A quick question on shaft driven boats

[cliffdale]

OK, I know all the pros and cons of efficiency and maintenance costs etc

Looking at KAD300s and 36 length', legs will make 40knots and the shaft version hits 30knots. I'm looking for info on displacement speed, if I make 6 knots will shaft driven boats match efficiency of leg driven boats at this speed? Or are the shaft versions always less efficient because of the drive angle?

 

[andy59]

I don't have facts to back up my opinion but I would say that similar hulls have the same displacement speed with either shafts or legs , my logic is that legs are more efficient at speed but shafts have more bite at slow speeds so they probably balance each other out .

 

[baylabayla3288]

My experience is that shafts are competitive or even more efficient at lower speeds, like for like. Also at lower planning speeds, say15 - 20 knots. Many outrive boats are almost unusable at displacement speeds due to difficulties in tracking straight. There are always exceptions but with outrives you want to go fast which is often not possible due to conditions.

 

[Seastoke]

OK, I know all the pros and cons of efficiency and maintenance costs etc

Looking at KAD300s and 36 length', legs will make 40knots and the shaft version hits 30knots. I'm looking for info on displacement speed, if I make 6 knots will shaft driven boats match efficiency of leg driven boats at this speed? Or are the shaft versions always less efficient because of the drive angle?

well shaft all day ,reasons speed 6 knots only in the marina , and legs no control at 6 knots ,2 if u want 6 knots why not get a raggi or single engine mobo ,bit of a daft thread if u ask me ,but u did not ,so good luck

 

[superheat6k]

Most boats that can do 30 or even 40 knots will rarely go above 20 -25 because few owners like being able to watch the fuel gauges actually move towards empty. Besides anything above 25 is going to be borderline dangerous in the Solent on a typical weekend.

Shaft every time and definitely for displacement speed cruising, which is most of my cruising now. Nice to have a blast for a few miles, but for me 8-10 knots, including a planned for favourable tidal flow off the plane is just great. I can steer effectively down to 4 knots or 3 knots with one engine in neutral, but without playing the engines.

 

[cliffdale]

well shaft all day ,reasons speed 6 knots only in the marina , and legs no control at 6 knots ,2 if u want 6 knots why not get a raggi or single engine mobo ,bit of a daft thread if u ask me ,but u did not ,so good luck

Reason for asking

We have just sold our raggy, swmbo hated it, previous boat was a targa 34.

I love sailing and more than happy to plod along at 5 knots with a fishing line trolling behind

Thinking about the French canals

Plus, im retired and have more time to enjoy plodding along than swmbo who has to work!

When shmbo is on board, we belt somewhere and anchor or tie up

 

[Seastoke]

Reason for asking

We have just sold our raggy, swmbo hated it, previous boat was a targa 34.

I love sailing and more than happy to plod along at 5 knots with a fishing line trolling behind

Thinking about the French canals

Plus, im retired and have more time to enjoy plodding along than swmbo who has to work!

When shmbo is on board, we belt somewhere and anchor or tie up

while she works ,never b happy but at 34 how can she retire

 

[cliffdale]

She has to keep me in the manner I am accustomed to.

God, if she read this i'm a dead man walking!

 

[Momac]

I would say at slow speeds there is probably nothing in it between shafts and legs as far a fuel consumption is concerned.
And if mostly going slow the fuel cost is possibly not going to be your greatest expense in a years boating.

 

[oldgit]

Claimed savings in fuel produced from various back to back tests between legs and shafts are always taken from the point of the top end performance of boat.
Very frequently at or near WOT, outdrives appear to give around 10- 15% saving from the boat tests available on the web.
The other advantage is greater top speed compared to a inboard arrangement.
However all these advantages are fine if you are able to use them.
If your boating only ever involves leaving your berth and seconds later spend the time at WOT until your tanks run dry,then all well and good.
However back in the real world..........
Start up to warm engine,getting out of your finger berth,creeping out of the marina,the slow run down to where ever the speed limit stops and then probably finding conditions are simply not suitable for that teeth rattling WOT 20 min blast,probably only round the the next bay, will most likely to be the norm.

Having owned a motley selection of outdrive boards in the past and God forbid, actually defended them on this very forum, It came as some suprise, after getting a shaft driven boat,there was virtually no difference in my fuel costs.
Really do miss peering over the stern wondering why the leg has not lifted/gone down or exactly where the iridescent sheen of oil/hydraulic fluid trailing behind me has come from.