Twin small outboards on inflatable

ifeoktistov · 06 March 2010, 05:35

Has anybody had any experience with small twin outboards on inflatable?
I have 8'8'' inflatable with Honda BF2 2HP outboard. It's not fast. I can barely make about 5 knots with just myself. But I love my outboard. I have an idea of twin mount. It's definitely going to add a redundancy. I'm having quite long trips along the shore from time to time. Rowing 5-10 miles will not be an option. Do you think twin 2HP is going to be twice faster? Is it going to be any faster/slower compared to 4HP outboard?

thornbackflound · 06 March 2010, 06:09

Quote:

Originally Posted by ifeoktistov

Has anybody had any experience with small twin outboards on inflatable?
I have 8'8'' inflatable with Honda BF2 2HP outboard. It's not fast. I can barely make about 5 knots with just myself. But I love my outboard. I have an idea of twin mount. It's definitely going to add a redundancy. I'm having quite long trips along the shore from time to time. Rowing 5-10 miles will not be an option. Do you think twin 2HP is going to be twice faster? Is it going to be any faster/slower compared to 4HP outboard?

Seems a small craft to be having twin engines,8'8,with two don t know what the weight would be ?,but if combined to the recommended weight for the transom,well,someone with more experience on here will be able to help you im sure,Happy sibbing

Channel Ribs · 06 March 2010, 07:25

Quote:

Originally Posted by ifeoktistov

Do you think twin 2HP is going to be twice faster? Is it going to be any faster/slower compared to 4HP outboard?

You will probably go slower

Get a lightweight 5 horse perhaps?

ifeoktistov · 06 March 2010, 15:31

Quote:

Originally Posted by thornbackflound

Seems a small craft to be having twin engines,8'8,with two don t know what the weight would be ?,but if combined to the recommended weight for the transom,well,someone with more experience on here will be able to help you im sure,Happy sibbing

The inflatable is rated for up to 5HP, 55lb (25kg). Honda BF2 is 27lb, so it's 54lb total.
Should be OK for transom. I'm just not sure about performance.

chewy · 06 March 2010, 16:34

Quote:

Originally Posted by ifeoktistov

It's definitely going to add a redundancy.

So will a set of oars.
I looked into putting twin 25's on my SIB and its not worth the hassle.

Channel Ribs · 06 March 2010, 16:41

Quote:

Originally Posted by chewy

So will a set of oars.
I looked into putting twin 25's on ym SIB and its not worth the hassle.

I used to be a huge fan of twin engines, but various threads over the years on here have opened my eyes.

If neither performance nor initial cost are an issue then yeah - great. Otherwise not so much.

chewy · 06 March 2010, 16:51

A 10hp 2 stroke should make it shift.

jwalker · 06 March 2010, 23:41

Ignoring the hassle of two throttles, you could probably steer using one engine. However, you won't get twice the speed from two engines. Also, if your single engine reaches maximum revs, as it should, then with a second one you will need to change the props for ones of greater pitch, you may find they aren't available. The wee Honda engines are noisy, I'm not sure I'd want to sit beside two of them at full chat. I used to run two 30hp on a five metre sib, it was a good arrangement.

spartacus · 07 March 2010, 20:38

Quote:

Originally Posted by malthouse

You will probably go slower

Get a lightweight 5 horse perhaps?

I'll second that. Two engines for day-to-day boating is trouble. You have the combined steering and throttle to contend with. Get yourself a light-weight 2-stroke or 4-stroke.

lightning · 08 March 2010, 07:24

If that Honda BF2 engine weighs 27lb you could get a Mercury/Mariner 6hp and that weighs 26lb.
I know you said you wanted to keep your existing engine, but you could use it as a spare....the Mercury 6hp on my 2.8m SIB will achieve 14.5mph and that's with two of us on board.

jyasaki · 08 March 2010, 16:38

Generally, if you take a motor of some hp rating, and add another similar motor, you get a max of about 75% of the total output in useable hp (twice the LU drag, added weight, etc.) at the props.

So your plan of using a pair of 2 hp motors would yield 3 hp or so at the props, which I suspect will not push a great deal faster.

Second the idea of upgrading to a more powerful single.

jky

phillnjack · 04 April 2010, 21:31

You will not gain anything worth noticing by fitting another engine the same.
If your getting 5mph now then adding a second might give you 6 but i doubt it.

A 4hp mercury will give you about 14 mph and a 6hp about 15mph but you could carry more people.

Ive got a 4hp mercury on a 9ft 6 meastral that will do 14mph flat out on the sea with one person onboard.

If you have a round transom (inflatable all the way round)you wont get as much performance as one with a solid transom .(points on the back)

Personaly i think you need a bigger engine, 2.2hp is not realy worth having unless your on a pond.

4 horse power mercury is good ,light, economical and very easy to get spares.
can pick them up reasonably cheap now as well.

or maybe a 5hp honda, heavy but reliable.

phill

9D280 · 05 April 2010, 11:43

Quote:

Originally Posted by jyasaki

Generally, if you take a motor of some hp rating, and add another similar motor, you get a max of about 75% of the total output in useable hp (twice the LU drag, added weight, etc.) at the props.

So your plan of using a pair of 2 hp motors would yield 3 hp or so at the props,

Errr 2 x 2 hp will give you 4Hp at the props!

Drag argument:
As a rule of thumb, Drag through water goes up approx at the square of your speed, and the cube of the cross sectional area of whatever you're dragging through the water. I would suggest that at 2-4Hp end of the scale and the size of gearboxes we're talking here, the "75% argument" will fall flat. I'll bet the 2hp has no F/N/R, which the 4 probably will, therefore give a substantial increase in cross section, and probably not be too far off the twin "slim" 2s. Then realise you're not going to pull 30+knots with either setup,and I bet if you do the maths it's nothing like double the drag at the sort of speeds you'll be pulling with 4Hp.

Weight, as shown, you can get a lot more HP for the mass, I won't re-iterate my usual reply, just search for other twin engine argum- er I mean discussions , but suffice to say you'd be surprised at how lardy a single can be, esp. when you take an aux into consideration.

Having said that, if you have a reliable 2Hp, and can find a suitable "big" lump, There's a good argument for keeping the 2Hp for trolling / aux & get an 6/8hp-ish for your main engine. The main thing to remember is that the arguments that apply at the 200+ horsepower end of the scale won't usually translate linerarly down to the "sub 20 Knot" end of the scale.

One thing I think we are all agreed on - rowing an inflatable any distance is just going to kill you.

lightning · 05 April 2010, 12:17

If you are going to get the 4hp Mercury, why not go for the 6hp one? It looks identical, weighs the same, and has 50% more power. The only difference is that it has an external fuel tank, but I would say this is an advantage in most cases as it holds 10 litres, which lasts absolutely ages....no need to carry a fuel can in the boat for refilling.

prairie tuber · 06 April 2010, 05:04

Another option (though pricey) if you want a backup motor is a Torqueedo electric outboard. It has it's own integral lithium battery on the head and folds up into a small bag.