Skeg question on the Sissy-Do

Canoes, Kayaks, Pedal power

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SamsBoat
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Skeg question on the Sissy-Do

Postby SamsBoat » Fri Oct 16, 2015 12:46 pm

In the 'Building the Sissy-Do' step-by-step that comes with the plans, under 'Skeg' it says, "If the larger motors are to be used, end this skeg about 15" forward of the transom." My question is why? and what constitutes a larger motor? The document states that a 3 hp is adequate while a 5 to 7 hp is enough push on this boat. Am I to assume if I use something less than a 5 or 7 hp I should run the skeg all the way to the transom then? Or?

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Roberta
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Re: Skeg question on the Sissy-Do

Postby Roberta » Fri Oct 16, 2015 1:49 pm

A skeg can cause turbulent flow of water if too close to the propeller. This is more critical of larger, more powerful motors. If you think you might want a larger engine in the future, make it shorter. I doubt it will make much, if any, difference for the smaller engine.

Roberta
Roberta "Queen of the Boat Builders"
Built Zip "Oliver IV", Super Spartan "Jimmy 70", and Torpedo "The Glen L".

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vupilot
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Re: Skeg question on the Sissy-Do

Postby vupilot » Fri Oct 16, 2015 5:31 pm

Think of prop cavitation like losing traction and spinning your tires when driving. The rpms will increase but you wont go anywhere until you back off and the prop bites undisturbed water again. The 15" clearance gives your prop room to always grab undisturbed water, even in sharp turns at high speed. I would give yourself the 15" clearance.

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Re: Skeg question on the Sissy-Do

Postby Moeregaard » Fri Oct 16, 2015 10:17 pm

I would stick to the suggestions on the plans. If you do the math, a one-inch-deep skeg presents a significant amount of lateral area and will be surprisingly effective. There is no advantage to be had by running it to the transom, and you'll probably find that the boat is reluctant to turn tightly if you do this. We terminated the skeg on our Zip 24" from the stern, per Mercury's directions, and still got some cavitation when turning. It's your boat, but if it were mine I would follow the plans.

-Mark Shipley

One Zip on the water and two Can-Yaks on the way....
A boat is just a wooden box with no right angles.

SamsBoat
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Re: Skeg question on the Sissy-Do

Postby SamsBoat » Mon Oct 19, 2015 3:55 pm

Roberta wrote:A skeg can cause turbulent flow of water if too close to the propeller. This is more critical of larger, more powerful motors. If you think you might want a larger engine in the future, make it shorter. I doubt it will make much, if any, difference for the smaller engine.

Roberta


Thank you Roberta.

SamsBoat
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Re: Skeg question on the Sissy-Do

Postby SamsBoat » Mon Oct 19, 2015 3:56 pm

vupilot wrote:Think of prop cavitation like losing traction and spinning your tires when driving. The rpms will increase but you wont go anywhere until you back off and the prop bites undisturbed water again. The 15" clearance gives your prop room to always grab undisturbed water, even in sharp turns at high speed. I would give yourself the 15" clearance.


Makes sense, thank you.

SamsBoat
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Re: Skeg question on the Sissy-Do

Postby SamsBoat » Mon Oct 19, 2015 3:59 pm

Moeregaard wrote:I would stick to the suggestions on the plans. If you do the math, a one-inch-deep skeg presents a significant amount of lateral area and will be surprisingly effective. There is no advantage to be had by running it to the transom, and you'll probably find that the boat is reluctant to turn tightly if you do this. We terminated the skeg on our Zip 24" from the stern, per Mercury's directions, and still got some cavitation when turning. It's your boat, but if it were mine I would follow the plans.

-Mark Shipley

One Zip on the water and two Can-Yaks on the way....


Thank you Mark. We actually have placed it on the boat 15 inches from the transom. It's something that has been bugging me for a while. Thought I'd confirm before coating the bottom.
Steven


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