Hull Speed

markpierce · Aug 5, 2013

dhiggins said:
I'm sad to say my boat has a full displ. hull. So I guess I'm stuck with the 8 kt. hull speed.

Yes. You have the wrong boat

... unless you're in a no-wake zone. :facepalm:

If it helps: I find if I look down at the water, it seems the boat is going faster than it is. :huh:

FF · Aug 8, 2013

HULL Speed is great mental masturbation but mostly useless for most marine motorists.

CO$T is the simple answer , boats that will burn minor fuel at 6K may double the burn going 7K and double it again attempting 8K.

For almost all displacement boats where the beam is about 1/3 the length , the square root of the LWL (underway) is the cheapest speed.

For a cheap 8K cruise a 64 ft LWL would be required.

Sailors may actually see or better hull speed IF its blowing hard enough,
tho most sail cruisers shorten sail for a better ride in 30K breezes.

Nomad Willy · Aug 8, 2013

dhiggins,
We can't make any really definitive response to your original post w/o seeing or knowing how you're boat is shaped below the WL especially aft. Most people don't really know the difference between a FD and SD hull and you (until we see otherwise) are probably "most people".

However if you were to say your transom is entirely out of the water we would know you have a FD hull. Even if it was double ended ... there are planing double enders so that wouldn't be conclusive.

Also if your boat can go more than one or 2 knots over hull speed it id SD. FD boats are not capable of that at all. I have a FD boat that requires doubling it's power to gain one knot over HS and I don't think doubling it again would gain another knot. To my knowledge it has never been done either.

Can you post a picture of your boat? There are instructions here on the forum on how to do that. Look at the top where it says "Forum Support"..... . I had to learn. When I started here I didn't know.

If your boat looks very much like either of these two boats in the stern it's probably FD. Ignore the keels. The rest of the boat (red bottom) is the definitive part. If it has a rather flat transom w about 6" or more submerged below the WL at rest it's probably not a FD boat. If the stern of your boat looks somewhat like the bow it is very likely a FD hull.

Your picture will tell if it's of the stern ... preferably out of the water.

brian eiland · Aug 11, 2013

manyboats said:
dhiggins,

Sounds like you have a semi disp hull if it's like a GB. The formula for HS is 1.34 X the square root of the water line length. Two feet of rake on the stem and the WLL will be 2' less than the overall length.

Semi disp hulls are much less sensitive to hull speed hydrodynamics and tend to respond more like boats called planing hulls. But some semi disp hulls like many DeFevers are quite close to FD hulls and their performance resembles FD hulls more closely. Your boat is probably a tad more like a planing hull than the DeFevers but still may be fairly close to a FD hull.

What this means to you is you can pay less attention to "hull speed" as your boat is more flexible. Like Mark says ... he runs a knot slower than HS. Actually that's the only reasonable option he has. Running HS on a FD hull is making a big wake and as Mark says burning TWICE as much fuel. Sure he can but most all knowledgable FD skippers will run one knot below HS.

Semi Displacement hulls can, rather gracefully, run one knot OVER hull speed as well as exactly at hull speed. They are designed to do so. The GBs are an excellent example. A 36 GB w 250hp will run 8 or 9 knots all the time. That's the glory of the SD hull. It goes faster and does it gracefully. Actually I'm quite sure a FD hull will burn more fuel at hull speed that a SD. Burning considerably more fuel and going more than HS is so desirable (since fuel is cheap) most all trawlers are SD as I think your's is.

It sounds like you've got a GB or a boat like a GB w a single engine. If you want (looks like it) to go over 7 knots there are lots of twin engined boat owners that are fixated on low fuel consumption that will probably trade for your more economical boat. Or do a good job of selling yours and buy a twin that will probably be cheaper. Most trawler owners and shoppers think single engined boats are better. You can use this to your advantage.

Edit:
Just read your OP. We need to know what kind of hull you have but if it's like a old GB yes you can put a bigger engine and go 8 knots or even 9 or 10 if you are willing to pay the fuel burn. If you have an Island Gypsy it may go faster than a GB. What have you got?

Hi Eric,
You seem to have a very considerable interest in this subject of hull speed.
I thought you might enjoy looking at this initial posting by a gentleman Mike D who was very well regarded, but has since passed away a number of years ago:
Hull speed - Boat Design Forums

Notes on hull speed and the gross misconceptions surrounding it. These notes concern displacement craft.

Codger2 · Aug 11, 2013

manyboats said:
Semi Displacement hulls can, rather gracefully, run one knot OVER hull speed as well as exactly at hull speed.

Not being a NA or a desire to be one, I have observed over the years that the above statement is true! At least my boat acts like that. It will go faster, however, with a different propeller or larger engine. I was told by Harvey Halvorsen a few years ago that they put a large engine in a 32' Island Gypsy and got her up to 20+ knots. :blush:

Nomad Willy · Aug 11, 2013

Walt re your boat going 20 knots ... yes she can.

Brian I think I've seen that but will study that or/and review. Yes I've always been fascinated w hull form and the dynamics. Thanks

Nomad Willy · Aug 11, 2013

Brian,
Very good indeed.especially the part about the nose getting ahead of the chin.

When I get home I'll bookmark that one to study. I don't understand enough about fourd's law. It's talked about a lot on BDesign so I need to understand it. Thanks for the link.

Brian I'm Easy Rider on BD. Would like to know your username there. PM

I have a thing called "effective hull speed" that I made up. I feel the effects of hull speed on hulls very full at the ends probably are different from hulls that are very "pointy" at the ends. A FD hull very full at the ends like some Dutch craft should have the ability to go faster (although needing more power) because the "effective" WLL is greater even though the physical WLL is the same.

Anybody see my point and have an opinion?

psneeld · Aug 11, 2013

manyboats said:
Brian,
Very good indeed.especially the part about the nose getting ahead of the chin.

When I get home I'll bookmark that one to study. I don't understand enough about fourd's law. It's talked about a lot on BDesign so I need to understand it. Thanks for the link.

Brian I'm Easy Rider on BD. Would like to know your username there. PM

I have a thing called "effective hull speed" that I made up. I feel the effects of hull speed on hulls very full at the ends probably are different from hulls that are very "pointy" at the ends. A FD hull very full at the ends like some Dutch craft should have the ability to go faster (although needing more power) because the "effective" WLL is greater even though the physical WLL is the same.

Anybody see my point and have an opinion?

Pretty sure that's Prismatic Coefficient....

here's one view of it...

"The prismatic coefficient (P.C.) is a technical term used to define how displacement is distributed along a hull, or how fine or full the ends of the hull are......"

Prismatic Coefficient - Fontaine Design Group-Super yacht Designer

like what Sceptic said....
"There is a poster here who thinks the performance of a boat is based almost entirely on the stern. He never shows any evidence that he knows anything about block, or prismatic coefficients." :socool:

Hull Speed

markpierce

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FF

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Nomad Willy

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brian eiland

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Codger2

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Nomad Willy

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psneeld

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