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I'm not at all comfortable carrying our 15 hp Johnson on a rail mount on our pushpit. So much so that we redesigned and rebuilt our davits to carry the dink and motor most of the time. As an added plus we have more space on the aft deck.
I'm not an engineer and I don't play on TV, however that looks to be a dicey arrangement. The engine weight needs to be on a vertical support. The engine weight is some 2 feet aft of the transom. The play is too much, you have 111 lbs on a 2 foot lever arm, moving an inch, something is gonna break at the wrong time.
It might be better if you can move the mount so that the weight of the engine pushes down on a vertical support. That would move it closer to the transom.
It might be better if you can move the mount so that the weight of the engine pushes down on a vertical support. That would move it closer to the transom.
For a guy who wants to avoid mistakes others have made, you’re starting to rack up quite a few repeats. Too large of a dink, heavy payload, big heavy engine. You’re heading for davits b/c when you finally get around to trying to use the stuff, that’s the only thing that will be practical. Typical newbie 
. One poor decision, forces another, then another, and so forth.
. One poor decision, forces another, then another, and so forth.
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That stern rail should hold a 250+ lbs. person who looses his footing with ease, I don't see your concerns about a 111Lbs. outboard.
The stern pics show that the load path to the hull by the lower arm under the seat vs the location of the motor mount is
the cause for the deflection. As an Aircraft Engineer I would look at adding a similar connection directly under the engine mount to the hull. It should reduce and possibly eliminate the deflection. There might need to be additional larger backing plate that ties the 2 aft lower arms together.
the cause for the deflection. As an Aircraft Engineer I would look at adding a similar connection directly under the engine mount to the hull. It should reduce and possibly eliminate the deflection. There might need to be additional larger backing plate that ties the 2 aft lower arms together.
Few people can lift and move about stuff much over about 60 to 70# w/o mechanical advantage; especially stuff of awkward shape. So, moving heavy, bulky stuff around on a sailboat with its all manner of obstacles is very difficult, even when the boat is at rest. Several folks advised Mr PP that a 15 hp 4-stroke for a 10-ft RIB carried on a 38-ft sailboat could cause handling issues. But, the replies were of the type that a 10 ft RIB is needed for our plans, a smaller engine won’t plane with all of the gear, and etc. So now, surprise, there might be a problem. That’s what we get here often from newbies—predetermined ideas; often immutable. I can’t tell you how to do it short of davits. I don’t know how. I have a 38-ft boat with a smaller RIB (9.5 ft) and smaller 4-stroke OB (5 & 8 hp) that I can stow below decks in a cockpit locker. That is, I take it off the rail when heading out on to the high seas, and STOW IT if it promises to rough out there. I wouldn’t do what you’re attempting to do, esp. lacking almost total experience doing it at all b/f heading out “loaded for bear” as people say.
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Do you actually use those pushpit seats? Can you do without them? You could redesign the pushpit and get the engine closer to the boat.
Have you considered mounting the engine bracket over to starboard either on the vertical stanchion of the seat, or perhaps on the arch?
From inside the boat, ya might epoxy some reinforcement plates to the attachment points to reduce transom flex from the weight. something like a 3/8" plate of G10 about 6" in diameter (no corners) would work. they'd have to be epoxy bonded to the inside to add significant strength. I'd worry about cracking the transom at the attach points of the seat frame. S JRT says, another strut, either in his location or making a triangle from the inboardmost leg/attach point to underside exactly under the motor mount..
This is exactly what I was thinking.... a stainless welder can do this easily. Also I must say I am not a big fan of the engine bracket as it puts the load in a small area at the top of the rail.. I have an edson engine bracket (bought at a consignment shop in Newport for $75. SCORE!!)... what I like about it is that it spreads the load vertically along the rail and does not create such a lever.
Good Luck
Greg
The problem lies in the design of the stern rail and lever forces. Weight carrying capacity is a distraction, the location of the weight is the big issue and it is very different for the seats and for the motor mount.
