Diesel v Petrol Glastron

[ontheplane]

Ari is spot on.

Nowadays the diesel will be 5-8k more in liklihood, so if you get 5-7k more for it when you sell it its only cost you £1k more..... Will you save £1k in fuel? Yes, probably in a few weekends!

Admittedly I have never run a 5.0GXi but is it really so much more economical than a carb?

 

[ontheplane]

Johhny - is it the Glastron that was on TheYachtmarket.com?? I've seen that one and it looks a super boat - very jealous

 

[QBhoy]

That money isn't 'spent', you get most or all (or sometimes more) of it back on resale compared to a petrol boat. If you spend it on the extra cost of petrol, it's gone.

Mmm. But the petrol is cheaper in the first place. I think they would deprecate and then settle proportionally. Especially an mpi or gxi petrol and being more efficient than any equivalent diesel I know of. Pound for pound.

 

[QBhoy]

Ari is spot on.

Nowadays the diesel will be 5-8k more in liklihood, so if you get 5-7k more for it when you sell it its only cost you £1k more..... Will you save £1k in fuel? Yes, probably in a few weekends!

Admittedly I have never run a 5.0GXi but is it really so much more economical than a carb?

Yeah. Certainly

 

[ontheplane]

Yes, but it’s what we said above, the petrol is cheaper to buy, then cheaper when you sell it - so overall cost the same, so the cost difference is the fuel difference......

The last big boat I had I sold for what I paid for it. A similar diesel at the time was 5k more - but when I sold it it was nearer 7k more. But my boat burned up to 20 gallons (not litres!) per hour flat our where a diesel would burn 8 or 9 gal/hr.

I used to do 100 hours a year so 10 gal hr difference x 100hrs = £1000 a season difference.

Keep the boat 4 years that’s 4K on fuel difference. Simples!

Even if you have to finance the boat, 10k at current interest rates of say 3% is only £300 a year difference.

The fuel is burned and gone forever whereas investing in the boat, the asset is still there.

 

[ari]

Especially an mpi or gxi petrol and being more efficient than any equivalent diesel I know of. Pound for pound.

If a petrol engine was more efficient than a diesel engine then maybe...

 

[ontheplane]

I struggle to believe any petrol, even multipoint injection is more efficient than a diesel.

It's not just that though - you can get diesel anywhere - petrol other than the solent is difficult and incredibly pricey at marinas.

 

[Whippersnapper]

I

I struggle to believe any petrol, even multipoint injection is more efficient than a diesel.

It's not just that though - you can get diesel anywhere - petrol other than the solent is difficult and incredibly pricey at marinas.

its not, I don’t understand why some people try and argue it is. My boat has a modern light weight common rail diesel which replaced the big block V8 that used to be specced. The builder says the fuel consumption is literally half at 25 knots. Here in New Zealand diesel is also cheaper than petrol so it’s a no brainer.

that doesn’t mean that for some day boat used cases small boats with petrol inboards don’t make sense they can. But as soon as you want to cruise diesel is the way to go.

 

[ontheplane]

I agree.

Where superb hole shot, high speeds and relatively light weights are concerned, a small to large petrol is the choice, but for cruising most of the day, diesel is still the only choice.

 

[QBhoy]

I

its not, I don’t understand why some people try and argue it is. My boat has a modern light weight common rail diesel which replaced the big block V8 that used to be specced. The builder says the fuel consumption is literally half at 25 knots. Here in New Zealand diesel is also cheaper than petrol so it’s a no brainer.

that doesn’t mean that for some day boat used cases small boats with petrol inboards don’t make sense they can. But as soon as you want to cruise diesel is the way to go.

I’m talking about mpi and gxi engines. I’m not aware of any diesel that is terribly more efficient when comparing hp do hp, rev range and weight of the engine. Then the hugest cheaper cost of servicing too. Haven’t looked in a while now, but certainly this used to be the case. The tamd and Kads didn’t compare too well to the petrols. And these mpi aren’t advanced engines in any way at all. But if you are including any petrol engine. The new mercury outboards are way ahead

 

[QBhoy]

I struggle to believe any petrol, even multipoint injection is more efficient than a diesel.

It's not just that though - you can get diesel anywhere - petrol other than the solent is difficult and incredibly pricey at marinas.

Totally agree about if you are in the sea and diesel being more available than petrol. I said that already ages ago. My point was that of a mpi or gxi using less fuel HP for HP than the equivalent diesel of their time. Like tamd and Kad...last I looked. May have changed now though. These mpi aren’t space age either. The new merc outboards are in a different league though.

