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Folks,

I know that the normal transom angle for a hard boat is anywhere from 10 - 15 degrees. When making a transom for a round raft, I am trying to determine what the transom angle should be.

My reason for asking is that many folks have recommended making the transom plate "vertical" (straight up/down). I'm guessing the reasoning is that the raft stern is going to dip/drop some just from the weight of someone sitting in the rear and the operation of the motor itself. Any drop in the stern naturally creates some transom angle. My concern is that if I'm just putting along at idle and perhaps use a tiller extension keeping more weight forward, its possible I won't get the dip/drop of the stern and therefore won't have the transom angle needed.

My outboard has 6 position trim, but of course only going "out ward".

Bottom line question is that I'm ordering some bend pipe for making my motor mount and am trying to decide what angle I should ask the pipe to be bent. 90 degrees is being recommended, but that would put the transom vertical. Any recommendation would be most appreciated. Thanks.
 

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Vertical or less than that. I mean canted forward at the top motor mount about ten degrees. Reason being, when the motor is under load, the boat will assume an arch shape because the bow is pushing in. The prop is on the end of a long lever and tries to climb up under the boat. My 1980 18' Avon came with a motor mount and it is angled about 20 degrees out at the bottom before you load the motor, then about ten, loaded, then you go full throttle, negative ten, (lower case climbing under boat.) This is a 9.8 merc. four stroke, about 75 pounds. You set the motor up to take wide open throttle, because you might need it. If the outboard is angled up a little at idle, it makes no difference. Prepare to be amazed at the drag a raft has at speed. It is gargantuan. The 18' Avon will barely go upstream into a 2 MPH current with the motor wide open. This is with maybe 1000 pounds on board. On an 18' NRS Kodiak cataraft, maybe a little lighter, it goes three times the speed. Rafts drag a shiteload of water along. Plan for it.
 

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Ditto what Catsailor said. About vertical but tilted forward to make up for what the motor pulls down is OK. I've been motoring rafts for 30+ years and have found that vertical is good, and there isn't that much else that can be done. I put my motor bracket out at it's farthest point (has six positions) and when I'm gunning the motor it pulls down some but, for the most part, with a raft you're gonna idle along somewhere mid-throttle. I'd put my transom vertical and then adjust the motor bracket out but realize there is no "perfect position" there is just a "most of time this I what it does" sorta position when it comes to rafts.
 

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motor

I want to add that once you have a motor, the other boats want to tie up to you. So conceptually, you want a motor angle that will be fine with the raft bow-on to a rock wall and at full throttle. See, in our minds, we think of the motor going and the raft going forward, thus unloading the strain. But in wind and waves, which is what we have it for, the beast just labors away and everything is straining and compressing. We had a motor try to go for a swim because we were two 1500# boats going straight into 2' waves with 40 mph of wind on the nose. It tried to rip right off the transom because you have a propeller screwing out 300 pounds of force on the end of a pretty long lever-arm. My guess is we were going 1 mph. So plan for that. It's not rocket science. A good motor mount is a wonderful thing. And I usually do tape a stick to the outboard handle so I can get away from the noise and steer up front.
 
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