twin engined boats economy?

[sundancer9000]

Im currently looking at buying a twin engined boat probably about two 200hp diesel engines. I was thinking how would the fuel economy be as in due to the fact that the engine revs will be lower to achieve the same speed as a single unit say a 200 hp unit? Is there a saving to be had here? Also a fireside expert lol,,informed me that i could use the P.O.S.H. method....Use one engine outward journey and the other engine for the home journey hence saving fuel and engine hours. Port out starbord home? Any help much appreciated as im sure my ignorance has humoured many here ! however as you can realise this fool isnt going to put up with fireside experts lmao at least i hope not ?

 

[blueglass]

not an engineer, but there is an issue with gearbox lubrication with some set ups. When using one engine don't forget the prop is still rotating on the sleeping engine as it is pulled through the water, and this can cause overheating of the gearbox. I do rest one engine at a time myself, having checked with qualified mechanics and on mine, hourly rotation is recommended - not your posh method.
There is a fuel saving to be made but only at lower cruising speeds.

 

[[2068]]

Im currently looking at buying a twin engined boat probably about two 200hp diesel engines. I was thinking how would the fuel economy be as in due to the fact that the engine revs will be lower to achieve the same speed as a single unit say a 200 hp unit?

You wouldn't find the same boat fitted with either two 200's or a single 200 that is going to get anywhere near the same speed, other than maybe a high performance rib. On something like an S29, Sealine offer either 2 * 190hp or one 370hp to get about 35kts. It would work with one 300hp, but you'd lose a couple of knots off the top speed. It depends on the boat, but twins can be anything from 10%-20% less efficient than the equivalent single due to the additional drag. Also, your service costs on the twin are double.

Is there a saving to be had here? Also a fireside expert lol,,informed me that i could use the P.O.S.H. method....Use one engine outward journey and the other engine for the home journey hence saving fuel and engine hours. Port out starbord home? Any help much appreciated as im sure my ignorance has humoured many here ! however as you can realise this fool isnt going to put up with fireside experts lmao at least i hope not ?

Using one engine at a time isn't easy although I have done it on long river stretches. For starters, it won't plane, so you're down to 6kts. Also, most twin setups only have one power steering pump, so steering is hard when that engine is off.

Having said all that, manouevring with twin engines is much easier. Plus you have a spare one.

dv.

 

[ulyden]

http://www.windy.no/performance_tests.html

Compare Winndy 33 with 2*D4260 against 1*D6 350 and see the difference!

You can also look at the 28 with 2*D3 160 against 1 KAD300 same power

 

[[2068]]

Hmm.

Same boat, same cruise speed, not much!
(Twin 260's at 31.8kts = 58L/H
Single 350 at 31kts = 54L/H)

 

[ulyden]

Yes! Small differnse 2*D4 against D6. To compare 520hp against 370 you can se the differnse in efficancy! The D6 uses 230g/kwh at full power and down to 210g/kwh at 2500rev propeller law 215g/kwh at 3000 revs. The same for
D4 260.

Much bigger difference on 2*D3 160 and KAD300! KAD300 is an old engine type!

 

[omega2]

We have twin 180's, in theory that should be 360, I reckon a single 360 will have the legs on us, due to the extra weight of not only the engine but the fuel tank as well, we carry independant 100 gallon tanks. Motoring on one engine is a drag in more ways than one, nothing to be gained in fact I reckon more fuel is being burned to maintain even a modest speed. The saving grace is get home ability, without a tow.

 

[Divemaster1]

Im currently looking at buying a twin engined boat probably about two 200hp diesel engines....... I was thinking how would the fuel economy be as in due to the fact that the engine revs will be lower to achieve the same speed ..........P.O.S.H. method....Use one engine outward journey and the other engine for the home journey hence saving fuel and engine hours. ........

All depends on boat configuration / engine combination etc... A Nelson 34 is VERY different from a Sealine 34.... a 200 Hp Volvo, is VERY different from a Detroit with same Hp.. so probably need more information.

However, POSH may not work... One engine may be sole provider of hydraulics, and there may be a split in which engine charges which batteries.... never mind the issues around gearbox lubrication..

What is certain is that a specific hull shape requires X Hp to be pushed through the water at Y speed (some minor differences with prop & rudder drag at higher speeds here). This means if you want to maintain Y speed, you will need to use X Hp ... that being from one engine or two. As Hp extraction and fuel consumption is directly linked, and with the above issues around hydraulics and charging, there is limited benefits in running on one, rather than two engines.... I'd rather have two well used engines, than two poorly stored.. (most leisure boat engines fails as a result of lack of use, not over use).

We run ours easily at 11 -1200 RPM (before turbo's kick in) for 8 - 20 hrs + (8 -10 knots) for maximum range, as well as at 1850 & 19 knots (we max out at 2300), for longer durations 5 hr + ... the latter is less fuel efficient, but is a compromise of speed, range and effectiveness.... it alldepends on the hull /engines / your expectations and the weather conditions..