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Author Topic:   Engine performance. Weight vs. hp formulas

Arch Autenreith posted 01-19-2003 10:24 AM ET (US)
Was is Boarders last night and just read this in some boating mag (but can't remember which one).
"In a planing hull a 10% increase in hp will increase speed 5%. A 10% in weight decreases speed 5%."
Using my engine as an example: 1980 80hp (powerhead) Mariner is 275#. Just say shaft hp is closer to 70. Present speed is about 36mph WOT. Montauk hull. The new Merc 60 4S is 248#.
Weight:
275 vs. 248 = 9.8% weight reduction (using above formula) = 38 mph.
HP.
60 vs. 70 = 7.1% hp reduction. (formula using current theoretic speed of 38 mph) =35.3 mph.
From the information above going from a heavier higher hp engine to a lighter lower hp rating equals about the same top speed ( 0.7mph slower).
There have been many discussions relating to hp vs. speed but I don’t recall engine weight being talked about as an equally important factor.
I remember Dick saying his top speed is about 35 (right, Dick?) with his 50 (same weight) but I didn’t realize the offsetting importance of weight reduction and the resulting fuel savings that will accompany.

Arch Autenreith posted 01-19-2003 10:25 AM ET (US)
Darn it. Should be posted in 'Performance' section.
triblet posted 01-19-2003 10:48 AM ET (US)
That's a 10% increase is SYSTEM weight (Boat,
motor, fuel, gear, occupants), not motor
weight.
Chuck
Arch Autenreith posted 01-19-2003 01:26 PM ET (US)
I agree, Chuck, system weight makes sense also.
But if I hang an engine on that's 50# lighter that by itself will have a direct affect on performance and gas usage.
Arch.
triblet posted 01-19-2003 02:06 PM ET (US)
Agree that a 50 pound lighter engine will
increase speed. Because it reduces the
system weight. Note that your example has
a change in weight of 27 pounds.
The horsepower reduction in your example is
(70-60)/70 = 10/70 = 14.2%, not 7.1%.
Do the math again. I think you'll find more
of a speed reduction.
Tom W Clark posted 01-19-2003 02:52 PM ET (US)
Arch,
Chuck is right. A boat like yours might weigh 1400 pounds with full fuel. Shaving 27 pounds off of it is a weight reduction of only 1.9%
If we interpolate from the theory that a 10% reduction in boat weight will increase boat speed by 5%, then you could expect to gain all of about .3 mph, and even that seems optimistic to my intuition.
Going from a 70 to a 60 is a reduction in horsepower of 14.3%, so now we would expect (based on your magazine's formula) your boat to only go 29.2 mph, which again, does not match up with what I would expect based on the comments of boat owners like Dick.
Simplistic rule-of-thumbs mean very little in the real world. They sort of fall into the class of one-size-fits-all things that I rarely find satisfactory. I think you'll be far better off just talking with people who actually run the motors you might be considering. That's the beauty of the FORUM.
Arch Autenreith posted 01-19-2003 03:42 PM ET (US)
Good gosh. That's why I did so poorly in school. My calculations were obviously waaaaay off due to the overall system weight (minus the engine weight savings). Sometimes I focus so much on one thing I can't see the forest...Good thing I have a sense of humor and can laugh at myself. Thanks guys.
Sal DiMercurio posted 01-20-2003 12:05 AM ET (US)
Your talking about a 23 year old engine that was rated at the flywheel at 70, lucky to be 62 hp at the prop when it was new.
The engines of today have so much more torque then they did years ago.
How do you explain my boat gaining 12 mph by putting an engine that is only 50 hp bigger then my 150, yet weighs about 50 lbs more.
My boat runs over 60 mph with just me in it, she gets 58 with 2 - 3 or 4 guys at 200 lbs each.
Don't worry about the weight of the engine, added weight means more hp in 2 strokes, can't say the same for 4 strokes.
Prop it right & she will push you the way she should.
50 lbs in any boat, will not alter the speed.
Sal

