Speed

[John_Madison CT]

So you're actually cruising at 20.1knots? That is much more inline with "normal". You're "SPEED" must be via paddlewheel, which is notoriously inaccurate.

DSCN0695.JPG

Tree I have attached a photo of the tach and speed of my normal crusing speed. Disregard the red lines on my instruments. Old school, I mark them that way as a quick reference to what the gauges are telling me. White needle past the red, take a gander.

[Serenity]

You are correct. At 3121 rpm's I am doing 20.1 knots. At 3793 rpm's I am doing 27.6 knots as per the GPS. I stand corrected I was off by 2 knots from my original statement that I almost hit 30 knots. As far as the wheel being inaccurate, both my units are quite close to readings. However, perhaps NSA is involved in GPS info. Nah, they wouldn't do that, Next time I go out, I am going to bring my handheld GPS and sit it on the engine box and see what I come up with. I still like my Albin 28 TE.

[John_Madison CT]

Fair enough! That's still excellent speed though, so be happy!

[Serenity]

Scan0004.pdf

I have attached a copy of a page out of Chapman's regarding speed. I have the same question again, how fast is the boat going? There are 2 speeds, one over ground (GPS) and through the water.

[Cape Codder]

The best/only way with an existing current is to run BOTH DIRECTIONS with GPS to get SOG ad the two speeds together and decide by 2 = true speed over ground. That is the "real" speed of your boat.

[RobS]

The best/only way with an existing current is to run BOTH DIRECTIONS with GPS to get SOG ad the two speeds together and decide by 2 = true speed over ground. That is the "real" speed of your boat.

Exactly.

[furball]

GPS gives you your true speed over ground, water or through the air. Current, wind etc are not relevant. Not in correcting GPS data. Running WITH a two knot current at 3200 rpm I get 16.2 knots on my GPS. Running AGAINST the same 2 knot current my GPS reads 12.2 knots. I'll get 14.2 knots if there is no current.
Paddle wheel or, SPEED THROUGH THE WATER, as the chart refers to it is the speed the hull is moving through the water, relative to the water, not the earth. In other words, if your GPS shows true speed over the earth as 10 knots but your moving into a 2 knot current, then your relative speed through the water is 12 knots not 10. An example where you can see how it relates to hull performance. Coming through kent narrows and into a current of about 7 knots, I can get the boat on plane but my GPS says I'm only going 5.9 knots.
Speed through the water is 12.9 knots but true GPS is still just 5.9.
John

[Serenity]

From my "research" GPS is time related. The only thing I do know, that the ship that I was on during my Naval career of 4 years was, cruised at 18 knots. Serenity does better than that, therefore all is good Jerry.

[Toolmanbob1993]

I know old thread,but it has peaked my interest.

So if i have read it all right.The only reason Serenity is running these speeds is because it is running the transmission with different ratio than most if us with Yanmars?and of course the prop

I am ready to take the plunge,i have located another transmission..i got the crew ready to pull the motor..

My question to this is when i plug the numbers into the boat diesel prop calculator with the 1.55 gear it never says these kind of speeds…i would hate to spend all that money for no speed gain at all?

So is there more to this story?

[gmberkley]

glad you brought this back up, I have never hit those speeds, not even close, so I'll be watching this post..............thanks

[gmberkley]

Saturday, August 12th, 2023, 3800 RPM's maxed out,..............15 knots, fairly flat seas, 1' or less,

Max states his RPM's are 3793 and doing 27.6 knots, clearly my boat is a dog.............something is drastically wrong

[Sisu22]

Same engine?

[Toolmanbob1993]

This was my best cruise the other day,i bumped it to 3900 one time but only saw 18 or so knots.. this was very light nowhere near the load when we go fishing

I have started researching and going down another rabbit hole. I have two huge transducers, and there’s a lot of talk that they rob a good one to three knots depending on boat.

So I think my next step is when I haul out to paint the bottom I’m gonna change those transducers

[Norseman]

Keep in mind that speed is not a good thing if you are "over-propped".
The Yanmars whether 300 or 315 are the same engine and produces the same HP
@ 3800 rpm. (Max hp @ 3,800 rpm, Max cruise @ 3,600 rpm)
To verify you are not over-propped wide open throttle should yield 3,950 to 4,050 rpm according to Yanmar.
If the boat is slow you may be under-propped.
The 2001 Albin 28TE I owned for 5 years would run 17.5 knots at the max cruise rpm with a light load after I tweaked the props a couple of times.
(Had 2 props)
Wot was up to 24 knots, but possible with a knot of current or so.
In other words, it may not be healthy to get good speed numbers if wot is 3,800 rpm, it could damage the engine in the long run.

[gmberkley]

Max speaks of Yanmar within this post, so I'm assuming he has a Yanmar, as I do, mine is an 6LPA-STP 315 HP Diesel. I run at 3,000 RPMs consistently, I only bury the throttle if I've been out for 6 plus hours fishing, and the engine has spent a lot of time idling while I drift, then I'll open it up on the way back to port for 3 - 4 minutes tops, then slowly back it down to 3,000 RPM's.