Cruising speeds

[Colin K]

Hello,
I wondered if I could pool from some of you more experienced motor cruiser owners? Not long had our Fairline Corniche which has twin TAMD 41a's fitted. 200hp each.
Bought her in January and took her home from the Solent to Ipswich all very enjoyable and fun however more interested in getting her home rather than putting her through her paces.
She did touch 25/26 knots when I opened her up a bit on that trip but now we have her loaded with Rib,fuel, water and my daughters make up case she seems to top out at just under 24 knots which I am pretty happy with.
We have nearly got her ready and and SWMBO is even keen! Had a sailing boat for years before and shown noooooo interest so bonus. "i think ha ha "
Hoping to do a lot of miles reaching as far as we can around the South coast and so I have got a bit of planning to do regarding fuel stops.
She seems to be happy cruising at displacement up to say 9 kts and I hope to get some early starts so I can travel a lot at this speed, I am presuming thats my most ecconomical speed?
Understand that if I want to go faster then she is ecconomically best at cruising speed and trolling the forums I think this is 13,14,15 kts ? Or when just on the plane.
I guess what I am after is whats the best way to get to this speed, do you open her up to near max (and I seem to peak at @3700rpm) and then back her off? I guess my naivety is that historically I have been used to getting smaller boats on the plane and you feel a definate "over the hump" feeling as she planes however with a bigger boat there is no "hump" as to speak and so I am curious whether if I get her to a certain speed she is definitely planing.
Thanks for any advise.

Cheers, Colin.

 

[Whitelighter]

Hi Colin,

A couple of observations:

For your hull at 36ft LOA true displacement speed will be less than 9 knots. Assuming a WLL I would guess 6.5-7knts as closer to the mark. Over that you are starting to push the hump and while it maybe comfortable you will be burning expontially more fuel for 9knts than you will be for true displacement, whatever that actually is.

As for cruising speed, for your boat 13/14/15knts is too low. The Corniche has a full planning hull and I doubt with its design and weight it will be on the plane much before 16knts. You don't mention if yours is shafts or outdrives but I would have thought a comfortable cruise in terms of boat movement and over all efficiency will be in the 18Knt-20Knt range

What you really need is someone who has had experience with the Corniche as they will give you real world experience rather than my theoretical judgement

 

[Colin K]

Hi Colin,

A couple of observations:

For your hull at 36ft LOA true displacement speed will be less than 9 knots. Assuming a WLL I would guess 6.5-7knts as closer to the mark. Over that you are starting to push the hump and while it maybe comfortable you will be burning expontially more fuel for 9knts than you will be for true displacement, whatever that actually is.

As for cruising speed, for your boat 13/14/15knts is too low. The Corniche has a full planning hull and I doubt with its design and weight it will be on the plane much before 16knts. You don't mention if yours is shafts or outdrives but I would have thought a comfortable cruise in terms of boat movement and over all efficiency will be in the 18Knt-20Knt range

What you really need is someone who has had experience with the Corniche as they will give you real world experience rather than my theoretical judgement

Hello,
Thanks for that, ah well I will get up an hour earlier to get some miles in before the family get up ha ha. Ive always liked the sunrise.
Sorry she is shaft drive I think she is at 19/20 kts at about 3300 reves from memory. She will have a clean bottom as I am being lifted the day before antifouled and back into the water props spinning ready for the off. Not that im keen!!

Cheers, Colin.

 

[James L]

Our boat is about the same length, same engines, but on outdrives and we'll just top 30 knots with a clean hull.
For economy, we go at either 7 knots or 25.

at 13 - 15 knots you can nearly see the fuel gauge moving. I would think 20/21 knots would be a good cruise if your max is 24, fast enough to get the hull well out of the water and when you hit a wave or wash you won't be knocked back off the plane.

If you have the time, you'll find this graph of revs to fuel useful, the lowest line is the one to use. Would be worth marking off the speed you get at each rev point and you can calculate your most efficient speed.
Not going to be 100% accurate, but will be close enough to help.

 

[rubberduck]

Cruising speed 80-90% of top speed.

 

[gery w]

Hi Colin are all your all your speeds taken from gps?
Best I've seen in my corniche is 25 knts s.o.g light.
My cruise speed is either 7 knts 1600rpm or 17knts 3100rpm

 

[Colin K]

Hello,
Thank you all for the advise, I would almost certainly have been cruising a bit slow and it looks like I should be looking for 20 kts or so. If I can pull 25 kts max then say 85% is 21 kts ish.
My speeds are off GPS and I use Navionics on my ipad which I think is reasonablly accurate? I will be lifting out before we set off in 2/3 weeks so will see what props I have on Gery, I know they are four blade.

Thanks again, Colin.

 

[gery w]

I had on 20x26 four blade when I bought the boat and could only manage about 3100rpm 20.5 knts wot so I had the props pitched to 20x24 to get 37-3900rpm depending on load.
Running the boat at 3500/3600 revs to get 21 knts sound sore on the engines.