Boat and engine efficiancy, drive type?

[Robg71]

Hi all
I have been doing lots and lots of research on various boats.
One thing that strikes me, is engine sizes on various similar design boats.

For example
Sunseeker rapallo. 36ft 2 x330 diesel. On shafts
Yet a similar size san remo, or martinique, gets a pair of engines down about 100 bhp. I also suspect they are down on torque as well. On outdrives. Duo prop.

Another example is the travado, lots of them with 400 bhp detroits,
Yet the Martinique 39 comes with kad 42, still down on power, with outdrives and duo props.

Are the stern drives that much more efficient, or have i missed something.
Just trying to get all the facts together in my head.
As looking at some boats with detroits, they are quite capable of demolishing 40 to 50 gallon of fuel per hour, for the pair.
I worked on a boat with detroits in the past and remember them being thirsty, but that doesnt change the performance difference between the two drive types, being over 100 bhp difference.

Any help appreciated
Rob

 

[MapisM]

Are the stern drives that much more efficient, or have i missed something.

In a nutshell, yes, they are.
Shafts are the less efficient propulsion system for a planing hull, they're widely used because of other advantages.

 

[Robg71]

So you can run 25 to 35 percent less power for the same performance??
And obviously with the gains that gives in fuel economy...

 

[hlb]

Outdrives will indead give better economy.

The down side is, they will not perform like shafts in a rough sea,

They also, will be no where near as manuverable in the marina.

They cost a fortune to service, whilst shafts cost little.

Other facts to bear in mind, boats are fitted with all sorts of different engines.

Some new owners pick engines far to small, to save cost. Only to find it uses far more fuel than an engine up to the job.

 

[Robg71]

Ahhh, more dilema's
Do i save up a bit more for the martinique 39, which is a good chunk more, but is cheaper to run, fuel wise, and has smaller engines in the first place, or stick to the original plan.
The san remo is also a lovely boat..

 

[Whitelighter]

Outdrives will indead give better economy.

The down side is, they will not perform like shafts in a rough sea,

They also, will be no where near as manuverable in the marina.

They cost a fortune to service, whilst shafts cost little.

Other facts to bear in mind, boats are fitted with all sorts of different engines.

Some new owners pick engines far to small, to save cost. Only to find it uses far more fuel than an engine up to the job.

You'll get a lot of the above. Fact is, some love shafts and hate outdrives, while others are the opposite.

Outdrives are inherently more efficient, but do cost a fair bit more to service.

As for rough weather handling, in my opnion that is just unfair/unfamiliar bias. The fact is most shaft drive boats are bigger than outdrive boats, so have more weight and are therefor more comfortable in bad weather.

Marina handling, its true that there are some outdrive boats that can be a bit of a handfull, but this is more to to with them being short tall flybridge boats and not to do with the drives. If you actually know how to drive an outdrive boat (which granted is different to a shaft drive boat) then there is no problem. I can put a outdrive powered sports boat anywhere HLB can put a shaft drive boat in the same conditions.

Where people come unstuck is trying to drive an outdrive powered boat like a shaft drive boat.

As with everything in like there are pros and cons to each type. The bigger the boat the more likely the boat will have shaft drives, sub 35ft they will almost all have stern drives and the 35-42ft range is where you have the choice.

 

[oldgit]

At displacement speeds.

It is not that difficult to get outdrives out of the water whereas it takes considerably worse conditions to get shafts clear of the water ?

 

[Robg71]

It is not that difficult to get outdrives out of the water whereas it takes considerably worse conditions to get shafts clear of the water ?

Im a diver, i can go and have a look at them

But i take your point

 

[gjgm]

Hi all
I have been doing lots and lots of research on various boats.
One thing that strikes me, is engine sizes on various similar design boats.

For example
Sunseeker rapallo. 36ft 2 x330 diesel. On shafts
Yet a similar size san remo, or martinique, gets a pair of engines down about 100 bhp. I also suspect they are down on torque as well. On outdrives. Duo prop.

Another example is the travado, lots of them with 400 bhp detroits,
Yet the Martinique 39 comes with kad 42, still down on power, with outdrives and duo props.

Are the stern drives that much more efficient, or have i missed something.
Just trying to get all the facts together in my head.
As looking at some boats with detroits, they are quite capable of demolishing 40 to 50 gallon of fuel per hour, for the pair.
I worked on a boat with detroits in the past and remember them being thirsty, but that doesnt change the performance difference between the two drive types, being over 100 bhp difference.

Any help appreciated
Rob

Also bear in mind what was technically possible back when those boats were made,as the outdrives couldnt handle the power- though I dont know if that was a limiting factor on these boats.I think it is only in the last few years they have managed to get 400hp through an outdrive, though I am sure someone more knowledgeable will correct me !

 

[Robg71]

Also bear in mind what was technically possible back when those boats were made,as the outdrives couldnt handle the power- though I dont know if that was a limiting factor on these boats.I think it is only in the last few years they have managed to get 400hp through an outdrive, though I am sure someone more knowledgeable will correct me !

Yes, i realise technology moves on. Especially where diesel technology has come from, and certainly from the likes of the first bristol diesel and the likes, that i have worked on. Its the torque that gives transmissions a hard time. The old adage, bhp sells, torque wins races.
Im looking for something around 36 to 40 foot, right in the middle ground. Is the smart money on the older boat, which is a good chunk cheaper, with the bigger capacity thirsty engines, on shafts, or the more expensive newer, smaller engines, that use considerably less juice, on out drives....
Bugger, i hate conundrums......
I may have a chat to the bank manager

 

[Deleted User YDKXO]

As for rough weather handling, in my opnion that is just unfair/unfamiliar bias. The fact is most shaft drive boats are bigger than outdrive boats, so have more weight and are therefor more comfortable in bad weather.

