Speed under motor

[BOAT]

The M performs quite well as a power boat UN-ballasted. If the sea is calm enough to run WOT then it's also calm enough to have an empty ballast tank. There will be a lot less spray if the ballast is empty.

If the dagger-board fell down at WOT the boat would get really squirrel-Ly really fast and might even tip over. The dagger-board should be considered very very dangerous when at high speeds under power and make sure it is very secure in the fully UP position at those times.

In a pounding sea your not going to be traveling at WOT so it's best to fill the ballast up full in choppy waters or swells. Really, the only time I ever use the boat UN-ballasted is in calm water under WOT.

If I am not going to be traveling at WOT I usually keep the ballast tank full.

[mastreb]

With my 50 I was at 8mph with ballast and 16 mph without. With the 90 it's at 20mph with ballast and 24mph without. So an almost-doubling of the Hp picked up 50% top speed with without ballast. Not terrible and well explained by others on this forum. I have to admit Victor's stories and videos at 30+ mph set my expectations too high. I'll blame it on the bumpy bottom paint too. Anyone know what the paint is really worth in top speed?

Your numbers (un-ballasted) are about what one would expect from the speed/square law, which states that it takes 4x the power to go 2x the speed through a drag inducing fluid. You might be 12% low, which would be losses due to less efficient prop for the speed, lower than advertised power output possibly due to fuel choices or altitude, or possibly higher than advertised output on your 50.

My guess is your 50 (which is the same engine as the 60) was putting out more than 50hp, and your 90 (which is the same engine as the 70) is putting out less. If your 50hp was putting out 4hp more than advertised, and your 90 is putting out 4hp less than advertised, then your numbers would be spot on.

Your numbers ballasted are dramatically affected by the drag envelope of semi-displacement operation. Just a few more hp or a few less pounds would have put you over the hump (literally the bow wave) and you would have jumped to 12 knots on a semi-plane.

http://en.wikipedia.org/wiki/Drag_(physics)

[BOAT]

"semi-displacement" operation - I did not even know it was possible - that's interesting -

I always act like I'm driving a displacement hull (submarine) whenever I have ballast in the tank. The only time I ever even try to achieve planing is with the gate valve open. Perhaps I have been underestimating the ability of the boat when fully ballasted?

[mastreb]

"semi-displacement" operation - I did not even know it was possible - that's interesting -

I always act like I'm driving a displacement hull (submarine) whenever I have ballast in the tank. The only time I ever even try to achieve planing is with the gate valve open. Perhaps I have been underestimating the ability of the boat when fully ballasted?

Depends on the motor. I use the term "semi-displacement" to describe the area of dynamic instability that occurs after the boat begins to rise on the bow wave but before it reaches the 50% weight mark where a stable semi-planing mode is reached. Speeds are between 8 and 12 knots. During this mode of operation, the turning moment of the boat oscillates between a "roll inward" as you would have on a plane and a "roll outward" as you would have in full displacement operation depending on the exact location of the bow wave on the waterline (mostly) and the existing roll moment of the boat. Because steerage is unstable, you should power through this mode in a straight line and as quickly as possible.

In Semi-displacement mode, the boat is "climbing the bow wave" constantly--literally going up a hill, but a hill that it pushes ahead of itself. It crests that hill when the bow wave goes past 50%, and just as in a car, everything gets better when its no longer climbing.

With many 50hp motors, a boat may simply not be able to get all the way up that hill with the ballast tank full, so it's stuck at 8 knots WOT. Empty the tank and once it gets over the hill it can do 16 knots.

My boat semi-planes just fine with ballast in, achieving about 14..15 knots. Since you have the same boat and motor, I suspect you'll have the same results. I get to 17..18 knots with the ballast tank open. It's MUCH drier and more fuel efficient to power with the ballast tank open. I usually operate 10% back from WOT on the throttle, letting the boat drift between 15 and 17 knots with the wave fronts, because it saves 20% of gas. That's "peak efficiency" at speed with an ETEC-60 (and most other motors as well).

[raycarlson]

Not sure what 10% back means, is that 550rpm less than wot or what, as virtually every outboard manufacturers fuelburn charts ive ever seen without a doubt always specify the 3500rpm range as the most efficient as it relates to GPH burn, MPG can be affected greatly by hull shape. But I would call 3500rpm 30% back from WOT. Just wondering what your definition of 10% applies to.

[mastreb]

Not sure what 10% back means, is that 550rpm less than wot or what, as virtually every outboard manufacturers fuelburn charts ive ever seen without a doubt always specify the 3500rpm range as the most efficient as it relates to GPH burn, MPG can be affected greatly by hull shape. But I would call 3500rpm 30% back from WOT. Just wondering what your definition of 10% applies to.

Hi Ray,

10% back is the best way to efficiently operate near the engine's maximum speed, not peak efficiency. I may have used quotes incorrectly, I meant "peak efficiency _at speed_" and by "at speed" I meant "as fast as the boat will go". I should have said "best efficiency at near maximum speed"

Easing off WOT by 10% allows you to eliminate losses due to pushing into wave peaks. It allows the boat to float back a little while going up a peak, but take full speed advantage of wave tops and downward motion into troughs. You lose less than 10% of speed, but gain 20% of fuel efficiency.

"Peak efficiency" for a motor pulling on a tensometer is likely is exactly where you describe, but on a hull in the water it will vary by hull characteristics. Sorry for the confusion.

[raycarlson]

Gottcha, at 3500rpm (most efficient according to tohutsu) mine will not be on plane. I need to keep at least 4200rpm to maintain a plane like attitude, but I think i'm back from wot at least 20% or more.

[SENCMac26x]

Question about the swing keel on my X when under power.

I normally leave about 6 inches down to help track but I was thinking when I go to WOT is that board being swung back into the trunk? Am Im risking damaging it?

What do other X owners do

[dlandersson]

Per the manual - anything over 8 knots - bring it up. My personal rule is 6.5.

Question about the swing keel on my X when under power.

I normally leave about 6 inches down to help track but I was thinking when I go to WOT is that board being swung back into the trunk? Am Im risking damaging it?

What do other X owners do

[dlandersson]

Per the manual - anything over 8 knots - bring it up. My personal rule is 6.5.

Question about the swing keel on my X when under power.

I normally leave about 6 inches down to help track but I was thinking when I go to WOT is that board being swung back into the trunk? Am Im risking damaging it?

What do other X owners do