Planing Hull at Disp Speeds

A planing hull, identified by it's straight and level (or fairly close to it) Quarter Beam Buttock Line (QBBL) requires considerably more power to be driven at displacement speeds than a full displacement (FD) hull. The question is "how much more"? We all know a (FD) hull requires about 3 to 4hp per ton. Many sail boats are underpowered FD hulls and most trawlers are underpowered planing hulls (like the GBs). Some trawlers are semi-displacement hulls like the CHB. I classify a trawler w 90% as much emersed transom as a full planing hull as still a planing hull such as the early Grand Banks boats. You can't convert a planing hull to a disp hull by putting a small engine in it and a big keel. Other boats in this same category are Island Gypsy, lobsterboats, Fleming, OA, Solo, NT. Willards and Krogens are unquestionally FD. A 40' Willard requires about 24hp to make 7 knots, 37hp for 7.5 knots and 48hp to make 8 knots. 60 to 70hp is all that's required. My 30' willard has 37hp and I could do everything I normally do w 30 to 32hp max. Yet I see most trawlers draging their broad squareish sterns through the water at the same speeds Willard's and Krogen's go at power loadings of 25 to 50%. Why would a NA design a broad beamed deep stern planing type hull to go at displacement speeds and how much more power is required to do that.

Easy Rider

 

Boy, I don't know, Easy, but I struggle with dragging that water behind me in my compromise semi-displacement hull any time I slow down - one can see it. I assumed that they put enough HP in those "trawler/yachts" to get on top. No? They probably get up and moving for the brochure numbers but get weighted down and never see plane again. Kinda silly. Tons of fishing boats around these parts that NEVER get on plane - "why not design them to be more efficient?", you ask. Good question.
Lucky to have big currents up here. I try to suggest a direction for my boat to go and let the moon do the work!

 

Natural navigator

Wishfull thinking

Here (=Norwegian nothern coast) the very same broad squared sterns are typical in the modern fishing vessels. (older fleet totally FD) Their uw hullforms are planned for the semidisplacement speeds and thats what they mostly do. Most of them have relatively short distances btw their fishing banks and piers. Their loads carried stay also quite reasonable so never seen any buried their sterns too deep..

 

you put more hp in a Grand Banks and it just goes faster....

Having been on some planning hulls that once planing could be slowed up to very slow speeds yet still maintain the plane it appears its in all those details that make boat do what they do.
Does the hull have bow lift?
does it get it from small/large hard chines
how much drag in the underwater gear
is there lift from the transom
is there dead wood
does the hull have any rocker
do the hull underwater sides have rocker ( dont know what you call that but a Formula 233 has it ) You can do this with a straight keel.
How is the weight distribition
large trim tabs can work wonders here....

 

Marketing the flat, deep and dragey stern is easy. People think they need extra speed to run for cover when it gets rough. Time and time again I hear that but I've found that when it gets rough I slow down always rather than speed up. Also people know that the same power as a Volkswagen won't be enough for their 10 ton boat. People are so familiar with one vehicle (cars) they just can't get out of the box. It's no mystery why people want more, or much more power than they need. Nordic tugs have planing hulls and most 32' NT owners go forth at about 7 knots. I want to know how much fuel they burn. That would tell me how much power is being used and therefore how much penalty they are paying for not having a full displacement boat. Personal testimony is too subjective and full of variables. Even perfect consumption numbers aren't perfect as diferent engines vary .. but not much.

Easy

 

"Why would a NA design a broad beamed deep stern planing type hull to go at displacement speeds" -- Is it the NA or the marketing department at work here? How many people really know what a semi-planing or displacement hull should look like? And if it is in the water, is the difference sufficiently visible?

"I struggle with dragging that water behind me in my compromise semi-displacement hull any time I slow down" -- Semi- displ., or semi-planing, are often rather ill-defined terms. Even in this middle somewhat "gray" area, the lower or higher end of the speed spectrum may be favored. The hull may be fundamentally a displ. hull with some compromise toward attaining higher speed or may be basically a planing hull with some gesture toward lower speed operation (smaller engine).

