2 or 1 engine

Danork · 13 July 2012, 08:18

Hi folk... I'm looking for some advice for my 4.6m rib.
Firstly has anyone ever heard of Elite Ribs?
Secondly, I removed the evinrude 40hp 2 stroke and had a go fitting a honda 20hp. Myself, some gear etc manage 19 knots which is not bad for a 20.
With 2 in it won't plane. At about 16 knots I get 3 miles a litre. Over 2.5 at max revs.
What I would like to do is either fit a second 20 (there is room) or get rid of the 20 and fit a 50 Honda. Any one got any advice...
Cheers
Danork

Poly · 13 July 2012, 08:38

OK - firstly go search the forum for something like 'twin versus singles', make sure you find some of 9d280's posts...

You need to decide if fundamentally you need/want twins (e.g. for safety) if you will actually be able to rig it with redundancy (e.g. fuel and electrics).

And if its worth any downsides... ...i'd have though 2 4stroke 20s on the transom was quite a weight...

Danork · 13 July 2012, 09:26

Thanks for that. I'm away today, but when I get back I'll read the twin v single posts. Yes, 2 x 20 will weigh more than the 50.
Can't believe how much a new 50 costs!
Cheers
Dan

Hightower · 13 July 2012, 10:07

Was there something wrong with the evinrude 40 you removed?

John Kennett · 13 July 2012, 11:44

Quote:

Originally Posted by Danork

Any one got any advice...

Sure. Put the 40hp 2-stroke back on!

9D280 · 13 July 2012, 16:26

What's your 20? You might be amazed at how lardy some engines can be - My old Suz DT25 (supposedly lightweight 2- stroke) clocked in at a mere 7Kg lighter than my current 60!

Assuming it's a new honda 50 you are thinking of, they are remarkably light for a 4- stroke. As are some of the new Yammies. But as Pol says, if you are out Orkney way, there may be an argument for living with a few extra Kg so you have a "20Hp aux" rather than a 4....

There is a fair amount to consider in this twin / single arena - but whatever happens, just remember that what may be true at 300+Hp is a whole different kettle of fish at the 50-ish HP end of the scale

Post back if your searches raise more questions.

Danork · 03 August 2012, 20:32

Well I've done a good bit of reading and had good advice from Clyde Outboards.
The impression I get is that 2 x 20hp will get me more safety but bad everywhere else. Fuel, weight, maintenance etc.
Since my last post I probably have covered more than 250 miles. Mostly quite remote sea areas. If my outboard packed up it would take a long time to get home with the 3.5 hp. 4 or more knot tides are everyday occurrence.
I keep on going back to the idea of the small twin idea. Would it be mad?
Do you think I would get 25knots if I get 19 now? Would non counter rotating props be a nightmare...
I like the idea of a 50 hp but the size of my RIB would preclude me ever using all the power 99.5% of the time.
I like the size of my rib and don;t think I will upgrade in size anytime soon. So I want to make this as sea safe as poss.
Thanks for your thoughts
Cheers
Danork

chewy · 04 August 2012, 06:38

Have you got twin fuel tanks, twin batteries and two completely independent systems so if one fails you can run the other?
Things like been able to link the batteries will help, also consider would one engine of a twin setup up get you back home or will you need fuel cocks so both tanks feed one engine?

Smithyyy · 04 August 2012, 07:26

Quote:

Originally Posted by Danork

The impression I get is that 2 x 20hp will get me more safety but bad everywhere else. Fuel, weight, maintenance etc.

Think that about sums it up really. Depends then on your personal preferences/priorities. Then if you choose to go for the safety side, the more else you can practically duplicate the more safety you get. So we have twin engines and separate batteries and electrics but with a good size built in fuel tank already in there, duplicating the fuel supply is a bit trickier simply for want of somewhere convenient to put another tank. It can all be done, just a question of how much effort you choose to put in

Clearly you burn more fuel with twins, on the other hand when you're not in a hurry you can poddle around on one and probably burn less than a larger single would

kubcat · 04 August 2012, 08:07

Most twin engined boats I have had do not plane on one engine so you probably will not go much faster with one of 20hp motors not working than using a 3.5hp.

I know you can plane with a single 20 now, but add the weight of a second 20 and rigging on the transom and perhaps another battery in the boat and it may be a whole different story.

