twin outbord on sib

[spainman]

Hello together,

My project is to make something similar to these photo.

Just to have a sib with show effect.

So One Question. Are the Outbord Long or Short shaft ?

I think these are long shaft any reason ?

Please help.

[jeffstevens763@g]

I would say long as the transom is a raised one above tube height

[office888]

Originally Posted by spainman

Hello together,

My project is to make something similar to these photo.

Just to have a sib with show effect.

So One Question. Are the Outbord Long or Short shaft ?

I think these are long shaft any reason ?

Please help.

Shortshaft (15").

[jeffstevens763@g]

Zodiac toobs 500mm? Transom raised by 100 MM I'd say 20 inch long shaft

[Lee1]

This was a project I was going to do but I never got round to it, there is an excellent video on youtube where I believe 2 x 3.5hp Tohatsu slider throttle engines were used, with the slider throttles you can slide up the power on both engines individually without a complicated double throttle remote cable, the stearing linkage that was used was also very easy with just a ball joint bar connecting the two handles of the outboards, you could steer both outboards as one by turning either of the outboards tillers.
It's a good project as you don't have to worry about engine failure! As your back up is already mounted and running!
I did wonder what kind of speed you will get, the video showed the boat planning I believe, I'm guessing twin 3.5hp would give more tongue and thrust that say a single 7hp? Maybe someone can elaborate more on this?

Good luck with the project and please post some videos of it in action

[office888]

Originally Posted by jeffstevens763@g

Zodiac toobs 500mm? Transom raised by 100 MM I'd say 20 inch long shaft

The pictured boat is a Zodiac Cadet 285S , circa about 1991.

400mm tube diameter. That's a factory transom, it takes a shortshaft.

[spainman]

So my Transom is for Short Shaft I know it.

In the Middel 38 Cm on the sides 39 Cm becaue the transom has in the middle like a small down curve.

So I have to put 2 short shatfs ? or two long shafts this is really my question.

Please help. Because I have to order the motors.

[Lee1]

Depends if your transom is the same hight all the way across, some transoms are cut out in the middle to take a 17-18inch single short/standard shaft outboard. If you have a cut out in the middle you may have to cut each side of the transom to meet the middle hight depending on how low the cut out is, ifs only an inch or 2 you should get away with using short/standard shafts. Double check by measuring the hight of the transom where the engines will be mounted to the water line.

This project is also about transom custom work as well as mounting the engines to move as one.

[spartacus]

When you say 'show effect', is this for actual use?

Twin engines on a SIB is pointless in my opinion. Controlling two engines will be nigh impossible.

I used to run a Zodiac GT 3.8m, aluminium floor with a Yamaha 25D standard shaft, 2-stroke. What a set-up. Great torque with two-up on board, frugal fuel efficiency, easy maintenance, and importantly it was easy to use in terms of manoeuvrability.

[spainman]

well it is realy a little bit grazy. but like in some youtube videos.

it works.

the transom hight in the middel with the cut is 38 cm and on the sides 38 cm.

so really the same.

[Lee1]

I'm trying to find the 2 x 3.5hp on a sib video. . . .
It did appear to work very well, the linkage was a very simple thread bar with ball socket mounted on each of the engine carry handle (the handles moved with the engine so it worked) I will keep hunting for the video. Here's a twin small engine set up on a non sib

https://m.youtube.com/watch?v=OTWhV_1hCIo

[spainman]

I have been searching and do not find the video with the sib and twin outbord

[Lee1]

Found it at last! Have a look at this if I could be sure this set up would give me the same thrust as a single 8hp I would use it for reliability, slider throttle outboards would have to be used though and it's a shame that recent models stop at 3.5hp!
You can see the transom in this video is the same hight all along except the unused corners not sure if this transom has been modded though.

https://m.youtube.com/watch?v=DxuowsHcYks

[jwalker]

Originally Posted by Lee1

Found it at last! Have a look at this if I could be sure this set up would give me the same thrust as a single 8hp I would use it for reliability,....

It will give you better thrust than a single motor because you will have twice the prop area in the water. Your top speed may not be quite as high because of the extra drag of the gearcase through the water. However, presuming your single engine is correctly proped, you will need props with a higher pitch when you run two of them because there is less load on each motor.

This gives you a problem if one motor fails because the single one left running will then be over proped. It does, though, allow you to keep going in a difficult situation.

[Landlockedpirate]

I did this as an experiment about 10 years ago. I used 2 x 2hp Yamaha/Mariner 2 strokes and compared it with a 4hp Yamaha 2 stroke. All were in good condition with clean and undamaged OEM props. All engines were short shafts.

Performance was really surprising. Using a Garmin hand held GPS on a Quicksilver 270 airdeck and 1 heavy person on board.

Single 4hp got onto the plane and topped out at 12.1mph

Duel 2hp wouldnt even get close to planing and maxed at 5.7mph

We put the difference down to the extra drag of the gear cases and the less efficient plastic props on the 2hp engines.

[Gobuchul]

Can I just ask - WHY!

[Lee1]

Originally Posted by Gobuchul

Can I just ask - WHY!

WHY NOT it's what playing around with and owning SIB's is about.

Re the x 2hp I think 2 x 3.5hp with metal props would perform a lot better

[chris123]

If you are going to go twin outboard you should do it properly:

http://www.rib.net/forum/f50/rnli-c-class-62313.html

[emoji1]

[spainman]

@ well i would likr to do it properly.

But the eralier rnli boats also had 2 tiller/ 2 men sterring.

I have a 3.10 german DSB / hypalon) sib so i will test 2x5 hp.

Sterring with one tiller because both are conected with a sterring bar like big ones. the only think is the throttle control and gears.

but with 2 5hp i think not a big problem.

[spainman]

https://www.youtube.com/watch?v=tYCG6GEyXSw