What's more important - HP or Torque?

As it was explained if you have a higher pitch which is the angle of the blade then the effective drive is increased at a fixed RPM.

Once you get nearer hull speed you'll get more "slippage" and you will waste more power trying to go a little bit faster.

By increasing the diameter and reducing the pitch I believe the engine will drive the boat slightly slower at tickover. By having a bigger prop I belive it has the ability to transfer more HP into the water so there will less slippage at speed. It should also be able to stop the boat better. Guess i'll still have to push in and out of gear when i want to go very slow as usual.

This has to be matched by maximum speed also hence if you look at the pitch on a gin palace it bears little resemblance to yacht prop

I am sure a greater expert will explain this more scientifically but i believe it will help a bit. I needed to change the prop anyway as the old engine is opposite handed to the new one. Worth checking on your boat I guess.
 
Wow. people are making heavy weather of this. HP is the maximum power available, usually at or slightly below maximum revs. But you don't want to be using maximum power. That is tiring and wasteful.
Torque is the "twisting effect" the motor can supply and is normally at much lower revs.
(as an aside, my 1600cc petrol engine in my trials car produces 82 bhp at about 5500rpm. But 92 lb/ft of torque at about 3000 rpm. It can crawl up hills at tickover.
The previous tuned 1040cc engine produced a similar bhp of about 85 (at nearly7000rpm) but only about 55 lb/ft of torque at about 3800rpm. It couldn't drag us up hills at low revs)
This illustrates that a low revving engine with torque will plod on producing usable power with minimum fuel consumption whilst a high revving motor with the same peak power will struggle to provide the thrust except at high and wasteful, in fuel terms, revs.

Getting a motor with good low speed torque you can them fit the right prop to keep the motor in it's torque band up to hull speed. You won't be over-revving the engine, but will be using it's natural strength (the torque) efficiently. Yes, over-boatspeed at tickover may be a slight problem ;)
 
So a 15% drop is HP is less serious than the 25% drop in torque? This is a big lady here, 35 feet and 13 tonnes :)

Just checked their site, the 70hp has max rpm of 2500, 58hp has a max of 3000. Shame the 70hp is so much more expensive!
 
The one possible next step, following on from Tranonas wise propellor-centric advice might be to have a word with those 'its only money' self pitching propellor manufacturers, perhaps...

Seems to me that as the world has moved on from those lovely huge huge two bladed props that can lay down some real torque and yet be aligned vertically when sailing , then a three blade folder might attain the theoretical match to the engines useable thrust when really needed, cruise economically at a different propellor 'pitch' and still sail, without enlarging the rudder aperture to swing a larger ' looks good on paper' propellor?

Give up beer for several Lents to pay for it all though?
 
Lakesailer is closer to the money, HP is Torque X RPM they are directly related but the type of engine makes a huge difference.

The problem is that different engine technologies have different characteristics and these in turn are determined to some extent by the calorific value of fuel and the geometry of the engine.

Petrol engines rev harder to get the HP diesels don't for a variety of reasons.

Lets stick to Diesel engines and not try to compare them with petrol it just makes it more complicated.

To work at their best diesel engines need a long stroke compared to the bore diameter. This is because its possible to get a longer slower burn from diesel fuel; partly because of the time it takes to inject the fuel after all the valves have closed and partly because diesel burns slower anyway - but has a higher calorific value so more pressure. The advantage this gives is more cylinder pressure over a longer time; which in turn means more torque. The disadvantage is that the pistons have to travel a greater distance in each stoke so they must travel at a high speed and accelerate and decelerate quicker than an over square (short stroke engine) these forces are not inconsiderable. As the speed (RPM) increases the loads on pins and bearings increase which when added to the increase in pressure in the bores will limit the Max RPM or the engine will destroy its self. Hence we have (relatively) low revving long stroke engines that deliver high torque at low RPM - EXACTLY the characteristics you need for boats.


So I would expect the lower Torque engine to have a shorter stroke relative to the bore diameter.
 
You need enough torque low down that the engine will reach the rpm where max torque is.
If you open the throttle and it bogs down at a lower rpm without producing enough force to get the boat moving, it will never produce enough drive.
You will be stuck in a useless part of the power/torque curves.
So it depends a lot on the propeller.
And also on the opposing force, i.e. what wind and sea you expect to drive into.
Or if you want to reach max power while the boat is aground perhaps?
Because of the action of the prop, boat speed is not directly tied to rpm like in a car.

