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Catalina/Capri 25/250 Sailor's Forums

General Sailing Forum

Motor Stress on Transom

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Rich G
Navigator

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226 Posts

Initially Posted - 05/18/2005 : 10:22:25

Is it better to tip the motor when resting at a mooring or slip, or is it better to keep it vertical?

I have a Suzuki 9.8 hp 4 stroke on there right now and it's a BEAR (~107 lbs.)! The transom is beginning to show some signs of wear that I think may be from the weight of the motor. Will there be less stress on the transom if I keep the motor upright?

Rich

Cerberus
Hull #4695
1984 SR/FK No Pop-Top

Edited by - on

John V.
Admiral

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USA
559 Posts

Response Posted - 05/18/2005 : 12:47:39

Hi Rich,

I keep the motor down at the dock and at anchor and only raise it while sailing. I suppose if you were docked in a very quiet location with no wave or tidal action the motor up would not put undue stress on the transom.

My reasons for leaving it down are practicle in that when I dock I am under power and the next time I use the boat I will again start out under power. So why go through the umghhhh of raising and then lowering the motor? As for stress, the heaviest portion of an outboard is the motor and not the shaft, and as you tip it up the weight crosses the false transom line puting the mount in better balance. I imagine that the highest stress on the mount bolts would be when under power when the asymetrical thrust will be trying to pull the top bolts out.

Check what kind of backing you have behind the motor mount and perhaps they could be beefed up a bit.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 05/18/2005 : 21:23:04

Hi Rich... That's why I put a Starboard plate between the bracket and the transom--extending well below the bracket to spread the forces that are inward against the transom. Thinking about your question, I suspect (but haven't proven) that tipping the motor might ease the stress slightly by moving the center of gravity of the engine forward, so there's less leverage on the bracket.

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Greg Jackson
1st Mate

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84 Posts

Response Posted - 05/21/2005 : 22:32:26

The forces that the motor mount place on the transom have two components. One is nearly straight down, this is a shear force against the bolts. The other force is a twisting force (technical term: moment) which is directly proportional to the mass of the motor times the horizontal distance between the transom and the center of gravity of the motor. If you could move the center of gravity of the motor directly above the transom, there would be no twisting force.

The twisting force is the most difficult. If tipping the motor up moves the center of gravity backword, then it gets worse. If tipping the motor up moves the CG forward, then it gets better.

I have a 110 lb Honda. Despite the weight, I have no problems trailering the boat with the motor in place. This is because I use a rope to pull the motor forward, tying it to the stern cleat. When the trailer hits a bump the motor pulls horizontal against the cleat, it does not submit any twisting force against the transom, only shear. The more you can limit the force to a simple shear. the less stress on the transom.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 05/22/2005 : 22:38:54

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Rich G
 Is it better to tip the motor when resting at a mooring or slip, or is it better to keep it vertical?<hr height="1" noshade id="quote"></blockquote id="quote">

Due to the tight quarters at my marina, most do not leave their motors sticking straight out so as not to present a hazard to someone (or themselves) while manuevering.

In the photo below, you can see what a hazard an outboard presents when it is sticking straight out. (My boat is the one with the black Merc)

I remember one time a powerboater drifted into an outboard sticking out and the motor mount was almost ripped from the stern of the parked boat.

Edited by - dlucier on 05/23/2005 14:46:37

Bubba
Admiral

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USA
542 Posts

Response Posted - 05/25/2005 : 22:01:07

Are we talking fresh water here? See that nice clean white motor in Don's picture? You leave it down all the time in salt water and it gets ugh-ly!

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La Dolce Vita
1st Mate

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USA
86 Posts

Response Posted - 05/26/2005 : 17:27:38

When I installed the 4 spring motor mount from CD I was also hoping that the verticle lift would be slightly higher.(Unfortunately it isn't.) I really dread to leave my motor sticking out but if I don't tilt it up from the plate down would be submerged in salt water. I would like to raise the motor just enough so it's above water level.The only way I can think of to accomplish this would be to stick a bar or some kind of wedge in there for it to rest on. Any suggestions?

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oldsalt
Admiral

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USA
578 Posts

Response Posted - 05/29/2005 : 18:31:20

Tipping your motor won't cause any damage to your transom and will save you a lot of trouble if you're in salt water. It's better to keep your motor out of the water rather than in. Of course you have to balance the benefit of keeping it out of the water against the chance that some knucklehead will rip it off your stern.

In my old marina, while showing off, some powerboat clown with a Sea Ray 33, who didn't even rent dock space there, slammed into my tripped motor while showing off by doing "donuts" inside the narrow slip area among rows of parked boats. The resultant damage was that one side of the aluminum motor mount bracket on the outboard was cracked in two and the prop was all bent. There was absolutely no damage to the Garhauer bracket or the transom. C 25 transoms are a lot stronger than many here believe.

In my present marina I rent a 30 foot slip so the outboard is pretty well protected while tipped since it's also protected by a piling.

PS. The Sea Ray clown payed for the damage since one of my neighbors took down his numbers.

Edited by - on

Rich G
Navigator

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226 Posts

Response Posted - 05/30/2005 : 10:45:32

Quote: Tipping your motor won't cause any damage to your transom and will save you a lot of trouble if you're in salt water. C 25 transoms are a lot stronger than many here believe.
_______________________________________________________________________________________________

I'm sure the motor itself won't cause any damage, but the transom had a nice sized crack in it when I bought the boat. The surveyor assured me that it was nothing structural, but should be repaired anyway. I had the yard repair it and it looked great; they glassed over the entire transom. Then, when we were out on our Maiden Voyage from Buzzard's Bay to NYC, a hairline begain to show itself again in the same spot.

Could be coming from the stress of the backstay (it's not split, it only runs to port side). Any thoughts?

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