how much faster with 2

[r mcdonald]

Dear Ribbers
I have tested my new rib with one 90 hp 2/st on the transom. Does any one know how much faster I would go with two 90 hp 2/st on the transom? Is there a rule of thumb formula?
The rib is 7.5m and I got 27kts wot,with minimal gear onboard.
thanks
rpm

[BogMonster]

The rule of thumb most often quoted on here in the past seems to be that twins lose ~30% of the power in other words if you fit two 75hp engines you would get about the same speed as a 100hp single rather than a 150hp single. So on that basis two 90s would get you about the same as a 130hp single. Not my experience just what I have read on here. Assuming your current top speed is within the engines WOT rev range, you would also need new props to increase the top speed by much.

Is the transom designed for twins? I don't know a lot about it but I think the transom cutout is usually designed to suit either singles or twins, so if yours is supposed to have a single, it might need some alterations? Is the transom rated to carry both the HP and the weight of two engines?

[GordyP]

If its speed your looking for you would be better with a big single engine, the twin set up is very good for shifting load and gives good redundancy to get you home if one engine fails, however there is increased drag with two engines.

[9D280]

Recent discussion on the matter- I can't be botered re- typing it all!

http://rib.net/forum/showthread.php?t=34755

Only thing is I noticed a typo in my reply - the drag goes up roughly as the square of the speed and the cube of the frontal area of the thing you're dragging through the water, not the other way round as I typed in that post......

If you already have a 90, a lot of manufacturers go to a bigger leg round that sort of HP, so you may find the drag equation isn't too drastic for two 90 legs, although you are getting into the "speed = significant to drag" region......


Form an economic point of view, I suspect buying another 90 vs trading the one you got in for a bigger engine might not be too far off the same overall cost...... remember your engine leg will have a long enough leg to hit the water at the deep bit of the V. It won't be a simple case of unbolt & move it to the side, even if you don't have a cut-out!


As Stephen says, your transom cut- out (or not) may be the deciding factor!

[jyasaki]

Originally Posted by 9D280

Recent discussion on the matter- I can't be botered re- typing it all!

Cut-and-paste works....

jky

[Robbie Diesel]

I would say 36-38 knots with the twin 90's. But thats just a guess.

[Channel Ribs]

A customer of ours has a 7.5m RIB with twin 90 two strokes, he gets 34/35kn max out of it.

Unless you have the money to spend on 2x servicing etc I suggest you trade up, twin engines are great but to make real sense you need two fuel tanks and two of everything else.

I tend to write off 50% of the second engine's HP, but as you can tell I am a one motor fella.

As an example we have a 90HP four stroke on our 6.25m boat, were I to add another 90 I would expect to get about 135HP "in the water" but at the cost of a whole load of weight. I would rather upgrade to a single 135.

[9D280]

Originally Posted by jyasaki

Cut-and-paste works....

Indeed it does, but the link gives the whole discussion in two clicks!

Originally Posted by malthouse

As an example we have a 90HP four stroke on our 6.25m boat, were I to add another 90 I would expect to get about 135HP "in the water" but at the cost of a whole load of weight. I would rather upgrade to a single 135.

And "expect" is the important word here!

Comparing twin 90s to a 135 of course you're going to be overly lardy - a quick spin onto a random OEM brochure I found on the PC here - (Johnson 2008) The 135 is the same engine as the 90, but runs at a WOT of 5900 instead of 5000 rpm so of course you are going to double the weight for naff all "real" extra power.

And if we are talking weight, for a 6.5+ metre rib, just how heavy is that aux you will need with the single? In a lot of cases it tips the balance in favour of the twins assuming you actually double the HP.......

As for drag, ideally you need data for the gearbx sizes - my Clamshell for example has the same size leg/ gearbox as my Suz 25! The Yam 50 it replaced had the same gearbox as the 90 - i.e. twice as big as it needed to be! Twin Clamshells will therefore create a lot less drag than twin Yam 55s........ Then if you start playing with Mariner "bigfoot" engines.....

All I'm saying is weigh up the facts before jumping to conclusions.



I still reckon if you have a single on there already, the cost of butchering the transom / converting the engine to short shaft will probably mean upsizing is the easier / cheaper option.

[r mcdonald]

Thanks for the replies
I designed the transom so I could have single engine 25 inch leg in the middle or twin engine 20 inch legs, So the transom doesn't need modifying. It will also take the weight of twins aswell.
I had a 90 on another boat so thats why I used that motor, just to see how the boat performed before I bought some new engines.
It seems adding another motor of the same HP adds about 20% on top speed due to drag of the second motor.
I think I will need to get some bigger engines.
Thanks again for your replies.
regards
rpm