When a person is sitting on the seat, the weight is centered over the middle of the seat. Roughly half of that weight will be supported across the front rail and to the 2 vertical supports and the rest through the middle support to the transom. The force on the transom will be horizontal because of the bend in the support, the vertical component will be smaller. The seats and rails work as seats and rails.
Hanging a motor off the rails, changes the dynamics. They are made more complicated because the bent supports act as compound levers which will multiply the lever effect (That's why crow bars and wreaking bars are curved.) Instead of the motor weight being vertical and supported by the supports, the weight of the motor causes a more horizontal force, the weight will try to pull the push pit away from the boat and not down onto the boat.
A rough analogy would be using a pry bar to remove a nail from a vertical wall. The pushpit acts like that pry bar and the nail can be pulled out. If you look at the center support, you will see that it resembles the shape of a pry bar.
Let's do some numbers, and they will be very rough numbers. A 200 lb person siting on a 1 foot wide seat will exert about 100 foot pounds of force on the forward edge of the seat (200# times .5 feet). Assume the motor weighs 100 lbs and is 12 inches from front to back. That will place the center of weight about 6 inches from the front of the motor. Calculating the force, yields, 100 lbs * 1.5 feet or 150 foot lbs. That's 50% more force on the rails than that exerted by a 200 lb person sitting on the seat. Now these numbers are really rough and don't account for the compound lever effects of the mounts. Add to this he momentum caused by bouncing around in waves and the forces on the pushpit get quite large.
I don't think adding bigger backing plates will make things better. The weight transfer to the transom and boat needs to be addressed so that either larger diameter rails are used or supports are reengineered to get the weight vertical and not horizontal. A mechanical engineer could easily calculate the vector forces and make better recommendations than a boatyard mechanic or some guy on the internet.
Good luck with this. The solution won't be cheap or easy.
When a person is sitting on the seat, the weight is centered over the middle of the seat. Roughly half of that weight will be supported across the front rail and to the 2 vertical supports and the rest through the middle support to the transom. The force on the transom will be horizontal because of the bend in the support, the vertical component will be smaller. The seats and rails work as seats and rails.
Hanging a motor off the rails, changes the dynamics. They are made more complicated because the bent supports act as compound levers which will multiply the lever effect (That's why crow bars and wreaking bars are curved.) Instead of the motor weight being vertical and supported by the supports, the weight of the motor causes a more horizontal force, the weight will try to pull the push pit away from the boat and not down onto the boat.
A rough analogy would be using a pry bar to remove a nail from a vertical wall. The pushpit acts like that pry bar and the nail can be pulled out. If you look at the center support, you will see that it resembles the shape of a pry bar.
Let's do some numbers, and they will be very rough numbers. A 200 lb person siting on a 1 foot wide seat will exert about 100 foot pounds of force on the forward edge of the seat (200# times .5 feet). Assume the motor weighs 100 lbs and is 12 inches from front to back. That will place the center of weight about 6 inches from the front of the motor. Calculating the force, yields, 100 lbs * 1.5 feet or 150 foot lbs. That's 50% more force on the rails than that exerted by a 200 lb person sitting on the seat. Now these numbers are really rough and don't account for the compound lever effects of the mounts. Add to this he momentum caused by bouncing around in waves and the forces on the pushpit get quite large.
I don't think adding bigger backing plates will make things better. The weight transfer to the transom and boat needs to be addressed so that either larger diameter rails are used or supports are reengineered to get the weight vertical and not horizontal. A mechanical engineer could easily calculate the vector forces and make better recommendations than a boatyard mechanic or some guy on the internet.
Good luck with this. The solution won't be cheap or easy.
There may be another problem. It may be just the photo angles, but it appears that the crane’s blocks are not positioned directly over the outboard. So when the OB is lifted from the dinghy transom and raised to the height of the rail, the OB will not be directly over the mounting pads on the rail, etc. It will have to be manually pulled over some toward the boat’s center. Not as easy as it should be, especially if the boat is moving. Rocking, for instance. You’d have to ease the fall in which case the heavy weight (OB) will pull away from you. A difficult “one-arm lift” where it is not easy to get leverage over.
