Optimal fuel economy 32' SD Trawler yacht

[Tonyp45]

I am retiring from a long sailing career and currently looking for a used displacement or semi displacement motor yacht suitable for continuous cruising/ live aboard in UK home waters for 8-9 months of the year. Trawlers such as the Corvette 32 appear to offer a good compromise of living space and seakeeping. GRP is a must for me however, which probably excludes some excellent full displacement steel boats.
I have looked at threads on fuel consumption but find my aging brain going numb at the facts and opinions presented. So a simple answer to a seemingly simple questions please :
What engine power - single or twin offers optimal economy for a vessel such as the Corvette 32 operating at displacement speeds 6-8 knots in low moderate seas?
Twin 120 hp seems more common on such boats but have seen twin 60 hp.
Thanks in advance.

 

[burgundyben]

My boat is 31ft and displaces 5 tons ish. I have a complex Kitchen rudder that adds a lot of drag. At displacement speeds, ie under 7 knots, I burn about 1 gallon per hour.

I think a 60hp boat would struggle more in a sea, praps best for rivers.

I reckon a Corvette 32 at 8 knots would burn about 3 gallons an hour.

A Mitchell 31 with 200hp will do 17 knots burning about 9 gallons an hour.

A Nelson 34 with a pair of 200hp engines will do 20 knots, about 18 gallons an hour.

If you really are content to trundle about at 6 to 8 knots then fuel really isn;t a big deal.

Most engines will deliver about 20 to 22hp per gallon per hour.

 

[superheat6k]

There is a host of information on the Corvette Motorboat Association website www.corvettemotorboat.wordpress.com

Apart from a few rare early boats all are twin engine. The very early boats had small 65HP, then the mid 80 boats were mostly 130 then from ~ 1987 most were built with ~200HP. The later 320's mostly had ~330HP.

My own has 210HP Cummins (1991), with many from that time built with TAMD41 Volvos 200HP.

The smaller engines are more suited to slow coastal, river and estuary cruising, and will struggle to plane at all even in flat water, but very economical at 6-8 knots plodding speed. With these they are not really SD boats. The 200HP offer a better compromise of ~ 14 - 16 knots cruise on the plane, with decent fuel economy below 10 knots. The hull is like the Grand Old Duke of York between 10-13 knots - neither up nor down.

In the four years I've owned my boat I expect ~ 1.5 MPG at a fast cruise increasing to ~ 4MPG ~8 knots. Note this assumes a clean hull and props.

 

[burgundyben]

Trev - that's 2 gallons an hour at 8 knots, seems ok to me.

 

[Tonyp45]

My boat is 31ft and displaces 5 tons ish. I have a complex Kitchen rudder that adds a lot of drag. At displacement speeds, ie under 7 knots, I burn about 1 gallon per hour.

I think a 60hp boat would struggle more in a sea, praps best for rivers.

I reckon a Corvette 32 at 8 knots would burn about 3 gallons an hour.

A Mitchell 31 with 200hp will do 17 knots burning about 9 gallons an hour.

A Nelson 34 with a pair of 200hp engines will do 20 knots, about 18 gallons an hour.

If you really are content to trundle about at 6 to 8 knots then fuel really isn;t a big deal.

Most engines will deliver about 20 to 22hp per gallon per hour.

Many thanks for that. The 60/65 hp boats I have seen were both twins

There is a host of information on the Corvette Motorboat Association website www.corvettemotorboat.wordpress.com

Apart from a few rare early boats all are twin engine. The very early boats had small 65HP, then the mid 80 boats were mostly 130 then from ~ 1987 most were built with ~200HP. The later 320's mostly had ~330HP.

My own has 210HP Cummins (1991), with many from that time built with TAMD41 Volvos 200HP.

The smaller engines are more suited to slow coastal, river and estuary cruising, and will struggle to plane at all even in flat water, but very economical at 6-8 knots plodding speed. With these they are not really SD boats. The 200HP offer a better compromise of ~ 14 - 16 knots cruise on the plane, with decent fuel economy below 10 knots. The hull is like the Grand Old Duke of York between 10-13 knots - neither up nor down.

