Seakeeping improvement as size increases?

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As always I am thinking about bigger boats.. Wondering about the ride as the LOA increases..

So in my current scenario we are in a 7m Merry Fisher 695.. If we compared the ride of our 7m boat to say an 8m (MF, Antares or similar), 9m and 10m boat how would the feel onboard while underway change?

I know the cruising speed would probably be lower as LOA increases but in the less than perfect conditions (swell and chop along the south coast) we often have to travel in where maximum cruise speed wouldn't be comfortably achievable would the bigger boats be significantly more comfortable anyway? As length increases does rolling, pitching, slamming and banging reduce?

Boats I am looking at as examples are MF 805/925 and Anrates 9/9.80 to stay with a similar style of boat that works well for us..
 
your cruise speed will increase generally, until you get so big that planing is impossible, your displacement speed is linked to waterline length - so displacement cruising improves a little, but the differences in ride at these sizes will mostly come down to the boat and hull design me thinks.
 
In my limited experience the beam is also a factor regarding a comfortable ride, it all about resistance to twist ie you end up with a more stable platform. In general longer boats are also wider. For me going from 10.4m x 3m to 13.6m x 4m meant SWMBO would go out in an F4
 
I had a MF645 (23ft) with Yam 115 hp which slammed a lot in lumpier solent conditions, requiring much slower speeds and 1 metre swells seemed to be about the limit. We got 29 knots out of her.
Our next MF855 (29ft) with twin Yam 150hp still slammed in the solent but was able to maintain planning speeds and coped with waves/swells of a couple of metres (when conditions deteriorated). We got 42 knots out of her.
Our current Bavaria Sport 45 (49ft) with twin VP D6 400hp has already coped admirably with 2 metre waves/swells and not really slammed yet. We rarely have to slow down. Haven't really tested top end but in region of 35 knots.
 
The modern builders trend to have 2 heavy power units hanging off the transom has everything to do with quick profits for the makers and nothing to do with the sea keeping qualities of the boat. You cannot now buy a new Merry Fisher with an inboard. All are outboard powered. Fast over the smooth but slammers in anything lumpy. The weight of a heavy 6 cylinder Volvo sitting low down in the boat driving a shaft stabilizes a boat nicely
in a sea but is a pricier to produce.
 
TonyR123 said:
Was reading another boating magazine this month and apparently the superyacht 'foners', 42m in length, has a top speed of 70knots!!
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I remember once when a tiny dot on the horizon started to loom up on me, we were in a narrow channel but I thought my 7m boat with 150hp on the back can do over 30mph I can out run her. No such luck I might as well be at anchor, I had a bad experience once in a super yacht's wake, so I prepared as best as possible. The super yacht passed me and guess what no wake! Just a perfect design, I guess, all her power goes into forward momentum.
 
You not moving up a great deal lengh wise .
Better look at hull form .like for like the deeper the Vee ,technically called deadrise at the stern ,the better the seakeeping and your ability to maintain a rate of knots up wind with out slamming and hence slowing down .
Take a look at ribs for example .
Modern low ish /moderate powered small boat hulls ,have flatter aft sections .This is the trade off flatter aft sections mean the builder can get sufficient lift from arguably a cheaper power installation .
Deeper V,s need more HP whch means more £ and equally usually more Kg ,so there's a balance .
It's tempting to go down the flatter aft section route .

Have you noticed @ boat shows ,they fit skirts around the underwater gubbins ? -IMHO. That's where you need to look 1st .

So a slightly longer but flatter aft section (deadrise ) boat will disapointed you .

By way of extreme example I anchored next to one of these last Y .
Just the two of us in a bay .
We both started up simultaneously and began the anchor wash as we up anchored .
I let him set off back to Cannes 1st kinds thought charitably I would give him head start .Sea state was choppy est 2m -3m wave s can not remember which direction .
Blimmey off he went like a scalded cat ,must have been WOT , once he got 1/2 a mile "up the road " so to speak I just could not catch him , he held his gap .

