How many knots while motoring?

[a64pilot]

Originally Posted by Chotu

YOU are getting one the first invites to go for a sail on my boat. Ha ha. Send me a PM with your email address and I’ll share some info with you about her. It’s basically a Gunboat, but lighter, hopefully, by reducing the weight of interior and components apart from the hull. It’s the least I could do for the help you’ve given on my threads. Assuming Covid is over by the time she’s completely done.

As to the second half of the post, I remember that all from the other thread. I wanted to see what speeds other sailing cats with auxiliary power were motoring at. Now I see mine is the same as everyone else’s so I don’t need to change anything. It’s a battle I don’t need to fight right now. Drops any look at my propulsion to the bottom of the list.

Although the boat smj is talking about doing 14 knots with a Honda 50 is more what I had envisioned with a pair of 30’s. My 7-8 knots will just have to do for now while I work on other things.

I think and mean no disrespect that lower speeds than your expecting will be realistic. Even if you could go 15 kts motoring, it’s very likely that your fuel mileage would be 1/3 what it is at half the speed, plus you would be running the snot out of the motors with the attending noise and vibrations.

You may can even sail at 15 kts, but I’d suspect that isn’t going to happen very frequently, and even then you may find slower is much more comfortable and easier on the boat and people on board.

I bought my boat really thinking that I just wouldn’t be able to live on something that averaged 6 kts. It is the only boat I have ever owned that can’t plane and make as a min 20 kts.
I really expected to be selling it, because I would screaming bored after hours at 6 kts.

But I’ve found 6 kts is fine, it’s actually pretty relaxing and you just get used to it.

I am not saying you will have a 6 kt boat, just that I have one, but if you don’t hit your expected speeds, don’t be disappointed, it’s not all about speed. Sure speed is nice, just it’s not always going to happen.
If you have noticed even the Multi’s that can motor much faster, usually don’t , they shut down one and motor at a more relaxed speed, cutting down the engine hours by half, and burn a whole lot less fuel, and sometimes fuel burn isn’t just saving money, it’s about range.
The whole sailing / cruising thing is seemingly best done at a relaxed pace, and without a schedule.

I would put tachs on your motors and motor at 3 to 4 thousand RPM, and whatever speed that gives you, get used to it.
Plus you may find that one engine will of course be slower but may be an acceptable speed, make your engines last twice as long, burn less fuel and require half the maintenance etc.

Sometimes slow is smooth, and smooth is fast.

[MicHughV]

it's an interesting topic and I never tire or reading all the different opinions...

as a side hobby, I run a twin engined fishing boat, and this realm is also swamped with "horsepower" wars.....two engines, in the old days, meant you were the top dog...not so today...at least 4 engines required now for bragging rights...and now you can get the " supercharged versions.....1,400 hp plus on your stern....but still doesn't guarantee you will catch more fish...but hey....if burning 70 gallons/hour don't bother you....go for it....

[Statistical]

Originally Posted by MicHughV

it's an interesting topic and I never tire or reading all the different opinions...

as a side hobby, I run a twin engined fishing boat, and this realm is also swamped with "horsepower" wars.....two engines, in the old days, meant you were the top dog...not so today...at least 4 engines required now for bragging rights...and now you can get the " supercharged versions.....1,400 hp plus on your stern....but still doesn't guarantee you will catch more fish...but hey....if burning 70 gallons/hour don't bother you....go for it....

Well there is the fact that power required to increase velocity increases by the cube of the velocity. To go 50% faster doesn't require 50% more power it requires (1.5^3) = 3.375 or 275% more power. To get 50% faster than that requires 275% more power than that so if you absolutely must go a bit faster than fast you pretty quickly get to some stupidly large numbers.

I agree with you that it doesn't make a lot of sense.

The same dynamic also applies for auxiliary motors on a sailing vessel just the numbers are smaller. To push a boat at 7 knots instead of 6 knots will require about 60% more HP (and properly sized prop to handle that power at cruising rpm). If you gotta go 8 knots it will require in the ballpark of 50% more HP than going 7 knots or more than double the power to go 6 knots.

[MicHughV]

yes, lost in all these various discussions is "sea state".....your average small boat is very susceptible to various wave conditions that must be factored in besides pushing the throttle down....

at the end of the day, it's a toss up....nice to have "more power" but it comes at a $$ cost...finding that " happy medium" is an elusive target...

a top dragster can produce 1,000's of horse power....but only for a few seconds...then needs to be rebuilt....but it can do the quarter mile in 4 seconds....

you can also do your quarter mile in 30 seconds in your VW, but the engine is likely to last much longer...

either car will get you to the end though.....

[MicHughV]

like a64pilot sez.....after a while, you learn to be quite content with plodding along at 6 knots...

[Chotu]

I’m sure I’ll hit my sailing numbers. This is the most similar boat to mine, exact same construction.

The only difference is the Gunboat has less waterline length and weighs a little more.

Please take a thorough read of this. It’s exactly my boat.


https://www.katamarans.com/gunboat-48/


I am making the 7-8 knots with one 30hp outboard on, which is what everyone else is making so I’m not going to mess with that at all. I’ll just live with it and do more sailing.

My boat also eats chop for breakfast. I can tell that already from the 1000 miles she’s already traveled. Not until big, steep, uncomfortable swells does it start moving in a way that’s no fun. It can eat a huge 70ft sport fisher’s wake for breakfast too. Doesn’t even move. The two hulls cancel all that out and when riding on the tops of chop, it just stays mostly still with some jerking around.

This isn’t a regular boat at all.

Here’s one that’s around 10% longer than mine.

Please thoroughly watch this video to understand.

https://m.youtube.com/watch?v=cveZ9UsxMt0

Cheat: it does 20 knots in the video and does 8 knots with just the self tacking jib up.

