
Is 35 @ 5200 with one or two passengers?What is the rpm and speed "loaded"?

 

[Frank Acampora]

Re: Prop for 96 Force 120 on 20 Trophy



Yes, it is higher than the manufacturer's specs but: Not to worry at 5600 RPM. You will not do any damage. Other models and years of Force engines are rated at 5500. Here is a 125 that is regularly run at 5900 RPM and 60 MPH.

Uh--- Yeah, the engine is rebadged as an 85 but is really a 125.

If you go up in pitch to a 19, RPM should drop by about 400 BUT hole shot will be less enthusiastic and top speed may actually drop a bit. If you go to a Ballistic 17 pitch prop, it is likely that your speed will stay the same but RPM will drop. Example: That 125 that turns 5900RPM with a cleaver stainless will drop to about 4500-5000 with the same pitch Ballistic stainless, but speed will remain at 60.

 

[jestor68]

Re: Prop for 96 Force 120 on 20 Trophy

Running the motor past it's recommended rpm range for any length of time will drastically shorten it's life.

The reason your '96 Force is still running well is because you paid attention to the rpm limit.

 

[steelespike]

Re: Prop for 96 Force 120 on 20 Trophy

Frank knows the Force motors inside out.
Frank, what do you think of 4 blade to reduce rpm and save hole shot?

 

[Frank Acampora]

Re: Prop for 96 Force 120 on 20 Trophy

Quite frankly, I have no experience with 4 blade props on a Force so I really can not comment. Once I did use a 4 blade composite prop on my 125 but I do not know the brand or pitch. I was simply testing to see if I could use it as a lightweight emergency prop to carry. While it did run the boat at a reasonable speed, it did not perform well. However, that could be because of unknown pitch.

As far as the comment about drastically shortening an engine's life: The exact same block, crank, and piston assembly is variously rated to run between 5250 and 5500 RPM max. 100 RPM over 5500 is not significant and is probably well within the tachometer error. So the engine could be running at 5500 or 5700 RPM at a tach reading of 5600. Again, there are no valve trains to "float" and cause engine damage. So 100 over rated max will in all probablility do no harm. There is a better chance of this engine breaking the semi-keystone top ring, which they seem prone to do for no reason.

The owner does not want to run the engine at this speed anyway, so it becomes a moot point, not worth arguing. If he has the time and money to experiment with various props, He very likely will gain some performance while lowering top RPM by trying a stainless prop. Figure a stainless prop will run 200 RPM faster, while cupping and going up to a 19 pitch will lower RPM by about 600 and he will be right in the recommended performance range and hole shot should be acceptable. This may not hold true for a Ballistic stainless because of the high rake, aggressive cup, and blade area. These props tend to load down Force engines more than the round-eared Michigan stainless.

The photos are simply to demonstrate the difference between the regular and Ballistic stainless. Although they are on earlier Force engines, the principle is the same.

I will admit: I really like the Ballistic. To read more about it, the OP should search for my post comparing it to other props.

I would expect the 17 pitch prop to be approximately correct on his 20 footer, but some of these later Force engines with Mercury ignition and lower units run surprisingly well. Thus a 19 pitch may not be out of the question. One point we have not addressed is : Is the tachometer accurate? Does the engine actually turn as high as the tach is indicating? Is speed by speedometer or GPS?

 

[Trophyman2]

Re: Prop for 96 Force 120 on 20 Trophy

Wow...Thanks for all the advice and info its greatly appreciated. As far as Tach and MPH accuracy I have been using the instrument guages. IF I can gain access to a GPS and diagnostic Tach I will update my post. The main reason for my post was whether or not I would do severe damage at WOT with my current set-up. Gaining a couple MPH at an RPM that is outside recommended specs is not important. I will continue to back off when I see the tach push up to 5500+ and trim out at a comfortable 30 MPH. If I have the opportunity to pick up a 19 Pitch prop to test then I will see how things go. Thanks for the help.