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I had a long trip on the Vineyard sound with steady winds, and while maintaining speeds between 4-6 mph observed the following

WHile running, broad reach, or close hauled I could not see any GPS variation with motor up or down (Merc 50 HP BF 41x11x3 prop)

ON a beam reach, I clearly saw .2-.3 mph increase in speed with the motor pulled up...I reproduced this 5 or 6 times in steady winds over the week.

POssible explanation
The effects of the motor are sensitive to heel angle , with no effect on a flat boat or 10-15 degree heeled boat, but in the 5-10 degree range it is important.



Y(Velocity)MV

Thanks for posting this data. I've always thought it should be a significant difference. I'm not sure why the broad reach or beating isn't affected though. Could it be a difference in weather helm on the different points of sail having a different effect on the water flow over the motor?

BB

That is odd. My results were quite different. 1/2-1+ knt difference in speed, not to mention the lighter helm feel with the motor tilted and disconnected.

Uh-oh, here we go again.
Just recently another M went sailing past me on the same tack, when I saw him gaining on me I trimmed the sails and corrected my course to his but he still easily passed me. When I looked back to see my wake I saw the engine down and promptly raised it gaining noticable speed, but it was too late I was already beaten. I have checked the difference in speed before and measured the gains to easily be 1 knot or more, but I don't measure any more because what I feel in speed change is convincing enough. I go to great lengths to improve my sailing speed which is why I am so fanatical about a clean hull. I pulled my boat out a week later to clean it as I was so dissapointed at being beaten at such a rate. The hull was no where near dirty enough to slow me that much but I cleaned it anyway. Probably the four blade prop I use slow speed more than three blades do, but there is a defnite difference dragging that skeg.

Im beginning to believe you M guys have a better release of flow on the transom, so the motor makes a difference while the X transom is turbulent, so little/no effect.

Four vs three blade prop is another difference.

I see no effect on any point of sail if I leave the motor in neutral (prop turns under flow) or freeze it in gear btw...

Definite difference with the prop in the water under sail. It's probably only that .2-.3kts,but every little bit counts when I'm trying to close on another boat.
I have a blue M,so we have a reputation to keep!

I have often heard that the drag of a "free" rotating prop is considerable, even though that may seem counter-intuitive. Maybe that is why keelboats with inboard diesels and prop shafts often have folding props.

?? kind of like a autogyro's passive lifting blades have enough "drag" to suspend the aircraft while the propulsion prop or jet pushes. ??

The prop presents the same area whether it turns or not , less the slippage factor when it turns. You also have to add the resistance of the gearbox when you spin the prop, which is minimal in the case of my Bigfoot (based on what I feel with the boat on the trailer)

..thats probably why there is no effect on speed locked vs spinning...it all comes out in the (prop) wash.....

Reasonably, anything in the water will cause a drag on the boat - even a line will, as we know. I've steered by trailing a warp once when the rudder cable broke on a trimaran I was trialing.

If the prop's in the water, the boat will be slower, althought perhaps imperceively w/o the accuracy of GPS, or a speedo.

I went to great lengths to lower my BF one bolt hole just to be able to get the monster up and tucked into the engine well under the seat and out of the water. I will sail without the engine attached to the rudders just for the feel, thouch that's not practical when getting ready to dock, anchor, etc.

The prop presents the same area whether it turns or not...

..thats probably why there is no effect on speed locked vs spinning...it all comes out in the (prop) wash.....

That's the equivalent of saying a pair of oars in the water provide the same propulsive force regardless of whether you're pulling with the blade crosswiswe to the flow or edgewise.

Don't know where you heard this, 'cause it's not so.

I worded that poorly indeed....the spinning prop should present less prop resistance, but wil have added resistance from turning the transmission or lower end...

I was trying to understand why I didnt see a diff between prop spinning and not spinning....

I worded that poorly indeed....the spinning prop should present less prop resistance, but wil have added resistance from turning the transmission or lower end...

I was trying to understand why I didnt see a diff between prop spinning and not spinning

....

Added resistance compared to what, the prop not in the water?

The part of the lower unit that's turning with the transmission in neutral can't be much; in any case, with the boat on the trailer, you can spin the prop with the gearshift in neutral and there is very little resistance.

The reason you don't see a difference is not because it's not there., it's because it's low enough to be difficult to accurately measure with the means at our disposal.

Large sail boats go to extremes to get more speed. They use folding props and other things. But the point is anything dragging in the water will slow you down. With the push of a button the motor is out of the water, so why would anyone want to sail with it down?

The reason you don't see a difference is not because it's not there., it's because it's low enough to be difficult to accurately measure with the means at our disposal.

Agreed.

so why would anyone want to sail with it down?

Because a seawall, 4 knot tide, and/or freightor is never more than 10 minutes away and the HUdson river wind can drop, turn 60 degrees, or suddenly turn and blow with the tide in a heartbeat, and Mr Mercury is the cure....I dont always have 30 seconds to drop motor and start to play with on the river

We sailed on Sunday and Monday, and both days were really light wind. I pulled the prop up numerous times and there was an obvious improvement in speed when I did that. The 14" prop is a drag in low wind conditions.