twin outboards on small boats

[Outinthedinghy]

Hello.
I'm going to put twin outboards on my dinghy for a bit more power to push a bit of current. what I am wondering is what the equivalent single outboard would be. For example, if I put twin 2hp outboards on it would that be the equivalent of a 4hp or something bigger, or smaller? I read somewhere that two 30hp outboards give the same 'power' as a single 75hp outboard but the person claiming this was selling two 30's so he would say that. The other thing is as the outboards are mass produced they will all have the same propeller rotation so would a pair of outboards result in really serious paddlewheel effect?
I'll know in a week or two but I'm curious. Has anyone else has tried it?
its an 8ft6 GRP dinghy and the transom is wide enough to get motors on without interfering with each other.

 

[sarabande]

The weight of a 4hp Mariner is 25 kg. A 2.5 hp mariner is 17 kg.

I think that carrying 10kg extra on the transom is going to be counterproductive and lift the nose too far into the air. You will have difficulties co-ordinating thrust and direction, too. Unnecessary fuel consumption is another area of concern.

 

[whisper]

Hi,
If it's just extra power you need then buy a single 5-6hp engine.
It would IMHO be :- a) lighter than 2x2hp, b) more powerful than 2x2hp, c) cheaper to service than 2x2hp and d) more fuel efficient than 2x2hp.

Apart from the greater security/ability to get you home, if you've got 2 engines, I can't see any other reason for taking the path you propose.

 

[gjgm]

I believe the general thought is that 2+2 is less than 4 when adding up twin outboards. Seems alot of trouble on a dinghy..

 

[Outinthedinghy]

Interesting point. The outboards I am using are Honda 2hp units, the early type which was water cooled. they weigh 12kg each. so that would be about the same weight as a new 4 by the sound of it. I do think you are right that the fuel consumption question is relevant. I have seen inflatables with twin outboards in fact I think they were once quite common with the RNLI. I assume there is some sort of advantage other than having a backup unit in case of failure, or is that the only reason to have twin engines on a small craft? I guess with an inflatable the fact is that it will plane with a single 30 on it and the other one tilted out of the water so if one engine fails you still get the boat into efficient operation and with two you get even more power.

 

[Davy_S]

[ QUOTE ]
I read somewhere that two 30hp outboards give the same 'power' as a single 75hp outboard but the person claiming this was selling two 30's so he would say that

[/ QUOTE ]
This is absolute rubish! A single 75 will leave two 30hp engines for dead! extra weight extra drag, simply put a 5hp engine on your dinghy, dont wast money messing about.

 

[Outinthedinghy]

sorry my reply was to 'sarabande'. thanks others for your comments too.

 

[Outinthedinghy]

here's where I read about the twin 30's. I see the word 'grunt' was used LOL. http://www.boatsandoutboards.co.uk/view/BHM677

 

[VicS]

Depends on the models concerned.

A single 4hp may be the same weight as two 2hps or it might be substantially less.

Eg I am looking at an old Evinrude brochure 2hp = 11kg but of two different 4hp models one is 23 kg while the other is only 15kg.

I don't see why two 2hp will give more power than a single 4 hp but matching the prop(s) to the duty might be a factor.

Some large outboard do come with the option of contra rotating props but not the small ones as far as I am aware.

How about

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[Tranona]

Don't think I have ever seen anybody put 2 *2hp motors or an 8ft6 dinghy. Totally unnecessary. One will achieve maximum hull speed easily. The engines are made for just this application. A 2hp may struggle a bit if the dinghy is heavily laden going into a chop, and 3.5's are probably a bit better here. However, anything more than that is a waste of hp.

If you want to go faster get one of the new shaped bottom inflatables which will plane with 6hp.

Twin engines are used on fast planing craft such as RIBs for maneouverability and back up.

 

[Lakesailor]

Have you considered the faff of refuelling two small outboards and the nightmare of neither of them starting? /forums/images/graemlins/grin.gif

Having said that, go on, give it whirl, If you have them anyway it won't cost you anything and could be a laugh.

 

[pcatterall]

used 2x30 hp OBs a lot on our 16 foot survey dinghy in the Nigerian delta. Infinatly better than the alternative ( a 55hp Homelite/bearcat) Faster ( but I suspect that the homelites were overproped) and at least there was a good chance of getting home at night!!

 

[PetiteFleur]

Two Honda 2HP are totally unecessaryon an 8' 6" dinghy. I also use an early Honda 2HP on my 8' 6" dinghy and I never have to use full throttle - even fully loaded with 4 people. Remember that your dinghy is not the planing type so fitting two outboards will not increase the speed by much - even against the tide.

 

[STEVEDUNSTABLE]

if you were around my area, all it would achieve is to say "OY.... LIGHTFINGERS, COME N NICK ME"