Stepping on the outdrive to get into boat?

[Fiat4Fun]

Had a interesting conversation yesterday. What is your thoughts about swimmers and skiers stepping onto of the upper drive on a alpha one to get onto the swim platform? Can this damage the drive, cause a leak on the transom, or any other ill effects? I have a 1986 Chris Craft with a 260 Alpha One, just wanted to see if that's a issue? Any advice is appreciated.

 

[tpenfield]

Might be tough on the feet.

 

[Scott Danforth]

the prop puts about 5X the force that your going to put on it climbing in (even if you weighed 500#)

 

[alldodge]

No issue, been doing it for years my self with my rinker

 

[harringtondav]

The boat doesn't care. My bud was using his as a ladder and his lower foot slipped off the anticavitation plate. Dang, that SS prop was sharp.

 

[Andy J]

the prop puts about 5X the force that your going to put on it climbing in (even if you weighed 500#)

Interesting, didn't think of that amount force applied to the transom from the prop. Guess the issue would be safely only, would hate to fall into the prop.

 

[Scott Danforth]

usually when your foot slips off, the anti-cav plate removes the skin from your shin prior to you hitting the prop...

 

[H20Rat]

Been doing it for year also. Just have to be very, very careful, lots of sharp pieces there.

Although I don't have any exact numbers, I'd wager that an average alpha connected to a v6 or v8 is probably putting out close to 3000# or more of thrust, and because of the prop being lower than the transom mounting, all of that is translating to downward twisting force. (same thing you stepping on the plate is doing)

Your outdrive/transom won't even come close to noticing you stepping on it, and if it does, it isn't seaworthy anyway.

 

[thumpar]

I would say to get a better ladder. Not because the drive can't take it but because it is so nice to have and not need to rely on the drive to get in. My old boat had a crappy 1 step ladder that would swing. It was not fun to use so we used the drive also. The new boat has 3 rungs and once down is fixed. It is so much nicer to get in the boat with a decent ladder.

 

[harringtondav]

Prop thrust is a lot of force. The link has a calculator. Also multiply your MPH by .89 to get Kts. Also use your planing speed to get max thrust coming onto a plane. Mine is around 23 mph and and 907 lbs of force at the prop. At my 50 mph WOT max speed, mine is 417 lbs.

The force on the lower gimbal ring pivot pin is greater than this by the lever advantage between the smaller dist. between the bell hsg. pivots and forward trim cyl. pivots, and the larger dist between the prop. center and the cyl pivots. I don't have my boat here to measure, but I'd guess this force is close to twice the prop force.

The link shows the science behind the calc. I imbedded an Excel sheet that does it for you. Double click on the docs cells to activate Enable editing. Excel. Input your boat's conditions in the yellow cells. https://www.boatdesign.net/threads/p...ulation.38283/

 

[porscheguy]

As has been said already, the danger isn’t to the drive. It’s the very real risk of injury should you slip while attempting to use it as a ladder.

 

[Fiat4Fun]

Cool, thanks for the replies. My buddy was trying to convince me that stepping on the drive would cause a leak between the gimbal housing and the boat. It is not used much, as the boat has a swim deck, but sometimes it's a good place to put your foot to lift onto the swim deck. I didn't realize that the thrust was so great from the prop!

 

[thumpar]

If stepping on it causes a leak you already have a serious problem.

 

[H20Rat]

Prop thrust is a lot of force. The link has a calculator. Also multiply your MPH by .89 to get Kts. Also use your planing speed to get max thrust coming onto a plane. Mine is around 23 mph and and 907 lbs of force at the prop. At my 50 mph WOT max speed, mine is 417 lbs.

The force on the lower gimbal ring pivot pin is greater than this by the lever advantage between the smaller dist. between the bell hsg. pivots and forward trim cyl. pivots, and the larger dist between the prop. center and the cyl pivots. I don't have my boat here to measure, but I'd guess this force is close to twice the prop force.

Using the bollard pull calculation T = 11.87 * ( SHP * D )^0.67, I get around 1950 pounds of static thrust that the transom/outdrive has to absorb. (my engine revs fairly quickly up to around 3000 rpm before the boat has any significant movement, and looking at dyno graphs, that is about 160hp at the crank, and assuming 11% loss at the shaft.)

?=11.87*(160*.89*14.25)^.67

1951

 

[Bondo]

that is about 160hp at the crank,

Ayuh,..... The Alpha drives are rated to 300 hp,.....

 

[harringtondav]

Using the bollard pull calculation T = 11.87 * ( SHP * D )^0.67, I get around 1950 pounds of static thrust that the transom/outdrive has to absorb. (my engine revs fairly quickly up to around 3000 rpm before the boat has any significant movement, and looking at dyno graphs, that is about 160hp at the crank, and assuming 11% loss at the shaft.)

?=11.87*(160*.89*14.25)^.67

1951

Yes, there is a lot of stress on the transom, especially at a WOT hole shot. I've been out of free body diagrams for nine years now, but know the gimbal hsg. and transom plate always force aft at the top, forward at the bottom, where most stern drive transoms are the weakest. Good thing those rear engine mounts share the thrust thru the engine, front mounts and stringers. I guess that hockey puck hard rubber in the mounts helps keep it all together - unless the transom gets mushy.