Speed

[Serenity]

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I have enclosed a photo of the prop I am currently using, It is an Dymex (Acme) 18x16x15.3. The prop has the same surface area as my old one, The new prop is on the right.

[Tree]

Thats an interesting shape Max - almost along the lines of a racing prop. Out of curiosity, what is your normal cruising speed at what RPM? General rule of thumb would be a 4 blade will give a better cruise but slower top end

[Cape Codder]

To my knowledge, a standard Hurth, with no trolling valve.

Interesting propeller design. I wonder how it would work on mine?

Last year H&H re-worked my 19x19 and feathered the cupping. Now reached max rpm with no lugging.
Yanmar warns that 6LP does not tolerate lugging well. May have lost 1-2 knots at various rpm's.

She spins up to 3900-3950 now with a little coaxing. Sounds pretty. Less smoke upon acceleration, and appears to have engine oil cleaner for much longer, which means less carbon to me.

26 knots top end

Comfortable cruise anywhere from 16 - 22kts.

[Serenity]

Tree, my normal cruising speed is between 3100-3200. That gives me 24 + knots. I only use Val Tech diesel, (no gas station fuel), only Yanmar lube and I installed a Walker Air-Sep air filter. If the weather holds up, I should be able to get a video.

[Cape Codder]

As for me...only ValvTect too. Rotella T, 15W-40. Clean bottom. 3100 is about 18kts.

[Serenity]

In regards to the speed of my Albin, I am using speed of the boat not SOG. At 29 plus knots my SOG is around 24-25 knots. Interesting thought, if the water is deeper does that have any bearing on SOG? IN other words, is SOG speed over the water or ground?

[Serenity]

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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.

[Russell]

The GPS gives you speed over the ground. A higher speed through the water suggests you are either running against a 3-4 knot current or your water speed transducer is not calibrated. If you go in the opposite direction the GPS should read higher than the speed through the water. If not, it is probably a calibration error.

[Tree]

Cheers for that Max. I'll have a better look tomorrow as I'm in bed now on my phone haha. I am convinced it's s gearbox thing and those with the hurth box and trolling valve must have a lower ratio. My buddies 2005 boat without that transmission will almost hit 25 knots also. Obviously in your case you've changed the rules with a different transmission again. Long term, I will talk to hurth to see what the options are but in the mean time with all of the input you an Bob have given and from what we know - propeller changes are on the horizon to get the best out of what's possible!

[Cape Codder]

Hmmmm,
We may be closer in speed than first thought.
SOG w/ GPS is the best constant to use.
Water-flow types are at the whim of current/tide and calibration.
With top rpm, SOG is usually 26 kts. (light load)
It's always interesting to see variation. One thing for sure. Props make the difference.

[halfmoon]

3793/1.56=2431 rpm at the prop. Most of our Yanmars will be turning 1896 rpm. Serenity is turning his prop about 500 rpms faster than we are at WOT. I hate to think of the cost of changing the Hurth reduction gearing, but that seems to be the key, not the prop. Serenity's prop is tuned for his higher rpms and he seems to have it right.

[Tree]

3793/1.56=2431 rpm at the prop. Most of our Yanmars will be turning 1896 rpm. Serenity is turning his prop about 500 rpms faster than we are at WOT. I hate to think of the cost of changing the Hurth reduction gearing, but that seems to be the key, not the prop. Serenity's prop is tuned for his higher rpms and he seems to have it right.

Just looking at this again this morning - Seems I have a 2.0:1 ratio box (HSW 600) and possibly Albins without the trolling valve gearbox are fitted with the HSW63A?

Quick research shows that the HSW63A could be converted to run a 1.5~:1 ratio and be rated at 363 bhp maximum input of 5000 rpms giving somewhat the same ratio as what Max is running. Since Max has the setup just right, those figures could be used for propeller manufacture.

This isn't something I intend to do for a while but it is something I would like to get more data and understanding with. Fuel over here in the UK even at a commercial rate is £0.73p a litre (around $4.50 per us gallon) gaining two knots over the course of a 12 month season is a big saving - possibly the cost of replacement transmission over a few years assuming fuel prices continue to rise. Besides - when you can cruise at 18 knots, why cruise at 16 etc!

[rcwhite]

Yanmar 315, 3793 rpm WOT w/75% Is this an accurate tach? If so, isn't
this slightly overpropped?

[Serenity]

The Yanmar is supposed to hit 3800. I have an Aetna electronic tach which has been photo tached.

I must admit I have been talking to people including RayMarine in regard to speed and GPS. I have spoken to professional captains as well. Speed is measured by the paddle wheel would on the transducer. IN my case I have 2, one transom mounted and the other thru hull. If I was at anchor in a 5 knot current, the "speed" of the boat should be 5 knots? As the water passes it will spin the paddle wheel to represent a 5 knot speed. The GPS will register 0 knots since the boat is not moving. Now lets assume that I am cruising at 20 knots with a 5 knot current. How fast am I going, 25 knots? It all depends what instrument you are using to measure speed. Now, if the situation were reversed, you are traveling against the current, how fast are you going. Will the GPS show 15 knots or 20 knots? I suppose the best way is to get a stop watch, a certified nautical mile a calculator and calculate speed vs time.

[rcwhite]

Before I purchased our 28 I spent a bit of time digging into the
various engine specs. One thing that came up on the Yanmar often
is it should be propped somewhere around 3900 - 4000 to
avoid overloading it. Mack Boring should be able to confirm/deny.
It is rated for full power at 3800 and continuous running at 3600. Our 28
would top out around 4000 although a couple of knots slower than
you are seeing.

Best method for determining speed is a run against the current recording
speed over ground and then turn around and run in the opposite direction.
Average the two readings and you have an good reference. In some
areas there are measured mile markers that can be used without a GPS
as well but not sure this is common any more:)