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Rescue Minor Corrective Surgery


Steve Day
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Hey Guys! I haven't posted in quite some time as I have been busy with things other than boatbuilding.

We have been using the Rescue Minor out on the Mobile Bay Delta and other places in the area, but we have been disappointed in the top speed of the boat. After thinking about it all of this time, I came up with the idea (duh) of tapering the stern log to better get "clean" water to the propeller. As you can see from the photos attached, only the outer part of the propeller is in "clear" water when we are up to speed. I will be working on the area to taper the area immediately in front of the prop to get better flow. We'll see what happens.

Steve

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Here we are after a couple of hours laying out and sanding with my new 7" sander/polisher. Not quite there yet, but well on the way. Other obligations are keeping me from continuing this afternoon, but I'll be on it again tomorrow. I'm hoping to be ready for epoxy when the weather warms up on the weekend. Supposed to be around 29 degrees tonight here in Lower Alabama.

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Got quite a bit done today. I took all the wood I dared to take off the stern log and shaped it the best I could. Got it smoothed out and the first layer of glass on it. Here are some shots of where she is now. You will note the cross-section at the stern is reduced a considerable amount. I hope it will help our speed.

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Steve,

Have you had a chance to weigh the vessel yet? You may then be able to apply Crouch's formula to determine the approximate vessel speed. Did you use plywood thicknesses as per the original Aitken's design. If you are only using a 20 Hp engine with a 2:1 knockdown gearbox, this may be your issue.

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Just haven't been near a scale near water to weigh the rig with and without the boat. We'll get around to it one day. The plywood is 3/8" vice the 3/4" called for in the plan, but we left the frames in. The weight is probably around where it should be. The fairing may help. If not we'll use it as is. Another option is to put more horsepower in it, but we don't want to go there right now.

Besides, it's kinda fun to try different things to see what helps and what does not. Gives us something to do in the off season.

Steve

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When I trimmed it this time, I was running into air on the inside of the hull at the forward end of the stern log. I figured that was probably far enough to go on this attempt. I made the necessary repair and applied epoxy and cloth to strengthen it and fair it as much as practicable. If I make progress with this modification, we'll look at what to do next, if anything. The prop does not have a whole lot of clearance between it an the hull, but larger diameter may be a possibility.

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  • 2 weeks later...

Larger prop is not a possibility! There is about 1/2" clearance to the hull.

The fairing did not increase top speed measurably. There seems to be less vibration, but that is entirely by feel and not verifiable quantitatively.

We are not going to be able to increase her speed without more horsepower. We will continue to look, but we are going to use the boat as she is and enjoy it.

It is really cool to go across the shallows where no one else goes. We have more fun watching peoples eyes bug out when we do that.

Steve

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  • 1 month later...

Shaft width tip clearance? 1/2" tip clearance? Damn that's way tight, even for a boxed keel boat. I consider the minimum tip clearance 10% of the wheel diameter and this is pretty tight, preferring 20%. At displacement speed you don't need huge gaps, but vibration and efficiency can be improved with some breathing room for that wheel. The thumping will go away too.

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