About 30 knots (out of 38 at that time of the season ) was all I could do an keep the crew happy ,comfatable etc .

How ever with a pair of 4615 + Hp and the extra L compared to my 730 hp and 14 m ,he was able to max out ,I could not .

So it's a fine combo of L as you infer ,
Deadrise as I infer
And Hp to create the missing lift from NOT having flat aft sections

Here,s a link to the B --stard boat that out ran me :) --- :):)
Take a look at the HP that's required

http://www.yachtworld.co.uk/boats/2017/Baglietto-46-FAST-3044334/Italy#.WLE6c2t5mK0
 
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We went from Weymouth to Torquay last year with an older inboard Merry 695 following us, we have a Rodman 870. The older 695 is probably around 2.5 tons whereas the Rodman is 5.5 tons so probably not the same comparison you require but regards length it is close.
We rounded the bill and headed across Lyme Bay which was lovely. Decided to fish a couple of wrecks en-route but midway across the wind had increased to a F4. A comfortable speed for us heading into that would have been 15ish knots. As we were leading/breaking water ahead of the 695 we had to reduce to speed to 10/11 knots so that they would remain in the water :-) . So I would say going from a 7m boat up to a 9 metre boat would make a largish enough difference to make it worthwhile however, as others have said the type of hull chosen would make fairly substantial difference to ride quality.
 
Thanks for all the replies.. Especially Chamaerops and maverickofpoole for the comparison details.. That's the detail I was looking for..

Might need to give some serious thought to selling the MF695.. Maybe list her and see what interest there is..
 
I went from a 585 Merry Fisher to a 7m Barracuda and the difference was obvious, especially crossing another boats wake there was less need to slow down. In our Swift Trawler34 the difference is amazing, we are comfortable in situations where I would normally tell passengers to hold on tight to something and don't put your tongue between your teeth.
Except in the rare case of a poorly designed hull, or a badly trimmed boat there is a direct comfort correlation between size of boat and size of the sea. This is regardless of hull type, size is everything.
 
The chief officer and I changed from a 25ft sports cruiser to a 34ft flybridge in 2014. Lengths overall . Both sterndrive.
Pleased we didn't go for an intermediate step to a 30 footer.
I would say the change has transformed the boating experience.
No doubt going bigger and mid engine would be even more stability .

The 34ft compared to the 25ft boat handles with greater stability and less drama - which is to some extent less exciting , less sporty. But when faced with the wake from a ship or a moderate sea a less dramatic response is welcome. Also love the flybridge and very rarely drive from the lower helm.
 
Bouba said:
I went from a 585 Merry Fisher to a 7m Barracuda and the difference was obvious, especially crossing another boats wake there was less need to slow down. In our Swift Trawler34 the difference is amazing, we are comfortable in situations where I would normally tell passengers to hold on tight to something and don't put your tongue between your teeth.
Except in the rare case of a poorly designed hull, or a badly trimmed boat there is a direct comfort correlation between size of boat and size of the sea. This is regardless of hull type, size is everything.
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I would put " hull type " in terms of deadrise more important then L . If we are only moving a couple of M ,s up .
We are not talking 8 to 80 m -if so then then L would be more important .
A 1M shorter rib with a 23 o deadrise will give a smoother ,faster upwind ride than a boat with a 16 o deadrise - both outboard -so similar CoG *

Here's the science
Effect of Hull Section on Impact Accelerations. It is rather obvious that the higher the section deadrise, the lower the impact acceleration, ** .Think of a knife Vs a tea tray hitting the water ,everything else being con- stant, the maximum impact acceleration is inversely proportional to the square of deadrise angle and hence decreases rapidly as deadrise angle is increased. Unfortunately, high deadrise hulls have higher drag in the water ,so need more Hp as illustrated in post # 8 above .

