Engines

[DavidP]

Hi,
Will two ,40HP outboards give the same amount of forward power as one single 80HP outboard, two 60HP the same as one 120HP, if not why, i seem to remember being told once it doesnt work like that ,if so can anyone explain and i can settle the argument at work.

Cheers ,
Dave.

 

[Guest]

I suppose they might be right as with 2 you have more weight, more drag in the water with 2 props, an 80 hp engine will weigh only about 1.25 times more than 1 40 hp engine so with 2 40hp engines you will have poor power to weight ratio than compared to 1 80 hp engine, I would rather have 2 anyway espacially on the sea, a better bet is to have 1 large engine on a rib type boat and say a small get you home engine on the transom off to one side, already pip;ed up with its own tank etc.

Paul js.

 

[adarcy]

I think pauljs is right but it's mostly due to the extera drag in the water than the weight.
I believe the rule of thumb is you only get about half i.e. 2 x 40 hp = 1 x 60 hp and gets worse if you have a really fast boat i.e. 2 x 150 may not = 1 x 225.
Obviously twins give security and more manoevrability but they are often set v close together. Paul's plan of one v big lump and a low powered get-you-home with completely separate fuel system is probably better but if you NEED 2 x 225.....

 

[Canboria]

When I water ski raced I ran a Phantom 21 in F2, single outboard, most used 2.5 fuel injected Mercs, top end around 75/80mph, F1 still used 21 foot boats with twin 2.5 Mercs, obviously boat was heavier, bigger fuel tanks, heavier lay-up in hull, more drag from gearboxes, 2 engine's are heavier than 1, top speed around 100 mph or just over, not a large increase in speed but you got there a lot lot quicker.

Now most twin rigs will run with short leg gear boxes from Inshore F1 circuit cats reducing drag considerably.

Derek