Newby.Engine power to sail boat speed

[Richard D]

I presume every boat due to its shape has a max speed through the water in normal conditions using sails only, how does one decide what power engine to fit ( assuming you need to fit replacement ) would a bigger power make the boat go faster than with sails and how do you determine the power needed. Do you get one that is over powered and run it lightly. I come from the comp motor sport industry and I was on a boat last week with 2.2 litre Yanmar that was only rated at 55bhp most modern 2.2 non turbo diesels turn out nearly double that, can someone explain.

Richard

 

[prv]

For every (non-planing) boat, there is a speed at which the speed/power ratio abruptly changes, such that it's not really worth trying to go faster by adding more power. This is called "hull speed", and trying to go faster just results in more noise and waves but not much more speed.

In the past, engines were seen as auxiliary, and probably couldn't get the boat up to hull speed. Now plenty of boats have big enough engines to reach hull speed under power in good conditions.

If you're motoring against wind and waves, then more power will be required. So it's not totally daft to fit an engine with more power than it takes to reach hull speed in calm conditions - although you won't go any faster on a still day, the extra power will keep you going fast when it's windy.

Actually deciding on the power to fit is a bit of a black art, and depends a lot on the propeller and gear ratio as well as just the power figure. Most people would go on what's normally fitted to similar boats.

As for why marine engines have low power for the volume, it's a combination of old designs (not worth doing cutting-edge development for a small market) and de-rating for reliability in adverse conditions (possibly poor cooling, running at high power for days).

Pete

 

[Richard D]

Thanks for that Pete, is this max hull speed a figure that you can get for any boat you are thinking of buying to give an idea of its perfoemance.

Richard

 

[prv]

Thanks for that Pete, is this max hull speed a figure that you can get for any boat you are thinking of buying to give an idea of its perfoemance.

It is (or can be) mathematically calculated from the hull form, and by far the dominant term in the equation is the waterline length. The longer the hole in the water, the quicker you go. Of course there are then lesser but still significant effects for deep chubby hulls vs fine sleek ones of the same waterline length.

As for actual published figures, I don't know. Might be available for some boats in some form. But the whole question of performance is a bit fuzzy really (no doubt frustrating to a motorsport mind )

Pete

 

[photodog]

Hull Speed formula...

v= 1.34 x Square root of Length of the waterline.

Were V is in Knots and LWL is in feet....

 

[Richard D]

Thanks Photodog, love something factual to go on.

Richard

 

[prv]

v= 1.34 x Square root of Length of the waterline.

Yep...

... except that the 1.34 is only an approximation for a "typical" boat. Increase for "fast" hull shapes and decrease for "slow" ones. How much to increase or decrease for a given hull is the subject either of serious mathematics or extensive experience

Pete

 

[Spyro]

Thanks for that Pete, is this max hull speed a figure that you can get for any boat you are thinking of buying to give an idea of its perfoemance.

Richard

For a non planing Max hull speed is usually worked out at 3.14 X square root of water line length.
Try googling max hull speed.

I was a bit slow with the typing there

 

[rob2]

Most sailboats are basically displacement hulls. They are limited to the speed the hull can achieve (based on waterline length) when the boat tries to climb its own bow wave. This type of hull requires surprisingly little power toreach its hull speed in flat water, but extra loads of waves and wind can be addressed by a bit of extra power. The reason turbodiesels are not generally used is that the transmission of the power is through a propellor, which demands a good torque output at such speeds, so a turbine offers very little and adds mechanical complexity.

A planing hull form has large flattish sections which allow the boat to "surf" up over the bow wave and can then use more power output to accelerate as the drag has been significantly reduced. Anyone who has ridden in a RIB planing at speed will have experienced how they are finely balanced with minimal grip on the water!

Hull speed can be calculated by the following formula:
v=1.34 × √LWL
where:
"LWL" is the length of the waterline in feet, and
"v" is the speed of the vessel in knots

Playing around with a prop calculator such as www.castlemarine.co.uk/pitch.htm can work out roughly what power engine and prop will suit a particular boat, although trial and error is usually involved in getting the absolute best out of everything. Even with a planing boat, it will depend on how heavily loaded the boat is!

Simplistic aproach, I know, but I hope it gets across the ideas.

Rob.

