Twinning a 45hp with a 50hp

hypergolic · 02 March 2008, 10:42

I own a 6.3m RIB with a pair of 45hp Honda outboards. The engines are over 10 years old, but are still running perfectly. Recently I did some damage to one of them and I've reluctantly come to the decision that it needs to be replaced.

My problem is that Honda stopped making 45hp outboards a long time ago, and the nearest current model is a 50hp. I'm thinking about keeping the remaining good 45hp, and replacing the broken 45 with a new 50.

I'd be interested to hear any opinions about whether twinning a 45 with a 50 is a bad idea?

Thanks

Sailmaker · 02 March 2008, 12:00

It's surely got to be a bad idea? Trying to match the power on both engines has got to be difficult and there must be a tendency to turn to one side or the other depending which engine is producing more power.

If you can't balance the power then you'll end up having some sort of permanent helm applied and that must mean that power is being wasted by trying to straighten up the course.

I fear it's time for a new engine(s).

hypergolic · 02 March 2008, 13:14

I'm not so sure the power difference between the two sides is such a big factor. I've often noticed that when I'm planing (say around 4,500 RPM) if one engine is 300 to 400 RPM below the other, there's no significant 'pull' to one side. Also, if the 45/50 setup produces a consistent pull to one side, it might be possible to adjust it out using the trim tabs.

danwells · 02 March 2008, 15:25

Quote:

Originally Posted by hypergolic

I'm not so sure the power difference between the two sides is such a big factor. I've often noticed that when I'm planing (say around 4,500 RPM) if one engine is 300 to 400 RPM below the other, there's no significant 'pull' to one side. Also, if the 45/50 setup produces a consistent pull to one side, it might be possible to adjust it out using the trim tabs.

ummm me personaly wouldent put two different powerded engines together but hah thats just me. if you want to put them together then go for it but like the ather fella said you may get a pull to one side.

Limey Linda · 02 March 2008, 16:32

Quote:

Originally Posted by hypergolic

I own a 6.3m RIB with a pair of 45hp Honda outboards. The engines are over 10 years old, but are still running perfectly. Recently I did some damage to one of them and I've reluctantly come to the decision that it needs to be replaced.

My problem is that Honda stopped making 45hp outboards a long time ago, and the nearest current model is a 50hp. I'm thinking about keeping the remaining good 45hp, and replacing the broken 45 with a new 50.

I'd be interested to hear any opinions about whether twinning a 45 with a 50 is a bad idea?

Thanks

Is one of the engines counter rotating?

spartacus · 02 March 2008, 17:48

There's a 4-stroke Honda 45HP (long-shaft/electric) at www.billhigham.co.uk/outboardwarehouse.html for £2,200.

hypergolic · 02 March 2008, 18:24

Spartacus thanks for the tip off on that used 45.

Linda, No one of em doesn't counter rotate. As a result there is more effort required to steer to one side than the other, but it's manageable. Despite the fact that they both spin the same way, the boat has no problem going in a straight line, and will happily do so with no hand on the wheel (not that I'd recommend the "look no hands" approach to RIB driving!).

Searider · 03 March 2008, 08:43

Worth doing some research I feel.

Are the engines essentially the same, just rated differently?
Perhaps the new 50hp is the old carburetter 45hp running on fuel injection?
I'm not an expert on Hondas, but the following should be true regardless of make:

If they have the same gear ratio and the same propellers on, if you run them at the same speed (assuming you have tachos and can do this) then the boat will "see" two engines each doing the same thing.

If the above is your situation then I would select the propellers so that the 45hp engine can reach its max rpm, so as not to overload it, leaving the 50hp a little lightly loaded.

Can't see why it wouldnt work.

Searider · 03 March 2008, 08:54

Have just looked on the Honda Marine website and you can download Owners manuals.

It looks like all 35/40/45/50hp engines use the same 808cc engine - so essentially the same.

9D280 · 10 March 2008, 11:57

And if it comes to that, just how accurate are the analouge tachos - you'll probably be within a similar percentage of "same" as with a "matched" pair.

Not had time to look, but if it's the same block, gears etc then I see no reason to move the carb(s) / ignition module off your old engine to the new one? (I say that not knowing what's knackered on the old engine)

What's actuaklly broken on the old engne? - Could you use spares of a siezed something else to rebuild it?

Searider · 10 March 2008, 15:00

Quote:

Originally Posted by 9D280

And if it comes to that, just how accurate are the analouge tachos - you'll probably be within a similar percentage of "same" as with a "matched" pair.

Would probably synchronise the engines by ear and just use the tachos as a guide.

9D280 · 11 March 2008, 09:54

Quote:

Originally Posted by Searider

Would probably synchronise the engines by ear and just use the tachos as a guide.

I rest my case!

yorfuoj · 31 October 2008, 19:19

using one 45 and one 50 = no problem, go !

As these motor are not counter-rotating, when you go straight line, the propellers are not pushing straight, but with an angle - the boat runs like a crab.
All single-motor boats go "crab", but twin-engine more, and one motor is pushing more than the other to counter-balance rhis angled-walk.

Possibly - probably - your boat would run better with 50 on starboard and 45 on port, as the props torque tends to push toward starboard - if you want to know, swap.