Boat stability question

[BruceK]

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.

I would imagine that is because when the naval architect designed the boat he had in mind outboards/ inboards/shafts and built the boat accordingly so that it balanced out and everything else people are opining here is just that. Their own little mantra or personal theory with a small minority of boats that can come with eithor / or choices. Or are you (plural) really going to look me in the eye and seriously say this boat because it has inboards and out drives and only ever came like that has a weight distribution problem the architect overlooked? Seriously? It's embarrassing is what it is.

 

[Bouba]

Perhaps the answer is to get whatever engine you want, but also get a Seakeeper

 

[Bouba]

I would imagine that is because when the naval architect designed the boat he had in mind outboards/ inboards/shafts and built the boat accordingly so that it balanced out and everything else people are opining here is just that. Their own little mantra or personal theory with a small minority of boats that can come with eithor / or choices. Or are you (plural) really going to look me in the eye and seriously say this boat because it has inboards and out drives and only ever came like that has a weight distribution problem the architect overlooked? Seriously? It's embarrassing is what it is.

I do remember that when IPS was first introduced, manufacturers were putting it on existing hulls. It was a while before a boat builder announced they have built a hull around the IPS.

 

[BruceK]

Perhaps the answer is to get whatever engine you want, but also get a Seakeeper

I'd say get a boat designed for the task. Not all boats are built equal and choice of engine and it's placement seems to be rather arbitary when looking at a premier selection.

 

[Bouba]

I'd say get a boat designed for the task. Not all boats are built equal and choice of engine and it's placement seems to be rather arbitary when looking at a premier selection.

But as hulls are getting ever lighter (and higher), surely it’s more critical

 

[BruceK]

And the architect would not be aware of this and build accordingly? That just defies credability.

 

[kolyo]

But as hulls are getting ever lighter (and higher), surely it’s more critical

Would "upgrades" like additional ballasting and/or different weight distribution help towards solving the problem?

Seakeeper - €€€ or $$$???

Edit: Got it Products | Seakeeper

 

[Bouba]

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

All of us here are old gits. But as Oldgit got the username before anyone else he probably is the oldest ?

 

[Deleted User YDKXO]

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.

You wont find any facts to support that because whether a boat is inboard, outboard, sterndrive or IPS has little or no bearing on how it behaves at anchor. First, just because the engine(s) are located aft doesnt mean that the CoG is necessarily located further aft because other weights may well have been pushed further forward in compensation (eg fuel/water/blackwater tanks, batteries, generator etc). Second, the longitudinal position of the CoG has no effect on how the boat rolls which is really the critical factor affecting how comfortable a boat is at anchor and the factors affecting that are the beam and weight of the boat, the hull design and the height of the CoG above the roll centre

 

[Bouba]

and the height of the CoG above the roll centre

Outboards hold their CoG very high, while an inboard much lower

 

[Deleted User YDKXO]

Outboards hold their CoG very high, while an inboard much lower

Yeah but once again you dont know what other weights affect height of CoG.

From personal experience, I can tell you that a Fairline Targa 48 rolls like a pig at anchor despite the fact that it has inboards and being a sports cruiser, has a relatively low CoG. On the other hand a Ferretti 46 rolls far less at anchor despite undoubtedly having a higher CoG due to the flybridge structure. The reason is entirely down to the beam. The Targa 48 has a relatively narrow beamed hull whereas the Ferretti is relatively wide beamed. Generally speaking, assuming hull design is similar, fat boats roll less. Probably the same with women

 

[kolyo]

...the longitudinal position of the CoG has no effect on how the boat rolls which is really the critical factor affecting how comfortable a boat is at anchor and the factors affecting that are the beam and weight of the boat, the hull design and the height of the CoG above the roll centre

OK, does this mean that if a stern or shaft driven boat is particularly stable by virtue of her hull design, beam size and appropriate CoG above the roll center would not suffer loss of stability in her OB variation.

 

[Portofino]

Yeah but once again you dont know what other weights affect height of CoG.

From personal experience, I can tell you that a Fairline Targa 48 rolls like a pig at anchor despite the fact that it has inboards and being a sports cruiser, has a relatively low CoG. On the other hand a Ferretti 46 rolls far less at anchor despite undoubtedly having a higher CoG due to the flybridge structure. The reason is entirely down to the beam. The Targa 48 has a relatively narrow beamed hull whereas the Ferretti is relatively wide beamed. Generally speaking, assuming hull design is similar, fat boats roll less. Probably the same with women

Hey Mike here is post #1 . Op is very specific, narrow in his question . Note the first 5 words .

“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.”

