WOT 3500 prop size 16'' x 19p 20' boat

[boxchevyman]

Hi all!
I have a 1975 20 foot Webbcraft runabout. I have a 233hp (351w) mercruiser with 1.32:1 geared out drive. Manual states WOT RPM is 3800-4200. This is my first I/O boat so this is all new to me. I have an aluminum prop that is 19 pitch and 16 inch diameter.

My problem is it takes a while to get on plane. Maybe it?s just me used to way smaller and lighter boats. But it will take half a minute to a minute to get on plane. Which is usually around 1900-2000 rpms. Wot is 3500 rpms trimmed properly. I cannot get it to turn any faster than that.

What I want is a great hole shot, good low end torque for big tubes. I was thinking about a four blade prop, what are your guys recommendations?

Thanks,
James

 

[SkiDad]

I would try a 15p blade. You should be able to plane most boats in 5-10 seconds. I think that gear ratio is holding you back so propping down is what you need.

 

[boxchevyman]

So something like a solas amita 15p x 14.5 would get me where i want?
Thanks!

 

[steelespike]

Prop size is written diameter X pitch 14.5 X 15.
Before any changes It would be good to know the wot rpm and gps speed lightly loaded.
Is the motor in excellent tune? Timing set right and advancing as it should?
Does the throttle open all the way?
Being a 40 year old boat I wonder if the foam under the floor may have absorbed water over the years.
Do you trim down for hole shot?

 

[boxchevyman]

Hi,
The gps speed app on my phone doesn't work. But i can tell you its feels like 35 mph or so (based on ridding in other boats with gps).

The engine is old but runs good. I have adjusted timing to be slightly more advanced than what is called for as it seems to like it. 15 degrees vs 10 base. The distributor does advance. The throttle opens just fine. Accelerator pumps works. Sparks plugs are a light tan.

Lets just go ahead and assume the boat is heavy. I do trim down for the hole shot, it really helps. With a light load it will pop on a plane in 10 seconds or so.

Basically this is a 1000 dollar boat that runs. If i can buy a 150 dollar prop that will make it give me more torque I'm all for it. I usually always run with a lot of people so I'm looking for good low end performance.

Thanks for your time.

 

[SkiDad]

So something like a solas amita 15p x 14.5 would get me where i want?
Thanks!

that should work - you might slightly overrev due to the difference in diameter so you want to watch that. or maybe look at a 16 pitch

 

[steelespike]

If a light load pops on plane in 10 seconds lightly loaded its close to normal.
if it's a 1,000 dollar boat it may be dragging a couple of hundred lbs of water right near the stern where it hurts hole shot.
If we assume a 15" prop makes 4200 rpm @ 12% slip we get about 39 mph.
If wee assume a 17" prop msakes 3900 at 12% slip we get about 41 mph.both will improve hole shot.
A 19" prop at 3500 and 14% slip we get about 41 mph. Keep in mind I'm guessing about slip and what rpm the different props may make.
It would struggle with a load.