Does it look really uncomfortable inside in the video at 20 knots?

Originally Posted by a64pilot

I think and mean no disrespect that lower speeds than your expecting will be realistic. Even if you could go 15 kts motoring, it’s very likely that your fuel mileage would be 1/3 what it is at half the speed, plus you would be running the snot out of the motors with the attending noise and vibrations.

You may can even sail at 15 kts, but I’d suspect that isn’t going to happen very frequently, and even then you may find slower is much more comfortable and easier on the boat and people on board.

I bought my boat really thinking that I just wouldn’t be able to live on something that averaged 6 kts. It is the only boat I have ever owned that can’t plane and make as a min 20 kts.
I really expected to be selling it, because I would screaming bored after hours at 6 kts.

But I’ve found 6 kts is fine, it’s actually pretty relaxing and you just get used to it.

I am not saying you will have a 6 kt boat, just that I have one, but if you don’t hit your expected speeds, don’t be disappointed, it’s not all about speed. Sure speed is nice, just it’s not always going to happen.
If you have noticed even the Multi’s that can motor much faster, usually don’t , they shut down one and motor at a more relaxed speed, cutting down the engine hours by half, and burn a whole lot less fuel, and sometimes fuel burn isn’t just saving money, it’s about range.
The whole sailing / cruising thing is seemingly best done at a relaxed pace, and without a schedule.

I would put tachs on your motors and motor at 3 to 4 thousand RPM, and whatever speed that gives you, get used to it.
Plus you may find that one engine will of course be slower but may be an acceptable speed, make your engines last twice as long, burn less fuel and require half the maintenance etc.

Sometimes slow is smooth, and smooth is fast.

[Chotu]

Originally Posted by MicHughV

like a64pilot sez.....after a while, you learn to be quite content with plodding along at 6 knots...

Unless you don’t have to.

I didn’t sacrifice all these years of my life and all this money to go 6 knots. LOL.

I’ll do 15-20 when conditions warrant it, or 6 knots when everyone else is motoring.


Then, when I absolutely have to turn a motor on to get into a harbor or when the wind is dead, I’ll live with the 7-8 knots I currently get from a single 30hp outboard.

[MicHughV]

well, if I had the moola...ie.....$$$$....for sure...I'd put a gunboat on my list of boats to see.....

but alas.....this is not the case...

...I'm actually thinking about getting another boat....diametrically opposed to a Gunboat....the venerable Hans Christian...I have thing about varnishing wood....a zen moment

and in my advanced age, there is something to be said about 9"high bulwarks...

[Chotu]

Originally Posted by MicHughV

well, if I had the moola...ie.....$$$$....for sure...I'd put a gunboat on my list of boats to see.....

but alas.....this is not the case...

...I'm actually thinking about getting another boat....diametrically opposed to a Gunboat....the venerable Hans Christian...I have thing about varnishing wood....a zen moment

and in my advanced age, there is something to be said about 9"high bulwarks...

I didn’t have the moolah either. So I put many years of my life, blood, sweat and tears into one.

[smj]

Originally Posted by MicHughV

well, if I had the moola...ie.....$$$$....for sure...I'd put a gunboat on my list of boats to see.....



but alas.....this is not the case...



...I'm actually thinking about getting another boat....diametrically opposed to a Gunboat....the venerable Hans Christian...I have thing about varnishing wood....a zen moment



and in my advanced age, there is something to be said about 9"high bulwarks...

9” bulwarks are a trip hazard for my size 17 feet! [emoji23]
Though I could see them as useful while heeled over.

[MicHughV]

well, I'm a post stroke person...so can't quite jump around like I used too...and my wife wants to keep me aboard...so the high bulwarks is a selling feature

I've always had a thing for these classic plastics....never thought them to be very practical, what with the long sprit, long keel, etc...but admired them nonetheless....

I've been on one offshore and there is no question it's a very able boat...

we'll see.....but I've started to look.....and going from past experience....from "look" to "purchase"....is usually a lengthy process...should be done by the time this C-19 thing has run it's course...

[MicHughV]

yep, me and moola have had a stormy relationship thru' the years....I make it, someone else takes it....and I must contend with the crumbs that fall to the floor....but it is what it is.......and I just make the best of it..... life is not always measured by the size of your bank account....in all other regards I've had a "full measure", so can't complain

[chris mac]

We aren't near that level of performance boat, however in the 1 season to bahamas and back we have sailed in double digit speeds close to a dozen times. Highest has been 13 knots(sub 20 kn winds, with gulf stream assist) In 20 to 25 kn winds, beam reach, we do 10 to 12 consistantly though. So I have no doubt you will hit 15 and over fairly easy
At 12 knots it does start to get pretty rough though. For us 9-10 is the best balance of speed and comfort

Originally Posted by Chotu

Unless you don’t have to.

I didn’t sacrifice all these years of my life and all this money to go 6 knots. LOL.

I’ll do 15-20 when conditions warrant it, or 6 knots when everyone else is motoring.


Then, when I absolutely have to turn a motor on to get into a harbor or when the wind is dead, I’ll live with the 7-8 knots I currently get from a single 30hp outboard.

[The Sun King]

I find that 2/3 of hull speed is a good economy cruise. Anything faster and the power and fuel consumption start going way up to get only a little more speed.

[HighTemp]

2019 Saona, 60 HP Volvos with folding props.

2500 RPM on one, 6.5 knts
2500 RPM on both 8.0 knts
WOT on both ( 2900 RPM ) 8.5 knts

Keep in mind we are live aboards and my wife appears to be a collector of questionable items with respect to usefulness........she is a heavy girl....the boat, not the wife.