prop choices

[Matthew A.]

I have a 17 foot 88' Bayliner Capri w/Force 85. The prop that I am currently using is a 3 blade, semi-clever, 17 inch pitch that I believe came stock on the motor. The blades have a few dings though one blade is slightly bent. So, I have set out to replace it.

I purchased a new 3 bladed 17x 13.75 cupped aluminum prop manufactured by Mercury which only allows the motor to reach WOT rpm of 3900 to 4000 rpms @trim. Any ideas on the pitch that might be needed to achieve the 500 to 750 rpm to get WOT between 4500 and 5500 rpm using a 3 bladed aluminum?

Help greatly appreciated.

 

[steelespike]

Re: prop choices

I think we could make a better decision if we knew your rpm and speed with both props.Without that reference the information doesn't mean much.It wouldn't hurt to describe the overall performance with each prop as well.Hole shot,top speed any ventilation problem.It would be good to check your compression and be sure you are getting WOT(wide open throttle)and are in top tune.Check your setup ,verticle height of the motor, antivent plate even with the bottom of the boat is a good starting point.If you could raise it 1 inch you could gain 2 to 3 mph and a couple of hundred rpm.
Cupping could knock some rpm off but generally can produce more speed and resistance to ventilation.Speaking strictly in terms of pitch 1" can make a change of 150 to 200 rpm.

 

[Matthew A.]

Re: prop choices

Thank you for replying SS. I will have to get back to you soon with the information requested as far as top speed with the cupped prop. Using the OEM prop 4500 rpm achieves roughly 27 mph according to the gps. The OEM prop tends to ventilate making somewhat tight turns above 23 mph or so if the motor is trimmed even slightly. I will also take a measurement of the height of the ventilation plate in reference to the bottom of the hull.

 

[Matthew A.]

Re: prop choices

If I remember correctly, the speed shown by the gps while running the cupped prop was 24 or 25 mph at 4000 rpm. As far as the height of the ventilation plate to the bottom of the keel, unfortuneately the plate protrudes about 1/2" lower then the bottom of the keel. Aside from installing a jack plate or drilling new mounting holes in the transom, I don't think there's much I can do about it, correct? Ran a compression test and got a reading of 140 psi on all three cylinders. I don't have the equipment to run a leak down test or I would have given that information as well.

 

[Frank Acampora]

Re: prop choices

Matt: That Mercury prop is KILLING you! Not only is the diameter 1/2 inch more than the stock prop, but it is cupped as well--no wonder your RPM dropped radically. Remember: your 85 is just a three cylinder engine--it does not have a lot of torque to turn a big prop. But that cup will help ventilation in turns some.

Now, if the original prop was that black painted one with the flared hub and (for lack of a better word) straight trailing edge: I never liked that prop and try to get rid of it if it comes on an engine I buy. It just does not perform well on Chrysler/Force engines. On my cuddy, my 125 had trouble getting that prop in a 17 pitch to 4,000 RPM. A change to Michigan (even though that "Semi-Cleaver" prop was probably made by Michigan) worked wonders.

On a 17 foot boat with a newly rebuilt engine at 140 PSI, I would expect a little more speed out of your rig even given the wrong prop. Go back over to Chrysler/Force and probably tomorrow I will post a few hints as to engine settings for your 85.

One other thing: I have some stock Chrysler aluminum 17s; They are a little better than that "Semi-Cleaver". --Send me a private message and maybe we can work something out.

By the way: Just for a rough comparison: That's a 90 on a 15 foot Glastron in my avatar. It turns a 19 pitch at 5,500 RPM and a 21 Pitch at 5,000 RPM. 40-42 MPH with each prop.