 

[Lakesailor]

It has to do with the length of the wave formed by the passage of the hull. A displacement boat has trouble escaping the wave by climbing out of the trough, over the wave, which a planing boat can.

This piccy shows the wave. As does this

Steamboats tend to have very long and slender hulls which allows them to escape the rule to a certain extent.
This one in this piccy is a modern version of a older boat and can do over 20 knots. In this shot it is doing less than 8 knots and the wave is only extended less than halfway along the hull.

 

[Tranona]

Talking about maximum hull speed is shorthand for estimating the speed at which the hull starts to meet resistance from its own waves. At that point the power required to overcome the resistance rises dramatically.

As photodog says the factor of 1.35*sq root waterline length is a good estimate - it is derived from empirical observations and the factor can vary with different hull forms. If you have excess power you can exceed this estimated figure (which mid 30s ft yachts is +/- 7 knots) by a small amount but for a dramatic increase in power. The "theoretical" speed is the same under sail and power, but can often be exceeded under sail alone particularly short term when surfing on waves.

Some sailing boats have flat bottom planing type hulls, similar to planing power boats. These can often get over their hull waves and plane at well above hull speed. See the test of the Seascape 18 in this months PBO as an example. However they won't do it under power.

So cruising boats normally have displacement hulls, and you actually need very little power to move them at low speeds, but power requirements rise rapidly as you get near hull speed. So my 37 foot boat has a theoretical hull speed of 7.4 knots. Its engine is rated at 29hp max and it will achieve hull speed at max revs. However it will cruise at 5.6 knots drawing only 20hp.

As to specific outputs for small diesels, they are mostly in the range of 25-30hp per litre at between 3000 and 3600 revs depending on manufacturer. So the 2.2l Yanmar you quote is typical. That engine can however be turbocharged up to 75hp, but for the reasons mentioned above high power is not required on displacement boats, so turbocharging is only used if the designer wants to keep the physical size of the engine down. Many simply fit a larger capacity engine to get more power.

The choice of engine for a boat is a function of LWL and displacement (primarily) but can be influenced by the size and type of propeller used. Engine power has increased in recent times, mainly because expectations of motoring performance have risen and the range of compact high (relatively) powered engines has increased.

So the boats you are looking at will have engines between 30 and 40 hp. More power will not get higher speeds, and higher power on boats of similar size usually reflects (but not always) greater displacement. Generally it is not wise to overpower aboat and run the engine at low loads. Small diesels are designed to run hard (typically 75% max power) and low speed running can lead to problems of bore glazing - as well as being a waste of money!. Engines don't wear out because they don't run high hours. Most have a life of 30 years+ and usually fail because the bits that make them work in the marine environment fail.

 

[snowleopard]

There was a brief mention of wind and waves above but I'll add emphasis. The minimum engine size to reach hull speed in flat water will leave you floundering when the wind gets up a bit. It is common with modern boats to be able to reach hull speed on half throttle. The rest of the power won't add much speed in flat water but will allow you to keep the speed up into wind and waves.

There are three ways to exceed hull speed: Planing as has been explained, surfing, i.e. temporarily achieving high speeds down the face of a wave and narrow hull forms. A hull with length:beam ratio of over 8 will generally be able to go faster than the theoretical maximum which is why multihulls can go so fast.

 

[Richard D]

Really appreciate the onfo guys it all makes sense. Has much reseaqrch been done into fuel consumption between the older and newer engines as I would imagine the likes of the old Thornycroft 35 bhp unit must a lot less economical than a modern Yanmar of similar hp. As I said I come from an automotive background and I wonderred if anyone has tried something like the latest Citroen/Peu 1600 Hdi which gives anything from 90 to 115 bhp, they would be so much lighter and compact and from a tech point of view easy to marinise.

Richard

 

[AntarcticPilot]

Really appreciate the onfo guys it all makes sense. Has much reseaqrch been done into fuel consumption between the older and newer engines as I would imagine the likes of the old Thornycroft 35 bhp unit must a lot less economical than a modern Yanmar of similar hp. As I said I come from an automotive background and I wonderred if anyone has tried something like the latest Citroen/Peu 1600 Hdi which gives anything from 90 to 115 bhp, they would be so much lighter and compact and from a tech point of view easy to marinise.