 

[ontheplane]

So petrol Engines (bear in mind this is AMERICAN Gallons - so multiply by

Fuel Economy

Engine	HP	Idle	1000	1500	2000	2500	3000	3500	4000	4500	4800	WOT
Volvo Penta 3.0 GL – 181 cid	135 hp	0.55	0.95	1.85	3.2	4.1	5.3	6.9	8.7	11.8		11.8 GPH
Volvo Penta 4.3 GL – 262 cid	190 hp	0.65	1.2	2.2	3.7	4.6	6.1	7.5	9.5	13	17.3	17.3 GPH
Volvo Penta 4.3 GXi – 262 cid	V6-225	0.69	1.3	2.4	3.8	5	6.75	7.5	10.3	14.5	18.1	18.1 GPH
Volvo Penta 5.0 Gi – 305 cid	V8-225	0.77	1.27	2.4	3.8	5	6.7	7.5	10.3	14.7	18.2	18.2 GPH
Volvo Penta 5.0 GXi – 305 cid	V8-270	0.82	1.3	2.5	4.1	5.8	7.5	10.4	13.7	17.8	21.4	21.4 GPH
Volvo Penta 5.7 Gi – 350 cid	V8-300	0.95	1.85	3.7	7	8.2	10.2	13.6	17.4	20.2	22.7	22.7 GPH
Volvo Penta 5.7 GXi – 350 cid	V8-320	1	1.85	3.8	7.3	8.5	10.2	13.9	17.8	20.6	23.6	23.6 GPH
Volvo Penta 6.0 – 364 cid	V8-380	0.82	1.3	2.3	3.8	6.1	7.5	9.9	12.7	15.4	18.7	30.6 GPH
Volvo Penta 8.1 Gi – 496 cid	375 hp	1.08	2	3.4	6.7	10.3	11.5	14.7	18	24.7	31.1	31.1 GPH
Volvo Penta 8.1 GXi – 496 cid	420 hp	1.15	2.1	3.7	7	8.9	11.2	14.9	18.2	26.3	34	34.0 GPH

So at 3000 rpm the 5.0GXi will use 28.3 litres per hr. I get that because of props gearing etc, the boat may be going a little quicker than with a diesel but it won't be much - the extra speed is mainly down to the petrol revving to 4800-5000 rpm and the diesel around 3750


See here


If you look - at 3000 rpm, the KAD is using 5 US GAL or 20 lit per hr and the petrol 7.5 US Gal or 28.3 litres an hr - about 50% more.

In reality the above figures are based on being at full load at each RPM - which you won't be as both will be throttled back a bit assuming you haven't got the props and everything wrong! But it's the only way to compare the two, and certainly my knowledge is that a diesel will burn a good 25-50% more than a petrol...... But if you look at MAXIMUM consumption (ok so in the petrol you will be going a lot quicker) Petrol 21.4 US GPH vs 10 US GPH - over double.

Apparently there is a constant, which is that to produce 1HP an engine needs to consume 0.5lb (0.2267 of a litre) of petrol but only 0.4 lb of diesel (0.18143 litre) of diesel - so to create 100 hp a petrol will use 22.67 litres per hr and a diesel 18.1 litres an hr - this is pretty much borne out by the above graphs give or take. And no matter how fancy the engine, it won't change by much,

Where it could move forward is turbocharging and downsizing - but the US boat market is way larger than Europe's and over there they won't drop CC's - they want bigger capacities.

And the Mercury Verado 200 uses 72 litres an hour flat out....... So similar to the inboard and nearly double a diesel......

So where on earth do you get your figures that the modern petrols are nearly as economical as diesels?

 

[QBhoy]

So petrol Engines (bear in mind this is AMERICAN Gallons - so multiply by

Fuel Economy

Engine	HP	Idle	1000	1500	2000	2500	3000	3500	4000	4500	4800	WOT
Volvo Penta 3.0 GL – 181 cid	135 hp	0.55	0.95	1.85	3.2	4.1	5.3	6.9	8.7	11.8		11.8 GPH
Volvo Penta 4.3 GL – 262 cid	190 hp	0.65	1.2	2.2	3.7	4.6	6.1	7.5	9.5	13	17.3	17.3 GPH
Volvo Penta 4.3 GXi – 262 cid	V6-225	0.69	1.3	2.4	3.8	5	6.75	7.5	10.3	14.5	18.1	18.1 GPH
Volvo Penta 5.0 Gi – 305 cid	V8-225	0.77	1.27	2.4	3.8	5	6.7	7.5	10.3	14.7	18.2	18.2 GPH
Volvo Penta 5.0 GXi – 305 cid	V8-270	0.82	1.3	2.5	4.1	5.8	7.5	10.4	13.7	17.8	21.4	21.4 GPH
Volvo Penta 5.7 Gi – 350 cid	V8-300	0.95	1.85	3.7	7	8.2	10.2	13.6	17.4	20.2	22.7	22.7 GPH
Volvo Penta 5.7 GXi – 350 cid	V8-320	1	1.85	3.8	7.3	8.5	10.2	13.9	17.8	20.6	23.6	23.6 GPH
Volvo Penta 6.0 – 364 cid	V8-380	0.82	1.3	2.3	3.8	6.1	7.5	9.9	12.7	15.4	18.7	30.6 GPH
Volvo Penta 8.1 Gi – 496 cid	375 hp	1.08	2	3.4	6.7	10.3	11.5	14.7	18	24.7	31.1	31.1 GPH
Volvo Penta 8.1 GXi – 496 cid	420 hp	1.15	2.1	3.7	7	8.9	11.2	14.9	18.2	26.3	34	34.0 GPH

So at 3000 rpm the 5.0GXi will use 28.3 litres per hr. I get that because of props gearing etc, the boat may be going a little quicker than with a diesel but it won't be much - the extra speed is mainly down to the petrol revving to 4800-5000 rpm and the diesel around 3750



See here
View attachment 104213

If you look - at 3000 rpm, the KAD is using 5 US GAL or 20 lit per hr and the petrol 7.5 US Gal or 28.3 litres an hr - about 50% more.

