How fast is fast enough

Bigplumbs

Bigplumbs

Well-Known Member
Joined
7 Nov 2015
Messages
8,221
Location
UK
Visit site
When I first started Boating I felt that I needed as much speed as possible and wanted to hit 50 mph (I have never achieved this). I now have several boats and the fastest I can achieve is about 37 knots in a 17 foot bow rider and also in my 4.5 m rib. I now realise that even this speed is not as much fun (apart from on the odd occasion) as just cruising along at between 18 and 22 knots on the plane. I think most boats have a sweet spot where they feel balanced and in tune and the engine sounds and feels 'happy''. So to any new boaters it is not all about speed it is more about that gliding along feeling in my book.

What do others think ?
 
Agree about the “ balance “ point where the boat feels right and engines feel and somehow sound “ happy “
I know scientifically the engines have no feelings or ears they cannot hear them selves .
Buy running them in the let’s call it in the 9/10 ths area for long periods imho just puts excess cylinder temps ,back to EGT ,s ( talking diesels ) ,hotter oil , strains the cooling systems internally etc etc .
Sure they can do it and do indeed but the lub and cooling systems are maxed out with little if any headroom for say a slight decrease in cooling power ,be it clogged coolers , worn impeller, loose belts , crap in the inlet side etc etc .
Oil film is pushed to the limit ,so you are really reliant on it now .Was it cheap or proper OEM spec ?
It’s at this point 9/10 th,s that oil quality counts and it’s filtration.OEM filters or cheapo eBay specials ?
Are those valves happy at 9/10 th,s rpm ? Is the cam overlap right ? Are the stems cooking oil on themselves? Rocker arms coping with the valve clearance ? Exhaust valves maintaining shape and fit at those elevated temps / pressures?

Back down a bit rpm wise then the pressure is off .
Fuel burn charts seem to be semi exponential in the sense there’s normally a short very steep bit on the rhs near the end of the “ rated “ rpm .
In 200 rpm I could be burning 100 or 140 L/ h , seems a huge jump at the end of the rev range to WOT .

Next element is the fit of the engines to the boat by the designer / builder and engine manufacturers.
Correct propping is vital so the engine sits comfortably in a well for want of a better word “ sweet spot “

That’s the cruise

It’s not the WOT ,never was .It always amazes me how Juno’s and jo public focus so much on WOT speed .
Take the FL 63 GTO , at its launch in Cannes 17 ( was in La Napoule btw for the MBY guys week before )
We ,this forum wasted huge amounts of time on was it 31.6 or maybe over 32 if better propped .
Pointless as nobodies gonna keep it on WOT all day .
I put Fwds a 6 knot arbitrarily decrease for seasons fouling and cruising stores so it’s a 25/26 knot boat in the real world.If that ?

My boats sweet spot is 1780 rpm .
Sounds nice turbos spinning , EGTs 560/575 ,depending on weight / fouling , comsuption 90 L / h , per side ,load tad under 80 % say 77 or something.Propper expensive oil taking care or the metal on metal bits .
Speed 27 /28 knots late season bit quicker after annual .

It’s fast enough is a busy area for us and mentally man maths about 30 ish miles an hour for the “ how long to X “ calcs

It’s an open and would not really want to go any faster breeze wise , mate was helming the other day and popped his head up over the windscreen and lost his hat and sunglasses .
Other thing is faster you go the shorter the fun time driving the thing .
 
Last edited:
Bigplumbs said:
I think most boats have a sweet spot
Click to expand...
Absolutely - you might even get rid of "most", in my experience.
And the differences can be huge, with P boats: I had a couple of them (a Molinari and a Fountain) which struggled to stay on the plane at anything below 22/25 knots respectively, and both really came into their own only at 30+ (the latter, all the way up to 60!).
My current boat is firmly on the plane at 15/16 kts, 22/23 @ 1800rpm being her sweet spot, burning almost exactly 7 lpm.
Otoh, sometimes I miss my old lady with her 8 kts/2 lpm sweet spot: that was as close to cruising in the silence of a sailboat as you can get on a MoBo! :rolleyes:
 
Interesting comments. We are now on our second twin diesel, the first one had total fuel burn data, the current does not.

