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 City-Data Forum Converting horsepower to force (electric, box, HP, Opera)
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04-11-2023, 08:00 AM
 Location: Redwood Shores, CA 1,651 posts, read 1,252,378 times Reputation: 1600

I need to mount a small motor on a piece of 2x4 wood. I want to know whether the wood can sustain the force generated by the motor.

I found these:

1 horsepower = 746 Watts = 746 Joules/Second

1 Joule = 1 Newton of force displacing 1 kg mass 1 meter

Does this mean 1 Joule = 1 Newton?

Is it possible to express the force in pounds that we are more familiar with? Because wood strength is described in pounds. An 8' 2x4 can probably sustain 200 lbs before it breaks.

04-11-2023, 08:30 AM
 12,512 posts, read 8,731,743 times Reputation: 34270
You've left out too much critical information and what you have is probably the least important. How big is the motor? How much does it weigh? How will it be mounted? Where will it be mounted? What is the motor operating?

If it really is a small motor, they you probably can mount it on a 2x4 or 2x6 which ever fits the bolt pattern. The real question is what do you mean by small?

04-11-2023, 09:23 AM
 Location: Redwood Shores, CA 1,651 posts, read 1,252,378 times Reputation: 1600
Quote:
 Originally Posted by tnff You've left out too much critical information and what you have is probably the least important. How big is the motor? How much does it weigh? How will it be mounted? Where will it be mounted? What is the motor operating? If it really is a small motor, they you probably can mount it on a 2x4 or 2x6 which ever fits the bolt pattern. The real question is what do you mean by small?
It's the tiniest motor, suzuki 2.5 hp outboard. Motor weight is 30 lbs. The locking mechanism is just 3" deep, so it can go on a 2x4 which is 3.5 inches. I will carve a notch so outboard cannot come off. The 2x4 will then be attached to a kayak, so the motor is by the side of the vessel near the tail.

If I truly use a 2x4 I think the strength will definitely suffice. But I am trying to reduce weight and bulkiness, thus the thinner the lumber the better. I want to know how much wood I can cut off and still be sufficient to sustain the max thrust of this little motor. My feel is even 2x3 can do.

04-11-2023, 04:50 PM
 23,483 posts, read 69,717,445 times Reputation: 48760
Quote:
 Originally Posted by RobertFisher It's the tiniest motor, suzuki 2.5 hp outboard. Motor weight is 30 lbs. The locking mechanism is just 3" deep, so it can go on a 2x4 which is 3.5 inches. I will carve a notch so outboard cannot come off. The 2x4 will then be attached to a kayak, so the motor is by the side of the vessel near the tail. If I truly use a 2x4 I think the strength will definitely suffice. But I am trying to reduce weight and bulkiness, thus the thinner the lumber the better. I want to know how much wood I can cut off and still be sufficient to sustain the max thrust of this little motor. My feel is even 2x3 can do.
Thrust isn't going to be as much of an issue as running into a submerged obstacle at any speed. Unless you have some sort of shear pin, when you hit a rock your mount is done and your motor takes a bath.

Aside from that, running any watercraft with propulsion mounted off to the side is a royal PITA. The further from the axis of the craft, the more it wants to go in a circle and you have to crab walk it to go anywhere near straight. No watercraft likes to crabwalk, IME.

I put an electric rolling motor on a Selvor inflatable, and made a battery box to fit in the back with a tail on it to mount the motor.

04-11-2023, 06:50 PM
 Location: Redwood Shores, CA 1,651 posts, read 1,252,378 times Reputation: 1600
Quote:
 Originally Posted by harry chickpea Thrust isn't going to be as much of an issue as running into a submerged obstacle at any speed. Unless you have some sort of shear pin, when you hit a rock your mount is done and your motor takes a bath. Aside from that, running any watercraft with propulsion mounted off to the side is a royal PITA. The further from the axis of the craft, the more it wants to go in a circle and you have to crab walk it to go anywhere near straight. No watercraft likes to crabwalk, IME. I put an electric rolling motor on a Selvor inflatable, and made a battery box to fit in the back with a tail on it to mount the motor.
Noted on both points. I'll check if Suzuki 2.5 has a shear pin. I think it should as this is a latest model.

