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Cruising speed

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Voyager
Master Marine Consultant

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USA
5296 Posts

Initially Posted - 07/09/2024 : 20:14:25

Last week, I spent a few days motorsailing for several hours at a time on Long Island Sound, and as I tried to make my best time and saving fuel, I finally realized something about the physics of pushing a sailboat along at hull speed using an outboard engine.
My engine mount has five positions on it: the top where I fully retract and tip up the bottom unit from the water, and the four other steps. I used to use step 3 to drive the boat, however, in choppy seas, I found that the prop came out of the water from time to time, spinning out the engine. So, just to be efficient, I began using step four on the mount to be sure that the prop ALWAYS made contact with the water.
Well, I found that when I�d get the boat up near hull speed, the bow would rise up on the bow-wave, and the stern would squat down, driving much of the long shaft part of the engine down into the water. Being stationary in the water, I realized that this obstacle was actually dragging and slowing down the boat! It then made sense to me that I should once again raise the mount to step 3.
Giving it a try, once the boat got up to speed and the stern squatted down, I found that the prop ended up in the exact right spot in the water without the rest of the engine digging in.
I found that I could gain a few tenths of a knot at the same RPM which doesn�t sound like much, but over the course of several hours, you can make a few more miles in a shorter time.
Motoring 30 nautical miles at 5.2kts takes five hours and 46 minutes. At 5.8kts, it takes five hours and five minutes.
Works for me and seems to save gas.

Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain � Milford, CT

Stinkpotter
Master Marine Consultant

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Djibouti
9039 Posts

Response Posted - 07/09/2024 : 22:26:46

It's fun to read an electrical engineer explaining mechanical engineering (fluid dynamics).

My question is, did you actually save fuel at 5.8 knots, or did it just feel like you should. I found on Passage that above about 5.5 knots, fuel consumption rose exponentially while time saved was minor (in your 30 miles, about 12%). Fuel efficiency seemed to be inversely proportional to the wake generated--when the bow raised, the transom dug and the wake grew. Of course I wasn't going to reach the "theoretical hull speed" of 6.3 under power, no matter the gas burned.

I became curious about THS and why it grows with the LWL... I've read some nonsense, but apparently it's a phenomenon having to do with bow wave length and angle due to the boat's speed, where the whole hull gets "stuck" in the trough of its own bow wake and can't go any faster unless it has enough power to climb out. The greater the LWL, the higher the speed to get the wave angle where this happens.

I've watched the slender antique "commuter boat" Aphrodite, with enough power to slice through the bow wave instead, reach amazing speeds without rising up and going on plane (with 3,000 hp burning 300 GPH). The hull speed formula (1.35 * sqrt(WLW)) does not apply to Aphrodite!

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 07/09/2024 22:38:04

OLarryR
Master Marine Consultant

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USA
3398 Posts

Response Posted - 07/10/2024 : 05:36:06

Bruce,
Interesting assessment ! My Garhauer mounting bracket has 2 lowered positions and I almost always use the lowest position since the other position will sometimes create cavitation from the prop being bit too high compared to the water level - Mostly, if I were to go into reverse or depending on the wave/rippes on the river that day. However, what you have mentioned got me thinking about what I experience. First of all, I never run my outboard at full engine speed///for whatever reasons. But what I will say is that as I approach max speed (for my hull), my mounting bracket, the lower end of the plate/pad/block that the outboard is mounted on, is slightly in or touching the water and so there is turbulence and drag created from the water and the mounting pad - I generally will run the outboard at about 3/4 engine speed which approaches but short of what would probably be my max speed...and that also minimizes the pad/block touching the water or whatever minimal drag that results. Now, whatever drag would be created would probably have minor but still some impact on gas efficiency even if I could get a bit more speed out of the hull. But I generally do not like to run at full throttle and besides that, I believe my max hull speed probably is reached before that max engine speed.

Anyway......interesting comment you have on this subject. In my situation, I wonder if I could use the next to lowest bracket position, which raises the pad/block a bit more...and then approaching max speed, my pad/block would not touch the water/create any drag...but probably a mute point since in that next to lowest bracket position, I risk cavitation depending on wave/ripples on the river.

Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html

Edited by - OLarryR on 07/10/2024 05:36:56

Steve Milby
Past Commodore

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USA
5877 Posts

Response Posted - 07/10/2024 : 07:05:33

While it seems like the stern is squatting, what's really happening is that the boat is climbing uphill. As boat speed increases to near hull speed, the bow wave moves aft. The boat tries to climb up the backside of its bow wave, but it doesn't have enough power to get all the way over it, so the bow is lifted slightly by the bow wave, and that makes it seem as if the stern is squatting. In this photo, you can see that the bow wave has moved aft about 2/3 of the way of the power boat https://media.istockphoto.com/id/139870470/photo/fast-drive.jpg?s=2048x2048&w=is&k=20&c=WRStf5-PBf2QOAlxZF4nCap1GqHizMMePc6Di1OW_dw=

As boat speed increases, the bow wave grows higher, making it even harder for the boat to climb over it. As the bow wave grows higher, the boat is pushing more water aside, and that uses more power. That power is provided by gas, so, it uses more gas to push that water aside.

If you can get the boat over the bow wave and in front of it, then the boat is pushed from behind by its own bow wave, and it begins to accelerate and get on plane. Here's a photo of a Laser planing. You can see that the bow wave has moved aft and the bow is hanging suspended over the water. When on plane, part of the bow is lifted out of the water, and that reduces the boat's wetted surface, which reduces drag. https://i.pinimg.com/originals/55/cd/6e/55cd6ece474b452afad18e23854afe01.jpg

I've never got a C25 on plane, except very briefly. It happens occasionally in a strong gust, under sail, with the assistance of a following wave to push the boat. My C&C 35 stayed on plane continuously for nearly 3 hours, and I believe it's possible on a C25 in ideal conditions. It takes nearly 30 kt steady winds on the beam, under sail, in relatively smooth waters. When I did it, it was along the western shore of the Chesapeake bay, sailing due south, with winds from the west. I believe a C25 would have done the same thing in those same conditions. I looked over the side and saw that the bow wave had moved aft all the way to the winches. Most of the hull was clearly ahead of the bow wave and boat speed was about 10 kts. In essence, the boat was on plane, surfing down the face of its own bow wave.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Edited by - Steve Milby on 07/10/2024 07:17:16

Voyager
Master Marine Consultant

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USA
5296 Posts

Response Posted - 07/10/2024 : 15:46:31

Essentially all I did was remove a big drag that opposed my forward progress by raising the engine. Not a fluid dynamics engineer, just using a little common sense. Once I removed the obstacle, I found that the boat moved a little quicker. My GPS and paddle wheel knot meter agreed on the percentage increase. I also found that my fuel consumption (1:15 hours per gallon) didn�t change while my speed increased.
While I was underway, the engine didn�t cavitate. Later, as I approached my marina, I dropped the engine into its default position to maneuver normally.