prop/torque

croz

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Feb 28, 2019
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Under average load a prop pitched to run engine at its peak torque RPM would have what effect? My goal is gas mileage at slowest plaining speed.
 

dingbat

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Nov 20, 2001
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Conditions have more of an effect than anything. Using the same prop and the same RPM, my fuel usage can double depending on conditions.
 

tpenfield

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Under average load a prop pitched to run engine at its peak torque RPM would have what effect? My goal is gas mileage at slowest planing speed.

I'm not sure anyone could truly answer that question. Lots of variables to consider, so it may situational as mentioned.

There is nothing about running an engine at the RPM of its peak torque when at minimum planing speed would mean that it is at its most fuel efficient point.
 

jimmbo

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Most boats get their absolute best MPG at idle speed. Worst is usually right before it is planing freely. After getting on plane the MPG will increase as the speed goes up, but it will peak at some point, then starts to go down again.

Props are chosen to get the engine in an RPM range which includes the RPM where the engine makes its peak HP. However Peak HP and Peak Torque occur at different RPMs, usually several Hundreds of RPMs apart. So trying to Prop an engine to run at its Peak Torque RPM will result in an Engine severely overpropped
 

QBhoy

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Totally hit the answer on the head in the above post. Spot on Jimmbo
if you are looking for economy at cruising speeds, you probably want to be only just into the rpm range of the engine. This will mean a higher pitch prop than what a good all round prop would normally be. Also mean slow acceleration.
Having tried a load of props on my boat, I can tell you that although you may get a prop that is good for low rpm cruising and also see high top end speeds...you also run the risk of it being a total dog of a thing for any other use.
A lugging engine by over prop not only runs the risk of engine failure...also or can almost certainly be worse on fuel too.
 

Texasmark

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Usually you prop for recommended operating rpm range at Wide Open Throttle with your average load. On 2 strokers I liked to prop at or slightly above the upper limit. Since most Mercs run about 1 cu in per hp, not that many cubes so you need the rpms to get your HP [HP = (Torque in ft-lbs x RPM)/5252] to drive the propshaft to get the desired thrust out of your prop.

Pull up some sales brochures on current boat-motor (engine) combos. You get curves on everything but torque. The curves tell you where your best MPG speed usually is....just above planing onset "generally". But as others have said that is very subjective, especially with the particular hull type.....best bet there I think would be to select a new boat brochure depicting the type of hull you have.

Deep V high deadrise at the transom hulls are arse heavy, tend to ride in the water (where hull drag is the worst) and need a lot of thrust to get up....but are soft riding..............trim tabs help to mitigate some of that. Semi V hulls with the flat/flatter transom area get better stern lift, get on plane faster and are easier to keep there but beat you to death in a chop. So you shoot for a compromise hull design.

Other thing is propping for speed vs for cruising (just above the onset of planing) if most of your boating pleasure is of such a nature. 4 blade or larger diameter, shallower pitched props help you to maintain that particular speed.......but can run you over the max rpms at WOT if you don't watch your throttle. Props tuned (on your boat) for speed (max top end with a light load) will keep you on the throttle (back and forth) as they load up at that "just above planing attitude" so the boat slips back into the trough requiring throttle. That puts the boat back on plane whereby the boat drag is reduced, the engine breathes easier, rpms increase and you are now above your desired cruising sweet spot.

If you have a prop shop handy, you can usually borrow a prop to buy and find the one that suits your needs. Usually it has to be returned in the condition in which you received and you are expected to make your purchase from them. Yes you will pay more but overall it will be your best money spent....no guessing, no spending money on what works only to try another might/might not work.

I guess I remember all that. Been gone for awhile. Sold my boat.
 

croz

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Feb 28, 2019
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Thanks for response and info! Just so you know, I dealing with the dreaded old 370/4cly Mercruiser on a 22' Starcraft Aluminum Islander. I am told that Merc did this motor to get a short Torque engine. I run a light load most of the time. I'm looking to go long distance per tank of fuel. Currently have a fourblade 20" pitch. The boat is aluminum and has trim tabs.
 

Bondo

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Thanks for response and info! Just so you know, I dealing with the dreaded old 370/4cly Mercruiser on a 22' Starcraft Aluminum Islander. I am told that Merc did this motor to get a short Torque engine. I run a light load most of the time. I'm looking to go long distance per tank of fuel. Currently have a fourblade 20" pitch. The boat is aluminum and has trim tabs.

Ayuh,....... Welcome Aboard,...... So what's the rpms at Wot the way it is now,..?? should be 'round 4200/ 4800 rpms,...

If you over-prop it to lower the rpms, you'll be luggin' the motor, at all rpms, which leads to detonation, 'n big holes in the piston's crowns,.....
 

Scott Danforth

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Under average load a prop pitched to run engine at its peak torque RPM would have what effect? My goal is gas mileage at slowest plaining speed.

fuel economy and boating are polar opposites. if you want fuel economy get a sail boat, kayak or row boat.

that being stated, you prop your boat so your motor hits the middle to upper end of the RPM range recommended by Mercruiser.

in I/O will burn 0.4# of fuel per HP commanded per hour.

your commanded HP will depend on the location of your throttle, the direction and force of the wind, the direction and force of any currents, the load in the boat, the position of your trim tabs, length of the boat, fresh water vs salt water, the cleanliness of the bottom, the condition of your prop and the angle of your drive.

since the HP will be a variable that changes by the moment, the only thing you can do is prop the boat properly, install a fuel flow meter, and play with trim tabs and drive trim until you get the best economy at that particular moment.
 

ahicks

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Sep 16, 2013
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I like the 4 blade plan for your boat/engine combo and how you plan on using it, but 20" of pitch sounds a bit much/too much prop. The little 4 bangers usually run 17 or 19" 3 blade props on runabouts, and maybe even smaller on a boat like yours.

What is your max rpm now? If it can't get past 4K when lightly loaded, I would back off on the pitch as required to turn that at minimum.
 
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