A physics of force question

Otter

Otter

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Here’s one I just can’t work out. I want to put an auxiliary outboard bracket on the very pointy canoe stern of our boat. The very pointy profile means it will have to go about 12 inches along which means it’s at a significant angle, the hull there is at about 45 degrees. I could offset this with a massive profiled piece of teak but I’d prefer not to. Instead I’m wondering if I angle the outboard so that it’s facing dead ahead i.e. turn it 45 degrees, will it create a turning effect or not?
 
I guesstimate that the turning force would be minimal, the prop would be offset from the centre line by about 12”? (About 6” hull offset and the same for the bracket, more if the axial length of the bracket is long pushing the outboard further out to the side. That may need to be addressed with a angled pad to get the bracket back inline with the boat).
There are lots of boats with transom hung rudders that have to offset their outboard by more than that.
My inboard drive shaft is offset by a couple of degrees, to allow removal by sliding it past the rudder, without any noticeable steering issue.
It’s probably there, but I never notice it.
 
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the outboard wlll be offset from the centreline of the boat, so even if its turned on the outboard bracket to point straight ahead, there will be a small turning moment/turning effect. depending on the size and displacement of your boat, it might not be much.
should be ok and little different from mounting an outboard on a transom with a transom hung rudder, where the outboard also won't be on the centreline.
 
Otter said:
Here’s one I just can’t work out. I want to put an auxiliary outboard bracket on the very pointy canoe stern of our boat. The very pointy profile means it will have to go about 12 inches along which means it’s at a significant angle, the hull there is at about 45 degrees. I could offset this with a massive profiled piece of teak but I’d prefer not to. Instead I’m wondering if I angle the outboard so that it’s facing dead ahead i.e. turn it 45 degrees, will it create a turning effect or not?
Click to expand...
As already said the fact that the OB is off centre will cause a slight turning effect. My transom mounted OB is about 12" off centre( due to transom hung rudder)
The turning effect is not normally noticed but it does affect the turning circle considerably. I can turn much more tightly one way than the other.
.
 
In my boat - a Drascombe Dabber - the outboard is in a well about 12" off centre to port. Motoring in a straight line the keel is not parallel to the course, the bows shall point a little to port and the boat procedes askew as certain dogs do when trotting, but the angle is so small that it can not be noticed, as well as the increase in resistance that is certainly there.
The proceding line is between the outboard turning centre and the general center of resistance view from above (if such thing does exist).
 
I had a Seagull on my old Six mounted on the starboard side, level with the cockpit, i.e. about ten feet ahead of the stern and nearly 4 feet off the centre line. Once started, the tiller was secured to a conveniently located cleat so that the prop shaft was parallel to the centre line. Steering was fine in open water once the boat was moving. But setting off from locks in the Crinan Canal could be interesting. Especially to the owners of other boats in the lock.
 
I would thinkl that the thrust being off set will not matter much. My o/b is also offset due to transom mounted rudder.
However looking at the outboard body as sitting in the cradle that attaches to the boat you will be applying thrust at an angle to normal straight ahead. Now of course the mount must be designed to take thrust with o/b turned hard either way so perhaps not a problem. I do think you will have problems steering with the o/b in one direction. But I guess you know that and can accept that problem. You may think about an additional clamp around the tube to the cradle like a big hose clamp to firmly attach against tilt and any stress against stops (all without seeing the o/b) So yes go ahead and fit it. ol'will
 
gardenshed said:
the outboard wlll be offset from the centreline of the boat, so even if its turned on the outboard bracket to point straight ahead, there will be a small turning moment/turning effect. depending on the size and displacement of your boat, it might not be much.
should be ok and little different from mounting an outboard on a transom with a transom hung rudder, where the outboard also won't be on the centreline.
Click to expand...
This makes sense, but can be counteracted by turning the o/b a tiny bit to port or left (you will lose a bit of thrust).

You will be able reach o/b. If have remote controls even easier.

A metal fabricator cold make a light mounting than teak perhaps if you pointed it the ‘pointy’ end; but remote controls may be needed ? Depending upon distance to reach.

I only read first 2 posts, so apologies if duplicated a reply.
 
Our first boat had an outboard in a well. It was on the port side. Having recently done my day skipper course we set off to get out of the marina. I put the helm hard over to spin her round to port. I gave a burst ahead and the offset thrust sent me spinning round to starboard.... straight into a pontoon. Once you were aware of it, the effect was really handy as it turned the boat beautifully in its own length.
I don't remember ever experiencing a noticeable turning force when motoring underway. Just when getting going
 
You will also be pushing the outboard sideways compared to its clamp, so would be well advised to bolt it to the mount if such mounting holes exist in the clamp. If not, I'd add some lateral supports to reduce the risk of it slipping sideways. (You can guess how I know and that just from steering forces!)

If considering a hinged raising mount do go up a size or two. Better really to get something very solid.
 
You should absolutely be able to trim the offset thrust and drag lines. The thrust angle will change with speed through the water as the drag and thrust values change. So you will need to be able to steer the outboard a little.
If fixed it should be pointing inwards a little but not the 45° you quote.
 
Pay attention to the mounting working loose. The motor will be held by nothing but friction (you could add a through-bolt). When you lift the motor it will stick out to the side a good bit (docking problem?). Is the location strong enough (no core) to withstand the pressure of the clamping bolts? And they will scar up the topsides.

I'd build a bracket.

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Re-turning moment. The outboards on the PDQ are about 4 1/2 feet off the center line, 8' forward of the transoms (cockpit wells). At steady speed forward you can feel it. Any accurate maneuvering at slow speed, like docking, is a nightmare. The engines cannot be canted to one side. The outboard on my F-24 is about 2 feet off the CL but square to the transom. The effect is very small, barely noticeable, and cocking the motor ~ 5 degrees to one side solves that. It's all about how far off the CL and cocking the motor.
 
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