As somebody who has done countless OB engine lifts from the dink to a stern rail, and vice-versa, it’s a process that requires balance, timing, and some degree of leverage. I’ve come to the conclusion that the dink should be firmly tied to the boat with the transom directly below the crane, so if you must push against the load as in stabilizing a swing for example, the dink does not scoot out from under you. It’s best if it’s a short, straight-up vertical lift from the transom to the rail mount. This is best done at the quarter, not directly astern. Attempting to hoist at the stern to an outboard “seat rail” from an offset crane a bit too far away will not work out well in my estimation, considering the OB weight. For one thing, the crane will want to swing outboard. It may be too long. The lifting crane should be short, inboard, with the blocks positioned directly over the load when lifting, if possible. (Curved stern rails make it hard to securely attach the rail mounting pad exactly where it should be.) Otherwise, the crew has to “manhandle” the OB onto the pad, as well as off of it.
Before you spend boo-coo bucks stabilizing the mount, if you do, I advise you to take the boat out and anchor it; say in Vinoy Basin there, and try out mounting and recovering the outboard with the set up you have in place to see what you’re up against. Next imagine lumpy seas and strong wind veering the boat all around while you and Mrs PP are struggling to get the OB up to get under way. It’s just not that easy. Next, you have to get the 10 ft dink aboard or be ready to tow, etc. Of course, if you decide to tow with the OB still attached, then that would be quicker.
Unfortunately, swapping a 15 hp for a only slightly less heavy 9.9 hp doesn’t really help that much with problems I describe the re:mounting & recovering. In the end, an OB that you can lift and handle (with facility) with one arm is best. Even the 5 hp four-strokes are heavy, 40 to 50#s. I’ve muscled one of those SOBs around for about 20 yr now. I have a virtually new Honda 8 hp with about 20 hr on it sitting out back on an outboard dolly (yeah, had to buy one of those to get the OB from the car to the boat!) since May. I put the “lighter” 5 hp Tohatsu on for my trip up the coast in June. Just took it off Sunday for servicing. So, regarding above, we’d be talking 30#’s or so, which is in the 2.5 hp to 3.5 hp range for four strokes. But then, problems solved, etc.
As somebody who has done countless OB engine lifts from the dink to a stern rail, and vice-versa, it’s a process that requires balance, timing, and some degree of leverage. I’ve come to the conclusion that the dink should be firmly tied to the boat with the transom directly below the crane, so if you must push against the load as in stabilizing a swing for example, the dink does not scoot out from under you. It’s best if it’s a short, straight-up vertical lift from the transom to the rail mount. This is best done at the quarter, not directly astern. Attempting to hoist at the stern to an outboard “seat rail” from an offset crane a bit too far away will not work out well in my estimation, considering the OB weight. For one thing, the crane will want to swing outboard. It may be too long. The lifting crane should be short, inboard, with the blocks positioned directly over the load when lifting, if possible. (Curved stern rails make it hard to securely attach the rail mounting pad exactly where it should be.) Otherwise, the crew has to “manhandle” the OB onto the pad, as well as off of it.
Before you spend boo-coo bucks stabilizing the mount, if you do, I advise you to take the boat out and anchor it; say in Vinoy Basin there, and try out mounting and recovering the outboard with the set up you have in place to see what you’re up against. Next imagine lumpy seas and strong wind veering the boat all around while you and Mrs PP are struggling to get the OB up to get under way. It’s just not that easy. Next, you have to get the 10 ft dink aboard or be ready to tow, etc. Of course, if you decide to tow with the OB still attached, then that would be quicker.