Yes the motor on one side will steer the boat to the other side. I haven't tried it on kayaks yet but since the vessel has an elongated profile I suspect it's only so slightly. I figure I'll just turn the motor to counter it. In any case will be using the motor very sparingly, just in those moments I'm just too lazy to paddle, and we should be only half a mile from shore at most.

I don't anticipate running the throttle at full power, but who knows; maybe we'll encounter a white shark some day. I just want to do the max stress test.

04-14-2023, 07:44 PM
 Location: The Driftless Area, WI 7,092 posts, read 4,900,353 times Reputation: 17405
Harry's right about the steering aspect. Also consider the effect of 30lb of weight mounted asymetrically on the craft. That will require counter weighting so you don't tip....

I also think you're confusing the power/torque generated by the motor with the forces affecting its mounting site- not equivalent...I'd be more worried that the mounting site on the kayak itself will give out long before a 2x4 will.

Kayaks have an advantage over canoes mostly in maneuverability and maybe weight-- at the expense of cargo carrying capacity....I'm not sure why you would need to motorize a kayak? Maybe a canoe with the flat stern to accept a motor would be more to your advantage?

04-14-2023, 09:42 PM
 Location: Redwood Shores, CA 1,651 posts, read 1,252,378 times Reputation: 1600
Quote:
 Originally Posted by guidoLaMoto Harry's right about the steering aspect. Also consider the effect of 30lb of weight mounted asymetrically on the craft. That will require counter weighting so you don't tip.... I also think you're confusing the power/torque generated by the motor with the forces affecting its mounting site- not equivalent...I'd be more worried that the mounting site on the kayak itself will give out long before a 2x4 will. Kayaks have an advantage over canoes mostly in maneuverability and maybe weight-- at the expense of cargo carrying capacity....I'm not sure why you would need to motorize a kayak? Maybe a canoe with the flat stern to accept a motor would be more to your advantage?
Yes I was just thinking about force vs. torque. Intuitively it's the force that break things. However the force to break a wood stick varies depending on where the force is applied; so from this angle it seems to be the torque not the force.

I am tempted to just do an experiment by hanging a 2x4 over the edge of a step, and see how much force from 1' out -- about the center point of the motor -- can break the stick.

My kayak fishing group despises using motors. But I think it's a safety equipment. Also I dive off the kayak; it is very exhausting; I don't want to waste energy on paddling.

The kayak plastic is very thick and very strong; I inspected it. I will also be using a big ol piece of backing plate for the mount. I feel the mount and the 2x4 wood are both much stronger than the force (or torque) involved. But still this can be a fun exercise.

When I'm done I'll post some pictures of the rig here.

04-15-2023, 03:41 PM
 Location: The Driftless Area, WI 7,092 posts, read 4,900,353 times Reputation: 17405
Instead of wood, consider getting some hard rubber engine mounting blocks at the auto parts stpre.
https://www.oreillyauto.com/detail/c...0/a2122?pos=10

04-19-2023, 11:22 AM
 12,512 posts, read 8,731,743 times Reputation: 34270
Suggest you look online for designs to mount motors to kayaks. There are a lot of them already, most designed to solve the problems being discussed here. Most of the ones for kayaks are designed around trolling motors. Most gas motors are too overpowered, thought that hasn't stopped people from doing it.

04-19-2023, 03:54 PM
 Location: Redwood Shores, CA 1,651 posts, read 1,252,378 times Reputation: 1600
Quote:
 Originally Posted by tnff Suggest you look online for designs to mount motors to kayaks. There are a lot of them already, most designed to solve the problems being discussed here. Most of the ones for kayaks are designed around trolling motors. Most gas motors are too overpowered, thought that hasn't stopped people from doing it.
Yes I already surveyed a bunch of designs, and came down to using 2x4. Some used plastic mount, some use aluminum bar. I am looking for the best balance between weight and strength and cost; and I think 2x4 is it. So this practice is just to ensure that the 2x4 can sustain the full throttle thrust of a 2.5 hp outboard. I can easily go to 2x6 if necessary.
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