Richard

Basically, the operating environment of a marine engine is VERY different from that of a car. The car engine is designed (presumably) to be responsive to the throttle, to operate with high efficiency and so on. The marine engine may need to operate at constant revs for hours on end, to provide a lot of torque at low revs, and above all, to be readily maintainable under less than ideal conditions! Most marine engines are therefore relatively low stressed engines, that will go on running almost no matter what is thrown at them. The RYA diesel course tended to concentrate on what you could get away with NOT having to keep a diesel running! A car engine which is (I guess) is a highly stressed common rail engine relying on an engine computer is likely to suffer from interesting failure modes due to salt in the electronics. It would also be difficult, if not impossible, to fix with minimum tools while at sea or at anchor remote from facilities.

As far as fuel efficiency goes; most people reckon about 1 litre per hour for every 10 BHP or thereabouts. I do about 2-3 litres per hour on a Volvo 2003; that seems about right.

 

[Tranona]

Really appreciate the onfo guys it all makes sense. Has much reseaqrch been done into fuel consumption between the older and newer engines as I would imagine the likes of the old Thornycroft 35 bhp unit must a lot less economical than a modern Yanmar of similar hp. As I said I come from an automotive background and I wonderred if anyone has tried something like the latest Citroen/Peu 1600 Hdi which gives anything from 90 to 115 bhp, they would be so much lighter and compact and from a tech point of view easy to marinise.

Richard

You will find specific fuel consumption of diesels varies very little, so fuel consumption is a direct function of power used. So using 25hp from an old Thornycroft (BMC taxi engine) will be very little different from using 25hp from a Yanmar. The rule of thumb is 1l fuel for every 10 hp used. So get the power curves of the engine, choose your revs/power for comfortable cruising divide by 10 and you get your hourly fuel consumption in litres.

The engines you mention are totally unsuitable for displacement boats. As already explained you only need 30-40hp for most smallish cruising boats and the industrial units used are fine for that. The high revs will also need gearing down as shaft speeds on a displacement boat are usually less than 1500rpm. Higher powered engines are marinised for use in planing boats, but are usually larger than that because the power range is 150hp upwards.

Not sure of the basis for your assertion they are easy to marinise. The weakest point of all high powered marinised engines is cooling. That amount of power creates tremendous heat as I am sure you know, particularly if it has a turbo. Keeping that lot cool in the confined space of a boat relying just on salt water as a cooling medium (remember no air flow) is quite a challenge!

 

[pcatterall]

For a non planing Max hull speed is usually worked out at 3.14 X square root of water line length.
Try googling max hull speed.

I was a bit slow with the typing there

And a bit inaccurate with the keys (transposition error?)

 

[misterg]

...I wonderred if anyone has tried something like the latest Citroen/Peu 1600 Hdi which gives anything from 90 to 115 bhp, they would be so much lighter and compact and from a tech point of view easy to marinise.

Richard

If you look on the cabin cruiser forum (sorry, motorboat forum) there are regular discussions about how much power can safely be wrung out of automotive derived engines (Volvo for example).

The crux is that on a (power) boat, an engine will need to deliver close to maximum power (maybe 80%) almost continuously for hours at a time with good reliability. In a car, it has to deliver maximum power for a few seconds as the lights turn green on a Saturday night, but spends most of its life at a much lower power output.

Andy

Edit: Search for posts about the Volvo D3 (which is the marinised D5 turbo diesel car engine) if you want to see some of the previous discussions. Note what Volvopaul & Spannerman say - they are both professionals.

 

[wotayottie]

I come from the comp motor sport industry and I was on a boat last week with 2.2 litre Yanmar that was only rated at 55bhp most modern 2.2 non turbo diesels turn out nearly double that, can someone explain.

Richard

The issue of bhp / litre is straightforward. Why extract 55 bhp from an expensively built high revving engine when you can get the same from a low tech cheap to make 2litre engine. Quite often they take the same approach with truck engines. if you like its the AC cobra versus ferrari 275LM comparison.

It's down to useage as well - a boat engine will often be running at the same revs putting out 2/3rds of its maximum output for 24 hours at a stretch.

 

[mitiempo]

Or Ford pickup vs Ferrari.

 

[Richard D]

What made me think about it was that the boat I was on had had a new Yanmar 55 and it cost over £6000. You can pick up an under 10k miles write off engine with all electrics and ecu for £750, gives you a lot of nroom for thought. The max torque on these engines os still sub 2000 so they would have the right power band
Richard