*CoG effects the trim .The more forward in true mid engined set ups ( ok rare these days with mid cabins ) the better natural trim .
Trim angle in a headsea also needs to be correct to maintain your speed .ideally zero trim so minimal drag slowing it down .its important to keep front pointy sharper bow sections in the water to slice /cut through the wave ,as oppose to the wave getting under the less sharp mid sections and tending to lift it and subsiquently drop it on the low deadrise /flat middle cum aft section - slamming and rattling out your fillings or traumatising your tongue - that bit I agree with you :)

So hanging the lumps other than in the middle ,ie at the back a la outdrives ,or further back V drives increase the probability of the hull momentarily getting out of trim in a chop ,and BANG .

You could as most do slow down of course ,after maxing out trim tabs .

Op wants to know how to go fast ,or put another way no be forced to slow down in a choppy headsea .

Just adding a couple of M ,next size up with no regard to hull form and really CoG ,as I said in post # 8 if it's actually a lower deadrise ,then he's got no where ,in terms of upwind ride /Speed .

** impact acceleration = slamming
Useful link below --- see the car anology ---
http://www.boats.com/boat-buyers-guide/what-hull-shape-is-best/
 
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Portofino said:
I would put " hull type " in terms of deadrise more important then L . If we are only moving a couple of M ,s up .
We are not talking 8 to 80 m -if so then then L would be more important .
A 1M shorter rib with a 23 o deadrise will give a smoother ,faster upwind ride than a boat with a 16 o deadrise - both outboard -so similar CoG *

Here's the science
Effect of Hull Section on Impact Accelerations. It is rather obvious that the higher the section deadrise, the lower the impact acceleration, ** .Think of a knife Vs a tea tray hitting the water ,everything else being con- stant, the maximum impact acceleration is inversely proportional to the square of deadrise angle and hence decreases rapidly as deadrise angle is increased. Unfortunately, high deadrise hulls have higher drag in the water ,so need more Hp as illustrated in post # 8 above .

*CoG effects the trim .The more forward in true mid engined set ups ( ok rare these days with mid cabins ) the better natural trim .
Trim angle in a headsea also needs to be correct to maintain your speed .ideally zero trim so minimal drag slowing it down .its important to keep front pointy sharper bow sections in the water to slice /cut through the wave ,as oppose to the wave getting under the less sharp mid sections and tending to lift it and subsiquently drop it on the low deadrise /flat middle cum aft section - slamming and rattling out your fillings or traumatising your tongue - that bit I agree with you :)

So hanging the lumps other than in the middle ,ie at the back a la outdrives ,or further back V drives increase the probability of the hull momentarily getting out of trim in a chop ,and BANG .

You could as most do slow down of course ,after maxing out trim tabs .

Op wants to know how to go fast ,or put another way no be forced to slow down in a choppy headsea .

Just adding a couple of M ,next size up with no regard to hull form and really CoG ,as I said in post # 8 if it's actually a lower deadrise ,then he's got no where ,in terms of upwind ride /Speed .

** impact acceleration = slamming
Useful link below --- see the car anology ---
http://www.boats.com/boat-buyers-guide/what-hull-shape-is-best/
Click to expand...
Well of course you are right...but on the water things are slightly different. In my example a boat slightly below 6m compared to one slightly over 7m. Here we have a mètre extra in length, half a mètre in width and double the weight on the water that is a huge difference. You will crash through waves that the smaller boat has to go over. Actual speed is hp but comfort at speed is mostly size, in the real world
 
Personal experience; 2 similar boats with the same hull design from the same manufacturer, Th 2 foot longer ( and many kg heavier) one was much better as far as sea keeping was conerned, Same weight at the stern ( V8 petrol/ DPS drive). As expected, the longer/ heavier one was not quite as fast though, not much in it ( both did over 45 mph- GPS).
 
I think with the MF type boats the outboard ones are planing and the inboard ones semi displacement (so won't have a deadrise). Anyone care to comment on what difference that would make?
 
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