Knees from Transom to the deck??

[dodl]

Hello,

Well i searched around and read a lot, but either it's a language barrier, or maybe my question wasn't really answered before

I have a small RIB (4,7m rigid inflatable) which is specified for a maximum of 70HP and 280lbs motor weight. Now i have 2 50HP mercurys which i'd like to mount on the transom. Makes 100HP and about 340-360lbs.

Since it is not too much i thought of simply putting some knees from transom to the deck. I'd like to put knees from transom to the hull, but that's not possible. There are no stringers and you can't reach the inside of the hull, except through a small (4") hole. The hull is extremly stable though.

Would you think it's sufficient to go with the knees to the deck? Deck is pretty stable. Transom is also perfect. No rot, and it's dry. I just wonder if a knee to the deck is any good, since i would think the deck was never designed to withstand such stress? Or do i overestimate (hope thats the rigth word ) the force, delivered to the transom by 2x50HP Outboards??

The boat ran fine with a single 90HP Yamaha at the previous owner. No stress cracks at the transom. Couldn't find the weight of the Yamaha though...

many thanks
martin

[Alan Power]

Welcome to Scream And Fly Martin. First what kind of boat is it and why 2 50's? A single 100Hp or 115 would weigh less!

My book makes the Yam 70 105.5kg (233lbs).
I wouldn't run knees to the deck unless the deck is fairly substantial, you would probably be ok with the expected weight you listed unless you plan on jumping big waves with it!

Another thing to considder is tie rods, long s/s threaded rod anchored to the top of the transom and running forward through the floor to the stringers. You will probably need to make brackets to fix the rods to the transom and stringers. If you can't fix to the stringers you could bolt to the floor if you put large backing plates behind.

Have you got a picture of the area where you intend putting the knees?

All this could be overkill though, I would hang the engines on the boat and see first.

Alan

[baja200merk]

sell the 2 50hp mercs on ebay and then go find yourself a v6 merc haha im just kidding. welcome to screamandfly!

you would have to run the knees through the deck and down to the bottom of the hull maybe install stringers... you should strenthen it this way if you plan on hopping waves with any motor, seems like a fairly simple winter project, and good insurance for your transom

my 02
kevin

[dodl]

Hello,

Well, thanks for your responses.

Sounds like a lot of work and maybe it's better to mount a 75 or 90HP engine.
To be honest, as soon as i have to strip the deck of, i get a bit frightened. That's a project a bit to big for me.

But thanks
martin

[Alan Power]

Martin, can you post a pic of the stern area of your boat, there probably isn't any reason to lift the floor! You should be able to cut slots in the floor for brackets to mount the tie-bars to, although you may need to cut an extra access hole or two
If you post a pic of what it's like I can draw the tie-bar arrangement for you, also what make and model is your rib!

Alan

[dodl]

Hello,

I will try to make some photos today. If the weather makes it possible
The Boat is an Avon Searider. A UK rescue boat, but with some modifications to make it a leisure boat (seats, windscreen)
I have 2 photos of the boat here. Unfortunately not of the stern aerea. They'll follow..





Oh, and yes, i also did not believe it's possible to do a twin engine setup with that narrow beam. But there are some of those boats driving around with 2x30HP. Just have to make a bigger cut on the transom. Thats actually specified by the manufacturer.

cu
martin

edit: The problem with going through the deck is, that the laminate is extremly thick in this aerea, since normally the Frame for the self righting bag ist mounted there. When used as a rescue boat, you have this Bag on the A-frame and if you capsize, you pull a rope, the bag gets filled with Gas and turns the boat over again. Thats the reason for the relatively strong deck laminate. Going through that does not sound very funny to me I'll upload some pic's today..

[sleepr]

neat looking boat , although I'd be inclined to just get a bigger single engine instead of trying for twins.

[dodl]

Hello,

I got ill last week and wasn't really able to do the photoshooting

In the meantime i heard of the previous owner, that the boat ran fine with a 90HP engine without any reinforcements. And i got some pics from the UK how to strengthen the transom. They only used a SS Plate and 2 SS struts from transom to deck.

For me this looks promising, and after the engine overhaul, i think i'll give it a try...
Just want to replace the struts with some "knees", to spred the load over a larger area.

@sleepr, i didn't want to get off topic here, in a Speedboat Forum.
The main reason i want 2 engines is safety, because i am doing long turns on the sea. I do't want to have to drive 35 miles at home, with a 5 HP auxillary

Thanks
martin

[baja200merk]

I do't want to have to drive 35 miles at home, with a 5 HP auxillary

Thanks
martin

get a 90hp 3cyl CARBED yammie and this will NEVER be an issue, they are awsome motors, if your really worried bout reliability get a new one that is like a left over still in the crate with 0 hrs ussually can get them for about 6 grand with controls and riging...

that will solve your reliability worries

kevin

cool rig man!