Information on Hull shapes?

We have narrowed down our hunt and I'm trying to sort out a short list of makes and models, some are very helpful and state the design of hull in the adverts, most don't...!!! Googling produces some results but most of the time nothing definitive and sometimes conflicting information.....

Is there any list of what boat has what hull shape, we fancy a displacement hull, it's sometimes obvious from the quoted cruising speed that the hull is a planing hull, some have pictures of the boat on the hard so you can see what it's got but some brokers list them with little or no information....

One more question, I understand the differences between the Displacement, Semi-displacement and Planing hull, where does the Modified "V" hull fit in and what are it's pro's and con's, again googling provides conflicting information??

Thanks

The V modified or deep is to smash into waves at High speed and be comfortable in a $7000 Stidd seat and not break your spine.

At usually 1/2 a mile per gallon its not what folks think of when going cruising.

However many higher speed boats like old Sport Fish at low speeds make fine cruisers.

The fuel burn may be a bit higher but the all glass hull removes the horrors of endless deck leaks and rework.

We plan to spend as much time as possible on the hook and away from marinas so I wanted the stability of a full displacement, did look at some sport fishing boats to convert but they all have planing hulls and I've read are not so stable, or is that more smoke and mirrors.... Wish I could buy some experience somewhere...

I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.

Haylands said:

One more question, I understand the differences between the Displacement, Semi-displacement and Planing hull, where does the Modified "V" hull fit in and what are it's pro's and con's, again googling provides conflicting information??

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You can consider a modified V as a planing hull.

(There may be exceptions, but I'm not aware of any.)


So there are various kinds of planing hulls: V, modified V, deep V, stepped, etc. Pro and cons for each.

-Chris

ktdtx said:

I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.

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These comercial trawler cat designs dont need stab's at anchor , and can cruise at 15 kts on smaller engines.

Maybe worth considering this type of design as you don't need much load carrying capacity on a cruising trawler.





Maybe something like this with catarmaran Hull form....?

If you want better stability on the hook, look for a boat with a comparatively flat bottom and hard chines. A pure displacement hull has neither a flat(ish) bottom or hard chines. Here is an example of what I am talking about. The Kady Krogan is a pure displacement hull and the Fleming is a semi-displacement. The displacement hull will role quite a bit more than the semi-displacement hull.

ktdtx said:

I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.

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Oh, right I see....

ranger42c said:

You can consider a modified V as a planing hull.

(There may be exceptions, but I'm not aware of any.)


So there are various kinds of planing hulls: V, modified V, deep V, stepped, etc. Pro and cons for each.

-Chris

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That makes sence, thanks

Rustybarge said:

These comercial trawler cat designs dont need stab's at anchor , and can cruise at 15 kts on smaller engines.

Maybe worth considering this type of design as you don't need much load carrying capacity on a cruising trawler.

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We did look at some but they don't seem to have as much space , and most were outside our budget...

TDunn said:

If you want better stability on the hook, look for a boat with a comparatively flat bottom and hard chines. A pure displacement hull has neither a flat(ish) bottom or hard chines. Here is an example of what I am talking about. The Kady Krogan is a pure displacement hull and the Fleming is a semi-displacement. The displacement hull will role quite a bit more than the semi-displacement hull.

Click to expand...

Oh, right, that makes sense, so does that apply to all makes? are semi displacement always more stable??

And a question to those who have experience of them all, how much of a difference is there between the hull shapes, is it a massive amount or is it something that doesn't really make that much difference..??

Thanks all for taking the time to reply to all these basic questions....

ktdtx said:

I think the full displacement hulls are actually considered less stable, especially in the roll axis-which is why you find most stabilizers on full displacement hulls.

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Agree. We have owned both and the FD rolled badly at anchor while our SD boats with chines did and do much better.

FD hulls are less stable innitially but many FD hulls like my Willard have considerable ballast (2 tons in our case) so in extreme conditions the FD boats are more stable. Look at some small fishing boats. They operate on a regular basis in seas that would scare the stuff out of most of us here.

But to many w trawlers innital stability is more important. But it usually also makes for a stiff boat.

Many trawler types spend most of their time when away from docks at anchor and that is when the SD with chines can be significantly more comfortable than many FD rollers. The FD when properly designed with proper distribution of weight can and is often a better ultimate sea boat. The question I raise with modern weather forecasting and modern electronics how often do we need an ultimate sea boat? For those who want to cross oceans I would advise FD. For the typical coastal cruiser its a toss-up. If roll at anchor and in small waves is important SD has an edge. For pack rats who really weigh a boat down FD tolerates that best. With fuel economy either can do the job if designed with that in mind.

eyshulman,
I suspect those that get FD boats do so for a combination of fuel economy and general seaworthyness. Not paticularly survivability. Our Willard is very seaworthy .. not tested by me and in 5 -7' confused seas can roll a lot. But w most seas a slight course change will take care of bad rolling. A fairly heavy FD boat can be a joy compared to the snap rolling SD types. One can frequently spend the whole day in lumpy seas that would send a SD boat heading for smoother water.

