Prop Sizing Question

[vdimenna]

Hello All,

Could anyone give me some guidance on whether or not I have the correct prop for my boat.
I have a 23 foot Wellcraft Walkaround fishing boat with a 1996 200HP Looper.
The prop is a stainless 14 1/2 x 19.
The boat will do about 40 MPH @ around 5,000 RPM with nominal weight in the boat (me and one other, 50 gallons of fuel, no gear...). Holeshot is great and it will get on plane without issue.
I just freshened up the engine (bored it, new wisecos..) and have around 20 hours on it.

It's my understanding that for this engine the RPM is on the low side.

Thanks in advance.
Victor

 

[crb478]

Re: Prop Sizing Question

I am not 100 percent sure, but I believe that with that engine you want to be closer to 5500rpm at wot. If you droped to a 17 pitch prop that would get you in the 5200 rpm range so maybe a 15 pitch would have you close to where you want to be.

Before changing props though how is you engine set up. If you can raise the motor you will also pick up speed and rpm's. Are you using the trim to get the best rpm and ride. How may rpm's were you getting before you freshened it up? Is the motor set up the same?

 

[steelespike]

Re: Prop Sizing Question

You might double check throttle opening.
It would appear a 17 ss would put you at about 5,400.The 19 does seem like a little too much.
If you have no venting problems you may be able to raise the motor.
Raise it to a point just before venting becomes a problem.An inch could net about 2 mph and some rpm no cost but your time.Be sure it pumps water.Then the 17 if it seems needed.Allways do one thing at a time and test.

 

[Silvertip]

Re: Prop Sizing Question

Moving 4 inches of pitch to pick up 500 rpm would not be my choice since very often, just 2 inches is sufficient to get the engine "over the hump" so to speak and it actually picks up the 500 RPM rather than the 150 - 200 we tend to consider a normal increase. Setup, load balance and other factors can also impact WOT performance so make prop choices your last change, not the first.

 

[vdimenna]

Re: Prop Sizing Question

I am using the trim to get the most of the setup. It got about the same speed and RPM before I freshened the engine. It is set up exactly the same.

I will start with moving the engine up. Other than making sure I have water pressure on my gauge, is there anything else I should look for so I don't hurt something?
Do I just move it one hole at a time?

Thanks
Vic

 

[Silvertip]

Re: Prop Sizing Question

How far you move it on the first attempt depends on the relationship between the AV plate and the bottom of the boat at this time. If it's flush or below, raise the engine so the AV plate is about an inch higher than the bottom of the boat. You then adjust one hole at a time up or down from there. Keep an eye on water pressure.

 

[vdimenna]

Re: Prop Sizing Question

I'm assuming the AV plate is the section of the lower unit that extends beyond the prop.
I'll first measure to make sure it's an inch or more above the bottom of the boat (I'm pretty sure it is). Provided it is, I will loosen the four bolts that hold the engine, jack the engine one hole at time until I have a water pressure issue or I reach the top hole. I know it's currently up one hole maybe two a thise time.

Thanks for helping me on this, I'm new to this sort of tuning.

Victor

 

[steelespike]

Re: Prop Sizing Question

Your watching water pressure and Ventilation(prop slipping).
Many tollerate a slight amount of venting to get the best setup.

 

[Silvertip]

Re: Prop Sizing Question

The AV plate (anti-ventilation) is the flat plate that sites directly above the prop.