Fore/aft Balance

Moderator: GreenLake

Fore/aft Balance

Postby GreenLake » Mon Jan 21, 2019 11:52 pm

This post isn't directly about racing, because what I discovered was under motor. However it sure is about "performance" in the below hull speed range.

I have a new motor (see here) and when I was testing it recently, it got up to 3.0/3.1 knots on my GPS in flat water under bare poles and a slight breeze off the stern quarter.

My crew was steering with the tiller, seated just about where the mainsheet block is located. By standing up on the cuddy, I was able to make the boat go faster, the GPS read 3.2/3.3 with an occasional 3.4.

I think this difference is entirely due to the different fore/aft balance and not, say, to my body acting as "sail". That would mean that in those conditions, the fore/aft balance is super critical. If most of you are like me, the wide expanse of bench going all the way to the rear is always tempting (and crowding the crew at the front of the boat doesn't always feel comfortable). But not moving all the way up comes at a cost.

On occasion, I will have to rerun this experiment to see whether ordinary weight distribution inside the cockpit is enough, or whether the effect requires one person to be on the cuddy. That would not be practical for races under class rules (but would be fine for the some of the no-handicap, more relaxed events I sail in during the summer).

Anybody else have any data or experience?
~ green ~ lake ~ ~
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Re: Fore/aft Balance

Postby sailor54 » Tue Jan 22, 2019 12:43 pm

Interesting question. The line I hear most often in my limited racing experience is 'keep the boat flat'. The more weight you put on the stern, the more drag you create. By moving some of the weight forward, you decrease the drag. Whether or not the redistribution of weight can be done inside the cockpit will depend on several factors and may require further experimentation.
If you can't tie good knots, tie lots of them.
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Re: Fore/aft Balance

Postby GreenLake » Tue Jan 22, 2019 7:29 pm

Data would be nice. I'd love to hear from other sailors. In the meantime, I'll keep experimenting.

For much lower speeds I know that sailing with the boat heeled helps (reduces the wetted surface). The effect seems noticeable even racing against unlike boats (we have our share of "drifters" during the season). However, with the motor I might use a truly calm day, take the sails down, go to a really low speed setting and see whether I get an increase in GPS speed by heeling the boat (and at what speed that stops working - when form resistance takes over from skin friction).
~ green ~ lake ~ ~
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Re: Fore/aft Balance

Postby tomodda » Wed Jan 23, 2019 12:19 am

GL;

No surprise to hear (read) that moving forward makes the boat go a bit faster. Except in planing conditions, I think you'd want to get as much of the DS's flat, fat aft sections out of water as possible. I write that with all my love to fat-bottomed girls, of course. :wink: But as soon as I get boat out of the paintshop/garage and into the water + get myself a GPS, I'll do some trialing.

Tom
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Re: Fore/aft Balance

Postby GreenLake » Wed Jan 23, 2019 3:44 am

Looking forward to your data.
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Re: Fore/aft Balance

Postby Signalcharlie » Fri Jan 25, 2019 9:51 pm

The motor thrust vector being well under the waterline can make the stern squat and increase drag, so by moving forward you helped counter that. We experienced that on a Capri 18, throttling up on the Suzui 6 dug the stern in to the point where water was coming in through the scuppers, boat felt like it slowed vs speeding up.
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1971 DS II #4624 "CYANE"
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Re: Fore/aft Balance

Postby GreenLake » Sun Jan 27, 2019 2:15 am

You realize that I'm talking about a 250 Watt electric? (*)

Yes, there's some small squatting effect and it's opposite to the lifting effect that you might get from pushing the boat forward by sail (mast being above the waterline). The combination may affect the ability to use experiments under motor to get conclusions about the effect of various weight distributions under sail.

I'm interested in getting answers for the low speed range, hoping that the effect of propulsion on fore/aft balance in that regime is swamped by weight distribution.

I know, I'll need to rig something to let me measure the immersion of the stern in real time. Because, for some reason, when I go to the back of the boat and lean over to check, I pick up this pesky systematic measurement error :)




(*) That's what I was told by the guy who builds it. Amazingly enough, that's 1/3 the power consumption of my 40# trolling motor yet the top speed is a tad higher.
~ green ~ lake ~ ~
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Re: Fore/aft Balance

Postby tomodda » Sun Jan 27, 2019 7:54 am

:lol: You should see the motor that GreenLake and I both use! Suffice to say that it puts out 1hp on a good day.

https://www.electricpaddle.com/

Great motor, weighs all of 14 pounds, 100% reliable, but I don't think that it causes any stern squatting when powered up.

For what it's worth and as far as I know, stern squatting is caused by applying more power to the boat than is needed for displacement hull speed. In other words, once your boat reaches hull speed, and additional power will just result in a bigger bow wave (that energy has to go somewhere), so your bow rides up and your stern digs in. And you get a bigger wake. That is with a pure displacement hull, like the Capri or any other keelboat. If you have a planing hull, you'll pop out of the water. Supposedly you can motor-plane a Daysailer, I think with a 5hp motor. Maybe that's the solution to GL's problem - trim tabs!

All kidding aside, what I always love about sailing is that we are all chasing the last tenth of a knot of speed. Going as fast as we can on slow boats! Tinkering is half the fun. Although my idea of trim tabs may be a bit much.... ;-)
Last edited by tomodda on Sun Jan 27, 2019 11:17 am, edited 2 times in total.
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Re: Fore/aft Balance

Postby Signalcharlie » Sun Jan 27, 2019 9:46 am

Oh goodness, I didn't see the link about the motor! Not stern squat then, and I forgot about the displacement vs heeling hulls.

Fore/Aft balance always an issue, my wife can get our Penobscot 14 to tack by just having me sit up by the mast and turn my big head :)
Kent and Skipper
1971 DS II #4624 "CYANE"
Small Boat Restoration blog http://smallboatrestoration.blogspot.com/p/o.html
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Re: Fore/aft Balance

Postby GreenLake » Sun Jan 27, 2019 10:23 pm

@tomodda: one hp ~ 750 W. So with our motors drawing 9.6 A at 23.5 V (per battery spec), they consume less than 1HP.

However, they produce as much forward propulsion as a trolling motor rated at 3 times the electric consumption (or nominally 1HP in energy used).

There's some serious optimization that went into those new motors, and the good news is that makes recharging a battery after use that much easier as it is smaller (but those considerations belong into the other threads . . .)

Too much wind on the water today to take measurements.
~ green ~ lake ~ ~
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Re: Fore/aft Balance

Postby tomodda » Sun Jan 27, 2019 11:28 pm

Hadn't gone thru the math... Obviously only a .3 HP motor. But, as you point out, it has amazing performance. For a space-age paddle, that is. I think you have to be a true engine-hater like me to appreciate it! Note - I only hate engines on sailboats, otherwise a Wankel, or a Stirling, or even a good ol' Flat-4 is a thing of beauty...

In other news, I FINALLY flipped my hull. A week late due to weather and health problems. Despite a few heart-stopping moments, all went well. I have some photos, will document it in a repair-section post.
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Re: Fore/aft Balance

Postby GreenLake » Mon Jan 28, 2019 4:59 am

Looking forward to a post in that section.
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