Most efficient fuel burn we witnessed on the Bavaria (40 foot twin D4 260 - according to the fuel data) after a service and new anti foul etc was circa 56 LPH with a speed of 22 knots and revs around 2,200. When we were on a passage (in South Devon), we always tried to hit that "data" sweet spot. At the end of the season, 22 knots was achieved at higher revs and around 75 - 80 LPH

The V42 (twin D6 370s) doesn't have total fuel burn data so, as alluded to above, I drive at a speed that feels and sounds comfortable and, believe it or not, that is around 22 knots and 2,200 revs, at the beginning of the (Med) season - but this drops off as the season progresses. However, everything feels "settled" at that speed.

That said, when we are about a mile from the marina, I do tend to give it one last blast, and try and hit north of 32 knots ... just for the thrill of it :cool: ... perhaps not such a bad thing that I cant see total fuel burn :ambivalence:
 
In the tender, I can just about crack 10kts with a 5hp engine, and it feels like I'm flying.

In the SC35, 21-22kts seems about right.
 
Mine is a little bit more complicated. The hull really comes into it's own at >24knts but the engines sound most relaxed when at 22 knts 3000 rpm and funnily enough again at 3000 rpm but at 14knts when in rough water (short chop) albeit that it can be more work on the throttles to keep it at those revs and speed in those conditions

Edit. the latter either wanting to go into kickdown mode and the root chargers start hunting or reach 16 knts when suddenly she will rapidly accelerate to 22 knts.
 
Last edited:
MapisM said:
Absolutely - you might even get rid of "most", in my experience.
And the differences can be huge, with P boats: I had a couple of them (a Molinari and a Fountain) which struggled to stay on the plane at anything below 22/25 knots respectively, and both really came into their own only at 30+ (the latter, all the way up to 60!).
My current boat is firmly on the plane at 15/16 kts, 22/23 @ 1800rpm being her sweet spot, burning almost exactly 7 lpm.
Otoh, sometimes I miss my old lady with her 8 kts/2 lpm sweet spot: that was as close to cruising in the silence of a sailboat as you can get on a MoBo! :rolleyes:
Click to expand...

7 litres a minute, bloody hell!
 
Preferred cruising speed is between 35 -40 knots . But also like a bit of WOT which should be 48knots .
Fuel burn is reasonable up to 3000 Revs above that its a bit high
 
I think most boats have a sweet spot where they feel balanced
Click to expand...
Every boat I've ever owned has had a sweet spot speed or more than one sweet spot speed where the engines are humming happily and the hull is loping along nicely and everything feels right with my boating world. And yes I get more pleasure cruising at these sweet spot speeds than blatting along with my pedal to the metal. Its a mechanical sympathy thing
 
And here we have it folks, Bigplumbs new project is putting a couple of Trent's in his boat, is that with or without afterburners?

Go for afterburners as you can cook your steaks in no time.
 
A question I’ve often asked my self after looking at what I’ve spent in the never ending and now given up on pursuit of 70mph GPS.
Boat and current engine have gone as far as reasonably possible in terms or dialling everything in.
Short of a supercharger or a bigger and thirstier engine....I’m done with it, haha.
 
Deleted User said:
Every boat I've ever owned has had a sweet spot speed or more than one sweet spot speed where the engines are humming happily and the hull is loping along nicely and everything feels right with my boating world. And yes I get more pleasure cruising at these sweet spot speeds than blatting along with my pedal to the metal. Its a mechanical sympathy thing
Click to expand...

Absolutely right......
 
yes all boats have a sweet spot, (or 2)

our 26ft Karnic, although I did 35kn with her many years ago,
she starts to feal uncomfortable above 25kn, (very sensitive for roll, in bumpy sea condition. )

agree that the excitement of speed is very relative,
many of my guests don't notice much difference between D speed 10kn and P speed 20kn on our Canados,

when I was young, I was interested in the max speed of a boat.
but actually never sailed much above 25kn with none of the boats I owned ( 19ftRib, 21ft, 23ft, 26ft)
the main raison for having the more powerfull version on the models above, is not max speed, but to get easyer on the plane,

thats why we choose the exceptionally powerfull non standard version of the Karnic, with 2 x 190Hp Diesel.
she goes very fast on the plane, even with 8 divers + gear onboard !
 
You must log in or register to reply here.

Share this page

Top