Boat and engine efficiancy, drive type?

[Divemaster1]

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Long post about 70's, '80's, '90s etc... diesel engine and propulsion lost.,,,,

Basically ...

early 80's family boat >35 foot > 20 knot = diesel shaft (amongst them Ford, Perkins, Detroit etc.... DD handled the bigger market V6,V8,V12,V16 above 350 Hp..)...

late 80's family boat >35 foot > 20 knot could be sterndrive, but anything heavier (bigger) meant 300 Hp plus and shafts as in above, but MAN and MTU started to move in to DD's market

90's saw rapid development at Volvo, Mercruiser and Yanmar and they managed to get above the 250 hp diesel sterndrives .... but once again bigger = shafts

Only the latter 10 years have seen 300 hp diesels on sterndrives...

Main reason for sterndrive efficiency is:

Less drag (less stuff in the water ... shafts, P Bracket and Rudder is drag ... and it gets bigger the faster you go)

Thrust Vector (sterndrive vector thrust in direction of travel, whilst shafts angles thrust down at an angle .... more so until Propellor tunnells came into play in the boat designs ... but drag is similar...).

Then we have surface and jet drives which we are not talking about now

Basically big, older, cheaper = Shaft = Slower & less efficient .... but you can get a lot of boat for relatively low cost ....

 

[Robg71]

Some great feedback here chaps. Just the sort of points and discusion i was looking for. So many thanks.

I'm still not sure i like the idea of keeping a pair of detroits fed at 25gph at full chat.... Each engine
Maybe i should stop beeing tight fisted..

 

[Divemaster1]

Some great feedback here chaps. Just the sort of points and discusion i was looking for. So many thanks.

I'm still not sure i like the idea of keeping a pair of detroits fed at 25gph at full chat.... Each engine
Maybe i should stop beeing tight fisted..

80's Detroits burn pretty much the same as a 80's four stroke ... about 200 gramme per HP hour when in tune...(trust me ... I have two... )... trouble is that leisure boats are ran at at peak power and often over propped and over loaded... Find a good one and a engine re-build will give you engines at factory spec....

 

[rafiki_]

Outdrives transfer the thrust to the transom, hence rear drive. Shafts transfer the thrust (via the propshaft..) to the gearbox, and hence to the engines and via the engine mounts to the hull. In the middle.

Not sure I understand. I think it is the props that provide the thrust. Yes, there is torque tranfer and vectoring, but without a prop in the water, you ain't goin' nowhere

 

[Robg71]

just to clear a matter up, a discussion im having at work,
In the bumf and adverts and indeed magazine reviews, when they talk about fuel figures, that is combined for both engines.... Is it??? I hope so rather than just one of em....
Ive got twenty quid on this....

Example
SS martinique 39 (1994), twin mercruisers diesels, at 270Bhp,
3000 rpm 33 knots @16gph

That puts the detroits and cats in to perspective for me.
20 plus gph per engine..... According to my figures...

 

[Deleted User YDKXO]

Not sure I understand. I think it is the props that provide the thrust. Yes, there is torque tranfer and vectoring, but without a prop in the water, you ain't goin' nowhere

I agree with rossavage. The prop may provide the forward thrust but that thrust is transferred to the hull through the propshaft, gearbox and engine. On most shaftdrive installations, the thrust is transferred to the hull through the engine mountings which are normally more towards the middle of the boat (unless there is something like an Aquadrive coupling in the way). With a sterndrive boat, the thrust is transferred to the hull through the transom plate

 

[Divemaster1]

just to clear a matter up, a discussion im having at work,
In the bumf and adverts and indeed magazine reviews, when they talk about fuel figures, that is combined for both engines.... Is it??? I hope so rather than just one of em....
Ive got twenty quid on this....

Example
SS martinique 39 (1994), twin mercruisers diesels, at 270Bhp,
3000 rpm 33 knots @16gph

That puts the detroits and cats in to perspective for me.
20 plus gph per engine..... According to my figures...

Diesel burn on a boat is all about how many HP you take out of the engine ... irrespective of speed achieved....

I have two detroits onboard, and if I take out about 620 HP (80% load), we will burn around 31GPH combined (21 Tonne boat)..

Suspect your fuel figures are for taking out about 80% load of the available 540 hp, bringing this down to 432 Hp combined... where the SS Martinique indeed will burn about 17 GPH (think 16 is optimistic sales talk and probably will be more..).

When I take out a similar ammount (460 HP) of our 1980's Detroits, we burn between 19 & 20 GPH combined.... but feel free to add another Gallon or two ....( not another 20 )... but we don't achieve 33 knots with our 21 Tonnes and Shaft propulsion...