Danork · 04 August 2012, 08:08

Thanks chewy and Smithyyy.
I know one engine will get me home as that is the engine I use as my main engine now.
I like the Honda 20s as they have a backup recoil if the elec starter should fail or battery die.
Anybody fancy hazarding a guess if 2 x20hp = about 30hp?
Thanks to all for the interest
Cheers
Danork

Danork · 04 August 2012, 08:14

Thanks kubcat... With another 75/80 kg person the speed is reduced to just on the plane around 13 knots.
I know there are many disadvantages and if I buy a brand new honda and the setup is total **** I am going to be very unhappy!
If I could try it out and then change my mind if it didnt go very well that would be great... But thats not how it works!
Thanks for the input
Cheers
Danork

spartacus · 04 August 2012, 09:22

I've had a starter motor fail on me before - but luckily I was still in the harbour, but that's what auxilliary engines are for. Because you're in Orkney there's a real chance you could be out and not see another boat, depending on the season. Look at the main engine and decide if you want the Honda 50hp, then look at an auxilliary back up as a suitable option, rather than a twin set up. For example a Tohatsu 5hp 2 stroke weighs 19kg, is relatively slim, and has its own integral tank, and can be connected to an auxilliary tank (with pre-mix).

Danork · 04 August 2012, 13:29

Thanks spartacus. All very helpful.
What ever choice I make, there will be a weight consideration. I know with another 75kg person aboard my speed drops considerably. If I get a bigger engine say 100kg and auxiliary, that will be about 120kg.
If I go for 2 20s twill be about the same. I don't need 2 batteries as they can be hand started. Which ever way I will be adding about 60kgs which is about the weight of the extra person. How much performance increase will I notice with 60kg extra aboard all the time?
I was looking at the traditional mariner 30 2 stroke (only for commercial new). weighs in at 51kg! Wow. Thats lighter than the 20 honda!
So many decisions!
cheers
Dan

Geoellis · 04 August 2012, 14:10

Quote:

Originally Posted by Smithyyy

Clearly you burn more fuel with twins, on the other hand when you're not in a hurry you can poddle around on one and probably burn less than a larger single would

This isnt really the case. Sure, two 20s will burn more than a single 20, but the OP replaced a 50. As such, the fuel burn of twin 20s will likely be close to the burn of the single 50.

chewy · 04 August 2012, 15:17

Quote:

Originally Posted by Geoellis

This isnt really the case. Sure, two 20s will burn more than a single 20, but the OP replaced a 50. As such, the fuel burn of twin 20s will likely be close to the burn of the single 50.

Twin 20's will never be as quick as a single 50 and I think the fuel consumption will be close.

Danork · 04 August 2012, 17:58

Thanks chewy.
I'm guessing equal to about 30hp?
If I could guaranty a gain in speed of say to 25 knots (from 19) ie 6 knots more, I would probably go for it.
I can get one shipped up here fairly painlessly.
Also, an unrelated question, how do I paste in a photo? Want to show my landrover rig towing my boat the vast distance to the slipway (less than a mile!)
Cheers
danork

Geoellis · 04 August 2012, 18:31

First of all, you're 10 hp less. Next, the drag of more running gear will slow the speed down. Finally, there's the weight consideration. Im not sure what this all translates into regarding speed loss numbers, but for a small boat there really is no rationale for twins over a single

clydeoutboards · 04 August 2012, 19:01

Hi

I am reading this with great interest and thanks for the kind words but thought I should just add one more consideration for the debate.

if it is 20 x 2 then we dont have a counter rotation engine option.

Dave

ps Danork, your maths are about right re 2 x 20hp = around an extra 10 hp in real terms but and here is the nasty rub. I would prefer 1 x 50hp chugging along than 2 x 20 hp at full tilt as the 20's will burn more fuel and you will be running them far harder than the 50hp.

oh and we still have the counter rotation problem.

PPS There is a nice searider 4.7 going cheap. The members on here can tell you more about these than me so again back of to them and i will read with interest.

Davie · 04 August 2012, 22:42

Have you got the space to double up the control boxes and thought how your going to get the steering cable to fit without it sticking out further than the beam of the tubes or are you going to use a baystar kit? More to this than bolting another engine on.