Outboard Engine Thrust

http://www.grc.nasa.gov/WWW/k-12/airplane/thrsteq.html

 

I agree. I think with a little research, he could make some reasonable guesses about HP required to reach a speed for that hull- assuming it isn't a scratch built design. With guesses on power at speed, then there can be guesses about thrust required. Garbage in-garbage out, though. Bad input makes for bad output, but it might illustrate a starting design range. It also doesn't cover other loading modes, like hitting the sand bar, nor the fact that the outboard thrust is cantilevered. But it is a start toward understanding the magnitude of the forces.

One more example to illustrate your point. The same LOA boat- a lake bass boat- will go not quite 2x as fast with a bit more than 2x the installed power (60 MPH with 225 HP). It needs something like twice the prop pitch to achieve the speed, but because it is a lower drag planing hull, versus a high drag planing hull like mine, the thrust is *about* the same. So you really DO need to have an idea about what the physics of the hull is doing in the water.

I don't have a 20 foot displacement example, but it would be very different in terms of thrust and top speed, obviously.

 

Why not use the breaking strength of the bolts holding the motor on as a design basis for the transom? After an event, you would still have a boat with a transom and a motor bracket, with the motor having been torn off at the tilt pivot. At least figure out what the breaking strength is, and compare it to the motors thrust, and think about why there would be such a discrepancy.

 

something else to throw in the pot !! shape increases strength . transoms on the back of boats are thick and heavy and made with glass lots ply wood and more glass along with brackets and knees and all manner of things !! right ??
Well i replaces all that with a solid glass 10 mm thick transom and increased the hp of the out board from a tired old 70 hp to a vee 4 yamaha 115 !! the transom has no wood what so ever , the out board is a 25 inch not a 20 like what was there before so its about half the weight and much more hp and alot more strain all on far less material !!
Its done simply by using shape to increase the ridged strength and by restitching just a small amount of the old splash well back on its made a neat and tidy transition from the old to the new plus lighter and stronger no knees and no brackets .
Think how do you Increase the ridged strength with out increasing the weight ??
Just add shape !!
Cars over the years have become lighter and stronger but use less materials !!,
Go see how many flat panels can you find on your car ?? non !! every panel has shape !!! simple ah !!
Making a bracket is all about where is the strain ??
what makes the strain ???
and what dirrection are all these strains??
side to side ??
up and down??
and not forgetting twist as well !!!
so how you going to incorrperate all those forces into a square box and keep it light and strong yet not weight a ton ?? its got a out board hanging off it . when the motor turns it has to force the whole boat to what ever side you want it to go . when you push the throttle lever down hard the motor is trying to twist and screw its self off the transom and push the bracket throught the transom . Then when you operate the trim its changing the forces in another direction and wanting to push the back down so the front of the whole boat comes up .
Man i am getting a sore head from all this twisting ,pushing ,shoving and a motor trying to rip its self off the back !!

 

The thrust from the engine is from the prop . This is 2 feet below the engine , therefore to top of the transom is being ripped backwards and the bottom is forced inwards.

If you run into a sand bar the engine kicks up.

Typical of amateurs is to over engineer.

 

Typical of amateurs is to fail connecting the motor to the vessel by a safety cable.

 

One thing i have learned over the years is to build strong its better to have a transom that will stand all the punishment that any motor can give and then some .
As for a stainless lanyard (safety strap) its a sissy strap for people that have not done there job properly! have seen them also rip out and motors go for a dive to the bottom more than once . a motor needs to break before the transom does !! do some drawings and see where the leverage is and then do some calculations as to how much force is involved in all directions not forgetting twist !! its scary !!

 

I agree on the engine Lanyard . Ive seen these 1/8 cables Ha what will that do. The control cable has more likeyhood of holding it.

If youre worried its going to fall off then you had better fix it.

 

Is that why I've recovered so many outboards from the sea floor.

 

About 2 years ago I did a little investigation into the stresses in a 5083 aluminum outboard bracket for my boat. Yield strength should be 240 kN for that material.

Ignore the 10 G bit- that is the study name. I analyzed it at 5G loading with a 375 lb /170 kg outboard with 2 kN of thrust. Something less than full output for my 100 HP rated outboard. I then analyzed it at 5G and no thrust. In either case, the 5G loading is worse at about 8000N, so the compressive stress on the lower mounting holes shows up nicely even without forward propulsion.

The pictures with the pinkish lines and load arrow at about propeller location are those with the outboard load.

The bracket is made of rather heavy 1/4" (6.4mm) aluminum, except where the motor mounts, that is 1/2" (12.7mm).

I would think it would be easier to see 5G vertical loads on the trailer than on the ocean. Even that might be a bit much. I may run this with different numbers later to see what just thrust does at modest G loads.

 

Good Idea, you could carefully back the boat up to a wall and softly with padding and a guage of some kind like a scale push on prop shaft where thrust is applied.


But I would not do that --I think your way over worrying about nothing --get some one to drive the boat and look and feel for stress or movement --I would expect none. Knees from the centre of the transom are important or get some one that know boats to look at it. He doent even need to touch it, Just look at it.

A god drain well gives strength as some one said put some shape in it.

 

Hell i didnt know they made 1000 hp outboards yet !!!
wow talk about over kill !!
Your going to need to add another axle under the trailer to take the extra weight!! When you shift the outboard you have to relocate the axle or move the pushing post at the front of the trailer forward to rebalance the rig again .
if you were thinking its going to add bouyancy it possibly wouldnt even support it own weight .
The bottom surfaces of the thing is way to close to the water coming out from the boats transom and it will suck its self down onto the waters surface and cause lots a drag and without those 1000 horses it will struggle to get on the plane . just to to rethink a few things and have a really good loo at what is on the market already Boats and water do lots a strange things believe me

Really nice drawing but !!!Must have had lots a fun !! pretty colours and everything !!.

 

Ive see a rigid inflatable with 5 x 300 Hondas. It came in the marina and I gave some advice about one counter rotation gearbox blowing.

What would you think that would be ? one engine blowing out gearboxes --same engine. And I mean blowing out --out of the case!!!!

You experts would know what that was in a jiffy.

 

Indeed. It is just a 5min hand calc!

How so??..your plots show a max stress of 75MPa (Von Mises) in both load cases

 

Glad you here ad hoc Can you explain this where engine thrust is reliant on hull size.

Im strongly of the opinion some one is being confused with acceleration and thrust