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Author Topic:   Engine Height Effect On Planing

mikemdd posted 01-08-2009 02:33 PM ET (US)
I did a search and read literally hundreds of postings regarding how raising the engine, all other things being equal, can benefit the time it takes to get on plane. I am not an engineer (by far), but many moons ago in basic math I recall that this is basically a force-vector issue (as I said, I am way over my head). Anyway, after doodling various vector drawings on a piece of paper, I can't undertand WHY raising the engine helps you to get on plane faster, given a constant trim angle. Does it have something to do with "leverage"...? Just curious as to WHY....in terms a normal person can understand.
Thanks
Mike
tedious posted 01-08-2009 02:46 PM ET (US)
Mike, I'm not an expert either, but from experience, anything that helps you keep the bow down helps improve time to plane.
I think you're right - it's about leverage. Think of two forces occurring at planing time - the force of the motor pushing forward, centered at the prop, and the force (resistance) of the hull pushing backward. Think of having a really long outboard that buried the prop down about 50 feet - when you hit the gas, you'd lift the bow and not much else. When you raise the motor you're doing the opposite, which keeps the bow down.
To get a bit more technical, you are raising the center of thrust relative to the center of resistance; that tends to keep the bow down. Or so it seems to me...
Tim
mikemdd posted 01-10-2009 09:36 AM ET (US)
Tim,
Thanks for your thoughts....it actually makes some sense to me.
Mike
L H G posted 01-10-2009 02:34 PM ET (US)
[Cited an article written by Mercury which says, "Raising the engine height may result in ...increase time to plane boat."]
Jerry Townsend posted 01-10-2009 10:36 PM ET (US)
Haven't really dwelled on this subject for some time - but a few comments in view of a bit of engineering and dynamics.
Tim (tedious) - your hypotherotical is one way to look at it - but now consider the axis of the prop being vertical - one way or the other. In other words - it depends on the thrust axis too. In fact, I suspect that it is more dependent on the thrust axis than the heigth - but I have not made that evaluation/analysis.
Now, before proceeding further - Larry - et. al - the words [Cited an article written by Mercury which says, "Raising the engine height may result in ...increase time to plane boat."] - which I have not read - are reasonable, from an engineering point of view.
Mike - your are right on - "... can't undertand WHY raising the engine helps you to get on plane faster, given a constant trim angle. Does it have something to do with "leverage"...? Just curious as to WHY....in terms a normal person can understand. ..."
Yes - in a nutshell - it is, in part, about leverage - AND the dynamics of the boat/engine system. But unfortunately - the TOTAL explaination is not simple where a "normal" person can understand it.
There are many forces acting on the boat/engine system including: o) the thrust and axis from the prop, o) the drag from the lower unit, o) the resistance of the boat/water interface, p) the gravitational forces and o) the buoyancy forces.
But, raising the engine, BY ITSELF - would tend to raise the bow - as that (raising the engine) would decrease the rotational moment about the center of gravity. But as alluded to before - the axis of the thrust angle is, of possibly greater importance. ---- Jerry/Idaho

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Author	Topic: Engine Height Effect On Planing
mikemdd	posted 01-08-2009 02:33 PM ET (US) I did a search and read literally hundreds of postings regarding how raising the engine, all other things being equal, can benefit the time it takes to get on plane. I am not an engineer (by far), but many moons ago in basic math I recall that this is basically a force-vector issue (as I said, I am way over my head). Anyway, after doodling various vector drawings on a piece of paper, I can't undertand WHY raising the engine helps you to get on plane faster, given a constant trim angle. Does it have something to do with "leverage"...? Just curious as to WHY....in terms a normal person can understand. Thanks Mike
tedious	posted 01-08-2009 02:46 PM ET (US) Mike, I'm not an expert either, but from experience, anything that helps you keep the bow down helps improve time to plane. I think you're right - it's about leverage. Think of two forces occurring at planing time - the force of the motor pushing forward, centered at the prop, and the force (resistance) of the hull pushing backward. Think of having a really long outboard that buried the prop down about 50 feet - when you hit the gas, you'd lift the bow and not much else. When you raise the motor you're doing the opposite, which keeps the bow down. To get a bit more technical, you are raising the center of thrust relative to the center of resistance; that tends to keep the bow down. Or so it seems to me... Tim
mikemdd	posted 01-10-2009 09:36 AM ET (US) Tim, Thanks for your thoughts....it actually makes some sense to me. Mike
L H G	posted 01-10-2009 02:34 PM ET (US) [Cited an article written by Mercury which says, "Raising the engine height may result in ...increase time to plane boat."]
Jerry Townsend	posted 01-10-2009 10:36 PM ET (US) Haven't really dwelled on this subject for some time - but a few comments in view of a bit of engineering and dynamics. Tim (tedious) - your hypotherotical is one way to look at it - but now consider the axis of the prop being vertical - one way or the other. In other words - it depends on the thrust axis too. In fact, I suspect that it is more dependent on the thrust axis than the heigth - but I have not made that evaluation/analysis. Now, before proceeding further - Larry - et. al - the words [Cited an article written by Mercury which says, "Raising the engine height may result in ...increase time to plane boat."] - which I have not read - are reasonable, from an engineering point of view. Mike - your are right on - "... can't undertand WHY raising the engine helps you to get on plane faster, given a constant trim angle. Does it have something to do with "leverage"...? Just curious as to WHY....in terms a normal person can understand. ..." Yes - in a nutshell - it is, in part, about leverage - AND the dynamics of the boat/engine system. But unfortunately - the TOTAL explaination is not simple where a "normal" person can understand it. There are many forces acting on the boat/engine system including: o) the thrust and axis from the prop, o) the drag from the lower unit, o) the resistance of the boat/water interface, p) the gravitational forces and o) the buoyancy forces. But, raising the engine, BY ITSELF - would tend to raise the bow - as that (raising the engine) would decrease the rotational moment about the center of gravity. But as alluded to before - the axis of the thrust angle is, of possibly greater importance. ---- Jerry/Idaho