The PO had a roll dampening device aboard so he must have anchored in somewhat exposed places.

[. One can frequently spend the whole day in lumpy seas that would send a SD boat heading for smoother water..[/QUOTE]

Agree and sailboats are a great example of that. Conditions that used to make for great sailing passages across the Georgian Straits are what I try to avoid. With my SD I pick my crossing weather and use that fuel guzzling speed my twins and hull allow to cross the 20k in less than 1.5 hours if need be. I have also found when things get moderately bumpy upping my speed makes for bumps but less roll that was also the case with my previous Lobster type. Once I get to quite water I go to my general cursing speed 9.2K at <5gal/Hr.

Thank you all for that, it's beginning to make more sense now....

We plan on island hopping up and down the Bahamas and Caribbean so I take it a SD hull will suit us best??

For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.

Science and theory are wonderful and I support their conclusions. However, I trust my eyes also. In eight years in the Eastern Caribbean we have seen one after another of full displacement motor boats, but only one semi-displacement motor boat, and that boat is in the charter fleet in the Virgins.

I am not counting local boats or boats over 54 feet (Flemings), nor the sport fish which show up for the tournaments.

Right now the seas between the Windward islands are averaging eight to nine feet on the beam with higher seas as you approach the island shelves. The last motor boat to pull into Saint Anne (southern Martinique crossed from St. Lucia). She is a Krogen 48 and had an "interesting ride".

The Bahamas are different as the travel is not always north south with trade winds from the east.

caltexflanc said:

For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.

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I will have a read, thanks...

Bay Pelican said:

Science and theory are wonderful and I support their conclusions. However, I trust my eyes also. In eight years in the Eastern Caribbean we have seen one after another of full displacement motor boats, but only one semi-displacement motor boat, and that boat is in the charter fleet in the Virgins.

I am not counting local boats or boats over 54 feet (Flemings), nor the sport fish which show up for the tournaments.

Right now the seas between the Windward islands are averaging eight to nine feet on the beam with higher seas as you approach the island shelves. The last motor boat to pull into Saint Anne (southern Martinique crossed from St. Lucia). She is a Krogen 48 and had an "interesting ride".

The Bahamas are different as the travel is not always north south with trade winds from the east.

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Sorry I don't fully understand this, does it make a difference when the boat is over 54"..??

And I thought the Krogen had a FD hull, do you mean a SD hull would have had even more trouble??

Please bear with me it's a steep learning curve...

If one intends to travel in areas where the seas are 8-9ft a well found sailing craft is almost always a better choice than an under 60 ft. power boat. Yes the sailboat is FD for the most part not sure how we would define high speed multihulls.

caltexflanc said:

For a good reference Dave Gerr's "The Nature of Boats" is an excellent resource for this type of thing. Gerr being an eminent naval architect, but he writes in layman's terms. Beebe's "Voyaging Under Power" also discusses hull form in some depth, though concentrates on the use of displacement hulls for long range cruising.

Click to expand...

Just ordered that book from Amazon and another by him that looked good.. Boat Mechanical Systems Handbook... Keep me busy for a few hours... Thanks for the info, they look good

eyschulman said:

If one intends to travel in areas where the seas are 8-9ft a well found sailing craft is almost always a better choice than an under 60 ft. power boat. Yes the sailboat is FD for the most part not sure how we would define high speed multihulls.

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Sailing is not for us I'm afraid, we are looking at a 50"+ (and probably nearer 60") power boat

Haylands said:

Just ordered that book from Amazon and another by him that looked good.. Boat Mechanical Systems Handbook... Keep me busy for a few hours... Thanks for the info, they look good



Sailing is not for us I'm afraid, we are looking at a 50"+ (and probably nearer 60") power boat

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Expect around 1 mpg at 7-8 knots in that size of boat; depending on the sea state.

Haylands, have you really read the book though? he covers this subject at some lengths. What's missing?

Rustybarge said:

Expect around 1 mpg at 7-8 knots in that size of boat; depending on the sea state.

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That's sort of what I expected...

Now, back to the original question, does anyone know where I can find out hull shapes of specific makes and models or is it just try google...???

Haylands said:

That's sort of what I expected...

Now, back to the original question, does anyone know where I can find out hull shapes of specific makes and models or is it just try google...???

Click to expand...

As a general rule of thumb: its the maximum speed of the hull.

7-8 kts: displacement.
15-20kts: s/d.
Over 20kts: planing.

But...

7-8kts boat which is 'also available' with big engine that gives you 40-50hp/ton is a s/d hull, or 80-100hp/ton planing hull.

Easypeasy!

Haylands said:

I will have a read, thanks...



Sorry I don't fully understand this, does it make a difference when the boat is over 54"..??