So what do you need?
A decent top speed in flat water.
Economical cruising.
The ability to drive into wind and waves.
Only you can decide how much wind and wave force you demand to be able to drive into.


The key is the efficiency of the prop in the different conditions.
 
The over riding desire is for a quiet, low vibration economical engine for passages where we can't sail or want to motor sail. We have a 31" three blade prop, a 2500 rpm diesel and a 3:1 PRM gearbox. We need about 40hp for the basics, everything above that is for foul wind, waves and tide, not inconsiderable factors in a long keel, high freeboard wheelhouse yacht.
 
Lazy Kipper said:
The over riding desire is for a quiet, low vibration economical engine for passages where we can't sail or want to motor sail. We have a 31" three blade prop, a 2500 rpm diesel and a 3:1 PRM gearbox. We need about 40hp for the basics, everything above that is for foul wind, waves and tide, not inconsiderable factors in a long keel, high freeboard wheelhouse yacht.
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Any modern engine will give the above., i do not think you need a 3:1 box.
my engine is rated 37.5hp @ 3000 RPM with max torque @ 2400 RPM with a PRM 150 box.
i would have thought 45hp was about right for your boat.
Why not ask Lindsey Rufford he is based @ Woolverstone ( Sea Power), a very genuine & well regarded chap
 
sailorman said:
Any modern engine will give the above., i do not think you need a 3:1 box.
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I believe a larger slower turning prop is more efficient - the bigger and slower the better. Usually space to swing one is the limiting factor. The disadvantage on a sailing boat is the drag from a large prop.
 
To summarise all the above and assuming the engine/gearbox match the prop reasonably well, the HP will determine the maximum speed of the boat while the rev range over which a reasonable amount of torque is available will determine how well the engine will cope with your rather large boat and prop as you ease off the throttle. I'd suggest you want to make sure you have adequate HP and then get the engine that offers the most torque lower down the rev range (i.e. the longest, flattest torque curve), assuming that the engine is running at or near it's best specific fuel consumption at the load at which you will be using it the most.

If you pop over to the MoBo you could ask LateStarter for guidance - he is the fount of all knowledge when it comes to marine (and other diesels). FWIW his mantra is "engines don't move boats, propellers do".
 
Tranona said:
Pretty sure he means 21" but no doubt Kipper will confirm.
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I agree, but the yard who worked on it said 31" and it has that stamped on it. It is huge. Once I'd discounted Bruntons because it cost the same as a new engine, I looked into Kiwi and they make a four blade equivalent. They didn't seem surprised by the size of the present one, said it matched the type of engine we have. Still, 31" is 2.5 feet!
 
Just doing some sums and you are probably right about the size if you have a 3:1 reduction - although ideally should be a bit smaller. The size might explain why you can get maximum boat speed at less than full power, and your 3 knot tickover!

You would not need a prop that size with a 58hp. If you kept a 3:1 reduction you would be looking at around 22". You may well find at that size a Bruntons is more accessible. If you change engines you will need a new prop anyway and a standard fixed 22" is more than £1k, so the incremental cost of a Bruntons is not so daunting!

You don't say what engine you are looking at, but suggest a Nanni is worth considering in that HP range.
 
I like the Nanni and they do a nice 62hp. The one I'm looking at though is a Mermaid MP58, it's £5k supplied and £1.5 fitting plus my new prop, whatever that turns out to be. The bruntons gets more complicated when you factor in £1.5k for the required shaft brake.
I'm guessing my gearbox ratio, it has 3.1 stamped in front of the serial number, but that could be something else, but it does make sense of the huge prop. Kiwi are now doing the K4 and that's a four blade prop which doesn't need a shaft break. All in all the Kiwi gives most of the benefits of the Bruntons and is £4.5k cheaper as a package. New engine, new feathering prop and all the benefits therein for £8k is a highly attractive proposition if, and only if, the 58hp is big enough. Some of the panel's views though cast doubt. I suspect that the engine I have has masses of torque in the low revs but the proposed new one will not deliver its big dollop of torque until much later in the revs.
 
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