Boat stability question

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kolyo

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Same model/hull type/size boats are increasingly offered by the manufacturers with an option of engine variety. Most often the choice is between an IB or OB. I'm interested how is this affecting the dynamics of the boat, particularly at anchor, in drift or at the inevitably lower speeds in rough conditions.
 
So long as you pick a motor which is within the recommended specs from manufacturer it shouldn’t be a problem.

lots of different reasons why you may prefer one or the other depending upon sise and use.
 
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May be I didn't put it clear enough but my question was which one will be more suited if the boat was used extensively in the above-mentioned conditions.
 
One of my boats is a 38ft Cigarette hull.

The beam is only 2.4m with a deep V as it is designed as an offshore racer.

Half way through the original build the owner wanted a pair of 2 Stroke V8 Evinrudes of 300hp each, being the largest outboards available around 1991.

They were mounted on 1.2m long pods attached to the transom.

The engines proved unreliable and I fitted two large Mercruiser V8s with stern drives. This is how nearly all other 38ft Cigarette boats were built.

The differences.

Surprisingly going from 600hp to 850hp didn’t increase the top speed by much. Maybe 5 knots.

With the weight of the engines much lower (outboard centre of gravity is much higher) and further forward the boat sits more level at rest and is more stable.

It is also better able to deal with large offshore waves, but that might be the extra 400kg in weight the new engines have.

With the more level attitude and the lower centre of gravity is is much better at low speed in big waves.

Hope this helps.
 
Back in the 1980/90s Fairline / Princess used to offer the 'same' boat with a variety of engine configurations:

  1. Princess 30 DS
  2. Princess 315
  3. Princess 33 (Mk1 and Mk2)
  4. Fairline Corniche
  5. Princess 35
  6. Princess 330
  7. Probably others but I'm 'geeked out'..

The general consensus is that shafts will give you better low speed handling and better, more balanced handling in rough weather as they were less stern heavy. The outdrive versions will provide higher top speed and probably better economy. There are literally thousands of web pages devoted to Outdrives vs Shafts and I'll not go into that here!
 
Kubcat said:
...With the weight of the engines much lower (outboard centre of gravity is much higher) and further forward the boat sits more level at rest and is more stable.

It is also better able to deal with large offshore waves, but that might be the extra 400kg in weight the new engines have.

With the more level attitude and the lower centre of gravity is is much better at low speed in big waves.

Hope this helps.
Click to expand...

It does help indeed. This kind of real life experience, based on trial an error, is most valuable for me. As I said, I'm interested in factors concerning boat stability, not speed or fuel economy.
 
Presumably you mean between outboard and inboard outboard stern drive configuration.
you’re right. It’s a thing we are seeing more and more these days.
The modern outboards are that good these days, that they are becoming a real alternative.
with the new mercury engines, the power they are putting out for the light weight they now are, is incredible.
I have a small fletcher 19 sports cruiser in the collection...they came with mostly inboard stern drives of between 3.0 140hp to 4.3l 190/220hp at their most powerful. The rarer option was the outboard version of up to a max of 135hp.
Mine has been blessed with a next time new 115hp mercury engine. With that on it, she will see speeds of only a few mph off what the 4.3 220 max inboard does. But using a fraction of the fuel. The smallest inboard of 135hp won’t come close to my outboard figures.
Having been on both inboard and outboard versions of these...the outboard just seems to have a much nicer attitude in the water. Down side is the loss of the full bathing platform that you’d get in the inboard version.
but the fuel consumption most of all is just not comparable between the two. These new outboard from merc are just that advanced for a marine engine, it’s ridiculous. 1.7 miles per litre of fuel at 25 mph cruising is just crazy.None of the inboard versions could come close to that. 45 mph gps at her best too. The 3.0 135hp inboard will only see late 30’s most times. It takes the 220hp V6inboard to see a couple mph more than the outboard with much less power. It’s a tough one I suppose. Some boats just look better with an inboard. But no doubt the performance gains, extra storage and economy have plenty to make you think.
 
Engines in the middle and as low as possible. Every other clever subsequent technical invention merely degraded stabilty.
You can go faster / save a tiny percentage of your total running costs / enjoy a cubic meter or two of extra internal space or have the best stability possible .
QED.
 
As per Oldgit ^^^ , He has nailed it , but you ask in your post #3
“May be I didn't put it clear enough but my question was which one will be more suited if the boat was used extensively in the above-mentioned conditions.”

Those conditions you set out in post #1 .
“I'm interested how is this affecting the dynamics of the boat, particularly at anchor, in drift or at the inevitably lower speeds in rough conditions.”

Weights not over the centre of lift ie hanging off the back like OB s and outdrives will make the ride worse , encourage proposing because at slower speeds the dynamic lift that kinda can compensate in a Ciggarette 50 with staggered stern drives or a Superhawk S0 with triple Yanmar stern mounted drives , just is not there in the quantities to make them as stable as a equivalent shaft drive boat .
Or any normal sterndrive boat in “ rough conditions “ going “ slower speeds “ This is different to them up and off cruising at design speeds whereby all the dynamic enhancements are working as they should .All planning boats get increased dynamic stability when running in the design range , but difference in total stability at speed ( you are not asking this ) is sill better in the central shaft drive boats .Markedly as the wave heights increase .They never loose that advantage in stability terms of having the main mass ( engines + tanks ) , the centre of gravity over or as close as possible to the centre of lift .CoL .
At speed in reasonable waves the design flaw s if you like of rear ward weight can be literary trimmed out .Plus any additional nuances like lifting strips and extra chine width etc can add loadsa extra dynamic stability, which it does .But again you are not asking this .

Moving the CoG rearwards away from the CoL will encourage proposing, more bobbing bout at anchor + “slow speeds “

Carlo Riva if still was with us would explain it better why he hardly ever went down the outdrive route .:) In small boats .
 
Oldest, I have a feeling you are absolutely right. However, I was surprised to see that the makers of an extremely stable boat, an inboard powered MJM, recently offered an outboard variety which seems to me just as stable??
 
I think Kolyo has asked a really good question, because logically an inboard must be best at anchor. But I can’t find any facts to support this.
 
Moving the centre of gravity from the middle to the stern of the boat can only have one outcome. Wether the boat moving or stationary.

Have no doubt that Volvo Penta are at this very moment working on something expensive,complicated and not very waterproof,to prove that merely applying a coat of green paint allows them to ignore the laws of physics.
Watch an outboard or to lesser extent outdrive boats corkscrewing when moving slowly or at anchor , when affected by wash, other than directly ahead or astern. ?
 
kolyo said:
Oldest, I have a feeling you are absolutely right. However, I was surprised to see that the makers of an extremely stable boat, an inboard powered MJM, recently offered an outboard variety which seems to me just as stable??
Click to expand...
In the new outboard powered boat, he has a Seakeeper stabilizer.
 
oldgit said:
...Watch an outboard or to lesser extent outdrive boats corkscrewing when moving slowly or at anchor , when affected by wash, other than directly ahead or astern. ?
Click to expand...

Off topic: OLDGIT, I apologize for calling you OLDEST, but I attribute this to the fact that I'm probably quite a few years (or may be decades) OLDER for my liking;)

Back on the subject: If by "corkscrewing" you mean "a floating cork behavior" the possible causes for such a phenomenon are of greatest interest to me since 90% of my tine spent at sea consists of moving slowly, in drift or at anchor.
 
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