Inboard hp / outboard hp

[oz-1]

Hello every one, This might sound like a very stupid question, but it has always baffled me. If you have a 10 hp outboard and a 10 hp inboard, are the engines producing the same hp, if the propellors are at the same depth in the water? I have an 8 hp 2 stroke Tohatsu on the back of my 22 foot, 3 ton sailboat. It seems to push the boat nicely at about 3 knots, but I mostly only use the engine to exit or enter our harbour. The engine only weighs 26 kilos and is very compact. A similar sized inboard would, I am sure, weigh a lot more and seems to be physically bigger. So again, are the two horse powers the same. I would really love to know the answer. Many thanks, Oz.

 

[Bobc]

Torque is more important, and a diesel inboard will produce more torque, meaning you can drive a bigger prop.

 

[Tranona]

Hello every one, This might sound like a very stupid question, but it has always baffled me. If you have a 10 hp outboard and a 10 hp inboard, are the engines producing the same hp, if the propellors are at the same depth in the water? I have an 8 hp 2 stroke Tohatsu on the back of my 22 foot, 3 ton sailboat. It seems to push the boat nicely at about 3 knots, but I mostly only use the engine to exit or enter our harbour. The engine only weighs 26 kilos and is very compact. A similar sized inboard would, I am sure, weigh a lot more and seems to be physically bigger. So again, are the two horse powers the same. I would really love to know the answer. Many thanks, Oz.

Little to do with the HP rating and all to do with the propeller. The inboard will have a reduction gear to give a shaft speed of between 1500-2000 rpm which enables it to turn a propeller of typically 12/13" diameter and pitch depending on weight of boat to move a displacement boat at its hull speed of around 6 knots. On the other hand most outboards are designed for planing boats and have higher engine revs, smaller reduction ratios and therefore higher shaft speed and smaller diameter propellers - usually 8/9 ". As you have discovered not very good for displacement boats. There are however a few outboards specifically designed for displacement boats with shaft speeds more suited to large diameter propellers.

 

[Alan S]

And of course inboards are usually diesel while outboards are petrol. That explains the size and weight difference.

 

[oz-1]

Thanks for the informative replies, I have learnt something new today, that the inboard ( diesel ) engine turns the prop shaft at a lower speed and spins a much bigger propellor than an outboard. My Tohatsu is a long shaft, sail drive model, but the prop is no where the size of an inboard engined one. Best regards, Oz.

 

[Chiara’s slave]

Thanks for the informative replies, I have learnt something new today, that the inboard ( diesel ) engine turns the prop shaft at a lower speed and spins a much bigger propellor than an outboard. My Tohatsu is a long shaft, sail drive model, but the prop is no where the size of an inboard engined one. Best regards, Oz.

You can much alleviate the difference with a better size and pitch of prop on your outboard. We are outboard powered, and have a 4 blade ultra low pitch prop, it made a world of difference.

 

[PabloPicasso]

I hooked my tohatsu 3.5hp to our club's work boat recently as club o/bs weren't working.

It pushed us 2 up gently around the harbour in flat water with light wind.

Applying throttle resulted in a lot of cavitation (i.e., o/b revs high, prop spins fast but doesn't grip the water as it is spinning in bubbles of air it has just created)

So the o/b was unable to transfer oprimal power effectively into driving the boat forwards. (It did okay at low revs to be fair)

Now it's possible a bigger prop with different pitch may have worked better, but the 3.5hp 2 stroke probably doesn't't have the torque needed.

There may be a prop of size and pitch to make better use of o/b power in such circumstances, but not worth it just for one off use.

 

[snowbird30ds]

Outboards are rated for max rpm with much less torque any lower, a diesel will give more torque over a bigger rpm range so the power will be in the useable range.

 

[Stemar]

IMO, an inboard diesel has two advantages over an outboard, even if the OB is properly sized - long shaft, and enough umph. The first is that the prop is well under the boat, so it won't lift out and lose drive in rough seas, the second is fuel economy.

 

[pete]

It's likely that the inboard will have a larger alternator for battery charging which will reduce it's output to the prop slightly but will deliver the power deeper in less aerated water.

 

[jamie N]

I've a GK24, which is in reality around about 2,000kgs. She's got a 6hp Mercury outboard on the back, and does get 5.5kts in flat sea and no wind. Against the wind, she doesn't really seem to suffer too much, pitching through waves isn't good news though. The boat's quite light, the lack of mass & torque really do affect the boat. I relax myself by saying that "If there's wind, I sail" (etc.), however, I reckon that in a normal life, a GK24/J24/Sonata type of boat is where the margin for whether an inboard or outboard is 'better' lies.
Larger boats will almost be undoubtedly better with an inboard, diesel at the moment, and smaller, lighter (racier) boats with a lighter outboard.

 

[Chiara’s slave]

I've a GK24, which is in reality around about 2,000kgs. She's got a 6hp Mercury outboard on the back, and does get 5.5kts in flat sea and no wind. Against the wind, she doesn't really seem to suffer too much, pitching through waves isn't good news though. The boat's quite light, the lack of mass & torque really do affect the boat. I relax myself by saying that "If there's wind, I sail" (etc.), however, I reckon that in a normal life, a GK24/J24/Sonata type of boat is where the margin for whether an inboard or outboard is 'better' lies.
Larger boats will almost be undoubtedly better with an inboard, diesel at the moment, and smaller, lighter (racier) boats with a lighter outboard.

Our displacement is very similar, but we are 30ft. Pitching is bad for us too, but we have 15hp, and can achieve about 8.5kn in flat water and no wind.

 

[William_H]

I think there is something horrible about running an outboard at full RPM so full HP especially the noise. ( so you probably never get your O/B top develop it's full HP)
On the other hand the inboard diesel is heavy will take up space and have a different kind of noise vibration.(especially single cylinder diesel) Plus you can not aveert propeller drag just minimise it with folding propeller. All more concerns re fouling.
To OP I say keep your o/b drive even though diesel might be better (more powerful) To me o/b drive on a sail boat is ok ol'will (to be honest I never use motor anyway)