Help with finding a good starting point for propeller

[Redtruck12]

recently purchased 1974 Duo (Starcraft) 17foot bowrider with 140hp Mercruiser. It is a Tri Hull
boat weighs about 1800 lbs without passengers, gear etc.
i have been looking for a simpleprop. Selector / pitch calculator to figure what pitch is likely best for general good performance.
i keep coming up with 21 or 23 for pitch (3 blade) ??? But the selectors I tried don't specify hull type
but I figure I am choosing for a similar boat.

my confusion starts there.
the boat came with 2 props, 1 fair and 1 badly beat up. BOTH ARE 15 PITCH ????????

am I calculating it wrong OR are the props I have wrong?

Any advice or direction would be greatly appreciated

 

[alldodge]

With both being 15 leans me to think, either there is something wrong with the boat
Water logged
Worn out motor
incorrect ratio drive

Or it used a 17 to 19 but it was sold before you got the boat.

I would look toward the 19 if (the big if) everything is original and running good. You could also put the fair 15 on and give it a spin and collect some data

 

[Bondo]

Ayuh,..... Standard pitches for yer drive run 15" to 23",....

Slap on a 19" pitch prop, 'n try it,....
19" is midway, 'n will work as an Ok backup prop, If it ain't the Right prop,...
A 19" prop is also easy to resell, if it's a way wrong pitch,...

 

[Scott Danforth]

first, you know for a fact that your boat weighs 1800# or you are guessing? the specs on the boat are 1655# dry weight.http://boatspecs.iboats.com/Duo__Vagabond_17__1974/bp/66b28171 and 1800# would simply be fuel added (no extra gear)

my 17' avanti was 1650# based on specs and $1850 with fuel and gear based on the scales. With me alone or a single buddy, i could run a 19P and hit 42mph. pulling skiers, we dropped down to the 15p as the 3.0 didnt have enough poop to pull.

first and foremost, weigh your boat. if your on a single axle trailer, use #850 for the trailer weight

then, with the 15"P, take it out and get real GPS speed and real verified RPM data. that motor should hit 4600 RPM and the most speed you will see with your hull will be 35-40 on a good day with the wind at your back.

because its a tri-hull which is not as efficient as a v-hull for pushing thru the water, 15p may be what you need.

 

[Sea Rider]

Try the 15 pitch prop that's in good shape on a wot spin as usually loaded on calm flat water cond and report max wot revs achieved with it. Need to know min-max wot rpm range factory stated to play with prop pitches all you want. Only then will know how much under or over propped engine is. A prop maximization afterwords is the way to go.

Happy Boating