WOT and prop size

[Vronsky]

WOT range for my DF115A is officially specified from 5000 to 6000 RPM
Can I safely increase the pitch of the prop untill WOT reaches 5000 RPM ??

WOT now is 5800: I want to reduce RPM for sailing at low speeds (speed limits!) for occasional long distance trips.
Pitch is now 19", am considering upping to 23", will reduce RPM by 800

THANKS,
V.

[wilbur1]

If at wot your only at 5800 you are about perfect. If you go to 23 you will only be at 5000 which is too low , you will be lugging engine and reducing fuel mileage and hurting the engine. I would stay where you are.

[Moonlighter]

If at wot your only at 5800 you are about perfect. If you go to 23 you will only be at 5000 which is too low , you will be lugging engine and reducing fuel mileage and hurting the engine. I would stay where you are.

100% agreed.

It is false logic to think you will get better economy or speed by increasing pitch like that. Simply doesnt work.

My Df115 is propped to max out at 6200rpm with a light load. Spot on.

[Vronsky]

I'm not concerned about economy here, just to reduce RPM at low speeds to get rid of annoying resonance in the hull.

If a 23" gives a WOT of 5000, it's within officlal WOT spec: where's the harm ?

[Moonlighter]

All dealers will recommend that the boat be propped to achieve close to the max recommended rev range.

Nobody who knows anything will recommend propping to achieve 5000rpm at wot. Nobody.

Hole shot will be poor. Economy will be poor, Speed will be poor. Throttle response will be poor.

Dont believe us - go ahead and buy the 23” and you will see.

A better course of action would be to address the root cause of the resonance. Something is loose or vibrating, best to find it and fix rather than compromise your engine’s life and boat performance.

[Vronsky]

I'm fully aware of all you mention, but this is not about getting best performance

if factory spec allows 5000 RPM WOT, why would it compromise engine life if the engine -for whatever stupid reason- is indeed propped for WOT at 5000 RPM ??

Anyone?

[D'oh999]

@ 5000 RPM WOT, the boat will have much more trouble getting on plane when needed. Your "Low RPM Speed" will increase but it won't help fuel economy as much as turning an extra 2-300 RPM to troll the same speed.

It's not like your car, that rolling down the Freeway @ 75 mph the engine is only using 150-175 hp to achieve. You are moving in water, sing all of your 115 HP to go 40 mph. Think of pulling a Parachute on your Freeway Run.

[Vronsky]

@ 5000 RPM WOT, the boat will have much more trouble getting on plane when needed. Your "Low RPM Speed" will increase but it won't help fuel economy as much as turning an extra 2-300 RPM to troll the same speed.

It's not like your car, that rolling down the Freeway @ 75 mph the engine is only using 150-175 hp to achieve. You are moving in water, sing all of your 115 HP to go 40 mph. Think of pulling a Parachute on your Freeway Run.

Yes, very true, but not so very different from motoring with a fully loaded boat, or towing another boat: very common & unproblematic practices

[D'oh999]

Yes, very true, but not so very different from motoring with a fully loaded boat, or towing another boat: very common & unproblematic practices

Well have at it then, a 23" will be fine, you are convinced. 115 hp is allows going to be 115 hp. 162n/m of torque can be changed. And Torque is a Curve, not a flat line.

I have 800 lbs in the Sportsman right now, plus me, 4 Dogs, food, gas. Tomorrow's trip with a 23" on a 140A that turns 6100rpm with just me will be over propped about the same amount as you are planning, causing heavier fuel burn. If I had a 21" pitch I would be swapping it for the trip in a heart beat. To get the motor back up into "It's Operating Range" rather than lugging it for 15 mile run.

[tsturm]

All dealers will recommend that the boat be propped to achieve close to the max recommended rev range.

Nobody who knows anything will recommend propping to achieve 5000rpm at wot. Nobody.

Hole shot will be poor. Economy will be poor, Speed will be poor. Throttle response will be poor.

Dont believe us - go ahead and buy the 23” and you will see.

A better course of action would be to address the root cause of the resonance. Something is loose or vibrating, best to find it and fix rather than compromise your engine’s life and boat performance.

Very good advise Moonlighter you can bet he will be replacing crank & bearings, rod's & bearings & pistons / rings. will be a good lesson!