Unfortunately, swapping a 15 hp for a only slightly less heavy 9.9 hp doesn’t really help that much with problems I describe the re:mounting & recovering. In the end, an OB that you can lift and handle (with facility) with one arm is best. Even the 5 hp four-strokes are heavy, 40 to 50#s. I’ve muscled one of those SOBs around for about 20 yr now. I have a virtually new Honda 8 hp with about 20 hr on it sitting out back on an outboard dolly (yeah, had to buy one of those to get the OB from the car to the boat!) since May. I put the “lighter” 5 hp Tohatsu on for my trip up the coast in June. Just took it off Sunday for servicing. So, regarding above, we’d be talking 30#’s or so, which is in the 2.5 hp to 3.5 hp range for four strokes. But then, problems solved, etc.
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Looks ok to me in flat calms seas.
How will it handle in Heavy seas?
Dynamic forces take over, and not just simple weight.
Simple solution...
By a smaller engine to swap out when out on High Seas Adventures.
Jim...
How will it handle in Heavy seas?
Dynamic forces take over, and not just simple weight.
Simple solution...
By a smaller engine to swap out when out on High Seas Adventures.
Jim...
Well I guess I could do a static calculation based on exact measurements, weights and distances, but the design solution will be the same
Like any boat owner that has pride in their vessel, I'd want it to match the original look and style so adding one arm under the vertical stanchion the motor mount is tied too will get the load to the stern. It might be better to move the additional arm inboard the same distance as the existing arm is offset to starboard too. This would be better because then the load will be equal to both lower arms. Thinking about this a bit more I'd try to make this work if there is space available inside stern. Backing plates inside with this approach is what I would still recommend, backing plates alone probable won't help the deflection.
Like any boat owner that has pride in their vessel, I'd want it to match the original look and style so adding one arm under the vertical stanchion the motor mount is tied too will get the load to the stern. It might be better to move the additional arm inboard the same distance as the existing arm is offset to starboard too. This would be better because then the load will be equal to both lower arms. Thinking about this a bit more I'd try to make this work if there is space available inside stern. Backing plates inside with this approach is what I would still recommend, backing plates alone probable won't help the deflection.
I can't imagine how much that would move around under heavy seas, we have a 4 hp 4 stroke mounted in the same location. I bought the mount the day we were leaving to bring the boat home. After that 12 day journey the motor has never been remounted, for a couple of reason, first I don't have a davit arm to raise and lower it, second when it was on the stern seat rail any wave would bounce that little bugger a lot. Looking at this problem I am thinking of putting a removable vertical support down to the top of the lazarette below. The lazarette hinged door is far more solid than the vertical under the stern rail seats that is for sure and it solves the problem of weight support. Is it ideal, no but I am sure it would solve the issue.
Call the folks at Garhauer Marine and tell them what you have, your concerns, and what you're trying to achieve.
They are very good at design & fabrication and can likely come up with something that would work without installing davits.
Frankly, I would not want to install a typical davit system on your stern and hang a 10 foot dink & 15 HP outboard off of it. To handle that large of a load, I'm thinking of an overhead arch mounted to deck and stern, with robust backing plates, and davit arms attached, to adequately handle the loads, especially in a seaway. ($$$$)
They are very good at design & fabrication and can likely come up with something that would work without installing davits.
Frankly, I would not want to install a typical davit system on your stern and hang a 10 foot dink & 15 HP outboard off of it. To handle that large of a load, I'm thinking of an overhead arch mounted to deck and stern, with robust backing plates, and davit arms attached, to adequately handle the loads, especially in a seaway. ($$$$)
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Amen
We don't feel the need need to plane very often, but we do need to hoist that engine on and off each time we deploy or retrieve the dinghy. Moved from a #60 Nissan 2-stroke to a #30 Suzuki 2.5 HP a few years ago and haven't looked back. That #30 cut allowed one-handed handling vs. 2-handed.(Edit - we have subsequently upgraded to a Torqeedo, but that's not a sustainable solution unless your boat has surplus electrical power generation from sun/wind).