Are my props too big? 22 by 20, 3 blades, a 1970 Eggharbor 37

[sdowney717]

I think they are Michigan wheels, typical left and right hand rotation.
I am including from the boat manual the page for the props, which seems all over the place and very generic.
My 22 by 20 props have about 2 inch tip to hull clearance

Boat is 20,000 lbs, Semidisplacement style hull, Velvet Drive 72c with 2.91 to 1 gear reduction, 1 3/8 shafts, twin engines 265 HP each
Engines can not barely get to 3000 rpm.
Boat is slow, typically cant get past 8 knots. It sure can pull weight though at slow speeds.

I saw an Eggharbor boat brochure once, and it said top speed around low 20's knots, so it should do better

So any opinions on the props. Manual says props need to allow for 3600 to 4000 rpm.

Propeller page, notice top speed column, my engines are M-392.
Seems odd set of info here for props.

 

[sdowney717]

Boat tends to just dig a deep stern hole and send bow up when pushing engines as hard as you can.

The other info regards the Bennet Trim tabs I left off the boat last haulout.
Do you think they were an OEM install?
Cause the boat used to do better years ago with them on from memory.
they are 42 by 9 and I made new ones of 42 by 12, but have to haul out to do that.

 

[BlueBell]

Cause the boat used to do better years ago with them on from memory.

How much weight has the boat gained since then?

 

[kapnD]

Haulout somewhere that can give you an accurate weight number.
Get a phototach to verify tach readings, and verify that motors and drivetrain are up to snuff.
VicProp has a free prop calculator, or better yet, work with a reputable prop shop.
Their vast experience in propeller matching is a very valuable tool, but worthless without solid data.

 

[sdowney717]

I keep hoping someone who has had this kind of boat, and they did make thousands of them decades ago, would post what they had.

 

[kapnD]

You could google search for owners groups and forums.
If it were mine, I’d be crunching the numbers.
Older boats, even those (few) that came from their factories correctly propped, gain weight and lose performance capabilities over the years, and a great many suffer more from sitting than overuse.

 

[BlueBell]

How much weight has the boat gained since then?

It's a very relevant question.

Good luck in your search.

 

[johneck]

The short answer is no, the props are not too big. Even at only 8-10 kts they are probably only using about 120 HP/shaft. The problem is likely somewhere in the engine, fuel, air, ... It also seems realistic to reach 20 kts with 530 total HP and a 20-25k lb boat. Trim tabs or interceptors might help some, but not at those low speeds. You need to find the missing horsepower first.