Boat vs Engine Size

davtt

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As someone new to the boating scene, I am somewhat confused as to the relationship of engine size to a specific boat size. To better explain my question:
As there is a maximum speed for a given length of boat (LWL) where the hull is a displacement hull, why do some boats have much larger engines than others for a given length? Are the benefits purely reliability an surely the increased weight must offset the advantages?

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Dave T
 

davtt

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In this specific instance, my sailing cruiser, but I guess this would apply to any displacement hull boat. I know that with a planing hull that the length rules do not apply as the boat 'rides over' the water.

Thanks and Regards,
Dave T
 

wpsalm

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Its not so much increased weight of the engines thats just balast also increased size of the prop added drag.. My little boat a 32 ft. double ended bathtub with sails was designed in 1920 and the designer reckoned about 7 hp would do the job. well in those days people sailed . an engine was only for times when the wind failed all together...Today the little boat is powered by a modern 40 hp engine which is maybe a bit much but I think of the extra horse power from a safety point of view might need to get into port against wind and tide might need to tow another yacht ..
 

charles_reed

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Engine size depends on a number of factors:

How close you want to travel to your max displacement speed

How easily driven the hull might be

Vessel windage

Sea state.

These factors mean that to drive the same hull to windward at full displacement speed in a F6, would require an engine of about x18 the power to move it at square-root-waterline' knots in still water.

In that I've left out how easily the hull is driven and assumed the prop/ratios are perfectly chosen for each engine.
 

davtt

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Hi Charles.

I think then that the answer is that the larger engine makes a difference when factors other that speed through the water are providing a slowing factor on the boat. The actual speed of the boat cannot be increased by a bigger engine, given that the other factors are the same?

Thanks and Regards,
Dave T
 

Rune

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Quite right. But just remember that if you increase the size of the engine you will need a bigger shaft and prop as well.
This again increases drag and raises the question if the prop can even fit without chopping the stern off.

Have one of the engine companies calculate it all for you.
Try Nanni’s www.nannidiesel.com/eintro.htm UK distributor. They will calculate everything from engine size, gearbox ratio, max hull speed, prop pitch…
 

kgi

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It pays to remember that on a modern deisel the max power is its one hour power, max continuos can be quite a bit less, eg yanmar 2GM 18hp max, at 3600 rpm, max continuos power 14hp at3400.........keith
 
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