In the four years I've owned my boat I expect ~ 1.5 MPG at a fast cruise increasing to ~ 4MPG ~8 knots. Note this assumes a clean hull and props.

Thanks for that as well as the Corvette link.

 

[Keith-i]

My Channel Island 32 with a single 265hp burns about 6 gallons/hr at 12kts and double that at 18kts. Less than 1 gallon/hr at 5kts. I can't remember the consumption at displacement speed.

 

[tillergirl]

My Mitchell 31 has a Perkins 6.354 (120hp) and burns about 1.4 gallons @ 7/8 kts at cruise.

 

[PCUK]

At 6 knots all boats of the size you want will be economical.

 

[omega2]

Why not take the trawl in and fit a mast and sails? the engines will use what they need!

 

[longjohnsilver]

My boat is 31ft and displaces 5 tons ish. I have a complex Kitchen rudder that adds a lot of drag. At displacement speeds, ie under 7 knots, I burn about 1 gallon per hour.

I think a 60hp boat would struggle more in a sea, praps best for rivers.

I reckon a Corvette 32 at 8 knots would burn about 3 gallons an hour.

A Mitchell 31 with 200hp will do 17 knots burning about 9 gallons an hour.

A Nelson 34 with a pair of 200hp engines will do 20 knots, about 18 gallons an hour.

If you really are content to trundle about at 6 to 8 knots then fuel really isn;t a big deal.

Most engines will deliver about 20 to 22hp per gallon per hour.

My 48’ displacement trawler yacht has twin 135hp engines. I reckon at my usual cruising speed of approx 7kts I’m only using about 40hp from each engine.

So for a 32’ displacement boat I’d think twin 60s would be more than enough.

 

[Lightwave395]

My boat is 31ft and displaces 5 tons ish. I have a complex Kitchen rudder that adds a lot of drag. At displacement speeds, ie under 7 knots, I burn about 1 gallon per hour.

A Nelson 34 with a pair of 200hp engines will do 20 knots, about 18 gallons an hour.

I always coveted a Nelson for when I'm too old to sail, sadly I think unless maths has given up before my sailing ability, nearly £600 in diesel to get from Yarmouth to Alderney and back takes it right out of my pension capability...

 

[Bouba]

I always coveted a Nelson for when I'm too old to sail, sadly I think unless maths has given up before my sailing ability, nearly £600 in diesel to get from Yarmouth to Alderney and back takes it right out of my pension capability...

Don’t you get cheap fuel in Alderney?

 

[burgundyben]

I always coveted a Nelson for when I'm too old to sail, sadly I think unless maths has given up before my sailing ability, nearly £600 in diesel to get from Yarmouth to Alderney and back takes it right out of my pension capability...

Maths wrong. Nelson 34 at 10 knots is probably about 5 gallons an hour, say five hours, so 25 gallons total. £125. Each way.

But at 7 knots , 2 gallons an hour, 7 hours, 14 gallons, £70. Each way.

I reckon chugging about in my displacement boat at 8 knots, my annual fuel bill is less than the cost of halyards, sheets, sail repairs, new sails and winches and rope jammers on a similar sailing boat over the same miles a year.

Keep the boat in the displacement speed range and they really burn very little.

 

[blampied]

consider an Aquastar 33 (we have one with a single 370 TAMD63P motor)
We live on it, it’s a great long distance tourer/ live aboard.
Just as happy at sea as on rivers and canals
low enough to fit under any bridge on the French canal network, but built on a proper sea going fishing boat hull.

we have taken her from the U.K. to Channel Islands, France channel coast, France Atlantic coast, France Mediterranean coast, and France Inland waterways, Spain, Gibraltar, Morocco, Italy, Monaco. etc.
at canal speed 4 knots it burns about 2 Ltrs an hour, slow sea speed 9 knots it burns about 20 ltrs an hour, quick sea speed 12 knots is about 40 ltrs. It will top out at about 20 knots but burning silly amount of fuel at that speed