On a shaft driven boat ... speed = $$$$$

 

[Robg71]

Thanks bery much for that DM1
I didnt realise, exactly how un efficient shafts are then. Its not the motors being especially thirsty, but purely the amount of power thats needed to get them to shake a leg. Same engines in to same boat with stern drives should be ballistic then, by comparison.

So in actual fact, all the talk of stern drives been expensive, is actually ****e in relation to savings in fuel costs. Especially if its only 500 per year service....

You could burn that off in a week

 

[Deleted User YDKXO]

So in actual fact, all the talk of stern drives been expensive, is actually ****e in relation to savings in fuel costs. Especially if its only 500 per year service....

Its when sterndrives go wrong that they become expensive, not routine maintenance. Think sophisticated gearbox immersed in salt water. Actually £500 per leg = £1000 for both is equivalent to quite a lot of fuel and you'd have to do quite a few miles to make that up. Look, no question that sterndrives will save you fuel but you have to set that against a potential big bill if you're unlucky plus arguably more difficult slow speed handling

 

[oldgit]

Especially if its only £500 per year service....

You could burn that off in a week in extra fuel .


IF..................................................................................................nothing else needs doing.
The heart sink moment is always about 10 secs after the boat lands on the hard.
All those steering fork seals/assorted /pipes/hoses/springs/UJ/Exhaust Rubbers/rams/gizmos to tell if you legs are up down or sideways etc etc.................
and that just the stuff you can see,no hint of stuff slowly disintegrating inside.........................not until you drain the oil that is.
Watch in despair as your back in on Saturday plans crumble into dust as the bills mount up.
Still you can always help you shaft drive mates crane themselves back into the water after a quick A/F and then zoom off into the distance in their "expensive to run" boats.
Corse outdrives are cheaper to run,they are always being repaired and never go anywhere.

Hint.The only man I know who fixes this junk is down in the Med looking for a blimming great big boat..does that tell you something.

 

[Divemaster1]

Thanks bery much for that DM1
I didnt realise, exactly how un efficient shafts are then. Its not the motors being especially thirsty, but purely the amount of power thats needed to get them to shake a leg. Same engines in to same boat with stern drives should be ballistic then, by comparison.

So in actual fact, all the talk of stern drives been expensive, is actually ****e in relation to savings in fuel costs. Especially if its only 500 per year service....

You could burn that off in a week

Shats are un-economical at higher speds ... no doubt about that, but when it comes to reliability etc., there is no comparison in my subjective view ... and id' hazard to guess that your service cost is for each leg...

Also remember that any time before the mid to late 90's ... andything above 250 HP and diesel was pushing the boudaries ... That combined with the artificial push by boatbuilders to achieve the magic 30 knot barrier, meant that the larger boats were fitted with engines which would achieve that somehow ... and with less than 20 pence per litre, economy may not have been the driving force for these high end luxury items...

Ask the people around here to tell you how much time they spend cruising in the high 20's, or how ofter the weather makes them slow down to a more comfortable 18 - 22, and you may find that the boats are not cruising at their max cruising speed... but way below that...

What you need to decide is "What do i want to use the boat for?" ..... and "what conditions will I encounter?"

If space is paramount, then speed / economy will suffer.... if you want a reasonable, fast boat wth limited accommodation go for something like a Sunseeker Tomahawk 37 or 41 ...

We went for space and sea capability, with loads of boat in the water (1.59M draft), as the coastline we were to cruise on, meant that seakeeping and solidity was P1, space was P2 ... speed was in principle immaterial (but wanted 20 knot min capability)....

Good luck in your choice and if you want more than 25 knot out of a family boat in the 40 foot range, then you have to go for something newer and more $$ with Sterndrives or accept that it will cost you more $$ to run those big diesels on shafts at those speeds...

 

[Robg71]

What, looking for one with shafts

Many thanks gents. I have learnt something new

DM1
Does your figure of 200g per hp work for most diesels, give or take a few percent.

 

[Robg71]

Well, i get out of this diving chamber in 10 hours, from ther a days sail to peterhead then home.
Then a week at home to recover and going looking in earnest.
My list its getting narrower by the day.
Tomahawk not enough accomodation.
Must have good see keeping,
As fancy doing around uk sometime. No rush
Would likeit to do thirty plus knots, but am not fussed about doing that every where. Its more for cruising and living away from home. Mobile caravan esque...

 

[Latestarter1]

Diesel burn on a boat is all about how many HP you take out of the engine ... irrespective of speed achieved....

