Motor Mounting Transom Plates

[Maria CS20 104]

??? Hey all, back on track and working the interior.  Got seat stringers set and done, cool how it's looking closer to the pics you guys share.  I am going to do some blue water sailing in the Gulf and in the Atlantic, but not without a motor on board.  I'd like to put two transom plates on, one each side to balance the weight and to ensure the option of mounting the motor on either side.  How wide should they be and how close to the center of the transom?  I put a 3.5 inch center plate on the transom and I think if I go with a 7.25 motor mount plate on each side it will look fine and provide me with what I am looking for in the end result.  The motor I am looking at will be a Honda 3.5 HP at least, no more than 4 HP based on reads in other attachments.  Ought to be sufficient for the weight and speed requirements.  On other thing, I read the plans and understand them but I want to put the hatch in the aft seat in the center and need to do the deck beams differently which is no big deal, but the spacing listed on the plans for the off-set hatch seem rather far apart.  I am not making a rear deck but am doing an aft seat in it's place, much like I've seen done by some others.  What would you recommend for beam and stringer placement?  Thanks All

Jim A

[wkisting]

Jim,

I think you only need to reinforce one side... just choose where you would rather situate your motor. I've never found "prop walk" to be much of a problem, though most folks tell you to locate it on the side that will negate that phenomenon. I instead opted to mount my motor so that the tiller handle/throttle of the motor would fall near the centerline, for quick easy adjustment from either side of the boat. I'm pleased with this arrangement.

For transom reinforcement, I used a slab of 1-1/4" thick laminated wood left over from making the CB. I made the slab about 1.5 - 2" wider than the bracket I intended to mount to it, so that the attachment holes would have plenty of wood around them to prevent tear out and spread out the forces more. Before doing any of this, I mounted the motor on a sawhorse and identified the perfect position (athwartships) for mounting it... which was as close to the centerline as possible, without risk of getting the rudder blade caught in the prop during a steep turn. Once you find this point, you'll be able to visualize exactly where to add the reinforcement. Also, I highly recommend attaching your bracket before you put on the aft deck (if including that option), as it is much easier than attaching it later--allowing more working room to do a nice job with sealant, tighten bolts, etc.

Most outboard brackets only mount on the outside surface, with nothing but bolts and washers to hold on the inside. If it were me, I'd get a piece of 1/8" thick stainless steel cut to match the external mounting plate, drilled with holes to match the bolt pattern. This will give you a hefty internal piece of metal that will allow the bracket to draw down evenly, with more dispersed forces than just using bolts and washers. It effectively sandwiches the transom, making a strong interface. It will only add about 3 - 5 lbs., I'm guessing, and would be well worth it in my opinion.

We've found our 3.5hp motor (Mercury 4-stroke) to be ideal for propulsion and I bet you'll be pleased if you go that route. Yesterday, on the lake, we put her at 1/3 throttle and she pushed us steadily along at 5 knots. Turn her up to half throttle and she'll do nearly 6 knots (if the CB is raised). Full throttle can probably eke out 7 or 7.5 knots, but it's not worth the extra noise and gas consumption. But there's power to spare for heavy winds, etc. I think it's the ideal motor size.

About beams and stringer placement... we opted to add a few extra stringers to support our seats because we were worried the 1/4" ply would have too much flex and crack the paint or seams. In retrospect, the worry was probably unwarranted, but we opted to add an extra stringer or two, allowing closer spacing, and our seats are now reassuringly stiff. Just make sure your stringer placement won't interfere with hatch placement, and you should be fine. Also, be sure the stringers aren't so deep that they will make it difficult to slide tall items through the hatch, if desired.

[wkisting]

Just went and measured.... the CENTER of our motor bracket is exactly 16" from the vertical centerline of the transom. Your motor may differ, but with our Mercury's turning radius, you can turn the prop and the rudder toward each other, and they will not foul each other. Of course, the distance behind the transom (depth of mounting bracket) may also cause this to vary.

[Edit to add: See pics of my mounting arrangement on my (long) thread at this link: http://www.messing-about.com/forums/index.php?topic=5967.285  My mount is a custom make, but the concept is the same as commercial kinds, except for the removable feature.]

[Terry Dunn]

On my CS 20 I put the hatch in the middle of the aft seat as well. It is OK most of the time.There have been a few occasions when it was not as convienient. Like when I put charts in that hatch and wanted to get them out while underway. My tiller does not lift up and down.

I have a 5hp tohatso 4 stroke long shaft  that I clamp directly to the transom. The center of the motor is 15.5" from the center of the transom. If I turn my motor hard and don't turn the tiller the propeller will strike the rudder. The transom where the motor is mounted is 1x6 + 1/4" ply across the top with 1x6 down to the bottom  sandwiched between 3/8" plywood on the inside and outside. The sheets for the mizzen foul on the motor when it is tipped up.

I went with 5hp for full gearshift, remote tank and enough power at 9,000 ft. We don't use the motor for local sailing so it does not get much use. Until I took it to Texas last year it had not been used for two years.

The 5 is 60 lbs. If I was doing it over, I would go for a motor 40lbs or less.