dose an 8HP outboard give the same push as an 8hp Inboard?

[trouville]

Do inboard Diesels push better than petrol inboards? And do Outboards push as well as inboards??

Would a 4 stroke push better than a 2 stroke? If the same boat with a 12 hp diesel gets up the Rhone would that boat manage to do the same with a 12 HP outboard?

 

[fireball]

The problem with the 8hp outboard is that it tends to be near the surface so as the vessel pitches so the prop gets near the surface - I would've thought not a problem on a smooth river - but a definate PITA for sea vessels ...

Had a 15hp outboard on a 22'er ... could get hull speed easily in the flat calm, go out in any little waves and it was all over the place.
Replaced with a Volvo MD5a (I think thats 5hp) could get to hull speed, just, in flat calm - but was hugely better in the waves!

 

[Channel Ribs]

And, just to confuse things further, not all 8HP outboards will pull the same; which is why some reviewers do a 'bollard pull' to see what oomph each brand has with a given prop etc.

The way an outboard is mounted will also make a difference, solidly bolted to a thick transom is better than flopping around on a sprung bracket.

 

[Guest]

It's down to thrust and RPM ... Diesels are generally workhorse jobs with high thrust and lower RPM ... able to just about pull anything .... Petrol usually for higher revving speed jobs ... with smaller props as they rev higher.

Outboards are generally propped for light load and high RPM ... except those designated saildrive such as the reduction gearbox Mercurys etc. So generally less thrust than inboards ... plus of course the tendency to "break surface" when boat starts pitching etc.

IMHO ..... fine weather ... anything goes ... Rough weather and to get home ? Inboard diesel.

 

[Bajansailor]

Following on from Nigel's post re thrust and rpm, here is (I think) quite a good example of how absolute power in hp or kw is meaningless on its own - what is important is how it is applied.

There is a parti boat here - about 100' long, 30' wide, weighs about 400 tonnes at least.
Looks like a multi-story car park.
Can take up to 600 punters on booze cruises along the coast.
Potters along happily at about 8 knots.
Her power? Two 250 hp Caterpillar diesels.
Suppose you put twin 250 hp 4 stroke (or 2 stroke) outboards on the back instead - what would happen?
Ermmm, nothing much - they would resemble egg whisks, in that they would churn up the water in a frenzy, but the boat won't go anywhere.
'But it is the same power!' I hear you say.
Yes, but it all comes down to how you apply the power, in order to achieve the required thrust and torque.
The Cat engines drive 4' diameter 4 blade propellers at low revs via 5:1 reduction gearboxes.
Whereas the outboard engines drive tiny (maybe 10" diameter) propellors at high revs.
The same principles apply to a little yacht, when comparing inboard diesels and petrol outboards of the same power.

 

[yoda]

What is the real question here? Power available from any engine has to be translated into thrust by effectively matching engine power, RPM and torque to prop pitch and diameter. Get that right and you can translate engine power to forward movement. position of the prop with respect to the hull shape will also play a large part. In general inboards are aranged to put the prop in the most effective place and any outboard may have to compromise on that. As for diesel against petrol power is power but in general 2 engines of the same power will end up with higher torque from a diesel. Now your boat like mine is a fine easily driven hull and a petrol engine will manage with no problem at all but you will have all the limitations associated with a petrol engine. You will however find that power for power the petrol engine is a lot lighter and smaller. I face the dilema that to replace my 16hp petrol engine I can only fit a 9HP diesel in the same space. I also can't face a £2500 bill for a new engine so will probably stick to the Dolphin.

Yoda

 

[trouville]

The rivers arnt rough but wash from down stream boats could cause the prop to leave the water.

Im just thinking of the Rhone and Rhine both 5/6 knots in places.Perhaps i could mount two outboards in case one breaks down? Add saildrive props to give extra push.Or perhaps a good 8 HP mounted behind the rudder on a frame to give maximum push.

I can see a large prop with a 3 to 1 reduction would be ideal but not when sailing! Do outboards have a 3or4 to 1 reduction option? That would be fantastic for a crusing boat and provide a hand break as well!

Best of all would be a small boat canal to by pass them or a trailor rental which dosent exist!!

Now im wondering about an 8HP 4 stroke with 4 to 1 reduction?If they exist if not they should.

 

[blackbeard]

As has already been said, it's not a simple question. With an outboard, much will depend where and how it's mounted. With some outboards different propellors can be specified and the choice will depend on the job it has to do.
One point which could be overlooked - on a sailing yacht with an inboard engine, the propellor is not necessarily optimised to performance under power. It may be a compromise between this and performance under sail, since a larger prop will slow the boat down more. The only way of optimising performance under power while retaining good sailing performance is then the extra expense of a folding/feathering prop. No problem with an outboard where the prop can be lifted clear of the water. Outboard will also have an advantage in water full of debris/plastic bags/ropes/general junk, since a foul can be cleared easily.
Have fun choosing ...

 

[Guest]

[ QUOTE ]
The rivers arnt rough but wash from down stream boats could cause the prop to leave the water.

Im just thinking of the Rhone and Rhine both 5/6 knots in places.Perhaps i could mount two outboards in case one breaks down? Add saildrive props to give extra push.Or perhaps a good 8 HP mounted behind the rudder on a frame to give maximum push.

I can see a large prop with a 3 to 1 reduction would be ideal but not when sailing! Do outboards have a 3or4 to 1 reduction option? That would be fantastic for a crusing boat and provide a hand break as well!

Best of all would be a small boat canal to by pass them or a trailor rental which dosent exist!!

Now im wondering about an 8HP 4 stroke with 4 to 1 reduction?If they exist if not they should.

[/ QUOTE ]

Prop has to be matched to engine and RPM ... especially on an outboard. OK a certain leeway is allowed to cater for hull / boat.

Also ... with reduction gearbox outboards designed as sailboat aux. engines ... such as the Mercury 7.5 & 9.9 HP jobs ... the box makes them very heavy. My 7.5 on my snapdragon was as heavy as a 9 or 10 hp std engine. But its thrust and capability on my Snapdragon 23 was amazing. But because of the weight and nearly falling in humping it on / off the bracket - I cut a cockpit based outboard well through the hull and mounted it there. Manouvreability was then truly incredible ... as I designed the well to alloow me to turn the engine about 50 deg either side ... this with the prop wash now over the rudder .... !!

 

[trouville]

Ive got a mariner 9.9 would that push me along better than an Evinrude 6HP Its certainly a lot bigger and a lot heavier