Is the Offset Motor on Rescue Minor Really Necessary?

[windmill4048]

ATkin's Rescue Minor, a tunnel hull Jersey Seabright skiff, has the motor offset to port by 5" at the flywheel.

The stated reason for this is to offset the torque thrust of the right hand prop. In theory the boat should be a little more efficient and faster due to carrying less trim of the rudder when running.

I noticed that none of Atkin's other tunnel hull skiffs has this offset. I've looked for unique features of the Rescue Minor design that might make the offset necessary and can't see any. It would seem that any efficiency improvement would be very small.

Am I missing something important? Is there real advantage to keeping the offset?

[mcdenny]

Getting the prop shaft out without removing the rudder?

[Wes White]

I believe it also helps counter the prop walk while backing down.

[Spokaloo]

Keep in mind that RM was one of the smaller boats of the fleet and on top of that was required to perform more severe service (wartime shallow draft rescue boat). When built as a larger, heavier craft, there wasnt as pronounced propwalk and the efficiency lay in the shape of the hull. On RM, her efficiency lay in the new lighter construction and materials used. This smaller boat probably produced more pronounced propwalk and being lighter didnt keep as steady a helm as something like LITTLE WATER.

You can always be the first to try it though!

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[Tylerdurden]

I think one of the other reasons is that in a man overboard situation the boat will continue to circle tighter and tighter allowing one to reach up and chop the throttle. The plans show the motor along this axis allowing more room at the seat. I am keeping the offset in mine for that and the reasons posted above. Robb White was very impressed with her abilty in backing. She is a small and light craft and with the offset she will keep better helm.
I was going for 24 HP on mine but I have settled for 18 because anything more is a waste with a 10" screw and economy will be slightly better at cruise.

[Spokaloo]

In talking with the limited resources who know the boat, Its my understanding that anything over 20 hp is just burning extra fuel as the hull refuses to go over Atkin's specified speeds. Excellent choice!

Im excited to see how yours comes together.

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[Tylerdurden]

Covering the shed now and finishing the woodstove. Hope to be building the strongback soon after. I promised myself I would learn how to post pictures by then.

[Spokaloo]

www.photobucket.com (http://www.photobucket.com)


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[Bill Perkins]

Windmill I just recently read Robb Whites' WB article on the Rescue Minor . In it he states that her prop is half out of the water when the boat is at rest .This would cause extreme prop walk at idle speed untill the tunnell fills with water at a certian speed .Same problem in reverse . Maybe the bigger boats draw enough to submerge their props when at rest and at idle speeds ?

I just looked here http://www.boat-links.com/Atkinco/Utilities/Everhope.html .It does look like Everhope and Little Water more fully submerge their props.

[Spokaloo]

Per Robb in some other articles, as well as some old Atkin info, state that as soon as the prop begins to spin, it fills the cavity immediately. Apparently Robb noted his boat to be the best boat in reverse (at idle in his articles in MAIB) he ever drove.

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[windmill4048]

Thanks for all your input. I'm now leaning toward leaving the offset in as designed.

It is a small, light boat. The displacement, as drawn, is 840 pounds. And while I had noticed that RM's prop shaft was on the waterline at rest I hadn't looked at the other boats carefully enough to notice that thier props lie deeper.

I'm using a 20 HP Kubota. That should be plenty of push for the boat.

Here's an interesting drawing of the boat.


http://i84.photobucket.com/albums/k37/windmill4048/Stern-11.jpg

[Bill Perkins]

What I took from Wes' dads ' Woodenboat article is that the tunnel is filled by the wake of the little canoelike hull beneath the main hull form . If this is right , it wouldn't be happening at rest or dead slow speed no matter how fast the prop spins . He states the boat is good in reverse , but that's with the offset prop.

[Wes White]

He thought that the stern wave of the little, double-ended hull filled the cavity at speed, but also that the prop-wash pretty much immediately filled it when idling.
I have been asked on the Atkins website if I thought that he would build it that way (offset) again, and I can't give a definitive answer. Every time I try to think about it, I remember a different statement that he made, and wind up changing my mind. I think that he couldn't decide either, and that it is probably six of one, a half dozen of the other. My personal recommendation would be to build it in.

[Sailor]

Anybody read Herrshoff's common sense of yacht design? He talks a little about why he offset the screw on several of his yachts. check it out it's interesting.

[TimmS]

I am just finishing a Rescue Minor and I did offset the shaft as drawn, I believe that most of what has been said above is true. The shaft can be removed without taking off the rudder, the offset engine makes a great space to walk through the starboard side, and from all that I hear it helps greatly at low speeds . See photos at http://www.schleiffboatworks.com/rescue.htm She will be launched next week and will be on display at the WoodenBoat show in Mystic, Timm