Multiple Outboards ..from Craig's thread

[Muskieman]

Okay, I've seen many boats with multiple Outboards and have also have been in a few... I never really questioned why.. I don't have much experience in bigger boats 22+' other than having piloted and docked couple. I know that OPP and Police boats are required to have 2 in case one breaks down.

 

... Now, I've also seen a lot of smaller ones like Craig_Ritchie recently posted.

 

I can see the advantage in a larger boat 22+' Cuddy and the whole shebang having 2x250 or more for lower end torque for some outtadaholitiveness to get to plane.

 

I'm trying to make myself understand

 

Say ..If I were to have a 17' Duck Boat and I wanted to have 60HP on the back I wouldn't go buy 2x30's...but I would if I'd have a 20' Barge right?

 

Let's say that I have a fully decked out 17.5 Princecraft just like Roy's with a 150 E-Tec.

 

#1 will top speed with 2 engines be greater than with 1 ?

 

#2 If the prop pitches are the same top speed should be the same?

 

#3 fuel advantage or disadvantage?

 

#4 would 2x75's perform as well as a 150, or better?

 

I don't see myself putting 2x15 on the back of my 14' to replace my 30.

 

It got me thinking is all.

 

Thanks gang!

 

 

Randy

[Bernie]

Hi Randy.

I have had two boats with twins. Gives the boat more torque for pushing and better hole shots but two 50's would not be as fast as one 100 all being equal.

Lower units create drag and will slow things down. Also the extra weight factor.

Burns more fuel too.

 

I loved the twins, they sound so nice when the harmonics make the boat hum. And having that built in "get her home" factor was assuring.

[Muskieman]

Thanks Bernie!

[DRIFTER_016]

Boats running twins are much more maneuverable in tight spaces (like the marina) than single engine boats. You can sit there, put one in forward, one in reverse and the boat will practically spin on a dime. You can also walk the boat sideways using just the controls.

This is especially important while trying to dock a larger boat, but is usefull on smaller boats (20+ footers).

[craigdritchie]

Bernie hit the nail on the head - total horsepower being equal, top speed with two 50 hp engines, for example, will be less than top speed with one 100 hp due to extra weight of the second engine and additional drag of the second lower unit. You'll also burn less fuel with the single 100 at wide open throttle, again due to less weight and drag.

 

Prop pitch will be different between twins and a larger single for a whole bunch of reasons. You would also want counter-rotating engines and props to eliminate what would otherwise be positively brutal steering torque.

 

Generally, the only reasons to use twin engines are:

 

(1) safety factor, which is a valid consideration if you often fish several miles from shore

(2) ability to troll much more slowly. Shut down one engine .... now you're pushing the same weight of boat with half the power .... you go more slowly

(3) MUCH better steering control, especially at low speed. You can literally spin a twin engine boat around in a circle by running one engine forward and the other in reverse .... turn 360 degrees on the spot without ever touching the steering wheel

 

The biggest reason you see multiple engines these days is that the big boats are rated for way more power than they can stuff into a single outboard. Some of the big centre consoles in my Miami pictures have 1,400 horsepower on the transom. Biggest outboard made right now is a 350, so if you want to max out the power, you need four of those bad boys to do it (for what it's worth, those four outboards are still lighter than a couple of huge inboards).