Its not about weight, its about where the weight is located. Outdrive boats have all their weight at the stern which means the bow is very light and IMHO is more easily pushed around by a sloppy sea. Shaftdrive boats have their engines/gearboxes located more towards the centre of the boat which means that the bow is pushed around less. IMHO thats why shaftdrive boats feel steadier in lumpy conditions

 

[gjgm]

Yes, i realise technology moves on. Especially where diesel technology has come from, and certainly from the likes of the first bristol diesel and the likes, that i have worked on. Its the torque that gives transmissions a hard time. The old adage, bhp sells, torque wins races.
Im looking for something around 36 to 40 foot, right in the middle ground. Is the smart money on the older boat, which is a good chunk cheaper, with the bigger capacity thirsty engines, on shafts, or the more expensive newer, smaller engines, that use considerably less juice, on out drives....
Bugger, i hate conundrums......
I may have a chat to the bank manager

My point was that they couldnt have got 330hp through drives on the rapallo, so it would have to be a less powerful engine on drives...
I thought detroits had a terrible reputation, but sounds like you know more about them than me anyway!
Personally, I think I would get shafts on an older boat just bcz I think you are more likely to have some grim bills on pretty old drives.

 

[Robg71]

My point was that they couldnt have got 330hp through drives on the rapallo, so it would have to be a less powerful engine on drives...
I thought detroits had a terrible reputation, but sounds like you know more about them than me anyway!
Personally, I think I would get shafts on an older boat just bcz I think you are more likely to have some grim bills on pretty old drives.

Ah, I see. Sorry, im being thick today.
The detroits we had were reliable, but they needed oil, and plenty of it. Two strokes, so not a common design for a high speed engine. They have been around since adam was a lad. Heavy, old, thirsty, leak, but sound hellish on full throttle
Hedemora were the worst i worked on, but they were a different class of engine. Ship i work on now uses wartsilia, but maybe a little big for my use

 

[MapisM]

I can put a outdrive powered sports boat anywhere HLB can put a shaft drive boat in the same conditions.

I've helmed outdrive sportboats for decades, pretty much anywhere between 20 and 40 feet, both single and twins, and with none of them I would dare competing with a twin shafts boat, when it comes to maneuverability. You must know something I don't, do tell.

Anyway, I assume that you're not considering sportboats with aggressive X dimension outdrives and surface props spinning inward. Ever tried one?

 

[Whitelighter]

No, just conventional duo prop drives ( unless you want to make it real easy and give me a set of bin lid alpha drives).

The thing I find is people with stern drive boats sticking to the 'don't touch the wheel and just oppose the engines' bull that really struggle. The beauty of stern drive handling is you can vector the thrust. If you don't use this feature then you are making it tough for yourself.

 

[MapisM]

Im looking for something around 36 to 40 foot, right in the middle ground.

You can surely find both alternatives in that size range, but the differences are more than the engine size alone.
Maintenance, seakeeping, efficiency, draught, internal accomodation, range, speed, responsiveness, maneuverability: these things (and possibly others I'm forgetting) are all affected by the propulsion choice, with the implied engine size and placement.
Rather than the propulsion, I'd try to start from the type of use planned for the boat, and the relative importance of each point - e.g. speed vs. comfort, etc.

 

[MapisM]

No, just conventional duo prop drives ( unless you want to make it real easy and give me a set of bin lid alpha drives).

LOL, naah, you can't fit cleaver props on alpha drives, you'd destroy them.
I was talking at least of Bravos with the sport master lower, or even better dry sump drives like the NXT and #6.
Boats equipped with them are, well, let's say interesting to maneuver.

Re. using the wheel to vector the thrust, of course that's something you can't do with shafts, but I wouldn't call it the "beauty" of outdrives.
In fact, that's not an additional option which you can count on (as frinstance with the thruster), but rather an additional level of complication you MUST deal with, to have a chance to place the boat where you want.

 

[volvopaul]

I'd look at a portofino400 on shafts, chris k on this forum has one, I can honestly say I have not driven an easier boat to handle, the first time he asked me to drive it was in the wind, off his mooring and out through the lock at island harbour, it was a treat to drive.

If it's a sun seeker you want thats the one, around that size and budget.

 

[Deleted User YDKXO]

Re. using the wheel to vector the thrust, of course that's something you can't do with shafts, but I wouldn't call it the "beauty" of outdrives.
In fact, that's not an additional option which you can count on (as frinstance with the thruster), but rather an additional level of complication you MUST deal with, to have a chance to place the boat where you want.

I'd agree with that. The necessity to steer as well as use the throttles on an outdrive boat at slow speed is an extra complication I don't need. Shaftdrive boats are much easier to handle at slow speed and the bow doesn't get blown about as much. Also outdrive boats tend to wander from side to side going forward at slow speed which means you need constant steering corrections and also IMHO they don't go astern as well as a shaftdrive boat. All in all, you can tell I'm not a great fan of outdrive powered boats

 

[volvopaul]

I recently drove a fair line sprint 21ft on the Trent, by about 10 mins I'd had enough! The steering system would be worn out in a year at that speed limit, and it was a duo prop leg.

An outdrive boat needs to be up to a reasonable speed to maintain a true course, a shaft drive at river speed is a delight, then again what's the point in a 200 hp 21 ft boat when you can do 6mph.