kicker motor brackets and transom stress

[johnyrude200]

I have a 1968-72 16’10 boston whaler. This is my first whaler, and to say the least, I push the limits of every personal boat I own. It’s pushed along with a 70hp 1993 OMC, and I put a 15hp 1990 OMC on it as a kicker/backup motor and it was mounted both with the stern clamps and the through hull screws on the port side. I take it off when trailering and use wing nuts to install during outings with the boat for peace of mind to know it wouldn’t fall off on it’s own or be easy prey for someone trying to steal it.

So here’s the issue – I have been running the boat through waves as high as 5′ (definitely an adrenaline rush) and have the kicker tilted up in the lockout position, but noticed a couple of weeks ago a hairline crack at the port aft edge of the transom where the motor was mounted. The hammer effect of the bow smashing against the next wave after coming down the backside of the last one has lightly damaged the back of the boat. My original concern was seeing the two flimsy lockout brackets on the motor fold like an accordion, but apparently the transom on my whaler gave up first.

Rethinking things, I realized the physics behind this repeated stress would eventually make any transom fail with this configuration. Recall my post on transom savers last year (and my background in physics) and the complete disagreement I received from some of the long term members here. Science and experience generally beats anecdotal opinion.

So I installed a kicker motor bracket on the starboard side, it sits about 3-4" lower with 4 mounting holes, but I still wonder if this really improves the application of stress on the corner of the boat? My hunch is there is no improvement to the situation on this boat, but I guess I’ll know in the next 5 weeks since my upcoming outings will be on the coast of maine, a large lake in NY, and going around NYC and long island.

I enhanced this bracket with some custom modifications to try and delete the original shortcomings. It was a garelick bracket rated for 15hp/75lbs. I read some old articles on whalercentral.com (great resource for these old boats, never as good as the AOMCI) and saw an article (not thread) about a $500.00 kicker bracket someone bought which is the equivalent of what I made in about 90 minutes using a $60.00 bracket. See the pictures below.

I’m thinking these old hulls weren’t designed to handle the stress of a kicker motor port or starboard of the main motor in rough waters. The splash well feature of these hulls seems to significantly reinforce the primary motor capacity of the vessel in general. A pre-73′ hull can take 115hp; then a 73-96 16/17 whaler is only rated for 90/100hp. Just look at the hull design.

[fleetwin]

I don’t really know what to say about the cracking hull, I’m surprised the engine’s swivel bracket and transom clamps are not showing signs of wear…I always thought the 9.9/15s needed a stronger set up/tilt lock support for trailering and auxiliary/kicker use. I don’t think you need such a larger kicker engine on that rig, a 6/8hp would do a great job and save a lot of weight/strain.

[johnyrude200]

My idea behind the 15 was getting stranded in ocean water in bad conditions. The 15 is just enough to get going headway speed in those conditions.

I used a 7.5 last year but that motor really struggled with the 17 whaler in calm waters. Prop was just too small. I have a 5 pitch on this 15 and dont bring it past 3/4 throttle and it doesnt seem to lug. Anything above thay t is a waste of a good engine.

[Anonymous]

Not sure what your last setup looked like but the new one would scare me. I don’t know what the motor weighs but that weight slamming down with a 20" or so moment arm is going to put one heck of a force on those top 2 thru bolts. I don’t know whether tilted up or down would make much difference although the CG might be a little closer to the transom tilted up. Probably not enough to make much of a difference. I’m guessing you investigated it and there’s some reason you can’t clamp it directly to the transom.

[johnyrude200]

I had it clamped to the transom on the other side before and that is what caused the transom fatigue. The lower the bracket is mounted, the more the stress would be placed on the stronger part of the transom.

The problem with these classic hulls is that the splash well is in the way for the bolts on the bottom if placed any lower.

My thought process is that at least the kicker bracket absorbs some of the stress with the set back as the motor bounces per se. Before with the motor mounted directly to the boat all it was doing is repeatedly hammering the top corner of the transom by flexing it.

Maybe these hulls just cant take a kicker mounted during turbulent water. I mean, ive been through 5-6′ waves with this boat, which means 10-12′ drop over the top of the wave into the trough between them.

[foot_doctor]

I’m afraid that your bracket is suspiciously weak. The fasteners are too small for my confidence, and the length of the brackets fastening it to the transom are too short from top to bottom. If I were given the task of designing a mount for your kicker, I would include two 1/8 x 1 1/2 inch angle iron brackets stretching from near the top of the transom to near the bottom. The angles would distribute the stress along a greater area. They would serve as mounts for the kicker mount. I would use no less than 5/16 inch diameter fasteners. The fasteners would extend thru the transom and be backed by no less than 1 1/4 diameter washers inside. In order to assure that the kicker would remain mounted, I would trade out the aluminum structure that now supports the engine, for a small wooden transom. The metal is too slippery, and it’s difficult for the transom clamps to initiate a meaningful purchase into the aluminum. The last would be an easily removable strut from the gear case area to the lower portion of the transom. Much like a trailering strut. Just my 30 cents. Have fun boating. R.T.

[billw]

Stupid question but why don’t you just put the kicker motor ON the transom? From what I recall about those Whalers, there is room for it; and you DO have a long shaft kicker….

I don’t ever remember a 16 having 115 hp capability. I thought 100 was always the max. In my area, there would have been someone who had tried it, especially since we had guys with 50s on the 13s that were only rated for 40….

And last but not least, I know you also have background in physical therapy. That’s good; ’cause you are gonna need it, driving a Whaler in 5 foot waves. They are great, great boats for inland fun but the shape of their bottoms were not conducive to a smooth, rough-water ride, to put it mildly.

Long live American manufacturing!

[johnyrude200]

I had it installed directly on the transom on the opposite side before and thats when i noticed the hairline crack. Used it that way since the beginning of this season. Pulled it when I saw the crack.

I may consider putting it back on the SB side of the transom directly again and constucting a mini transom brace as suggested by the last guy.

This kicker bracker I put together is a little too wobbly for my tastes….

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