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Author	Topic: Engine performance. Weight vs. hp formulas
Arch Autenreith	posted 01-19-2003 10:24 AM ET (US) Was is Boarders last night and just read this in some boating mag (but can't remember which one). "In a planing hull a 10% increase in hp will increase speed 5%. A 10% in weight decreases speed 5%." Using my engine as an example: 1980 80hp (powerhead) Mariner is 275#. Just say shaft hp is closer to 70. Present speed is about 36mph WOT. Montauk hull. The new Merc 60 4S is 248#. Weight: 275 vs. 248 = 9.8% weight reduction (using above formula) = 38 mph. HP. 60 vs. 70 = 7.1% hp reduction. (formula using current theoretic speed of 38 mph) =35.3 mph. From the information above going from a heavier higher hp engine to a lighter lower hp rating equals about the same top speed ( 0.7mph slower). There have been many discussions relating to hp vs. speed but I don’t recall engine weight being talked about as an equally important factor. I remember Dick saying his top speed is about 35 (right, Dick?) with his 50 (same weight) but I didn’t realize the offsetting importance of weight reduction and the resulting fuel savings that will accompany.
Arch Autenreith	posted 01-19-2003 10:25 AM ET (US) Darn it. Should be posted in 'Performance' section.
triblet	posted 01-19-2003 10:48 AM ET (US) That's a 10% increase is SYSTEM weight (Boat, motor, fuel, gear, occupants), not motor weight. Chuck
Arch Autenreith	posted 01-19-2003 01:26 PM ET (US) I agree, Chuck, system weight makes sense also. But if I hang an engine on that's 50# lighter that by itself will have a direct affect on performance and gas usage. Arch.
triblet	posted 01-19-2003 02:06 PM ET (US) Agree that a 50 pound lighter engine will increase speed. Because it reduces the system weight. Note that your example has a change in weight of 27 pounds. The horsepower reduction in your example is (70-60)/70 = 10/70 = 14.2%, not 7.1%. Do the math again. I think you'll find more of a speed reduction.
Tom W Clark	posted 01-19-2003 02:52 PM ET (US) Arch, Chuck is right. A boat like yours might weigh 1400 pounds with full fuel. Shaving 27 pounds off of it is a weight reduction of only 1.9% If we interpolate from the theory that a 10% reduction in boat weight will increase boat speed by 5%, then you could expect to gain all of about .3 mph, and even that seems optimistic to my intuition. Going from a 70 to a 60 is a reduction in horsepower of 14.3%, so now we would expect (based on your magazine's formula) your boat to only go 29.2 mph, which again, does not match up with what I would expect based on the comments of boat owners like Dick. Simplistic rule-of-thumbs mean very little in the real world. They sort of fall into the class of one-size-fits-all things that I rarely find satisfactory. I think you'll be far better off just talking with people who actually run the motors you might be considering. That's the beauty of the FORUM.
Arch Autenreith	posted 01-19-2003 03:42 PM ET (US) Good gosh. That's why I did so poorly in school. My calculations were obviously waaaaay off due to the overall system weight (minus the engine weight savings). Sometimes I focus so much on one thing I can't see the forest...Good thing I have a sense of humor and can laugh at myself. Thanks guys.
Sal DiMercurio	posted 01-20-2003 12:05 AM ET (US) Your talking about a 23 year old engine that was rated at the flywheel at 70, lucky to be 62 hp at the prop when it was new. The engines of today have so much more torque then they did years ago. How do you explain my boat gaining 12 mph by putting an engine that is only 50 hp bigger then my 150, yet weighs about 50 lbs more. My boat runs over 60 mph with just me in it, she gets 58 with 2 - 3 or 4 guys at 200 lbs each. Don't worry about the weight of the engine, added weight means more hp in 2 strokes, can't say the same for 4 strokes. Prop it right & she will push you the way she should. 50 lbs in any boat, will not alter the speed. Sal