View Full Version : belaying torque applied to frame
08-07-2011, 04:41 PM
I am re-powering a 16' day cruiser with 40 hp outboard motor. To overcome a reverse transom angle to provide the required 12 degrees, thus obtaining the correct "tuck", a small transom extension is required. My question regards torque applied to the frame of the extension: Will there be so much torque from a modern 40 hp outboard that twist will be a factor on a well constructed wooden transom extension? How can twist be calculated? Oak framing, bs1080 ply and marine epoxy will be used in the construction as well as properly engineered joinery.
08-07-2011, 06:35 PM
Was this boat originally powered with an outboard? If this wan an inboard cruiser the transom might not be up to hanging a modern OB on it. There will be a lot of torque on the new bracket that will be transferred to the old transom.
Photos would help.
08-07-2011, 08:59 PM
Along with the photos, a sketch of the planned extension would help. But clearly, you're dealing with lots of stress and lots of vibration. Even if it appears to hold up well initially, you could be stressing the hull excessively. All this stress needs to be transferred from the transom to the rest of the boat, too. It may call for adding some knees, or replacing small knees with more substantial ones.
08-07-2011, 09:24 PM
Typically, outboard transoms are rigidly braced to the keel with a large knee and to the sheer stringer with substantial quarter knees, plus other bracing as needed. Plus, the transom itself is heavy and stiff to resist the thrust and torque of the motor.
It sounds to me like you're talking about a fairly vertical transom, not originally built for an outboard.
08-07-2011, 09:39 PM
One can add a simple wooden wedge under the engine to change engine tuck. No engineering required,unless of course it does not have those knees.http://farm6.static.flickr.com/5223/5669305361_4ef239f4a7_z_d.jpgthe wedge can go either way
08-09-2011, 08:55 PM
Thanks for all the helpful replies! The original plan in 1965 relied on the old style outboard bracket which, if I'm correct, allowed more "tuck" than today's motors. TerryLL, thank you for pointing out the need for 1/4 knees on the sheer stringer. It is substantial and will accept them. Verticle? No, worse...and yet it was designed for an old style outboard. Wobox, I will also increase the size of the main knees (they are box knees originating from the keelson). wizbang 13, good photo...I'm interested in the Panther hydraulic tilt mechanism. Do you think it would adapt to my aplication?
Once again, I am grateful for your input. I will add photos of the proposed changes at a later date...
08-09-2011, 09:15 PM
The Panther ,(model55) is electric (12v dc) not hydraulic. About $500.It adds 5% negative til (tuck),and as you can see , I tucked it WAY in with the wedge.
I think a T n T bracket would help most any boat. It is like adding 20 % more hp and makes faster hole shots and adds safety. Wizbang (the boat) is un safe without one.
Photos of your set up would help with definitive answers.
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