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Old 01-12-2016, 00:35   #46
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Re: Boat Speed problems with my Hunter 386

This does seem a little slow, if my math is right. My Hunter 27 (1978) was getting between 4 and 5 kts GPS in the ICW and the St. John's River (Florida) this last spring as I brought her home from purchase, and when we hauled her out in August from the freshwater river, the three inches of barnacles were hollow shells (fresh water apparently killed them and the minnows and bluegills cleaned them out of the shells!), and mussels and other marine life were there too. We used a pressure washer which pulled the vast majority of the growth off (as well as a ton of sponge creatures) and eventually we got to the bottom paint, and then even it was falling away.

We then sanded and chipped until the hull was clean, and then started applying bottom paint. I do have a 3 bladed fixed prop, and it was not moving (engine had a broken valve spring so was not running), and I also used a sailboat shaft outboard (also an 8 hp motor like the Yanmar SB-8 inboard that is in there still) to propel her because the PO or his PO had removed the running rigging (which along with the dorm fridge, microwave, AC unit, sail suite, etc., was all still on board for the transit, along with something like six full 5 gallon gasoline cans on deck).

So even with the extra outboard drive shaft submerged and lots of cavitation (as the bracket is mounted just a touch too high and the outboard was not precisely enough angled, AND mounted to one side!), tons of bottom growth, extra weight within the cabin, two 300+ pound 6 foot new "sailors" (hey, we are working on it!) in attendance, and odd currents along the way that took us up to 5.2 knots at a high and down to 4 knots at a low outside of the St. Augustine area (Matanzas Inlet is a booger at tidal change) at varied times in increments of tenths of a knot, we were not far from the hull speed of theoretically 6+ knots on our 22 foot waterline length.

You SURELY should be getting better than that. Sailboat Data shows you have a 32 foot waterline length. You can paste the link below into your browser in another window and it will show some stats in case someone thinks the numbers I am giving need recalculating.

HUNTER 386 sailboat specifications and details on sailboatdata.com

My math shows about... 9 knots and change... for your theoretic hull speed and maybe closer to ten, I think. I have to do the exact math, but the square of the cube root of hull waterline length should give you the answer for "guessing" about the correct fluid dynamic solution for your hull speed maximum. If you get to 9 knots, you are definitely approaching where you SHOULD be, approximately, at max motoring speed (and theoretically the upper potential limit for your sailing speed, if you are not surfing a wave).

Of course, it also depends how close to wind you are sailing, etc., hence the motorized nature of this test, which should limit the windage variance issue to an extent. This formula is an approximation, and I don't recall precisely where I first heard it, but it was in a book on building boats, I am fairly sure, and I have seen it here on these forums as well. You can also find it online in various sites related to how vessels achieve their motion through the water without planing (this math is for displacement hulls like most sailboats utilize).

I will post back when I manage to get Equinox back into the water again from the yard, if you like, but I bet I am at 6 knots solidly by then, maybe a tad more (7? 7.5??). My theoretical hull speed maximum is something like 6.8 or some such. It will be interesting to see how much each engine affects this, and even what happens if I try to run both engines at once. I imagine I will get to hull speed then plant the bow firmly into the water and just be wasting horsepower and tranquility at that point rather than climb over my bow wave. But we shall see!

All that said, this mathematical tomfoolery only gets us an approximation, because I don't think it fully takes into consideration density of the medium penetrated, volume of hull under water (wetted area), our three bladed fixed propellers, my extra outboard shaft running down beside the rudder, my bent rudder post, the difference between keels (I have a shoal keel), etc., and those may be close enough to negligible to partially cancel out, but accuracy of it is negatively affected by drag caused by growth of critters on the hull, and that drag, as others noted, will make a difference.

Your mileage may differ slightly, that is my unsolicited opinion, worth precisely three times what you paid for it (or is that a third??).
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Old 02-12-2016, 14:51   #47
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Re: Boat Speed problems with my Hunter 386

Quote:
Originally Posted by SailingFan View Post
My math shows about... 9 knots and change... for your theoretic hull speed and maybe closer to ten, I think. I have to do the exact math, but the square of the cube root of hull waterline length should give you the answer for "guessing" about the correct fluid dynamic solution for your hull speed maximum. If you get to 9 knots, you are definitely approaching where you SHOULD be, approximately, at max motoring speed (and theoretically the upper potential limit for your sailing speed, if you are not surfing a wave).
I don't know where you got that formula from, but the usual formula for hull speed of this hull type is 1.34 X √LWL

In this case, with 32ft LWL, that gives a max displacement hull speed of 7.58 knots.

That's a lot higher than what Ryall is getting now ... but it's way below the 9 to 10 knots you suggested.

Note that below that limit, the relationship between speed and energy is not linear. The last knot or so before hull speed requires a lot of extra energy. So a reasonable expectation of a decent cruising speed (e.g. reach in moderate winds with all sail set) might be about 6 knots.

Ryall does need to get that hull cleaned, and possibly to learn more about sail trim. But speeds of 9 to 10 knots will be attainable only when surfing
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Old 02-12-2016, 16:13   #48
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Re: Boat Speed problems with my Hunter 386

TwoLegged, I am going to do some digging, because the additional digging I did a moment ago confirms your formula, and not the one I remember. I am going to have to look late tonight and see where I got the formula I have. I know I did not come up with it on my own, and I rarely get numbers I read someplace wrong in memory. Very rarely, in fact. It bothers me because I so rarely have that issue.

Regardless, I agree that he is slower than he would be with a good hull cleaning. Thanks for your input, because it tells me I need to do some more digging and find out where the number I have came from, and also I have found there are additional formulas out there though they are somewhat more complicated. Still, I would offer that wetted area is also very important, as drag will definitely affect final speed relative to horsepower (whether mechanically or hydrodynamically produced) applied, thus whether theoretical hull speed maximum can even be reached.

I also located this gem, something I had not anticipated anyone famous was toying with officially, until I just now saw it:

CRUNCHING NUMBERS: A Better Way to Estimate Hull Speed
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Old 03-12-2016, 15:01   #49
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Re: Boat Speed problems with my Hunter 386

Hull condition is important, but a question back to your sail trim:

Is your mainsail mast furling? Most Hunters of that size (I have the 380 2 years older than yours) have a mast furling main. With trim, you also have to flatten the luff from the sail with your boom vang in combination with the mainsheet (probably on the traveler on your arch). The bend in a B&R rig creates a good amount of luff. I have seen boom vangs set wrong preventing your ability to drop the boom enough.

And, let your topping lift get a bit loose.

Also avoid pulling the mainsheet over too far with the traveler. There's a tendency to overuse that 4.5' of traveler and pull the clew way over to point higher into the wind, but the headsail cannot accommodate what you are trying to do.
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