Hull Attitude

Does anyone know why all the latest motor yacht's seem to ride with the bow so high in the air that the skipper can hardly see over it ?

 

Bad design and no control over weights and centers during construction, in particular, are the usual culprits. Another could be the owners' thirst for power and speed, and trying to put too much power into the yacht than it can handle. Trim increases as the hull tries to rise over the hump on the way to plane.

Eric

 

I had thought that it was something along those lines, the boats that I am refering to are typically those from the major UK / EU manufacturers, should they not be designing better boats

 

Given that I assume you're talking about the likes of Fairline, Princess, Azimut etal - all of whom produce boats with good reputations, I would suggest that the most likely culprit is in fact the owners.
Some would be so heavily laden with junk (the boats I mean!) that they simply no longer have sufficient power to perform as they were designed, but most are simply operating their boats with ignorance. They assume that if they barge along at 15 or 18 knots they will be using less fuel than if they were doing 25. As a result the boats operate in semi-displacement mode (where planing hulls are probably at their worst) and consume enormous quantities of fuel.

 

Ah I think that would explain alot, yes I am refering to those makes and the boaters in question are likely to be the culprits, especially those on the Thames and in Monaco, probably have a few tons of booze on board

 

It's mostly a matter of owners not having a clue what they're doing. Few boaters bother to learn the physics behind their hull- if they did, they'd know that a planing hull is much safer and more efficient on a full plane than in semi-displacement speeds. There are also some yachts that are just poorly balanced, or under/over powered, and so are hairy to handle, but in my area at least it's mostly an uneducated-owner issue.

 

That ... or they are practicing for wakeboarding

 

It'a too easy to toast the operator although they are often partly to blame. I think the most important culprits are poor design and too much weight.

I recently was asked to go out with my brother in law in a boat he had just bought. It's a V hull runabout of about 18-20 degrees deadrise, about 18 feet LOA with a 100hp outboard. This boat did not drop its nose on plane until running above 22 MPH and top speed was about 28 MPH. From 7 Mph to 22 MPH she was wallowing with big wake, big fuel use and poor handling. Nothing the operator could do would solve that problem and I would rather choose to be ashore than out on the water in such a doggy boat..

The trim angle of a planing or semi-planing boat will always adjust to provide the dynamic lift required to account for the loss in immersed buoyancy by the boat trying to get on top of the water. Dynamic lift varies directly as the sine of the trim angle between the aft bottom surface and horizontal. More weight = greater trim angle. Less weight = flatter running angle. As Eric said, weight distribution also has a lot to do with it, but in a boat built heavy in the first place or had so much stuff added, it's difficult to find enough loose stuff to move.

 

l.c.b. at 40/48% l.w.l. and l.c.b. at 35/38% l.w.l.

Have a look at the static trim of the boat...you can easily immagine how it will be by running....

The boat needs more volume aft in order to shift the l.c.b. aft closer to l.c.g.

Have a look on my web about ASD hull shape for high speed craft (+40 knots)...

 

Tom,
I agree that these boats are heavy - particularly by your standards! (and mine too...). Sadly people insist on having a floating condo when they go boating - everything from aircon to trash compactors, so the reality is a heavy boat.
But, to be fair, the designers brief is generally not to provide a lightweight boat that will slip along comfortably at 15 or 18 knots. It's to design a big beefy boat that will thump its way through a slop at 25 or 30 knots, carrying all & sundry. And at doing this, most excell.
There is a fine line between giving the owner what he/she wants regardless of its effect on performance and trying to convince people that often less is more....
In a custom boat situation, it is a luxury that the designer is more likely to enjoy. But for the designer of production boats it's a different matter: the (wealthy) masses are paying for luxury and they damn well expect to get it!....

 

Will, Of course I agree with you on the matter of a designer needing to give the client what they will pay for. If they are unwilling to do that they will usually not get the customers. Still, the designer will have to educate his client on many matters regarding what is possible and good performance can be one of the education topics. After all, if the client knows that his dream waterborne condo is going to be a poor performing fuel hog, they might opt to give up at least one of the Jacuzzis.

I watch hordes of snow bird yachts going south in the fall and north in the spring on the Intra Coastal Waterway on my sailing water every year. The wake from some of these can be impressive a mile away from the offending beast. Often the first thing I notice is a big white bow wave and then that the stern is almost submerged by the stern wave. I don't care that the guy can afford the boat and its fuel bill, I still hate to see such waste or be personally inconvenienced by their excesses. They can often soak up more fuel in a single fill-up than I will use in a decade or two.

End of rant.

 

If the boaters want to cruise at 15-20 knots, isn't it wrong by the sales person to sell a boat designed for 45 knots?

 

I guess therein lies the big debate, why do people buy gas guzzlers that can do 30 knots when most of their lives they never go faster than 15 knots because any faster will spill their tea / champagne and or blow their hair out of shape.

So, are there any manufacturers out there that offer a basic boat that is lighter and cheaper than the everage floating condo and is reasonably light on fuel ?

 

This is a Norwegian project, still on the drawing board, it's a new design based on an old classic:
http://www.nes-sledeverksted.no/trebatskole/rita_35.htm

 

antonfourie - I think you miss the point. People buy boats that can do 35 knots because they can! Pipping the Jones's with a bigger, faster and more comprehensively equipped boat is nothing new....
And take the wife out on a shopping expedition for a new boat that costs more than the house that you just told her you couldn't afford to renovate. Then try and tell her that she's better off doing without a hair dryer and a microwave. And that showering very second day isn't really dirty.....
Simple fact is that these builders are producing the boats that people want to buy.
Personally, I'm in the lightweight brigade - the KISS principle is at the top of my list.
Then again, I do like a shower every day....