here is the info.. What more do you need?

[malayan]

I have a 17' Maxum Open Bow 2550 lbs with me in the boat & Trailer on scale alone. ........ Subtract the average trailer weight.

I have a 85 HP FORCE Outboard
Mounted now is a #072127 Michigan Prop 13 1/2 Dia 15 Pitch keep for spare only (badly worn out).

I am acheiving 4200-4500 RPM @ 30 MPH (Just me in the boat)

Planes pretty easy starting at about 20-25 MPH with the Stingray Hydrofoil Stabilizer... then increases speed to 30MPH @ 4200-4500 RPM.
(should I be using this stabilizer?..Came with boat).


Throttles fine & on plane all seems fine.

Should this be correct to use this boat & prop for tubing and Knee Board?
(Have not bought my tube & Board gear yet either).

or should I change the pitch?

One last question please...
Can I pull Knee board & tube from the mount on the stern of my boat or do I need to install a Board Tower?....if so
I am a fabricator by trade so maybe I can weld my own for cheaper

Just need specs for hight & mounting suggeestions.

Thanks..You guys are great just to sit and (read)learn from

 

[hwsiii]

Re: here is the info.. What prop do I need?

Re: here is the info.. What prop do I need?

Malayan, I am assuming you are using a speedometer and not a GPS for your speed, because if you are using a GPS that prop is a theoretical 16" pitch with 12% prop slip. Otherwise at 15" pitch and 30 MPH you are showing a 6% prop slip, which is not normal for an aluminum prop, especially with 4,500 RPM at WOT. Be sure and check your tachometer for accuracy and tuneup your motor if it has been awhile since this was done, before you buy a new prop, and check for height on the anti ventilation plate above the bottom of the boat. It should at the least be even with the bottom of the boat.
MY prop picker software shows you should be using a 14" Pitch prop, but with the RPM at 4,500 RPM I am going to recommend a 13" Pitch because you want to use it for tubing and kneeboarding. Using your 4,500 RPM as the base for your 15" prop to attain close to WOT RPM I actually show you should be using a 12" Pitch prop at 5,344 RPM. But you said the prop was worn out and my Prop Picker shows you should be using a 14" Pitch, that is why I am recommending a 13" Pitch. Running the numbers I show with an aluminum 13" Pitch you should be getting right at 5,000 RPM which is definitely on the LOW side of your recommended WOT RPM but I have to assume there is some other problem not identified yet and you will get more RPM than I show, unless I am wrong on the information I have for your motor. These are the numbers I show for your 85 Force, recommended WOT RPM 4.500 to 5,500 RPM and a gear Ratio of 2 to 1. I used a base weight of 2,700 pounds with 3 people and assorted gear.If these numbers are wrong everything I say will be wrong, so be sure and check this for accuracy please.

Prop Slip

Prop Pitch 13

Prop Pitch 12

Prop Picker

H