Semi displacement/ Modified V Hulls

[tedted]

Hello Folks, I was wondering about the terms "semi displacement" and "modified V" hull.

Being an ex sail boater I'm very familiar with the "displacement hull" The boat I'm considering is a "modified V". A term I'm hot quite clear on. Most of the trawlers I've seen list their hulls as displacement or semi displacement. The cruiser yachts around my parts fall mostly into the semi displacement category I believe. Hence their ability to plane provided they apply enough power. So where does a modified v fall into the mix?

From the looks of the bottom of the boat I'm considering (Heritage West Indian 36) I'd call it a semi displacement. But it's listed everywhere as a modified V. The forward sections to my eye appear very sail boat like. (Probably thanks to Charlie Morgan's eye). Very rounded and a lot more sculpted than the twin gas cruisers I see. The transom ends in a very shallow V. I don't see much difference in the grand scheme between it and a semi displacement hull. So, is this all just PR hype from builders or is there really something profound going on here. My guess is it's just minor variations of the same thing???

Also, if there is a big difference what effect does it have on fuel economy? Also,this boat has somewhat rounded chines, which I thought was rather odd. Especially when it is considered a "V" hull. I suspect there isn't much difference between the "semi displacement" and this "modified V" in sea keeping. Any thoughts?

 

[psneeld]

The gross generalization....


Planing (fast, lightly loaded),


Semi-displacement (sorta fast -mid teens MPH say - more load carrying capacity)


Displacement (slow - speed is abut the square root of the waterline length, heavy possibly and greater load carrying capacity than others)


Modified vee is a type of planning vessel. Each category of hull has variations on it allowing different improvements such as speed, wave handling, handling in general, roll, etc)


Don't put much faith in advertising brochures, brokerage listing descriptions, even what it is called by the owner or friends....they all could be using terminology that has been bastardized since Noah.


Describing part of the shape of the hull isn't the full story. It most likely was designed to be powered to meet a particular performance range and thus really the designer would be the best decribe the hull, though it could have been modified or the boat repowered and used differently. Some hulls are right at the transition point and whether up or down a "hull type" can't be clearly delineated except by some formula number picked by random....making them fall into a category different than a hull that is almost identical.

 

[Nomad Willy]

I don't really know what a modified V hull is.

But as Freed Z says .. here's my take.

That said all the boats I've seen called mod V were of boats having a lot of twist in the bottom. Lots of deadrise "V" fwd and less aft .. frequently lots less and even in the case of the Mainship 34 the bottom eventially bocomes flat at the transom.

The V hull was just a V hull until the "deep V" came along w constant deadrise running most of the length of the hull. That was 1961. Since that time I've heard the expression modified V and it seems to be always referring to regular V hulls. The real "modified" V hull is the deep V but in nominclature it seems the old standard V hull has become the modified V. Not modified at all. So I think it was sort of a "well that's a deep V so what's that?" .... refering to a standard V hull. Someone started saying mod V and like many things it stuck.

 

[Baker]

I think the important thing between V hull and semi planing is that the forward part of a V Hull creates a significant amount of lift whereas a semi hull creates almost no lift forward. So in a semi hull, the forward part of the hull goes THOUGH the water instead of lifting out of it. Watch a Planing V hull go through the water(larger boat). The forward part of the boat is out of the water with a higher deck angle. A semi boat will run fairly level and stay completely level in a turn while on plane. A V hull will bank into the turn.

Operationally......A V hull is more efficient on plane...for the reasons above. You are not dragging half of the boat THROUGH the water. A V hull is not that fun off of plane in that it is fairly tender since they do carry V(deadrise) all the way aft. Large V hulls aren't so tender simply because of size and weight...but it is proportional as the same size and weight semi boat would be less tender.

Semi hulls are likely (nominally)more efficient at displacement speeds and don't roll as much because of the flat after sections that resist roll. Semi hulls also provide a better ride on plane because the forward part of the boat goes THROUGH the weather instead of lifting over it and potentially pounding.

These are all just generalizations. There is a lot of grey areas over types of hulls. I have never seen a picture of the bottom aft end of a Heritage West Indian so I don't know what kind of hull it would be. I am thinking Morgan knew what he was talking about. But almost none of them were powered to plane(160hp Perkins) so I am curious why he designed it that way.

 

[FF]

The full displacement style is for ex sailors that want to be up on the boat , not down in the boat , and arev willing to go displacement speeds .

The others are an attempt to go faster and perhaps not suffer the rough ride speed causes.

1/2 to 1 NM per gallon, attempt to be on top of the water.

3 - 6 NM per gallon displacement it is.