 

[kolyo]

Generally speaking, assuming hull design is similar, fat boats roll less. Probably the same with women

Absolutely true re women. I'd be happy if you are right about boats too since I do prefer beamy ones.

 

[Hambleite]

With regards to slow speed handling a scenario I can comment on as I experience this everytime I go out. My boat is a 6.5m Scorpion rib with a 150hp outboard. When travelling at 4-6 knots and if there is extra weight (people) sat on the rear jockey seats the boat will tend to 'squirrel' as it travels - the bow oscillates in yaw. However, if those same people move towards the centre of the boat then the boat will track a lot straighter, requiring less steering inputs and concentration. The boat has deep vee with an exceptionally fine bow entry.

When pootling up and down the river in calm waters this doesn't really matter but if the balance was fixed (heavy outboards that you can't move) and I was planning to be travelling in more challenging conditions then this would be a consideration.

 

[Bouba]

Can I ask a follow up question? On a IPS boat, the pods not only bring the weight very low (below the hull) but they look just like stabilizing fins (they are even at the same angle). So, are IPS boats more stable at anchor (and slow speed)?‍
I know a IPS boat rocks like crazy when docking?

 

[Portofino]

Can I ask a follow up question? On a IPS boat, the pods not only bring the weight very low (below the hull) but they look just like stabilizing fins (they are even at the same angle). So, are IPS boats more stable at anchor (and slow speed)?‍
I know a IPS boat rocks like crazy when docking?

Not really the hierarchy is central weight distribution, that CoG being as close to the centre of buoyancy and just to complicate Mike F point ( compensation of rear weight OB / OD by balancing tanks Fwds etc ) what mass you do have the more of it you can physically place near the centre the better @ anchor to avoid the dumb bell effect .In fact to visualise think yacht the weight of the mast and keel are the same dumb bell effect in the vertical plane . This having the majority of weight at each end in the horizontal plane is what I mean .

As far a any semblance to fins , such small moveable items like fins or drives or any kind , or rudders on a shaft drive are effectively pretty useless static , ie not moving .Remember fins work when working moving to conta the boat rolling .
They are ineffective against pitching for and aft in a bow to wind / wave @ anchor situation .

Digressing the two specific advantages of IPS are in certain speed ranges usually lower to middle they may be more fuel efficient, buts that’s just the reflection of lower cubic capacity pumping out lower Hp .The more Hp the more hydrocarbons are burnt up .
Space saving , the owners mid cabin thingy being larger because the machinery is further aft .
Thats it .
I have not included manoeuvring as a any 1/2 decant “normal “ helms person can execute practically the same parking etc .That’s a training issue only , you do not need IPS per se to park a boat .

Buying a IPS for extra conferred stability @ anchor is imho a fools folly .

 

[Portofino]

With regards to slow speed handling a scenario I can comment on as I experience this everytime I go out. My boat is a 6.5m Scorpion rib with a 150hp outboard. When travelling at 4-6 knots and if there is extra weight (people) sat on the rear jockey seats the boat will tend to 'squirrel' as it travels - the bow oscillates in yaw. However, if those same people move towards the centre of the boat then the boat will track a lot straighter, requiring less steering inputs and concentration. The boat has deep vee with an exceptionally fine bow entry.

When pootling up and down the river in calm waters this doesn't really matter but if the balance was fixed (heavy outboards that you can't move) and I was planning to be travelling in more challenging conditions then this would be a consideration.

Yep it’s the rearward weight and wave hitting it not perpendicular to the bow .So in a bow quarter wave the “fine entry “ lifts out too much and as the rear of the boat ( still in the water ) rolls over slightly off perpendicular , kinda rolls off the wave as it passes under it twists and the fine bow drops 15 degrees or what ever from when it lifted out .Because it’s fine , the finer it is it acts as a mini fwd keel and the track now alters 15 degrees , so the helm then turns the wheel to get it on track .
If it’s lifts with some 10 degree turn already in the same dir as the added up roll rotate and drop 15 degrees it’s now 25 degrees off plot .
Going slow in a short sharp confused chop where by its huge dynamic stability aids are pretty inefficient at low speeds .

 

[QBhoy]

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 .

I get what you’re saying about the outboard away over the back...but I know for sure that the inboard version of my boat has a 4.3l v6 engine close as dam it near the back with an alpha one outdrive right off the back. They will weigh around 300-400kgs I’d guess. My outboard weighs 165 kgs. The result is that as far as I can tell, the outboard version has a better attitude going through the chop and keeping the sharp bit of the bow in the water at displacement speeds better. The heavy inboard has a more bow up attitude usually and causing more of a slam harsh ride in the rough.

 

[kolyo]

QBhoy, are the hp ratings of either version the same? I read somewhere that an OB powered boat needs less power.