In reality the above figures are based on being at full load at each RPM - which you won't be as both will be throttled back a bit assuming you haven't got the props and everything wrong! But it's the only way to compare the two, and certainly my knowledge is that a diesel will burn a good 25-50% more than a petrol...... But if you look at MAXIMUM consumption (ok so in the petrol you will be going a lot quicker) Petrol 21.4 US GPH vs 10 US GPH - over double.

Apparently there is a constant, which is that to produce 1HP an engine needs to consume 0.5lb (0.2267 of a litre) of petrol but only 0.4 lb of diesel (0.18143 litre) of diesel - so to create 100 hp a petrol will use 22.67 litres per hr and a diesel 18.1 litres an hr - this is pretty much borne out by the above graphs give or take. And no matter how fancy the engine, it won't change by much,

Where it could move forward is turbocharging and downsizing - but the US boat market is way larger than Europe's and over there they won't drop CC's - they want bigger capacities.

And the Mercury Verado 200 uses 72 litres an hour flat out....... So similar to the inboard and nearly double a diesel......

So where on earth do you get your figures that the modern petrols are nearly as economical as diesels?

But you are comparing 270hp to 170hp there. That’s not too relevant really. I have a mpi v8. It will cruise at 2200 rpm and mid 20’s approximately mph gps, using around 17/18 litres per hour. Show me a Diesel engine of around 300hp from the same era that will do that. She will also rev to around 5150 rpm and admitted using about 80 odd litres per hour at that...but will be doing about 65-67 mph gps. Again...show me a diesel equivalent that will do that ?
You’d need to compare that 5.0 gxi to a much larger Kad 6 cylinder to be fair.

 

[Whippersnapper]

I was talking about modern engines. Modern lightweight common rail diesels aremjst

But you are comparing 270hp to 170hp there. That’s not too relevant really. I have a mpi v8. It will cruise at 2200 rpm and mid 20’s approximately mph gps, using around 17/18 litres per hour. Show me a Diesel engine of around 300hp from the same era that will do that. She will also rev to around 5150 rpm and admitted using about 80 odd litres per hour at that...but will be doing about 65-67 mph gps. Again...show me a diesel equivalent that will do that ?
You’d need to compare that 5.0 gxi to a much larger Kad 6 cylinder to be fair.

must be a very small boat if you can do mid 20s at 2200rpm. I would have thought 3000 plus for a petrol engine. My 30 footer does about 12knots at 2200rpm and it will be producing a shed load more torque at those revs than a petrol v8

 

[ontheplane]

Also is your petrol engine from that era?? You can't compare a brand new mpi with a 20 year old diesel thats ridiculous.

Also your boat does 65mph so thats where petrol do come into their own. A diesel doesn't need to produce as much power as its the torque that moves boats - so a 200hp diesel is the equivalent for cruising as a 260hp petrol, but obviously not equivalent for acceleration or top speed.

 

[ontheplane]

And as said above, we were talking cruising boats - to be getting mid 20mph at 2200 rpm you must have a light narrow boat - can't see it being a 26' 2500kg cruiser to do that.

 

[ontheplane]

So from here a D4. 260 HP

http://www.darthaven.co.uk/userfiles/products/files/Technical data D4-260I.pdf

At 2000 rpm (fair comparison as the prop / gearing will be higher due to diesels increased torque) the vp d4 would be burning 9.8 litres an our, just under half what the petrol burns.

Fact is, unless you are comparing a brand new petrol engine with a 40 year old diesel, then the diesel will always be much more efficient.

There is not a petrol made that can match a diesel for efficiency - certainly in the boating world.

 

[jrudge]

I am surprised there is any debate as to which is more efficient. Diesel will always win. The flip side is they are heavier And cost more.

 

[ontheplane]

Correct - there isn't a debate as far as I know!

I agree Petrols have their benefits - lighter for a given HP in general (although not sure that's true with the newer high powered 4cyl diesels). They are cheaper to buy and tend to give better top speed and hole-shot.

However that's where the benefit ends. For heavy cruising boats, there is no point in them - I know, been there done that bought the T-shirt.

 

[QBhoy]

I was talking about modern engines. Modern lightweight common rail diesels aremjst

must be a very small boat if you can do mid 20s at 2200rpm. I would have thought 3000 plus for a petrol engine. My 30 footer does about 12knots at 2200rpm and it will be producing a shed load more torque at those revs than a petrol v8

She’s an 18ft apex stepped Kevlar hull with a pad.