I have two detroits onboard, and if I take out about 620 HP (80% load), we will burn around 31GPH combined (21 Tonne boat)..

Suspect your fuel figures are for taking out about 80% load of the available 540 hp, bringing this down to 432 Hp combined... where the SS Martinique indeed will burn about 17 GPH (think 16 is optimistic sales talk and probably will be more..).

When I take out a similar ammount (460 HP) of our 1980's Detroits, we burn between 19 & 20 GPH combined.... but feel free to add another Gallon or two ....( not another 20 )... but we don't achieve 33 knots with our 21 Tonnes and Shaft propulsion...

On a shaft driven boat ... speed = $$$$$

I tried to stay out of this discussion as it mixed up drive efficiency with engine efficiency which is a nonsense and I violently agree with 95% of Alf's comments particulary regarding the statement regarding fuel burn and HOW MUCH YOU TAKE OUT OF ENGINES. The old propellers move boats not the engines stuff again.

The other 5%...........All about him being a stroker man which I respect, which most certainly am not, but each to their own.

 

[gjgm]

What was the root cause of the problem that resulted in the "explosion"?

Sorry..
well Volvo came down from Rugby or wherever they hang out and I think the conclusion was a tooth or more had come off one of the cogs or something and jammed the gearings, and the only way out for the power was to fire parts through the casing. Looked like something from WW2.

 

[Divemaster1]

Well, i get out of this diving chamber in 10 hours, from ther a days sail to peterhead then home. .......... Its more for cruising and living away from home. Mobile caravan esque...

Which boat are you on ?? a SS7 or any of the other DSV's ??

So speed should not be of essence, seakeeping, space and reliability is... then there is the question of canvass or hard-top / Flybridge ... this could go on and on ...

 

[rafiki_]

Sorry..
well Volvo came down from Rugby or wherever they hang out and I think the conclusion was a tooth or more had come off one of the cogs or something and jammed the gearings, and the only way out for the power was to fire parts through the casing. Looked like something from WW2.

Thanks, but there must have been a reason for this, manufacturing quality, poor maintenance, abuse etc. Of course, the same thing could happen with a gearbox.

I think all would agree that transmissions must be

1. Specified correctly to carry the engine max torque, and lifed to suit.
2. Serviced to spec
3. Inspected regularly and rectified when subject to abuse
4. Installed correctly in the boat
5. Oil cooled appropriately

There are probably several other points too, and they all apply to sterndrives, pod drives and gearboxes. With a sterndrive, clearly the sevicing is more important and more costly than with a std shaftdrive gearbox.

I personally think that Merc sterndrives are easier to check than VP's, as there is a gear oil reservoir so you can keep an eye on the oil. Single props are cheaper and easier to maintain than DP's. If (when) you ding a prop, there is 1 to repair, not 2, and the drive is more complex in a DP than SP. DP's give you better performance.

I will stand back and wait to be shot down now.

 

[Divemaster1]

..............
The other 5%...........All about him being a stroker man which I respect, which most certainly am not, but each to their own.

... Like the engines by default .. their simplicity, parts cost and maintainability suits me ....

I did choose delivberately & at a budget, and if you are buying something 20 year old plus, you need to look at the mechanics piece... and at around 400 HP plus in the early '80's there were limited choice... and you do not want a throwaway block if you can avoid it...

Newer boat would have been a different choice ... and if I ever wanted to re-engine, I would choose a low tech four stroke... like a Cummins CTA...

 

[Latestarter1]

... Like the engines by default .. their simplicity, parts cost and maintainability suits me ....

I did choose delivberately & at a budget, and if you are buying something 20 year old plus, you need to look at the mechanics piece... and at around 400 HP plus in the early '80's there were limited choice... and you do not want a throwaway block if you can avoid it...

Newer boat would have been a different choice ... and if I ever wanted to re-engine, I would choose a low tech four stroke... like a Cummins CTA...

Straight to my soft underbelly!

 

[Nick_H]

I think all would agree that transmissions must be

1. Specified correctly to carry the engine max torque, and lifed to suit.
2. Serviced to spec
3. Inspected regularly and rectified when subject to abuse
4. Installed correctly in the boat
5. Oil cooled appropriately

6. Not permanently immersed in salt water ...

I have sterndrives on the Windy and i'd never dream of having them again on any boat, unless I was dry stacking it, and even then I wouldn't be keen

At least with the higher fuel consumption of shafts, you spend the money when you use the boat, so the value and the cost are more related. Sterndrives cost even when you can't use the boat due to weather, circumstances etc, and they're also much more likely to ruin a holiday, with all the associated hassle, disappointment and costs.