And I thought the Krogen had a FD hull, do you mean a SD hull would have had even more trouble??

Please bear with me it's a steep learning curve...

Click to expand...

55 feet was used as my cut off point as it excludes the Fleming 55 which is semi-displacement. And I am not familiar with the hull shapes of a couple of other boats that show up every year that are in the 60 foot plus range. I have even less knowledge of the 100 footers. 95%+ of the cruising boats here are in the 34 to 54 foot range.

Krogens are FD hulls. The FD hull is considered more seaworthy which is the reason it is the favorite hull form for ocean crossing.
Almost all cruising sail boats in the 34 to 54 foot range are full displacement, there are a few designed for speed with semi-displacement hulls but these are specialty items.

To simplify your choice, if you are crossing an ocean, coming to the Eastern Caribbean, or the south Pacific, get a full displacement boat. If you are coastal cruising, including an occasional trip to the Bahamas a semi-displacement hull may have advantages of speed, twin engines and multiple choices.

Although full displacement boats do tend roll more than SD boats at anchor and under power in small to medium waves on the beam (when comfort is an issue), this may not necessarily be the case in very large swells when safety is more of an issue than comfort.

Just wondering, Mr H.. with a boat of that size, would you not consider some sort of stabilizer system?

My boat rolls like a b**** in beam seas, without the sails raised; At anchor it is also can somewhat rolly, but the bilge keels help somewhat. Underway they do not help much.

With a larger FD non-sail boat, I would definitely consider either active stabilizers or paravanes if planning on regular cruising in open water.

"With a larger FD non-sail boat, I would definitely consider either active stabilizers or paravanes if planning on regular cruising in open water."

PLAN B IS A SLIGHT CHANGE OF HEADING

In open water a 5-deg change is only 5 miles off after 60 nm.

Perhaps an extra hour after 12 ,

Thanks you all for the words of wisdom....

The problem we have is we don't really know what we will be doing, the plan is to live aboard for 6 months a year to get away from the cold winters in the UK, I broke my back in 1990, it's been screwed back together, I have some nerve damage in my right leg, I've now had two new hips and a left knee that's been patched up twice and will probably need replacing soonish... it doesn't stop me doing much, I just have to do things differently... My wife is a cancer survivor and clear at the moment, we are lucky in that we can just about afford to do this now at the age of 52.... Things like this change your outlook on life and we do tend to live for the moment... we just want to get on with it...

Trouble is, we have never been boat people, probably because of circumstance more than not wanting to...

Our idea is to start off in Florida and head East, then south stopping at every opportunity to enjoy the local area then moving on when we're bored.... When we hit Venezuela we will turn round and come back.. If we buy a boat somewhere else then obviously the plan will be amended slightly... We will have no agenda so don't "have" to go anywhere at a specific time... We would find somewhere local for the boat to be put on the hard for the 6 months that we will return to the UK...

We don't know how long it will take to get to the bottom as we haven't a clue how long we will want to stay in places.... We don't fancy living in Marinas and would stay out on the hook for as long as possible....

With all that in mind and a recent trip to look at boats and see what I need (being 6"5' doesn't help either) to be able to service/fix it we have come to the conclusion that we want a 50-60ft motor boat, we don't want a fast one as the trip is as important as the location, we need one that is easy to get off and on a tender when on the hook.

We want one that is as comfy as possible, we appreciate that all boats are a compromise but want to go for the most comfortable if we can...

So what type of boat would suit us best, I don't mean the specific make and model, just the design?

There has been one last spanner in the works, our eldest daughter is now getting married early next year so we are postponing the boat buying till after then so we can concentrate on one thing at a time... That does give me the opportunity to do loads more research, but reading only tends to create more questions than answer them and nothing can replace experience... Amazon are making a fortune out of me at the moment...!!

Thanks again....

Regarding rolling while at anchor...

"We plan to spend as much time as possible on the hook and away from marinas so I wanted the stability of a full displacement, did look at some sport fishing boats to convert but they all have planing hulls and I've read are not so stable, or is that more smoke and mirrors.... Wish I could buy some experience somewhere...
__________________
Pete"


Other than manyboats briefly mentioning roll dampening devices I'm surprised there has been so little said about "flopper stoppers" for reducing roll while at anchor. They do a pretty decent job of dampening the roll while at anchor whether it be a FD or SD hull. Here is a link to a popular brand of flopper stopper:

Stabilizer

There are many other similar products and with a simple google search it will lead you to most of them.

Note: If wanting passive stabilizers for while underway and while at anchor I would look at installing paravanes.

Thanks they look like good systems, I'll have a read...

While you are logged into Amazon, pick up a copy of "Sell Up and Sail" from your countrymen Bill and Laura Cooper. They retired on a boat that they modified to their requirements which included some medical issues. Good read as well as informative.

For S&G's you could also try my personal favorite, "Away from it all" by Sloan Wilson. A well written view of life afloat by a world class writer.