Is it possible that planing hulls predominate, even when FD or Semi-D may be more efficient, because the boat yards are more familiar with the full planing hull? With its more constant cross sections, it may also be easier to build or to fit on a trailer. Having less immersed volume at the stern and a smaller B/L ratio may require changes to the interior layout which most customers don't appreciate.

 

As I've stated elsewhere, the original GB's were simple, hard-chined displacement hulls. The move to ever larger engines was a result of customer demand, that got to the point where GB had to change the boat to suit. One only has to look at Nordhavn, who forged a significant place in the market by producing heavy, offshore capable displacement boats. Over time, they added 'aft nacelles' to their deep buttocked, round-transom boats in order to allow them to travel at slightly higher speeds. All as a result of market forces. For many customers, who run less than 100hrs a year, the ability to travel 50% faster - albeit it at a horrendous increase in fuel use - is a more important consideration than the ultimate pursuit of efficiency.
I'm not aware of any simple means of establishing the difference between the drag of a planing hull and a displacement hull at displacement speeds. I would generally allow around 10% as a rough rule of thumb, though clearly the differences can be much more, or less, than this depending on the vessels in question

 

and
too many people want flat transoms so stern to mooring works and or you get to have a swim platform
otherwise canoe stern is better for just about everything at sea I reckon.

 

Will,
How much power do you think it take to drive a 36 and a 42' GB at 7 knots?

Easy

 

LOL,
If you use Mark's lunar navigation trick about 10 or 15 hp ought to do it for half the day. A lot of water goes up and down the hill in the Inlet!

 

Well.... funny you should ask.....
Based on the published displacement figures for both boats (26,000 & 34,000 lbs respectively) and assuming waterline lengths of 34 and 40 feet, the boats will be operating at S/L's of 1.20 and 1.11
What's interesting is that as a result, they will both require about the same power... 38hp (at the prop)

As they approach their respective hullspeeds this will increase to about 54 and 71 hp, again, at the prop

 

When was this? I worked for American Marine out in Hong Kong in the 70's and the wooden Grand Banks could certainly be described as 'wide transom and flattish buttocked'. Although most 42's left with twin Lehman Ford 120HPs, (flat out 10.5 kts, half revs; 8kts), the yard demonstrator had the larger Thorneycrofts with variable pitch propellor and planed very easily.

However at displacement speeds, all these GBs were 'interesting' in large quartering Pacific seas. That shape (essential as these boats were marketed on the attractiveness of the large aft state room), and small rudders could never be described as 'seaman like' at displacement speeds in these conditions. Unfortunately it was our standard operating conditions in winter off the eastern side of HK!

 

Mere volume sells Da Boat ,
if its pretty inside and large enough to echo,

ITS A HIT!

Seaworthy , Seakindly , Efficient , all unimportant to the look.

NA's are hired by boat yards to create boats for boat yards to SELL.

You will need to be very selective if , Seaworthy , Seakindly , Efficient are what you desire.



FF

 

Am I correct in my assumption that when traveling at lower speeds if you place the trim tabs down , depressing the bow, you are lifting the stern?

Sometimes it seems like I can tease out a little improvement with my tabs at slower speeds measured as fuel burn on a flow meter. Balancing the added drag of the tabs against the lift perhaps?

 

Makes sence to me, tho I have nothing to back it up. I've often wondered if I could trim my boat at displacement speed to get the transom out of the water enough to make a difference. Little things...(it's mostly a passenger boat) put a platter of ceviche on the forward table, put a movie on forward, move fish or fuel forward (all in nice weather, obviously), and slow down. I don't have a fuel flow meter on the new power yet so don't know.
I think Easy got most of his answer with actual GB numbers there.